[Weekly Topic] Discuss the main design criteria for mechanical parts (2011.06.20-06.26)

Design criteria for mechanical parts: The design of mechanical parts is subject to numerous constraints, and the design criteria represent the requirements that must be met during the design process. I. Technical Performance Criteria Technical performance includes all aspects of a product’s functionality, as well as its manufacturing and operational conditions; it refers both to static and dynamic performance. For example, the power that the product can deliver, efficiency, service life, strength, stiffness, friction resistance, wear resistance, vibration stability, thermal properties, etc. Technical performance criteria refer to the requirement that relevant technical performances must meet specified standards. For example, vibration generates additional dynamic loads and variable stresses; especially when its frequency is close to the natural frequency of the mechanical system or its components, resonance occurs. In such cases, the amplitude increases sharply, which can lead to rapid damage of the components or even the entire system. The vibrational stability criterion is to keep the relevant vibration parameters of mechanical systems or components, such as natural frequency, amplitude, and noise, within specified allowable ranges. For example, the heat generated during machine operation can lead to thermal stress and thermal strain, and may even cause thermal damage. The thermal characteristic criterion is to keep various relevant thermal parameters (such as thermal stress, thermal strain, temperature rise, etc.) within specified limits. II. Standardization Criteria The main standards related to mechanical product design include: Conceptual standardization: Terms, symbols, units of measurement, etc., used in the design process should comply with standards ； Standardization of physical form: The structural format, dimensions, performance, etc., of components, raw materials, equipment, and energy sources should all be selected in accordance with unified specifications. Method standardization: Operating methods, measurement methods, test methods, etc., shall be implemented in accordance with relevant regulations. The standardization criteria mean that all actions throughout the entire design process must meet the aforementioned standardization requirements. The standards related to mechanical part design that have been issued can be classified, in terms of their scope of application, into **three categories: national standards, industry standards, and enterprise standards. In terms of mandatory use, they can be divided into those that must be implemented and those that are recommended for use. III. Reliability Criteria Reliability: The probability that a product or component will be able to perform its specified functions within the expected lifespan, under specified operating conditions. The reliability criterion refers to the requirement that the designed products, components, or parts must meet specified reliability requirements. IV. Safety Criteria The safety of machines includes: Component safety: This refers to the ability of components to withstand specified external loads within a given time period without experiencing issues such as fracture, excessive deformation, excessive wear, or loss of stability. Overall machine safety: refers to the requirement that the machine operates without failure under specified conditions and can fulfill its overall functions properly. Work safety: refers to the protection of operators, ensuring their physical safety and mental well-being, etc. Environmental safety: Refers to the absence of pollution and harm to the surrounding environment and people caused by the machine. (

Reply to 1# 3968668: 1. The strength criterion requires that the working stress σ of mechanical parts shall not exceed the allowable stress. The typical calculation formula is: (3-16) σlim – the limiting stress. For brittle materials subjected to static stress, it is taken as their ultimate strength; for plastic materials subjected to static stress, it is taken as their yield limit; for materials subjected to variable stress, it is taken as their fatigue limit. S – Safety factor. 2. Stiffness criterion: Mechanical parts undergo elastic deformation when subjected to loads, and stiffness is the ability of a material, mechanical part, or structure under external forces to resist deformation. The stiffness of a material is measured by the amount of external force required to cause a unit deformation in it. The stiffness of a mechanical part depends on its elastic modulus E or shear modulus G, its geometry and dimensions, as well as the nature of the external forces acting on it. Analyzing the stiffness of mechanical parts is an important task in mechanical design. For certain components that require strict limitation of deformation (such as aircraft wings and machine tool spindles), stiffness analysis must be conducted to control such deformation. We also need to control the stiffness of the components to prevent vibration or instability. Furthermore, like springs, it is necessary to control their stiffness to a reasonable value in order to ensure their specific function. The stiffness criterion requires that the elastic deformation of a part under load does not exceed the allowable elastic deformation. The expression for the stiffness criterion is (3–17). y represents the amount of elastic deformation, such as deflection or longitudinal elongation (shortening); it is the corresponding allowable amount of elastic deformation. The elastic deformation amount of a part can be determined through theoretical calculations or experiments, while the allowable deformation amount depends on the purpose of the part and is determined based on theoretical analysis or experience. 3. Heat resistance criterion: Due to factors such as friction, the temperatures of mechanical components and lubricants generally increase during machine operation. Excessively high operating temperatures will lead to a decrease in the lubrication effect; at the same time, they can cause thermal deformation of the components, as well as a reduction in their hardness and strength, and may even result in damage. At high temperatures, metal mechanical parts may experience bonding or sticking ； Non-metallic mechanical parts such as plastics may soften or even melt, and in some cases, thermal stress can also be generated. The heat resistance criterion generally aims to keep the operating temperature of mechanical components below the allowable value, in order to ensure their proper functioning. Its expression is given by equation (3–18). To improve heat dissipation and control temperature rise, measures such as water cooling or air cooling can be employed when necessary. 