What is mechanical design?
Mechanical design is the design of parts, components, products, or systems of a mechanical nature. For example, mechanical design includes the design of various machine elements such as shafts, bearings, couplings, gears, and fasteners.
Mechanical design is the process of conceptualizing, analyzing, and calculating the working principle, structure, movement, transmission of forces and energy, materials, and shapes of individual parts, lubrication methods, etc., according to the requirements of the machine and transforming them into concrete descriptions to be used as a basis for manufacturing.
Mechanical design is an important part of mechanical engineering, the first step in the production of machinery and the most important factor in determining its performance. The goal of mechanical design is to design the best possible machine under a variety of constraints (e.g. materials, processing capabilities, theoretical knowledge, and calculation tools), i.e. to make an optimized design.
Optimal design requires a comprehensive consideration of many requirements, generally the best working performance, the lowest manufacturing costs, the smallest size and weight, the most reliable in use, the lowest consumption, and the least environmental pollution.
These requirements are often in conflict with each other and their relative importance varies depending on the type of machinery and its use. The designer’s task is to weigh up the priorities according to the specific situation and to take into account the overall situation so that the designed machinery has the best overall technical and economic effect.
In the past, design optimization relied mainly on the designer’s knowledge, experience, and vision. With the development of basic mechanical engineering theory and value engineering, systems analysis, and other new disciplines, the accumulation of technical and economic data on manufacturing and use, as well as the promotion and application of computers, optimization gradually abandoned subjective judgment and relied on scientific calculations.
The design of industrial machinery, especially the overall and whole system of machinery design, must be attached to the relevant industrial technology, and difficult to form an independent discipline. Therefore, the design of agricultural machinery, mining machinery design, pump design, compressor design, turbine design, internal combustion engine design, machine tool design, and other specialized mechanical design sub-disciplines.
What are the mechanical design categories?
Mechanical design can be divided into new mechanical design, inherited mechanical design, and variant mechanical design in 3 categories.
New type of mechanical design
The application of mature science and technology or after the experimental proof is feasible new technology, design of new mechanical design in the past has not been.
Inherited mechanical design
According to the use of experience and technical development of existing machinery design updates to improve its performance, reduce its manufacturing costs or reduce its operating costs.
Variant mechanical design
To adapt to the new needs of the existing machinery for partial modification or addition or deletion and the development of different from the standard type of variant products.
What are the steps of the mechanical design?
The quality of a machine is basically determined by the quality of the design. The role of the manufacturing process in the quality of the machine essentially lies in achieving the quality specified in the design. The design phase of a machine is therefore the key to its quality.
The mechanical design process discussed refers only to the design process in a narrow technical sense. It is a creative working process and at the same time one that makes use of as much of the existing successful experience as possible. A good combination of inheritance and innovation is required to design a high-quality machine.
As a complete machine, it is a complex system. A scientific design process is necessary to improve the quality of the design. Although it is not possible to list a single procedure that is valid in all cases, the design procedure for a machine can be basically as follows, based on the long experience of people designing machines.
The following is a brief description of each mechanical design step.
First. Mechanical design planning
In the mechanical design planning phase, the requirements of the machine to be designed are fully investigated and analyzed.
Through this analysis, the functions of the machine are further defined and constraints are set out for subsequent decisions in terms of environment, economy, processing, and time frame.
On this basis, the comprehensive requirements and details of the design task are clearly written out, resulting in a design brief that serves as a summary of this phase.
The mechanical design brief should include, in general, an estimate of the machine’s function, economy, and environmental friendliness, a general estimate of the manufacturing requirements, basic usage requirements, and an estimated time frame for the completion of the design task.
At this point, these requirements and conditions can also generally only be given as a reasonable range, not as an exact figure. For example, they can be determined in terms of requirements that must be met, minimum requirements, desired requirements, etc.
Second. Mechanical design program design
Depending on the working principle, a number of different specific solutions for the actuator can be drawn up.
For example, in the case of thread cutting, it is possible to use a workpiece that only moves in a rotary motion and a tool that moves in a straight line to cut threads (e.g. cutting threads on an ordinary lathe) or to make the workpiece immobile and the tool rotate and move to cut threads (e.g. machining threads with plate teeth).
This means that even for the same principle of operation, there can be several different construction solutions.
There are of course also a number of options for the engine section. Due to the widespread availability of electricity and the development of electric drag technology, it can now be said that the majority of stationary machines prefer electric motors as the prime mover section.
Thermodynamic prime movers are mainly used for transport, construction, or agricultural machinery. Even when an electric motor is used as the prime mover, there are still options for AC and DC, high and low speeds, etc.
The options for the transmission section are even more complex and varied. For the same transmission task, a variety of mechanisms and combinations of different mechanisms can be used.
Therefore, if Ⅳ, the number of possible solutions for the prime mover part, N2 and N3 represent the number of possible solutions for the drive part and the actuator part respectively, the overall number of possible solutions for the machine Ⅳ is Ni x N2 x N3.
The above is only a discussion of the three main parts that make up the machine. Sometimes it is also necessary to take into account the configuration of auxiliary systems, which will not be discussed in this book.
Out of such a large number of options, only a few are technically feasible. These few options are evaluated in terms of their technical aspects as well as economic and environmental aspects.
There are many ways of evaluating them, so we will now take economic evaluation as an example. When evaluating on the basis of the economy, it is important to take into account both the economy of design and manufacture and the economy of use.
If the machine has a more complex structural solution, the design and manufacturing costs will be relatively higher, but its functionality will be more complete and its productivity higher, so the economy of use is also better.
Conversely, a simpler, less functional machine will cost less to design and manufacture but will cost more to use. When evaluating the design and manufacturing economy of a structural solution, it is also possible to use the cost per unit of efficiency to express this.
For example, the cost per unit of output, the cost per unit of product, etc.
When carrying out machine evaluation, the reliability of the machine must also be analyzed as an indicator for evaluation. From the point of view of reliability, it is often unwise to blindly pursue complex structures.
Generally speaking, the more complex a system is, the less reliable it is. In order to increase the reliability of complex systems, it is necessary to add parallel backup systems, which inevitably increase the cost of the machine.
Environmental protection is also an important aspect that must be carefully considered in the design. Technical solutions that have an adverse impact on the environment must be analyzed in detail and technically mature solutions proposed.
The evaluation of the options leads to a final decision and the definition of a schematic diagram of the mechanism movement sketch on which the next technical design steps will be based.
In the design phase, the relationship between borrowing and innovation must be correctly managed.
Successful precedents of similar machines should be followed, while previously weak links and parts that do not meet the existing task requirements should be improved or radically changed.
We should actively innovate and oppose conservatism and copying the original design, but also oppose the two wrong tendencies of seeking new and abandoning the reasonable original experience.
Third. The technical design of mechanical design
The objective of the technical design stage is to produce sketches of the general assembly and sketches of the component assembly.
Through the sketch design determine the shape and basic size of each component and its parts, including the connection between the components, parts, and components of the shape and basic size.
Finally, the working drawings of the parts, the component assembly drawings, and the general assembly drawings are drawn.
In order to determine the basic dimensions of the main parts, the following must be done.
1. Kinematic design of the machine
Determine the parameters of the prime movers (power, speed, linear speed, etc.) according to the defined structural solution. Then do kinematic calculations in order to determine the kinematic parameters (speed, velocity, acceleration, etc.) of the moving elements.
2. Calculation of the dynamics of the machine
In conjunction with the structure and kinematic parameters of each part, the magnitude and characteristics of the loads applied to the main components are calculated. The loads derived at this point are only nominal (or nominal) loads acting on the parts, as they have not yet been designed.
