1. What are the failure modes of mechanical parts?
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(i) Overall fracture (ii) Excessive residual deformation (iii) Surface damage of parts (iv) Failure caused by damage to normal working conditions
2. Why do threaded connections often need to be prevented from loosening? What is the essence of anti-loosening? What are the anti-loosening measures?
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Answer: Generally, threaded connections can meet the self-locking conditions and will not loosen automatically, but under vibration or impact loads, or when the temperature changes greatly, the connecting nut may gradually loosen. The main reason for thread loosening is the relative rotation between the thread pairs. Therefore, in actual design, anti-loosening measures must be adopted. The commonly used measures are mainly the following: 1. Friction anti-loosening---maintain the friction between the thread pairs to prevent loosening, such as adding spring washers and double nuts against each other; 2. Mechanical anti-loosening---use stop parts to ensure anti-loosening, and slotted nuts and cotter pins are commonly used; 3. Destruction of thread pair anti-loosening---destroy and change the relationship of the thread pair, such as the impact method.
3. What is the purpose of tightening in threaded connections? List several methods for controlling tightening force.
Answer: The purpose of tightening in threaded connection is to make the bolt generate pre-tightening force. The purpose of pre-tightening is to enhance the reliability and tightness of the connection to prevent gaps or relative sliding between the connected parts after loading. An effective way to control the tightening force is to use a torque wrench or a fixed torque wrench. When the required torque is reached, just lock it; or use the method of measuring the elongation of the bolt to control the pre-tightening force.
4. What is the difference between elastic sliding and slipping in belt drive? When designing V-belt drive, why should the dmin of the small pulley be limited?
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Answer: Elastic sliding is an inherent characteristic of belt drive and is inevitable. When there is a tension difference and the belt is an elastic body, elastic sliding will occur. Slipping is caused by overload and is a form of failure. It can be avoided and must be avoided. Reason: Slipping occurs on the small pulley. The greater the external load, the greater the tension difference on both sides, which leads to an increase in the elastic sliding area. When elastic sliding occurs in the wrap angle, slipping occurs. Elastic sliding is a quantitative change, while slipping is a qualitative change. The small wheel has a small diameter, a small wrap angle, and a small friction contact area, so it is easy to slip.
5. Why is the allowable contact stress of gray cast iron and aluminum iron bronze turbines related to the sliding speed of the tooth surface?
Answer: Because: The main failure form of gray cast iron and aluminum iron bronze turbines is tooth surface bonding, and bonding is related to the sliding speed, so the allowable contact stress is related to the tooth sliding speed. The main failure form of cast tin bronze turbines is tooth surface pitting, which is caused by contact stress, so the allowable contact stress has nothing to do with the sliding speed.
6. State the common motion laws, impact characteristics and application occasions of the cam mechanism follower.
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Answer: Uniform motion law, uniform acceleration and uniform deceleration motion law, simple harmonic motion law (cosine acceleration motion law);
The uniform motion law has rigid impact, which is used in low speed and light load occasions;
The uniform acceleration and uniform deceleration motion law has flexible impact, which is used in medium and low speed occasions; the simple harmonic motion law (co4 sine acceleration motion law) has flexible impact when there is a rest interval, which is used in medium and low speed occasions, and no flexible impact when there is no rest interval, which is used in high speed occasions.
7. Briefly describe the basic law of tooth profile engagement.
Regardless of the position of the tooth profile in contact, the common normal line drawn through the contact point must pass through a certain point on the center line to ensure that the transmission ratio is constant.
8. What are the methods for circumferential fixation of parts on the shaft? (Point out more than four methods)
Circumferential fixation: key connection, spline connection, interference fit connection, set screw, pin connection, expansion connection
9. What are the main types of axial fixation methods for parts on the shaft? What are the characteristics of each? (Point out more than four methods)
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Axial fixation: Shoulder, shaft ring, shaft sleeve, shaft end baffle, elastic retaining ring, shaft shoulder, shaft ring, shaft sleeve are fixed reliably and can withstand large axial forces; elastic retaining ring fixation can withstand small axial forces; shaft end baffle is used to fix shaft end parts.