4. Vibration stability criterion: When the frequency of the excitation equals the natural frequency of the object, its amplitude is at its maximum; the greater the difference between the excitation frequency and the natural frequency, the smaller the object’s amplitude. When the frequency of the excitation is close to the natural frequency of an object, the amplitude of the forced vibration becomes very large. This phenomenon is called resonance. Vibration stability refers to the quality of mechanical components in avoiding resonance during machine operation. To extend the lifespan of the machine and prevent damage to the shafts and the machine itself, it is necessary to verify the vibration stability of the shafts, especially those in high-speed machines. The vibration stability criterion requires that the natural frequency of mechanical components be offset from the frequency of the excitation to prevent resonance. Let the natural frequency of the components in the machine that are subjected to excitation be f, and let the frequency of the exciting force be fp. It is generally required that fp < 0.85 f or fp > 1.15 f (3–19). Changing the stiffness and mass of the mechanical components can alter their natural frequencies. Increase the stiffness of mechanical components and reduce their mass to raise their natural frequency ； Reducing the stiffness and increasing the mass of mechanical parts decreases their natural frequency. Sometimes, the speed of the machine needs to be adjusted while it is running to prevent resonance. The main reason for resonance in the shaft is that, due to uneven mass distribution within the material, along with manufacturing and installation errors, there is a deviation between its center of mass and its center of rotation. As the shaft rotates, inertial forces are generated, and these forces cause the rotor to undergo forced vibrations. The speed of the shaft at which resonance occurs is called the critical speed. At the critical speed, the frequency of this inertial force is equal to or several times the natural frequency of the rotor, resulting in resonance. 5. Life expectancy criterion: To ensure that a machine operates properly over a certain period of time, it is necessary to set requirements regarding the life expectancy of its mechanical components during the design phase. It should be noted that during the machine’s service life, parts can be replaced; in other words, the lifespan of certain mechanical parts can be shorter than that of the machine itself. The lifespan of mechanical parts is primarily influenced by material fatigue, wear, and corrosion. To avoid failures caused by part fatigue, such as fatigue fracture, the fatigue strength should be calculated based on the fatigue limit corresponding to the service life of the mechanical parts. That is, based on the life requirement and taking into account factors such as the part’s rotational speed, the fatigue limit at stress cycle count N is calculated using equation (3-6); this value is then used in the strength condition equation to determine the fatigue strength. When the fatigue strength is satisfied, it can be ensured that the number of stress cycles before failure of the mechanical part meets the life requirement. Wear is generally inevitable. Under certain conditions, corrosion is also inevitable, such as the corrosion of bridge structural components and buried steel pipelines. During design, the main goal is to ensure that mechanical components do not suffer excessive wear and corrosion over their lifespan. The mechanism by which wear occurs has not yet been fully understood, and there are many factors that influence wear. Generally, the wear resistance of friction pairs is improved based on the principles of tribological design. The main measures include: reasonably selecting the materials for the friction pair ； Select lubricants and additives appropriately ； Control the operating conditions of the friction pair, such as pressure, sliding velocity, and temperature rise. To date, there is no practical and effective method for calculating corrosion life; measures are usually taken in terms of material selection and anti-corrosion treatment. If corrosion-resistant materials are used, surface coating, spraying, phosphating and other treatments are employed. 6. Reliability criteria: Reliability is the ability of a product to perform its specified functions under specified conditions and within a specified period of time. The quality of a product generally should include performance indicators and reliability indicators. The performance indicators of mechanical products refer to the technical specifications of those products, such as the power, torque, working force, and operating speed of the machinery. If there are only performance metrics and no reliability metrics, the performance of the product cannot be guaranteed either. For example, an advanced aircraft with low reliability is bound to experience frequent failures, which disrupt normal flight operations and increase maintenance costs; it may even lead to serious accidents. The reliability of a product is measured by the reliability R(t). Reliability is defined as the probability that a product will perform its specified functions under specified conditions for a specified period of time. Reliability is a function of time. There is a batch of n identical products that start operating at t=0. Over time, the number of defective units no(t) increases, while the number of functional units ni(t) decreases. At any given time t, the reliability of the products is given by equation (3–20). If the reliability R(t) of a product after 3000 hours of operation is 0.96, this means that 96% of the products can continue to function properly for more than 3000 hours; for a specific product, the probability that it will function for 3000 hours is 96%. Failure