3. Design of the working capacity of the part
Having known the main parts subject to the size and characteristics of the nominal load, you can do the initial design of parts and components.
Design based on the working capacity guidelines shall refer to the general failure of parts, components, working characteristics, environmental conditions, etc. reasonably formulated, generally having strength, stiffness, vibration stability, life, and other guidelines.
By calculation or analogy, the basic dimensions of the parts and components can be determined.
4. Design of component assembly sketches and general assembly sketches
According to the basic dimensions of the main parts and components, a sketch of the component assembly and a sketch of the general assembly are designed. The sketches need to be structured with regard to the shape and dimensions of all parts.
In this step, the structure and dimensions of the parts need to be well coordinated, and the structural process of the designed parts and components needs to be considered comprehensively so that all parts have the most reasonable configuration.
5. Checking of the main parts
There are some parts, in the above-mentioned step 3) due to the specific structure is not determined, it is difficult to carry out a detailed calculation of the working capacity, so only preliminary calculations and design.
After the sketches of the component assembly and the general assembly have been drawn, the structure and dimensions of all parts are known, as are the relationships between the neighboring parts.
Only at this point can the loads acting on the parts be determined more precisely and the details of the factors affecting the working capacity of the parts be determined.
Only under these conditions is it possible and necessary to carry out precise calibration calculations for important or complex components in terms of shape and force.
Based on the results of the checks, the structure and dimensions of the part are repeatedly modified until they are satisfactory.
In the various steps of technical design, the optimization techniques developed over the last 30 or 40 years have increasingly demonstrated their ability to optimize the choice of structural parameters.
Some new numerical calculation methods, such as the finite element method, allow excellent approximations to quantitative calculations for problems that were previously difficult to calculate quantitatively.
For a few very important, complex, and expensive parts, model tests are used when necessary, i.e. a model is made according to the preliminary design drawings, and through tests, weak points or excess cross-sectional dimensions are identified, and the original design is modified by strengthening or reducing it accordingly to achieve perfection.
Mechanical reliability theory is used in the technical design phase to evaluate whether the designed parts and components and their parameters meet the reliability requirements from a reliability point of view, and to make suggestions for improving the design, thus further improving the quality of the machine design. These new design methods and concepts should be applied and promoted in the design so that they can be developed accordingly.
After the sketch design is completed, the basic dimensions of the part can be determined according to the sketch industry, the design of the working drawing of the part. At this point, there are still a large number of details of the structure of the part to be refined and determined.
When designing the working drawing, take full account of the processing and assembly process of the part, the inspection requirements, and implementation methods of the part during and after processing.
Some details must be returned to for recalibration if they have a worthwhile effect on the working capacity of the part. Finally, a working drawing of all the parts, except the standard parts, is produced.
The component assembly drawing and the general assembly drawing are redrawn according to the structure and dimensions on the finalized working drawing of the part. This exercise allows for the detection of possible hidden dimensional and structural errors in the working drawings of the parts. This work is commonly referred to as paper assembly.
Fourth. Technical documentation
There are many kinds of technical documents, commonly used are the design calculation manual of the machine, the instruction manual, the standard parts list, and so on.
When preparing the design calculation manual, it should include all the conclusive content of the program selection and technical design.
When preparing the manual for the use of the machine for the user, the user should be introduced to the range of performance parameters of the machine, the use of operating methods, routine maintenance, and simple maintenance methods, the catalog of spare parts, etc.
Other technical documents, such as inspection and qualification sheets, schedules of purchased parts, acceptance conditions, etc., are prepared separately, whether required or not.
Fifth. The application of computers in mechanical design
With the development of computer technology, computers are increasingly used in mechanical design, and many highly efficient design and analysis software have emerged.
The use of this software can be used in the design phase of multiple program comparisons, which can be different including the large and very complex programs of structural strength, stiffness, and dynamic characteristics of accurate analysis.
It is also possible to build virtual prototypes on the computer and use virtual prototype simulation to validate the design, thus enabling the feasibility of the design to be fully assessed at the design stage.
It can be said that the widespread use of computer technology in machine design has and is changing the process of machine design, and its advantages in terms of improving design quality and efficiency are hard to predict.
The design process for machines has been briefly described above. Broadly speaking, at any time during the manufacture of a machine, there is a possibility that the design may be modified for process reasons. When modifications are required, certain approval procedures should be followed.
After the machine has left the factory, a planned follow-up survey should be carried out; in addition, the user will give feedback to the manufacturing or design department on problems that arise during the course of use.
Based on this information, the design department, after analysis, may also make modifications to the original design or even change the model. These tasks, although part of the design process in a broad sense, are at a different level and will not be discussed in this book.
However, as a design worker, one should have a strong sense of social responsibility and extend the horizon of one’s work to the whole process of manufacture, use, and even end-of-life utilization, repeatedly and continuously improving the design in order to continue to improve the quality of the machine and to better meet the needs of production and life.
Sixth. Mechanical design stages are described separately
(1) Mechanical design planning stage
After the new machine is designed according to the needs of production or life, the planning stage is only a preparatory stage. At this point, there is only a vague concept of the machine to be designed.
(2) Mechanical design scheme design stage
This stage plays a key role in the success or failure of the design. In this stage also fully demonstrated the characteristics of the design work as more than one solution (scheme).
The functional analysis of the machine is to carry out a comprehensive analysis of the requirements that must be met, the minimum requirements, and the desired requirements in the machine functions proposed in the design task, i.e. whether these functions can be achieved, whether there are contradictions between several functions, whether they can be substituted for each other, etc.
Finally, the functional parameters are determined, which serve as the basis for further design. In this step, the possible contradictions between needs and possibilities, ideals and realities, development goals and current goals, etc. are dealt with appropriately.
Once the functional parameters have been determined, possible solutions can be proposed, i.e. possible options. The search for solutions can be discussed separately in terms of the prime mover, the drive, and the actuator sections. The more common approach is to start with the actuating part.
When discussing the actuating part of a machine, the first question is about the choice of the working principle. For example, when designing a machine for manufacturing screws, the working principle can be either by turning the threads on a cylindrical blank with a turning tool or by rolling the threads on a cylindrical blank with a thread rolling die.
This presents two different working principles. The different working principles will, of course, result in a fundamentally different machine design. In particular, it should be emphasized that new working principles must be constantly researched and developed. This is an important way of developing design technology.
Seventh. Steps in machine design
Before the design begins, the design task is formulated. When the design task is complex, a three-stage design is generally used, i.e. preliminary design, technical design, and working drawing design.
When the task is relatively simple, such as the new design of simple machinery, general machinery inheritance design, or variant design, the design will be done to the depth of technical design at the beginning, after review, modification, and approval to do the working drawing design, and become a two-stage design.
In the preliminary design stage of the three-stage design, the main steps of the design are: to determine the working principle and basic structure type, motion design, design of major parts, and components, drawing preliminary general drawings, and preliminary design review.
In the technical design stage, the main steps are modification of the design according to the review, design of all parts and components, drawing of new general drawings, and technical design review.
In the working drawing design stage, the design is modified according to the review comments, all working drawings are drawn up and all technical documents are developed. For batch or mass-produced products, a final design is also carried out.
At each step of the design, it may be found that some decision in the previous step is unreasonable, which requires folding back to that previous step, revising the unreasonable decision, and redoing the subsequent design work.
1. Formulating the design task
This is the first step in the mechanical design process. The mechanical design task is based on customer orders, market needs and new scientific findings.
The mechanical design department uses various technical and market intelligence to draw up a list of possible solutions, compare their advantages and disadvantages, discuss them with the management and the user, and set a reasonable target for the design task.
This is particularly important for new mechanical designs. Failure to meet the objectives will result in serious economic losses and even total failure.
2. Determine the working principle and basic structure type
If the mechanical design task is not clearly defined, the first step of the design is to determine the overall scheme, that is, to determine the working principle to be applied and the corresponding structure type.
For example, in the design of a high-powered marine diesel engine, the first step is to determine whether to use a two-stroke, double-acting, crosshead, low-speed diesel engine, or a four-stroke, single-acting, medium-speed diesel engine.
Another example is the mechanical design of crushing machinery for coarse rock crushing, the first step is to determine whether to use a jaw or gyratory crusher with extrusion and bending as the main crushing action or a single or double rotor impact crusher with impact as the main action.
3. Motion design
Once the overall mechanical design has been determined, it is then necessary to apply the knowledge of mechanics to select the appropriate mechanism to obtain the required motion solution.
The jaw crusher mentioned above relies on the oscillation of the moving jaw plate to crush, bend and split the rock entering the crushing chamber, while the oscillation of the moving jaw plate can be simple with a double elbow plate mechanism or complex with a single elbow plate mechanism.
In new mechanical designs, it may be necessary to combine a new mechanism to obtain the required movement solution, which is often a difficult task. Therefore, the designer generally tries to apply the motion solutions given by existing and proven mechanisms.
4. Structural design and preliminary general drawing of the mechanical design
After the motion design, the mechanical designer begins to design the structure, calculate the main mechanical parts of the force, strength, shape, size, weight, etc., and draw the main parts, and parts sketch.
At this point, if it is found that the original structure chosen is not feasible, it is necessary to adjust or modify the structure. Consideration should also be given to any areas where overheating, excessive wear, or vibration may occur.
In this step, the designer will find contradictions in the shape, dimensions, and proportions of the various parts by sketching them. In order to strengthen or improve one aspect, the other may be weakened or worsened.
At this point, the priorities must be weighed and coordinated to achieve the best overall effect. Once the sketches have been repeatedly revised and found to be initially satisfactory, the preliminary general drawings and cost estimates can be drawn up. The preliminary general drawing is drawn strictly to scale and sufficient views and sections are selected.
5. Preliminary examination
Preliminary general drawing, the need to ask for the type of machinery experienced in the mechanical design, manufacture, and use of personnel and representatives of the user or commissioned design units for a preliminary review.
Review results such as the design are not applicable (such as weight, volume is too large, the cost is too high, the reliability of the structure has doubts, etc.), it is necessary to re-movement design, or even change to another working principle and the basic structure type. In most cases, the design is improved by certain measures.
6. Technical design
According to the preliminary review, the design is modified and all parts and components are drawn up.
Precise stress analysis is carried out on the main parts and components, the shape and dimensions of the parts are corrected according to the results of the analysis, and materials and heat treatment are specified.
Determine the machining accuracy of the parts and the assembly conditions of the components and assemblies. Complete lubrication design, and electrical design (drives and controls).
Redrawing of general drawings and, in some cases, mock-ups for certain important and mass-produced machinery.
Submission of the completed technical design for a second review.
7. Drawing of working drawings
After making final changes based on the comments from the second review, official parts drawings, component assembly drawings, and general assembly drawings can be drawn up, and technical documents such as parts lists, lists of wearing parts, and instructions for use can be prepared.
The person in charge of the design should pay attention to the coordination of dimensions between parts, check the tolerance fit between coupled parts and review the strength and stiffness of certain parts. It is very important to start checking the drawings after the part drawings are completed.
A carefully checked drawing will ensure smooth assembly after machining. The most reliable method of checking is to redraw a general assembly drawing based on the completed parts drawing, where all inconsistencies will be apparent.
In the drawing of parts at the same time also need to carry out two work: one is the process audit so that the parts facilitate processing and reduce manufacturing costs; the second is the standard audit, so that the structural elements of the parts, dimensions, tolerances and technical conditions of heat treatment, as well as standard and common parts in line with the provisions of the standard.
8. Trial production and finalized design
For single-piece or small-batch production machinery, the design drawings completed through the above steps can be put into formal production.
For machines produced in batches or in large quantities, prototypes must be trial-produced before formal production, and functional tests and appraisals should be carried out.
After passing, batch trial production will be carried out according to the mass production process. Problems that arise during batch trial production may require corresponding modifications to the design before it can become a finalized design that can be used in formal production.
What are the mechanical design constraints?
The design of a mechanical component has a number of constraints and the design criteria are the constraints that the mechanical design should meet.
Technical performance criteria
Technical performance includes all aspects of the product’s function, manufacture, and operating conditions, and refers to both static and dynamic performance. For example, the power, efficiency, service life, strength, stiffness, resistance to friction and wear, vibration stability, thermal characteristics, etc. that a product can transmit.
Technical performance guidelines mean that the relevant technical properties must meet the specified requirements. For example, vibration can generate additional dynamic loads and variable stresses, especially when its frequency is close to the inherent frequency of the mechanical system or parts, resonance will occur, and when the amplitude will increase sharply, which may lead to rapid damage to the parts or even the whole system.
The vibration stability criterion is to limit the mechanical system or parts of the relevant vibration parameters, such as the inherent frequency, amplitude, noise, etc. in the specified allowable range.
Another example is that the heat generated by a machine during operation may lead to thermal stress, thermal strain, and even thermal damage. The thermal characteristics criterion is to limit the various relevant thermal parameters (e.g. thermal stress, thermal strain, temperature rise, etc.) to within the specified limits.
Standardization mechanical design guidelines
The main standards relating to the design of mechanical products are broadly
Standardization of mechanical design concepts: the nomenclature, symbols, units of measurement, etc. involved in the design process should conform to the standards.
Physical form mechanical design standardization: the structural form, size, and performance of parts, raw materials, equipment, energy, etc. should be selected in accordance with the unified regulations.
Standardization of mechanical design methods: operating methods, measurement methods, test methods, etc. should be implemented according to the corresponding regulations.
Standardization mechanical design guidelines are in the design of the whole process of all behavior and are to meet the above standardization requirements.
The mechanical design standards related to the design of mechanical parts that have been issued can be divided into three levels: national standards, industry standards, and enterprise standards in terms of the scope of application. In terms of mandatory use, they can be divided into those that must be implemented and those that are recommended for use.
Mechanical design reliability guidelines
Reliability:
The probability that a product or component will perform its specified function under the specified conditions of use and within its expected lifetime. A reliability criterion means that the product, component, or part should be designed to meet the specified reliability requirements.
Mechanical design safety guidelines
The safety of a machine includes the below:
Component safety:
It means that the component does not break, deform excessively, wear excessively or lose stability, etc., under specified external loads and for a specified period of time.
Complete machine safety:
It means that the machine is guaranteed to be fault-free under the specified conditions and to achieve the total function properly.
Work safety:
It refers to the protection of the operator, to ensure personal safety and physical and mental health, etc.
Environmental safety:
It refers to the environment around the machine and people do not cause pollution and harm.
Mechanical design methodology
The purpose of mechanical design methodology is to elevate design thinking to a rational process so that design can follow a certain logic and so that more designers can make good designs. It broadly comprises some of the following elements.
- Dividing the stages of mechanical design into very detailed stages, so that each stage becomes a disciplined and rational thinking activity.