rate refers to the probability that a product will fail within a unit of time in the subsequent phase, by the time it has been in operation for t. It can be shown that its mathematical expression is given by equation (3–21). By separating the variables and integrating both sides, equation (3–22) is obtained. The relationship between a component’s failure rate and time is generally illustrated in Figure 3-13. The efficiency loss curve can be determined through experimental methods. The failure rate curve reflects the failure rate of a product over its entire lifetime. As can be seen from the failure curve, failure can generally be divided into three stages. Figure 3-15: Phase I is the early failure phase, and the curve is decreasing in shape. In the early stages of product use, the failure rate is high and declines rapidly. The main reasons are defects arising from design, manufacturing, storage, transportation, etc., as well as human factors such as improper commissioning, running-in, and startup. When these failures caused by congenital defects occur and the equipment begins to operate normally again, the failure rate tends to stabilize. Every effort should be made to prevent premature failure of components, thereby reducing the failure rate and the time t0 in the early failure stage. Stage II is the random failure stage, during which the failure rate increases slowly. Failure is mainly caused by accidental factors such as unexpected overload, misoperation, and sudden natural disasters. Since the causes of failure are mostly accidental, it is referred to as the accidental failure stage. Reducing the failure rate during the period of accidental failures can increase the effective service life; therefore, it is important to focus on improving product quality and to carry out proper use and maintenance. Stage III is the damage and failure stage, during which the failure rate is increasing. The failure rate increases significantly after t1. This is caused by factors leading to wear and tear, such as product aging, fatigue, abrasion, creep, and corrosion; hence it is referred to as the wear-out failure period. To address the causes of failure at this stage, it is necessary to pay attention to inspections and monitoring, as well as carry out repairs in advance, so as to prevent the failure rate from rising. 7. Accuracy criteria: For high-precision mechanical parts, mechanisms, or equipment, it is required that their motion errors be below the allowable values. For example, in precision machinery, certain accuracy requirements must be met for things such as the linear error of guide rails, the radial runout error of spindles, and the angular error in gear transmissions. Based on the functional requirements of machines and parts, appropriate tolerances and fits can be selected—that is, precision design can be carried out—and these can be correctly indicated on the drawings. The error of the mechanism can also be calculated using the tolerance values specified in the part diagram, and then compared with the required precision of the mechanism.

1. The strength criterion requires that the working stress σ of mechanical components not exceed the allowable stress. The typical calculation formula is: (3-16) σlim – the limiting stress. For brittle materials subjected to static stress, it is taken as their ultimate strength; for plastic materials subjected to static stress, it is taken as their yield limit; for materials subjected to variable stress, it is taken as their fatigue limit. S – Safety factor. 2. Stiffness criterion: Mechanical parts undergo elastic deformation when subjected to loads, and stiffness is the ability of a material, mechanical part, or structure under external forces to resist deformation. The stiffness of a material is measured by the amount of external force required to cause a unit deformation in it. The stiffness of a mechanical part depends on its elastic modulus E or shear modulus G, its geometry and dimensions, as well as the nature of the external forces acting on it. Analyzing the stiffness of mechanical parts is an important task in mechanical design. For certain components that require strict limitation of deformation (such as aircraft wings and machine tool spindles), stiffness analysis must be conducted to control such deformation. We also need to control the stiffness of the components to prevent vibration or instability. Furthermore, like springs, it is necessary to control their stiffness to a reasonable value in order to ensure their specific function. The stiffness criterion requires that the elastic deformation of a part under load does not exceed the allowable elastic deformation. The expression for the stiffness criterion is (3–17). y represents the amount of elastic deformation, such as deflection or longitudinal elongation (shortening); it is the corresponding allowable amount of elastic deformation. The elastic deformation amount of a part can be determined through theoretical calculations or experiments, while the allowable deformation amount depends on the purpose of the part and is determined based on theoretical analysis or experience. 3. Heat resistance criterion: Due to factors such as friction, the temperatures of mechanical components and lubricants generally increase during machine operation. Excessively high operating temperatures will lead to a decrease in the lubrication effect; at the same time, they can cause thermal deformation of the components, as well as a reduction in their hardness and strength, and may even result in damage. At high temperatures, metal mechanical parts may experience bonding or sticking ； Non-metallic mechanical parts such as plastics may soften or even melt, and in some cases, thermal stress can also be generated. The heat resistance criterion generally aims to keep the operating temperature of mechanical components below the allowable value, in order to ensure their proper functioning. Its expression is given by equation (3–18). To improve heat dissipation and control temperature rise, measures such as water cooling or air cooling can be employed when necessary. 