- Storing successful or good mechanical designs and establishing a design database for future reference or adoption in design.
- Introduce the concept and method of value engineering in the mechanical design work, and make a trade-off between function and cost for the contradictions in the design, in order to obtain a good use effect.
- Adopt the knowledge of tribology, vibration, fracture mechanics, finite element method, reliability design, optimization mechanical design, system engineering, ergonomics, and other emerging disciplines in the design to improve the scientific nature of the design and reduce blindness.
- Extend the scope of mechanical design work forward to market prediction and backward to after-sales service.
- Use computer-aided design to reduce the amount of mechanical design labor, and improve mechanical design speed and design quality.
The future of mechanical design
In the future mechanical design is bound to permeate industries such as semiconductor manufacturing, bioengineering, nanotechnology, and robotics, contributing to the development of society while constantly improving itself and making further innovations in theory.
(1) Further systemic approach
That is, starting from a systems viewpoint, mechanical products are seen as a system or whole, relying on computer technology to achieve coordination between man, machine, environment, and each other.
Specifically, the total system is decomposed into a number of sub-systems, using a variety of modern design theories and methods, with the pursuit of system optimization as the goal of coordinating the design and matching of the sub-systems.
(2) Deepening intelligent mechanical design
With the progress and development of science and technology, mechanical design should increasingly take into account the factors of intelligence. A large number of mechanical design content can be established by establishing a model to describe the behavior of various working conditions of mechanical products, the model can be solved to predict the performance of the product, and the rationality, and optimality of the design.
For example, various types of vehicle performance evaluation of intelligent decision-making systems, gearbox design expert systems, and fault diagnosis systems have been applied in the development and design of new vehicles.
(3) More attention to green thinking
Green design technology is a technology for mechanical designing products in their life cycle in accordance with the requirements of environmental protection, highest resource utilization, and lowest energy consumption.
The mechanical designer is required to consider the environmental attributes and basic properties of the product from the whole cycle and to always base the design on human physical and mental health, environmental protection, etc. At the same time, the product designed is required to be recyclable and to cause minimal damage to the environment.
What are the modern mechanical design methods?
Professional modern mechanical design
Computer software developed by mechanical design and computer professionals, which can reflect and describe the various mechanisms of damage, failure, and destruction of mechanical products under actual working conditions, which can quantitatively analyze and calculate the dynamic behavior of mechanical parts and machinery, and which form fixed design procedures, are professional modern design methods.
For example vibration analysis and design, tribological design, thermodynamic heat transfer design, strength and stiffness design, temperature field analysis, etc.
All these software are developed on the basis of traditional mechanical design methods with the application of computer technology.
For example the analysis of the dynamic characteristics of mechanical devices with Pro/M software and the analysis of stresses with ANSYS software are good examples of this, laying the foundation for accurate judgment of the reliability of devices and the selection of design parameters.
General modern mechanical design
To meet the high requirements of mechanical product performance, a large number of computer technologies are used in mechanical design to assist in design and system analysis, which is a common modern design method.
Common mechanical design methods include optimization, finite elements, reliability, simulation, expert systems, CAD, etc. These methods are not just for mechanical products to study, but also their own scientific theories and methods.
1) Optimisation mechanical design
The basic idea is to establish a mathematical model that reflects the engineering design problem and meets the requirements of mathematical planning according to the theory, methods, and standards of mechanical design, and then use mathematical planning methods and computer computing technology to automatically find the optimal solution to the mechanical design problem.
It is a modern mechanical design method formed by the combination of mechanical design theory and optimization mathematics and electronic computers.
2) Simulation and virtual mechanical design
Computer simulation is a comprehensive technology that uses the computer as a tool to “build a model of an actual or imaginary system” and to experiment with the dynamic operation of the model under different conditions.
The essence of virtual technology is to use computer-supported simulation technology as a premise to simulate the entire product development process and its impact on product design in real-time and in parallel, to predict product performance, product manufacturing costs, product manufacturability, product maintainability, and disassembly, etc., so as to improve the success rate of product design at once.
This approach not only shortens the product development cycle but also achieves a shorter distance between product development and the user.
3) Finite element mechanical design
This method uses mathematical approximations to simulate real physical systems (geometry and load conditions). By also using simple and interacting elements, i.e. cells, it is possible to approximate a real system with a finite number of unknowns to an infinite number of unknowns.
It can be used not only for the solution of complex non-linear and non-stationary problems in engineering, but also for static and dynamic analysis of complex structures in engineering design, and can accurately calculate the stress distribution and deformation of complex shaped parts, becoming a powerful analysis tool for strength and stiffness calculation of complex parts.
4) Fuzzy mechanical design
It is a mechanical design method that applies fuzzy mathematical knowledge to mechanical design. There is a large amount of fuzzy information in mechanical design.
For example, in the mechanical design of mechanical parts, the safety factor of the part is often taken from a conservative point of view, taking larger values without economy, but there is a large fuzzy interval within its permissible range.
The development of mechanical products often encounters a variety of fuzzy problems at various stages, and although the characteristics, nature, and requirements of these problems for planning vary, the fuzzy analysis methods adopted are similar.
The most important feature is that the impact of each factor on the mechanical design outcome can be analyzed quantitatively and comprehensively, resulting in a comprehensive quantitative indicator that can be used as a basis for selection decisions.
525 list of Mechanical design taboos
After talking about the mechanical design methods and steps, we will discuss the notes and taboos in mechanical design.