4. Vibration stability criterion: When the frequency of the excitation equals the natural frequency of the object, its amplitude is at its maximum; the greater the difference between the excitation frequency and the natural frequency, the smaller the object’s amplitude. When the frequency of the excitation is close to the natural frequency of an object, the amplitude of the forced vibration becomes very large. This phenomenon is called resonance. Vibration stability refers to the quality of mechanical components in avoiding resonance during machine operation. To extend the lifespan of the machine and prevent damage to the shafts and the machine itself, it is necessary to verify the vibration stability of the shafts, especially those in high-speed machines. The vibration stability criterion requires that the natural frequency of mechanical components be offset from the frequency of the excitation to prevent resonance. Let the natural frequency of the components in the machine that are subjected to excitation be f, and let the frequency of the exciting force be fp. It is generally required that fp < 0.85 f or fp > 1.15 f (3–19). Changing the stiffness and mass of the mechanical components can alter their natural frequencies. Increase the stiffness of mechanical components and reduce their mass to raise their natural frequency ； Reducing the stiffness and increasing the mass of mechanical parts decreases their natural frequency. Sometimes, the speed of the machine needs to be adjusted while it is running to prevent resonance. The main reason for resonance in the shaft is that, due to uneven mass distribution within the material, along with manufacturing and installation errors, there is a deviation between its center of mass and its center of rotation. As the shaft rotates, inertial forces are generated, and these forces cause the rotor to undergo forced vibrations. The speed of the shaft at which resonance occurs is called the critical speed. At the critical speed, the frequency of this inertial force is equal to or several times the natural frequency of the rotor, resulting in resonance. 5. Life expectancy criterion: To ensure that a machine operates properly over a certain period of time, it is necessary to set requirements regarding the life expectancy of its mechanical components during the design phase. It should be noted that during the machine’s service life, parts can be replaced; in other words, the lifespan of certain mechanical parts can be shorter than that of the machine itself. The lifespan of mechanical parts is primarily influenced by material fatigue, wear, and corrosion. To avoid failures caused by part fatigue, such as fatigue fracture, the fatigue strength should be calculated based on the fatigue limit corresponding to the service life of the mechanical parts. That is, based on the life requirement and taking into account factors such as the part’s rotational speed, the fatigue limit at stress cycle count N is calculated using equation (3-6); this value is then used in the strength condition equation to determine the fatigue strength. When the fatigue strength is satisfied, it can be ensured that the number of stress cycles before failure of the mechanical part meets the life requirement. Wear is generally inevitable. Under certain conditions, corrosion is also inevitable, such as the corrosion of bridge structural components and buried steel pipelines. During design, the main goal is to ensure that mechanical components do not suffer excessive wear and corrosion over their lifespan. The mechanism by which wear occurs has not yet been fully understood, and there are many factors that influence wear. Generally, the wear resistance of friction pairs is improved based on the principles of tribological design. The main measures include: reasonably selecting the materials for the friction pair ； Select lubricants and additives appropriately ； Control the operating conditions of the friction pair, such as pressure, sliding velocity, and temperature rise. To date, there is no practical and effective method for calculating corrosion life; measures are usually taken in terms of material selection and anti-corrosion treatment. If corrosion-resistant materials are used, surface coating, spraying, phosphating and other treatments are employed. 6. Reliability criteria: Reliability is the ability of a product to perform its specified functions under specified conditions and within a specified period of time. The quality of a product generally should include performance indicators and reliability indicators. The performance indicators of mechanical products refer to the technical specifications of those products, such as the power, torque, working force, and operating speed of the machinery. If there are only performance metrics and no reliability metrics, the performance of the product cannot be guaranteed either. For example, an advanced aircraft with low reliability is bound to experience frequent failures, which disrupt normal flight operations and increase maintenance costs; it may even lead to serious accidents. The reliability of a product is measured by the reliability R(t). Reliability is defined as the probability that a product will perform its specified functions under specified conditions for a specified period of time. Reliability is a function of time. There is a batch of n identical products that start operating at t=0. Over time, the number of defective units no(t) increases, while the number of functional units ni(t) decreases. At any given time t, the reliability of the products is given by equation (3–20). If the reliability R(t) of a product after 3000 hours of operation is 0.96, this means that 96% of the products can continue to function properly for more than 3000 hours; for a specific product, the probability that it will function for 3000 hours is 96%. Failure rate refers to the probability that a product will fail within a unit of time in the subsequent phase, by the time it has been in operation for t. It can be shown that its mathematical expression is given by equation (3–21). By separating the variables and integrating both sides, equation (3–22) is obtained. The relationship between a component’s failure rate and time is generally illustrated in Figure 3-13. The efficiency loss curve can be determined through experimental methods. The failure rate curve reflects the failure rate of a product over its entire lifetime. As can be seen from the failure curve, failure can generally be divided into three stages. Figure 3-15: Phase I is the early