First. Structural design to improve the strength and stiffness of the mechanical design
- Avoid the distance between the force point and the support point is too far away
- Avoid cantilever structures or reduce the length of cantilevers
- Do not ignore the beneficial effects that can be produced by working loads
- Avoid frictional transfer of forces in parts subjected to vibratory loads
- Avoid unbalanced forces in the mechanism
- Avoid considering only a single path of force transmission
- The influence of the deformation of parts on the distribution of forces during operation should not be ignored
- Avoid large tensile stresses in cast iron parts
- Avoid bending stresses on thin rods
- Avoid excessive stiffness in parts subjected to shock loads
- parts subject to variable stresses to avoid excessive surface roughness or scratches
- Avoid residual tensile stresses on the surface of parts subject to variable stresses
- Variable load parts should avoid or reduce stress concentrations
- Avoid local structures affecting strength by being too close to each other
- Avoid pre-deformation and workload generated by the same direction of deformation
- Wire rope pulley and reel diameter can not be too small
- Avoid wire rope bending too many times, pay special attention to avoid repeated bending
- When lifting wire rope and reel coupling to leave a margin
- Can not transfer the force of the middle parts should try to avoid the force
- Try to avoid the installation of the additional force generated by the axis of misalignment
- Minimise the force acting on the foundation
Second. Structural design to improve wear resistance of the mechanical design
- Avoid the same material to form a sliding friction pair
- Avoid the thickness of the white alloy wear layer is too large
- Avoid increasing the requirements for the whole part in order to improve the wear resistance of the surface of the part
- Avoid the scrapping of large parts due to local wear
- When using white alloy as bearing lining, attention should be paid to the selection of the material of the shaft tile and the design of the structure of the shaft tile
- The lubricant supply is sufficient and the working surface is covered
- The lubricant tank should not be too small
- Do not allow the filter to filter out the additives in the lubricant
- The size, position, and shape of the oil groove of the sliding bearing should be reasonable
- The amount of grease added to rolling bearings should not be too much
- Increase the wear margin for the wearable surface of the parts
- Pay attention to the adjustment of the parts after wear and tear
- The speed and pressure difference between the points on the same contact surface should be small
- Use dustproof devices to prevent abrasive wear
- Avoid the formation of step wear
- Sliding bearings should not be sealed with contact oil
- The easily wearable parts should be protected
- Automatic compensation of wear structure can be used for wear-prone parts
Third. Improve the accuracy of the structure design of mechanical design
- Try not to use the structure scheme which does not conform to the Abbey principle
- Avoid the mutual superposition of the error generated by the amount of wear
- Avoid the superposition of processing errors and wear
- The driving force of the guideway should act on the center of pressure of the two guideways so that the moments generated by the friction of the two guideways balance each other
- For the requirements of high precision guide, should not use a small number of ball support
- In the reduction chain requiring accuracy of movement, the last transmission ratio should take the maximum value
- The number of nuts and buckles in the measuring spiral should not be too small
- The axial runout of the spiral bearing must be strictly limited
- Avoid the unreasonable combination of bearing accuracy
- Avoid unreasonable configuration of bearing radial oscillation
- Avoid tightening screws to affect the accuracy of the rolling guide
- When the gap between the push rod and the guideway is too large, it is appropriate to use the sine mechanism, not the tangent mechanism
- The accuracy of the sine mechanism is higher than that of the tangent mechanism
Four. Consider the ergonomics of the structure design issues of mechanical design
- Reasonable selection of operating posture
- Reasonable values should be used for the ratio of table height and human body size of the equipment
- Reasonable placement of adjustment links to enhance the applicability of the equipment
- The manipulation, control, and display devices of the machine should be arranged in the most reasonable position in front of the operator
- displays are of a reasonable form
- The lettering on the instrument panel should be clear and easy to read
- The size and shape of the knobs should be reasonable
- The keys should be easy to operate
- The force required to operate the handle and the range of hand movement should not be too large
- The shape of the handle should be convenient for operation and force
- Reasonable design of the size and shape of the seat
- The material and flexibility of the chair should be reasonably designed
- No excessive noise in the working environment
- The light level of the operating site must not be too low
Five. Consider the structural design of heat, corrosion, noise, and other issues of mechanical design
- Avoid the use of low-efficiency mechanical structure
- The size of the lubricating oil tank should be large enough
- The return fluid of the diversion system should be cooled
- Avoid exposure of high-pressure vessels, pipes, etc. to the hot sun
- Parts exposed to high temperatures should not be made of rubber, polyethylene plastic, etc.
- Precision machinery box parts should not be arranged inside the oil tank, so as not to produce thermal deformation
- For longer mechanical parts, consider the temperature change in size change, which can be free deformation
- Hardened material’s working temperature should not be too high
- Avoid moisture condensation caused by high-pressure valve bleeding
- Boxes with high thermal expansion can be supported in the center
- The flange of the bolt connection is used as the connection of the pipe, when one side is exposed to sunlight, the temperature and elongation of the two sides are different, resulting in bending
- The structure in contact with corrosive media should avoid having slits
- The liquid in the container should be able to be removed cleanly
- Care should be taken to avoid mechanochemical wear on the contact surfaces of the shaft and hub (micro-abrasion)
- Avoid corrosion-prone screw structures
- When the steel tube is connected to the copper tube, it is easy to produce electrochemical corrosion, so a section of the tube can be arranged for regular replacement
- Avoid structures susceptible to corrosion
- Pay attention to avoid the impact of the heat exchanger pipe micro-abrasion
- Reduce or avoid the impact and collision of moving parts to reduce noise
- High-speed rotors must be balanced
- The mass of impacted parts should not be too small
- To absorb vibrations, parts should be strongly damped
Six. Casting structure design of mechanical design
- Strive for simple parting surfaces
- Casting surface to avoid concave
- Surface tabs should be concentrated as far as possible
- Large castings should not have small projections on the outer surface
- Improve the structure that hinders the starting mold
- Avoid large and thin horizontal surfaces
- Avoid shapes that produce large internal stresses
- Prevent deviations in the molding from having an adverse effect on the appearance
- Use a structure that is easy to decore
- Minimise the number of parting surfaces
- Strive for uniform wall thickness of the casting
- Use strengthening ribs to make the wall thickness uniform
- Consider the solidification sequence to design the casting wall thickness
- The inner wall thickness should be less than the outer wall thickness
- Casting wall thickness should be gradually transition
- The angle between the two walls should not be too small when intersecting
- The inner cavity of the casting should make it easy to make a core
- No or less use of core support
- Try not to use the core
- The hole side of the casting should be raised
- The structure of the casting should facilitate the removal of the core sand
- The core design should help to improve the quality of the casting
- Holes in castings should be pierced as far as possible
- Reasonable arrangement of reinforcement ribs
- Ensure free shrinkage of the casting to avoid defects
- Pay attention to the force of the ribs
- Consider structural stability in the setting of ribs
- Eliminate unnecessary rounded corners
- Turn large into small and complicated into simple
- Pay attention to the reasonable force transmission and support of the casting
Seven. Forging and stamping parts structure design of mechanical design
- Free forging parts should avoid conical and wedge-shaped
- Simplify the coherent form
- Avoid using ribbed plates
- Free forged parts should not be designed with complex tabs
- There should be no tabs inside the forged parts of the free forging
- The size of the parting surface of the forging should be the maximum size of the part and the parting surface should be flat
- The shape of the forged part should be symmetrical
- The drop-forged part should have a suitable radius of rounded corners
- The forging should be suitable for the release
- The shape of the forged part should be as simple as possible
- The shape of the stamped part should be as symmetrical as possible
- The partial width of the part should not be too narrow
- The depth and shape of the bosses and holes should be required
- The design of the stamped part should take into account the material-saving metal processing micro letter, the content is good and worthy of attention
- The shape of the stamped parts should avoid large flat surfaces
- Bending parts should avoid wrinkling at the bend
- Pay attention to the design slope
- Prevent hole deformation
- Simplify the unfolding diagram
- Pay attention to the support should not be too thin
- Thin plate bending parts in the bend to have a cut
- Pressure rib can improve the stiffness but directional
- The shape of the drawn parts to strive for simple
- The convex edge of the drawn parts should be uniform
- The use of the notching process can simplify the structure
- The dimensions of stamped parts should take into account die wear
- The dimensions of stamped parts should take into account the stamping process
Eight. The structure of welded parts blank of mechanical design
- Reasonable design of the shape
- Reduction of edges and corners
- Use of nesting cuttings
- Section turns should not be arranged with weld seams
- Welded parts should not simply imitate castings without regard to their characteristics
- The shape of the section should be conducive to reducing deformation and stress concentration
- Correctly select the position of the weld seam
- Do not let the welding influence zone is too close to each other
- Pay attention to the forces on the weld seam
- The arrangement of the strengthening ribs of the weld should be reasonable
- Reduce the force of the weld seam
- Reduce thermal deformation