failure phase, and the curve is decreasing in shape. In the early stages of product use, the failure rate is high and declines rapidly. The main reasons are defects arising from design, manufacturing, storage, transportation, etc., as well as human factors such as improper commissioning, running-in, and startup. When these failures caused by congenital defects occur and the equipment begins to operate normally again, the failure rate tends to stabilize. Every effort should be made to prevent premature failure of components, thereby reducing the failure rate and the time t0 in the early failure stage. Stage II is the random failure stage, during which the failure rate increases slowly. Failure is mainly caused by accidental factors such as unexpected overload, misoperation, and sudden natural disasters. Since the causes of failure are mostly accidental, it is referred to as the accidental failure stage. Reducing the failure rate during the period of accidental failures can increase the effective service life; therefore, it is important to focus on improving product quality and to carry out proper use and maintenance. Stage III is the damage and failure stage, during which the failure rate is increasing. The failure rate increases significantly after t1. This is caused by factors leading to wear and tear, such as product aging, fatigue, abrasion, creep, and corrosion; hence it is referred to as the wear-out failure period. To address the causes of failure at this stage, it is necessary to pay attention to inspections and monitoring, as well as carry out repairs in advance, so as to prevent the failure rate from rising. 7. Accuracy criteria: For high-precision mechanical parts, mechanisms, or equipment, it is required that their motion errors be below the allowable values. For example, in precision machinery, certain accuracy requirements must be met for things such as the linear error of guide rails, the radial runout error of spindles, and the angular error in gear transmissions. Based on the functional requirements of machines and parts, appropriate tolerances and fits can be selected—that is, precision design can be carried out—and these can be correctly indicated on the drawings. It is also possible to calculate the error of the mechanism using the tolerance values specified in the part diagram, and compare it with the required precision of the mechanism

1. Strength criterion 2. Stiffness criterion 3. Heat resistance criterion 4. Vibration stability criterion 5. Service life criterion 6. Reliability criterion 7. Accuracy criterion

1. Strength criterion. It is required that the working stress σ of mechanical parts does not exceed the allowable stress. 2. Stiffness criterion. Mechanical parts undergo elastic deformation when subjected to loads, and stiffness is the ability of a material, mechanical part, or structure to resist deformation under external forces. The stiffness of a material is measured by the amount of external force required to cause a unit deformation in it. 3. Heat resistance criterion. Due to friction and other factors, when a machine is in operation, the temperatures of its mechanical components and lubricants generally rise. Excessively high operating temperatures will lead to a decrease in the lubrication effect; at the same time, they can cause thermal deformation of the components, as well as a reduction in their hardness and strength, and may even result in damage. At high temperatures, metal mechanical parts may experience bonding or sticking ； Non-metallic mechanical parts such as plastics may soften or even melt, and in some cases, thermal stress can also be generated. The heat resistance criterion generally aims to keep the operating temperature of mechanical components below the allowable value, in order to ensure their proper functioning. To improve heat dissipation performance and control temperature rise, water cooling or air cooling can be employed when necessary. 4. Vibration stability criterion. When the frequency of the excitation equals the object’s natural frequency, the amplitude of the object is at its maximum; the greater the difference between the excitation frequency and the natural frequency, the smaller the amplitude of the object. When the frequency of the excitation is close to the natural frequency of an object, the amplitude of the forced vibration becomes very large. This phenomenon is called resonance. Vibration stability refers to the quality of mechanical components in avoiding resonance during machine operation. 5. Life expectancy criterion. To ensure that the machine operates properly over a certain service life, it is necessary to set requirements regarding the lifespan of its mechanical components during their design. It should be noted that during the machine’s service life, parts can be replaced; in other words, the lifespan of certain mechanical parts can be shorter than that of the machine itself. The lifespan of mechanical parts is primarily influenced by material fatigue, wear, and corrosion. 6. Reliability criteria. Reliability is the ability of a product to perform its specified functions under specified conditions and within a specified time period. The quality of a product generally should include performance indicators and reliability indicators. The performance indicators of mechanical products refer to the technical specifications of those products, such as the power, torque, working force, and operating speed of the machinery. If there are only performance metrics and no reliability metrics, the performance of the product cannot be guaranteed either. 7. Accuracy criterion. For high-precision mechanical parts, mechanisms, or equipment, it is required that their motion errors be below the allowable values. For example, in precision machinery, certain accuracy requirements must be met for things such as the linear error of guide rails, the radial runout error of spindles, and the angular error in gear transmissions. Based on the functional requirements of machines and parts, appropriate tolerances and fits can be selected—that is, precision design can be carried out—and these can be correctly indicated on the drawings. The error of the mechanism can also be calculated using the tolerance values specified in the part diagram, and then compared with the required precision of the mechanism.