- Rational use of profiles to simplify the welding process
- Weld seam should avoid processing surface
- Consider gas diffusion
- Stamped parts can be used instead of machined parts
- Use plate bending parts to reduce the weld seam
Nine. Mechanical processing parts structure design of mechanical design
- Pay attention to reducing the size of the blank
- Machining surface and non-machining surface should not be flush
- Reduce the length of the processing surface
- Different machining accuracy surface to be separated
- Change the complex shape of the parts into a combination of parts to facilitate processing
- Avoid unnecessary accuracy requirements
- Easy access and exit of the tool from the machined surface
- Avoid machining closed spaces
- Avoid inaccessibility of the tool to the workpiece
- Do not use a structural shape of the part that is not suitable for the shape of the tool
- To take into account the influence of casting errors
- Avoid the combination of several parts to be machined
- Complex machining surfaces should be designed on the outer surface and not on the inner surface
- Avoid chamfering of complex-shaped parts
- Stop fits for non-round parts must be avoided
- Avoid unnecessary supplementary machining
- Avoid unflappable part structures
- Avoid part structures without a measuring base
- Avoid shocks and vibrations in machining
- Avoid drilling holes in inclined surfaces
- Do not produce partial undrilled holes at the bottom of the through-hole
- Reduce the number of tools used to machine the same part
- Avoid multiple fixing during machining
- Pay attention to the possibility of machining more than one part at a time
Ten. Structural design of heat-treated and surface-treated parts of mechanical design
- Avoid disparity in wall thickness between parts
- The size of parts requiring high hardness (integral hardening treatment) should not be too large
- Avoid sharp corners and sudden dimensional changes
- Avoid asymmetrical structures
- Avoid quenching open-shaped parts
- Avoid too complex structures of hardened parts
- Avoid parts with too low stiffness, resulting in quenching deformation
- Use partial quenching to reduce distortion
- Avoid holes too close to the edge of the part
- High-frequency quenching gear block should be a certain distance between the two gears
- The surface of plated steel parts should not be too rough
- Plated mating parts should be considered for plating thickness during machining
- Attention to electroplated parts reflective unsuitable for certain working conditions
Eleven. Consider the mechanical structure design of assembly and maintenance of mechanical design
- Avoid having to remove other parts when dismantling a part
- Avoid fitting two mating surfaces at the same time
- To leave the necessary operating space for disassembling and assembling parts
- To avoid failure to work properly due to incorrect installation
- Use special structures to avoid incorrect installation
- Use symmetrical structures to simplify the assembly process
- Flexible sleeves should be installed with a guide section
- Difficult to see matching parts with a guide section
- In order to facilitate the installation by a robot, the use of clips or an internal locking structure
- The head of the fastener should have a smooth straight edge for easy pick-up
- Parts should have the necessary chamfers on the mounting area
- Parts fed by the automatic loading mechanism should avoid tangled laps
- Simplify the assembly movement
- A machine should be reasonably divided into parts
- Minimise the amount of assembly work on site
- Use standard parts as far as possible
- Parts should be easy to remove after damage to recover materials
Twelve. The design of the threaded coupling structure of mechanical design
- The height of the top nut is different at the same time, do not install the opposite
- Anti-loosening method should be reliable
- Subject to the bending moment of the screw structure, should minimize the thread force
- Avoid bending stress on the screw
- Use threaded parts for positioning
- Screws should be arranged in the part of the coupling with the greatest stiffness
- Avoid excessive deformation of the jointed parts when tightening the nut (or screw)
- Flange bolts should not be arranged directly below
- The bolt spacing of the side cover should be considered for sealing performance
- Do not make threaded holes through to prevent leakage
- The threaded hole should not penetrate two welded parts
- For deep screw holes, the corresponding tabs should be designed on the part
- The head of the fastening bolt of a high-speed rotating body should not protrude
- Avoid intersecting screw holes
- Avoid bolts passing through chambers with temperature variations
- Do not arrange ground bolts near the concrete end of the foundation
- Shear bolts should have a large contact length
- Consider sufficient spanner space when tightening nuts
- The bolt diameter, spacing, and thickness of the joint of the flange structure should be selected appropriately
- To ensure the space for the installation and removal of the bolt
- Fastening screws should only be added in the direction of the non-load bearing
- Aluminium gaskets are not suitable for use in electrical equipment
- Screws with plating on the surface should have a plating margin before plating.
- The hole edge of the screw hole should be chamfered
- When there is a risk of bruising the threads at the top of the screw, there should be a cylindrical end to protect the threads
- Fixed with more than one countersunk head screw, each countersunk head can not be tight
Thirteen. Positioning pins, coupling pin structure design of mechanical design
- The distance between the two locating pins should be as far as possible
- Symmetrical structure of the parts, positioning pins should not be arranged in a symmetrical position
- Two positioning pins should not be arranged in two parts
- The pin holes of matching parts should be machined simultaneously
- The pin holes of hardened parts should also be matched
- The locating pins should be perpendicular to the joint surface
- The pin must be easy to pull out
- The pin should not be installed on the interference surface
- The assembly of pins that are not easy to observe should be carried out using appropriate measures
- The installation of locating pins should not make the disassembly of the parts difficult
- Avoid unbalanced forces when using pins to transmit force
Fourteen. Structural design of bonded parts of mechanical design
- Two cylindrical butt joints should be added casing or an additional internal connection column
- Improve the structure of the bonding joint to reduce the force on the bonding surface
- The larger part of the peeling force using enhanced measures
- Different characteristics of the bonding structure from cast and welded parts
- Bonding for repair cannot simply be bonded, but increase the bonding area
- repair heavy parts in addition to bonding, should be added waveform key
- Repair of cracked parts in addition to bonding, other measures should be taken
Fifteen. Key and spline structure design of mechanical design
- The radius of the bottom rounded corner should be large enough
- Both sides of the flat key should have a tighter fit
- When two flat keys are used for parts on a shaft, high machining accuracy is required
- When using two oblique keys, they should be 90 to 120 degrees apart
- When using two half-round keys, they should be on the same bus line in the axial direction
- When using flat keys on the shaft to fix two parts respectively, the keyway should be on the same busbar
- Do not open the keyway in the weak part of the part
- The length of the keyway should not be opened to the stepped part of the shaft
- Hook head oblique keys should not be used for high speed
- Long shafts with keyways on one side are easily bent
- Flat keys and set screws cause eccentricity of the shaft parts
- Flat keys for tapered shafts are parallel to the axis as far as possible
- When several parts are strung on the shaft, it is not advisable to use the key connection separately
- Special attention should be paid to the strength of the end of the spline shaft
- Pay attention to the distribution of the stiffness of the wheel and the Yi, do not make the torque only by part of the spline transmission
Sixteen. Design of interference fit structure of mechanical design
- The mating parts must be easy to fit
- interference fit parts should have a clear positioning structure
- Avoid pressing into two mating surfaces at the same time
- The interference fit parts should be considered for easy disassembly
- Avoid fitting more than one interference fit into the same fit size
- Pay attention to the influence of the working temperature on the interference fit
- Pay attention to the influence of centrifugal force on the interference fit
- To consider the two parts with interference fit after assembly, other dimensional changes
- Tapered fits cannot be positioned with shoulders
- The taper of the tapered fit should not be too small
- In the cast iron parts embedded in the small shaft easy to loosen
- Stainless steel bushings can loosen interference fits due to temperature effects
- The interference fit of the shaft and hub, with a certain length of the surface
- When the interference fit and the key are used together, the key slot should be installed first
- Do not make two holes of the same diameter for interference fit
- Avoid interference with the set of an asymmetric cut
Seventeen. Flexible transmission structure design of mechanical design of mechanical design
- Belt drive should pay attention to increasing the small wheel wrap angle
- The two axes in the up and down position of the pulley should make the drape of the belt conducive to increasing the angle of the package
- Small pulley diameter should not be too small
- The speed of the belt drive should not be too low or too high
- The center distance of the pulley should not be too small
- The center distance of the belt drive should be adjustable
- The belt should be easy to replace
- The belt should not be cantilevered when the belt is too wide
- Tension the belt drive by self-weight, when the self-weight is not enough to add auxiliary devices
- Pay attention to the parallelism of the two axes and the central position of the pulley
- Small pulleys for flat belt drives should be made slightly convex
- The working surface of the belt pulley should be bright and clean
- Half-crossed flat belt drive should not be reversed
- High-speed pulley surface should be slotted
- The installation requirements of synchronous belt drive are higher than those of ordinary flat belts
- Timing belt pulleys should consider the installation of retaining rings
- Increase the radius of the rounded corners of the top of the belt teeth and the top of the wheel teeth
- Positive deviation of the outer diameter of the synchronous belt should be adopted
- The chain drive should be tightly edged on the top
- When two chain pulleys are arranged up and down, the small chain pulley should be on top
- A chain should not be used to drive multiple sprockets on a horizontal line
- Pay attention to the influence of flexural drive tension change on the bearing load
- The chain should be lubricated with a small amount of oil.