1. The strength criterion requires that the working stress σ of mechanical components not exceed the allowable stress. Its typical calculation formula is: http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image002.gif (3-16). σlim – the limit stress; for brittle materials under static stress, this value is taken as their strength limit, for plastic materials under static stress it is their yield limit, and for materials under variable stress it is their fatigue limit. S – Safety factor. 2. Stiffness criterion: Mechanical parts undergo elastic deformation when subjected to loads, and stiffness is the ability of a material, mechanical part, or structure under external forces to resist deformation. The stiffness of a material is measured by the amount of external force required to cause a unit deformation in it. The stiffness of a mechanical part depends on its elastic modulus E or shear modulus G, its geometry and dimensions, as well as the nature of the external forces acting on it. Analyzing the stiffness of mechanical parts is an important task in mechanical design. For certain components that require strict limitation of deformation (such as aircraft wings and machine tool spindles), stiffness analysis must be conducted to control such deformation. We also need to control the stiffness of the components to prevent vibration or instability. Furthermore, like springs, it is necessary to control their stiffness to a reasonable value in order to ensure their specific function. The stiffness criterion requires that the elastic deformation of a part under load does not exceed the allowable elastic deformation. The expression for the stiffness criterion is given at http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image006.gif (3–17). y represents the amount of elastic deformation, such as deflection or longitudinal elongation (shortening); it represents the allowable amount of elastic deformation. The elastic deformation amount of a part can be determined through theoretical calculations or experiments, while the allowable deformation amount depends on the purpose of the part and is determined based on theoretical analysis or experience. 3. Heat resistance criterion: Due to factors such as friction, the temperatures of mechanical components and lubricants generally increase during machine operation. Excessively high operating temperatures will lead to a decrease in the lubrication effect; at the same time, they can cause thermal deformation of the components, as well as a reduction in their hardness and strength, and may even result in damage. At high temperatures, metal mechanical parts may experience bonding or sticking ； Non-metallic mechanical parts such as plastics may soften or even melt, and in some cases, thermal stress can also be generated. The heat resistance criterion generally aims to keep the operating temperature of mechanical components below acceptable levels, thereby ensuring their proper functioning. Its expression is given in http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image008.gif (3–18). To improve heat dissipation and control temperature rises, measures such as water cooling or air cooling can be employed when necessary. 4. Vibration stability criterion: When the frequency of the excitation equals the natural frequency of the object, its amplitude is at its maximum; the greater the difference between the excitation frequency and the natural frequency, the smaller the object’s amplitude. When the frequency of the excitation is close to the natural frequency of an object, the amplitude of the forced vibration becomes very large. This phenomenon is called resonance. Vibration stability refers to the quality of mechanical components in avoiding resonance during machine operation. To extend the lifespan of the machine and prevent damage to the shafts and the machine itself, it is necessary to verify the vibration stability of the shafts, especially those in high-speed machines. The vibration stability criterion requires that the natural frequency of mechanical components be offset from the frequency of the excitation to prevent resonance. Let the natural frequency of the components in the machine that are subjected to excitation be f, and let the frequency of the exciting force be fp. It is generally required that fp < 0.85 f or fp > 1.15 f (3–19). Changing the stiffness and mass of the mechanical components can alter their natural frequencies. Increase the stiffness of mechanical components and reduce their mass to raise their natural frequency ； Reducing the stiffness and increasing the mass of mechanical parts decreases their natural frequency. Sometimes, the speed of the machine needs to be adjusted while it is running to prevent resonance. The main reason for resonance in the shaft is that, due to uneven mass distribution within the material, along with manufacturing and installation errors, there is a deviation between its center of mass and its center of rotation. As the shaft rotates, inertial forces are generated, and these forces cause the rotor to undergo forced vibrations. The speed of the shaft at which resonance occurs is called the critical speed. At the critical speed, the frequency of this inertial force is equal to or several times the natural frequency of the rotor, resulting in resonance. 5. Life expectancy criterion: To ensure that a machine operates properly over a certain period of time, it is necessary to set requirements regarding the life expectancy of its mechanical components during the design phase. It should be noted that during the machine’s service life, parts can be replaced; in other words, the lifespan of certain mechanical parts can be shorter than that of the machine itself. The lifespan of mechanical parts is primarily influenced by material fatigue, wear, and corrosion. To avoid failures caused by part fatigue, such as fatigue fracture, the fatigue strength should be calculated based on the fatigue limit corresponding to the service life of the mechanical parts. That is, based on the life requirement and taking into account factors such as the part’s rotational speed, the fatigue limit at stress cycle count N is calculated using equation (3-6); this value is then used in the strength condition equation to determine the fatigue strength. When the fatigue strength is satisfied, it can be ensured that the number of stress cycles before failure of the mechanical part meets the life requirement. Wear is generally inevitable. Under certain conditions, corrosion is also inevitable, such as the corrosion of bridge structural components and buried steel pipelines. During design, the main goal is to ensure that mechanical components do not suffer excessive wear and corrosion over their lifespan. The mechanism by which wear occurs has not yet been fully understood, and there are many factors that influence wear. Generally, the wear resistance of friction pairs is improved based on the principles of tribological design. The main measures include: reasonably selecting the materials for the friction pair ； Select lubricants and additives appropriately ； Control the operating conditions of the friction pair, such as pressure, sliding velocity, and temperature rise. To date, there is no practical and effective method for calculating corrosion life; measures are usually taken in terms of material selection and anti-corrosion treatment. If corrosion-resistant materials are used, surface coating, spraying, phosphating and other treatments are employed. 6. Reliability criteria: Reliability is the ability of a product to perform its specified functions under specified conditions and within a specified period of time. The quality of a product generally should include performance indicators and reliability indicators. The performance indicators of mechanical products refer to the technical specifications of those products, such as the power, torque, working force, and operating speed of the machinery. If there are only performance metrics and no reliability metrics, the performance of the product cannot be guaranteed either. For example, an advanced aircraft with low reliability is bound to experience frequent failures, which disrupt normal flight operations and increase maintenance costs; it may even lead to serious accidents. The reliability of a product is measured by the reliability R(t). Reliability is defined as the probability that a product will perform its specified functions under specified conditions for a specified period of time. Reliability is a function of time. There is a batch of n identical products that start operating at t=0. Over time, the number of defective units no(t) increases, while the number of functional units ni(t) decreases. The reliability of the products at any given time t is given by http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image010.gif (3–20). If the reliability R(t) of a product after 3000 hours of operation is 0.96, it means that 96% of the products can continue to function properly for more than 3000 hours; for a specific product, the probability that it will function for 3000 hours is 96%. The failure rate refers to the probability that a product will fail within a unit of time in the subsequent phase, by the time it has been in operation for t. It can be shown that its mathematical expression is given by http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image016.gif (3–21). By separating the variables and integrating both sides, we obtain http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image018.gif; further calculations yield http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image020.gif (3–22). The relationship between the failure rate of a component and time is generally shown in Figure 3-13. The efficiency loss curve can be determined through experimental methods. The failure rate curve reflects the failure rate of a product over its entire lifetime. As can be seen from the failure curve, failure can generally be divided into three stages. http://jpkc.tjpu.edu.cn/2007/jxsj/cool/jishe/3.3.2.files/image022.gif Figure 3-15: Phase I is the early failure phase, and the curve shows a decreasing trend. In the early stages of product use, the failure rate is high and declines rapidly. The main reasons are defects arising from design, manufacturing, storage, transportation, etc., as well as human factors such as improper commissioning, running-in, and startup. When these failures caused by congenital defects occur and the equipment begins to operate normally again, the failure rate tends to stabilize. Every effort should be made to prevent premature failure of components, thereby reducing the failure rate and the time t0 in the early failure stage. Stage II is the random failure stage, during which the failure rate increases slowly. Failure is mainly caused by accidental factors such as unexpected overload, misoperation, and sudden natural disasters. Since the causes of failure are mostly accidental, it is referred to as the accidental failure stage. Reducing the failure rate during the period of accidental failures can increase the effective service life; therefore, it is important to focus on improving product quality and to carry out proper use and maintenance. Stage III is the damage and failure stage, during which the failure rate is increasing. The failure rate increases significantly after t1. This is caused by factors leading to wear and tear, such as product aging, fatigue, abrasion, creep, and corrosion; hence it is referred to as the wear-out failure period. To address the causes of failure at this stage, it is necessary to pay attention to inspections and monitoring, as well as carry out repairs in advance, so as to prevent the failure rate from rising. 7. Accuracy criteria: For high-precision mechanical parts, mechanisms, or equipment, it is required that their motion errors be below the allowable values. For example, in precision machinery, certain accuracy requirements must be met for things such as the linear error of guide rails, the radial runout error of spindles, and the angular error in gear transmissions. Based on the functional requirements of machines and parts, appropriate tolerances and fits can be selected—that is, precision design can be carried out—and these can be correctly indicated on the drawings. The error of the mechanism can also be calculated using the tolerance values specified in the part diagram, and then compared with the required precision of the mechanism.