- The center distance of the chain drive should be adjustable
- The direction of the chain spring should be adapted to the running direction of the chain
- The belt and chain drive should be covered
- The diameter of the rope wheel should not be reduced arbitrarily
- Repeated bending of the steel rope should be avoided
- The designer must strictly stipulate the steel rope end-of-life standard
- Steel rope must be regularly lubricated
- The surface of the reel should have a rope groove
Eighteen. Gear transmission structure design of mechanical design of mechanical design
- Gear arrangement should be considered in favor of shaft and shaft bearing force
- Herringbone gears of the two directions of tooth combination point (A) should enter the mesh first
- Gear diameter is small and should be made into the gear shaft
- The root circle diameter of the gear can be smaller than the diameter of the shaft
- The width of the pinion should be greater than the width of the large gear
- The gear block should consider the distance cut out by the tool when processing the gear
- The coupling of gear and shaft should reduce the machining during assembly
- Pay attention to ensuring consistent gear stiffness along the width of the teeth
- Use the uneven deformation of the gear to compensate for the deformation of the shaft
- Split large gears should be separated at the absence of spokes
- The surface hardening layer of the gear teeth should not be interrupted
- Bevel gear shafts must be fixed in both directions
- Both large and small bevel gear shafts should be able to be axially adjusted
- Combined bevel gear structure in the bolt to be free from tension
Nineteen. Worm drive structure design of mechanical design of mechanical design
- Worm self-locking is not reliable
- Cooling fan should be installed on the worm gear
- The direction of the heat sink outside the worm gear reducer is related to the cooling method
- Worm gears are more severely affected by heat than worm wheels
- The position of the worm gear is related to the speed
- The worm gear stiffness is not only determined by the force applied during the operation
- The complexity of the force on the worm drive affects the precision of precision machinery
- The force of the worm drive affects the flexibility of rotation
Twenty. Reducer and transmission structure design of mechanical design of mechanical design
- The transmission should strive to form a component
- The transmission ratio of the primary transmission should not be too large or too small
- Transmission of high power should be used to divert transmission
- Try to avoid the use of vertical reducer
- Pay attention to the pressure balance inside and outside the gearbox
- The box surface should not be gasketed
- The vertical box should prevent oil leakage from the splitting surface
- There should be enough oil in the box and replace it in time
- Planetary gear reducer should have an equal load device
- Gearbox moving gears should have a neutral position
- Gearbox gears should have rounded teeth
- Friction wheels and friction stepless transmissions should avoid geometric sliding
- Soft material for active friction wheels
- Conical friction wheel transmission, the compression spring should be mounted on the small conical friction wheel
- The design should seek to increase the force transmission path and turn the compression force into internal force
- The mechanical characteristics of the stepless transmission should be matched to the working machine and the prime mover
- The bus line of the working cone of the pulley with CVT is not straight
Twenty-one. Design of transmission system structure of mechanical design of mechanical design
- Avoid motion uncertainty of the hinged four-bar mechanism
- Pay attention to the dead point of the mechanism
- Avoid side thrust on the guide rail
- The limit switch should be set on the member with a larger stroke in the linkage mechanism
- Pay attention to the transmission angle must not be too small
- The pendulum of the cylindrical cam of the swing follower should not be too short
- Correct arrangement of the guide position of the offset follower disc cam moving the follower
- Balance of the planar linkage mechanism
- Design of intermittent motion mechanisms should take into account the coefficient of motion
- Analysis of the reliability of the locking device using the instantaneous stop section
- Selecting the type of gearing, considering cylindrical gears first
- When machinery requires reversal, the motor reversal can generally be considered
- The starting performance of the prime mover must be considered
- The crane lifting mechanism shall not use a friction drive
- For the requirement of slow moving mechanism, the spiral is better than a rack and pinion
- Use the standard gearbox with a large transmission ratio instead of bulk transmission
- Use reduction motors instead of prime movers and transmissions
- Use of shaft-mounted reducers
Twenty-two. Coupling clutch structure design of mechanical design of mechanical design
- Reasonable choice of coupling type
- Balance of the coupling
- Couplings with sliding friction should pay attention to maintaining good lubrication conditions
- High-speed rotating couplings must not have protrusions outside
- Use couplings with convex shoulders and grooves for alignment, taking into account the disassembly of the shaft
- If the two ends of the shaft require synchronous rotation, flexible couplings with elastic elements should not be used
- Do not use cross-slide couplings at both ends when the intermediate shaft has no bearing support
- Single universal coupling cannot achieve synchronous rotation between the two shafts
- Do not use the jacket of the gear coupling as a brake wheel
- Pay attention to the lubrication of the gear coupling
- Note on nylon rope coupling
- Precautions for shear pin-type safety clutches
- Do not use oil-lubricated friction disc clutches for rapid disengagement
- Multi-disc friction clutches should not be used when working at high temperatures
- The clutch operating ring should be mounted on the half-clutch connected to the driven shaft
Twenty-three. Design of the shaft structure of mechanical design
- Minimise the stress concentration in the abrupt change of the shaft section
- To reduce the stress concentration in the interference fit of the shaft
- Pay attention to the influence of stress concentrations caused by the keyway on the shaft
- To reduce the difficulties of assembly and disassembly of interference-fit parts
- The starting point of the assembly should not be at a sharp angle, and the starting point of the two fitting surfaces should not be assembled at the same time
- The positioning of the parts on the shaft should be done using shoulders or rings
- Blind holes into the interference fit shaft should be considered to discharge air
- Reasonable arrangement of shaft parts and improved structure to reduce the force on the shaft
- Use of load shunting to improve the strength and stiffness of the shaft
- Use of central equidistant drives to prevent torsional deformation differences at both ends
- Improved surface quality and fatigue strength of shafts
- Reasonable setting of multiple keyway positions on the shaft
- The lower wall thickness of the keyway in hollow shafts should not be too thin
- Easy machining of the keyway on the shaft
- It is difficult to drill long, thin holes in the shaft
- Cut threads on the rotating shaft to facilitate the loosening of the fastening nut
- Ensure the correct installation of the stop washer on the shaft
- Ensure dimensional differences in the compression or clearance of the shaft and the mounting parts
- Avoid axial forces on elastic rings
- Hollow shafts save material
- Do not operate the shaft at the same or close to its inherent frequency
- Keep flexible couplings for high-speed shafts as close to the bearings as possible
- Avoid zero bearing reaction forces on shafts
- Do not connect small shafts directly to the shaft end of large shafts
- Sufficient hardness is required on the surface of the journal
Twenty-four. Design of the plain bearing structure of mechanical design
- To make the lubricating oil can enter the friction surface smoothly
- Lubricating oil should be introduced into the bearing from the non-load area
- Do not make the full ring oil groove open in the middle of the bearing
- Split shaft tile joints should be open oil groove
- Make the oil ring fully oiled and reliable
- Do not block the oiling holes
- Do not form a non-flowing area of lubricating oil
- Prevent sharp edges or angles that cut the oil film
- Stepped wear occurs
- Do not make linear contact between the thrust end faces of the shaft shank
- The thrust bearing should not be in full contact with the journal
- The starting of high speed rotating shaft of heavy load large machinery needs high-pressure top shaft system bearing
- Bear heavy loads or high-temperature rise bearings should not be hollowed out in the middle of the contact surface of the bearing housing and the shaft shank
- Do not use bearings that cannot be assembled or disassembled, such as shingles or bushings
- Reduce the edge pressure on the bearings supporting the intermediate wheel and cantilever shaft
- Choose self-aligning plain bearings if the bearing housing bore is not centered or if the axis is deformed after being loaded.