It’s frustrating – why hide things like this? The topic of the main design criteria for mechanical parts is too broad; it probably can’t be covered in just a few sentences. The main aspects are: 1. Strive for standardization, 2. Ensure that the mechanical and chemical properties of the parts meet the required standards. 3. When the usage requirements are met, minimize the processing requirements as much as possible. 4. Cost considerations: when the requirements can be met, use materials that are as low-cost as possible. Wait

1. Strength criterion. It is required that the working stress σ of mechanical parts does not exceed the allowable stress. 2. Stiffness criterion. Mechanical parts undergo elastic deformation when subjected to loads, and stiffness is the ability of a material, mechanical part, or structure to resist deformation under external forces. The stiffness of a material is measured by the amount of external force required to cause a unit deformation in it. 3. Heat resistance criterion. Due to friction and other factors, when a machine is in operation, the temperatures of its mechanical components and lubricants generally rise. Excessively high operating temperatures will lead to a decrease in the lubrication effect; at the same time, they can cause thermal deformation of the components, as well as a reduction in their hardness and strength, and may even result in damage. At high temperatures, metal mechanical parts may experience bonding or sticking ； Non-metallic mechanical parts such as plastics may soften or even melt, and in some cases, thermal stress can also be generated. The heat resistance criterion generally aims to keep the operating temperature of mechanical components below the allowable value, in order to ensure their proper functioning. To improve heat dissipation performance and control temperature rise, water cooling or air cooling can be employed when necessary. 4. Vibration stability criterion. When the frequency of the excitation equals the object’s natural frequency, the amplitude of the object is at its maximum; the greater the difference between the excitation frequency and the natural frequency, the smaller the amplitude of the object. When the frequency of the excitation is close to the natural frequency of an object, the amplitude of the forced vibration becomes very large. This phenomenon is called resonance. Vibration stability refers to the quality of mechanical components in avoiding resonance during machine operation. 5. Life expectancy criterion. To ensure that the machine operates properly over a certain service life, it is necessary to set requirements regarding the lifespan of its mechanical components during their design. It should be noted that during the machine’s service life, parts can be replaced; in other words, the lifespan of certain mechanical parts can be shorter than that of the machine itself. The lifespan of mechanical parts is primarily influenced by material fatigue, wear, and corrosion. 6. Reliability criteria. Reliability is the ability of a product to perform its specified functions under specified conditions and within a specified time period. The quality of a product generally should include performance indicators and reliability indicators. The performance indicators of mechanical products refer to the technical specifications of those products, such as the power, torque, working force, and operating speed of the machinery. If there are only performance metrics and no reliability metrics, the performance of the product cannot be guaranteed either. 7. Accuracy criterion. For high-precision mechanical parts, mechanisms, or equipment, it is required that their motion errors be below the allowable values. For example, in precision machinery, certain accuracy requirements must be met for things such as the linear error of guide rails, the radial runout error of spindles, and the angular error in gear transmissions. Based on the functional requirements of machines and parts, appropriate tolerances and fits can be selected—that is, precision design can be carried out—and these can be correctly indicated on the drawings. The error of the mechanism can also be calculated using the tolerance values specified in the part diagram, and then compared with the required precision of the mechanism.

This post was last edited by Black gold on 2011-6-26 08:37: 1. Strength criterion 2. Stiffness criterion 3. Heat resistance criterion 4. Vibration stability criterion 5. Service life criterion 6. Reliability criterion 7. Accuracy criterion

1. Static strength criterion, 2. Fatigue strength criterion, 3. Tribological technology design, 4. Stiffness, 5. Reliability.