- Do not allow relative movement of the bearing housing and the shaft tile.
- To make the bimetallic bearing in the two metals attached firmly
- Ensure a reasonable running clearance
- ensure the clearance required for thermal expansion of the shaft during operation
- Consider clearance adjustment after wear
- Prevent instability in cylindrical bearings used at high speeds and under light loads
- Use bearings with good anti-vibration properties for high-speed and light load conditions
- Oil bearings should not be used for high-speed or continuous rotation purposes
- Sliding bearings should not be combined with sealing rings
- In the bearing cover or the upper half of the box lifting process do not make the shaft tile off
Twenty-five. Rolling bearing shaft system structure design of mechanical design
- Consider the design of bearing dismantling
- Bearing inner circle radius and shoulder radius
- Combination of a pair of angular contact bearings
- angular contact bearing in series with the combination
- Angular contact bearings should not be combined with non-adjustable clearance bearings in pairs
- The combination of bearings should facilitate uniform load sharing
- ensure that the expansion or contraction of the shaft is required due to temperature changes
- Consider the temperature change and thermal expansion of the inner and outer ring when the combination of tapered roller bearings
- Shafts requiring high axial positioning accuracy should use adjustable axial clearance bearings
- It is not advisable to use a rolling bearing to support the swim wheel and intermediate wheel
- In the two machine seat holes different hearts or in the load after the axis deflection deformation conditions in the use of the shaft to choose a spherical performance of the bearing
- Consider the difficulties of mounting intermediate bearings when designing multi-pivot bearings for equal-diameter shafts
- Not suitable for high-speed rotating rolling bearings
- Shafts requiring high rigidity of support should use bearings with high rigidity
- Rolling bearings should not be used in combination with sliding bearings
- Grease-lubricated roller bearings and dustproof, sealed bearings are easy to heat up
- Avoid filling with excessive grease, do not form grease flow end
- Angle contact bearings lubricated with grease should be installed on the vertical shaft to prevent grease from being removed from the lower part of the bearing.
- Use grease lubrication to avoid oil, grease mixed
- Oil lubrication should be noted when the problem
- Bearing box shape and rigidity of the impact
- Bearing seat force direction should be pointed to the bottom of the support
- The holes of the bearing on the housing should strive to simplify the borehole
- For the inner and outer rings can not be separated bearing in the housing hole should be easy to install and remove
- Should not use the method of axial fastening to prevent the bearing with the surface creep
Twenty-six. Sealing device structure design of mechanical design
- Static sealing gasket can not be installed between the wire
- Static coupling surface should have a certain degree of roughness
- The width of the contact surface of the high-pressure vessel seal should be small
- Gaskets should be added when sealing with edges
- The O-ring seal should have a protective ring when used for high-pressure sealing
- Avoid O-ring edge protrusion being cut off
- When the position of the shaft center in contact with the seal changes frequently, contact seals should not be used
- Correct use of skin ring seal
- Do not rely on the threaded rotating gland to compress the packing of the seal
- When there is more packing, the packing hole is not deep enough to compress
- To prevent packing hair
- Different parts of the seal should be supplied with oil separately
- When using oil to lubricate the sealing device, keep the oil level at a certain height
- When there is a gap in the seal, the gap in the multi-layer seal should be staggered
Twenty-seven. Oil pressure system and pipe structure design of mechanical design
- Piping arrangement should be easy to disassemble and inspect
- The strength of Y-joints for large-diameter pipes is poor
- To avoid mixing air in the oil pressure pipeline
- Drainage should be noted at low points in the pipeline
- Discharge pipes should avoid interfering with each other due to combined flow
- The pipeline should be smooth and avoid disturbance when merging
- Stress caused by the expansion and contraction of the pipeline should be avoided
- Parts of the piping system that require frequent operation and observation should be easy to operate
- Pipeline joints should not be left or right threaded
- Pay attention to the design of pipeline support
- Do not move equipment when disassembling and assembling pipelines
- Pay attention to the lagging phenomenon of hydraulic and pneumatic equipment
- Avoid additional stress on the hose
- When the pressure of the medium in the hose is impulse change, the hose should be fixed
- To consider the oil supply when starting and stopping
- The internal relief valve of the oil pump should not be commonly used
- Cooling water contamination will reduce the cooling capacity
- Prevent condensation on the surface of cooling water pipes
- Prevent the handwheel of the valve from turning during vibration
- Difficulty in opening large-diameter globe valves
- To prevent the safety valve from opening when the media ejected to injure people
Twenty-eight. Frame structure design of mechanical design
- Reduce the number of cores
- Avoid the use of core support to prevent leakage
- Change the inner cavity structure to ensure the strength of the core iron and facilitate sand cleaning
- Pay attention to the small size of the parts
- Improve the cooling of the casting
- Simplify the molding of the casting
- Improving the structure and eliminating the need for cores
- Prevent deformation of the casting frame
- The structure at the throat should be reinforced
- Pay attention to strengthening the torsional rigidity of the base
- Change the casting into a punching and welding structure
- Change forging parts to the cast-forging-welding structure
- Reduce the wall thickness to save metal
Twenty-nine. The structural design of the guide rail of mechanical design
- In general, it is not suitable to use double V guide rails
- The table supported by the guide rail, the driving force should make the two guide rails balance the resistance moment
- Table and rail should be “short on”
- Double rectangular guide rails should be considered to adjust the clearance
- The distance between the guiding surfaces should not be too large when the temperature of the guideway changes significantly
- The pressure plate of the guideway should be fixed with good contact, stable and reliable
- The pressure plate should have a size division
- Steel guide rails should not be fixed with slotted countersunk head screws
- The inlay should be adjusted without gaps
- The guiding surface should be unchanged
- Inserts should be installed on the unstressed surface
- Avoid casting defects in the guideway
- The supporting part of the guideway should have a high stiffness
- The screws fixing the guideway should not be placed at an angle
- Avoid deformation of the guideway caused by tightening the set screws to affect the accuracy
- Ball guides should have sufficient hardness
- The roller guide should not be too long
- Try to avoid the use of scraping research guide
- To prevent the rolling parts out of the guide, install the limit device
- Reduce the adjustment work of the guide installation
- Note that the matching guide surface can be mutual research
Thirty. Spring structure design of mechanical design
- The spring should be necessary to adjust the device
- Helical compression springs should have a certain margin when subjected to the maximum working load
- Tension springs should have a safety device
- Combination of spiral spring rotation should be the opposite
- Attention should be paid to the direction of the force of the helical torsion spring
- Attention should be paid to the wear and lubrication of the plate spring pin
- Ring springs should be considered for their resetting
- Springs should avoid stress concentrations
- The springs of automatic loading should avoid winding each other
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