10. Why must closed worm transmission be thermally balanced?
Worm transmission has relative sliding and large friction. Because closed worm transmission has poor heat dissipation and is prone to bonding, thermal balance calculation is required.
11. In the calculation of gear strength, what are the two strength calculation theories? Which failures are they respectively aimed at? If the gear transmission is a closed soft tooth surface transmission, what is its design criterion?
Answer: The contact fatigue strength of the tooth surface and the bending fatigue strength of the tooth root are calculated. The contact fatigue strength of the tooth surface is for the fatigue pitting failure of the tooth surface and the bending fatigue strength of the tooth root is for the fatigue fracture of the tooth root. The gear transmission is a closed soft tooth surface transmission. Its design principle is to design according to the contact fatigue strength of the tooth surface and check the bending fatigue strength of the tooth root.
12. What are the functions of the coupling and the clutch? What is the difference between the two?
Answer: The function of the coupling and the clutch is to connect the two shafts so that they rotate together and transmit torque. The difference between the two is that the two shafts connected by the coupling cannot be separated during work. The two shafts can only be separated by disassembling the parts after stopping, while the clutch can separate or connect the two shafts at any time during the operation of the machine.
13. Explain the necessary conditions for oil film bearing?
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Answer: A wedge-shaped gap must be formed between the two surfaces in relative motion; 2. The two surfaces separated by the oil film must have a certain relative sliding speed, and the direction should ensure that the lubricating oil enters from the large port and exits from the small port; 3. The lubricating oil must have a certain viscosity and the oil supply must be sufficient.
14. Briefly describe the meaning, characteristics and application occasions of the bearing model 7310.
Answer: Code meaning: 7-angular contact ball bearing; (0)-normal width, 0-can be omitted; 3-diameter series is the medium series; 10-bearing inner diameter is 50mm.
Features and applications: It can withstand radial loads and unidirectional axial loads at the same time, has a high limit speed, and is generally used in pairs.
15. In the transmission system composed of gear transmission, belt transmission, and chain transmission, which transmission should generally be arranged at the highest speed level? Which transmission should be arranged at the lowest speed level? Why is it arranged in this way?
Answer: Generally, the belt drive is arranged at the highest level and the chain drive is arranged at the lowest level; the belt drive has the characteristics of stable transmission and buffering vibration absorption, so it is placed at the high-speed level, which is beneficial to the motor; the chain drive is noisy when working and is suitable for working at low speeds, so it is generally arranged at the low-speed level.
16. What is the cause of uneven chain drive speed? What are the main influencing factors? Under what circumstances can its instantaneous transmission ratio be constant?
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Answer: 1) The main reason for uneven chain drive speed is the polygonal effect of chain drive; 2) The main influencing factors are: chain speed, chain pitch and number of sprocket teeth; 3) When the number of teeth of the large and small sprockets is equal z1=z2 (that is, R1＝R2), and the center distance of the transmission is exactly an integer multiple of the pitch p, the instantaneous transmission ratio is constant, that is, it is always 1.
17. In the cylindrical gear reducer, why is the small gear tooth width b1 slightly larger than the large gear tooth width b2? When calculating the strength, is the tooth width coefficient ψd calculated according to b1 or b1? Why?
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Answer: 1) In order to prevent the axial misalignment of the large and small gears due to assembly errors, which will lead to a decrease in the meshing tooth width and increase the working load, the small gear tooth width b1 should be slightly larger than the large gear tooth width b2; 2) The tooth width coefficient ψd is calculated according to the large gear tooth width b2; because the large gear tooth width b2 is the actual contact width when a pair of cylindrical gears are meshed.
18. Why should the small pulley diameter d1 ≥ dmin and the driving pulley wrap angle α1 ≥ 120° in the speed reduction belt transmission? The recommended belt speed is usually between (5 and 25) m/s. What will happen if the belt speed exceeds this range?
Answer: 1) The smaller the diameter of the small pulley, the greater the bending stress of the belt. Therefore, in order to avoid excessive bending stress of the belt, the minimum diameter of the small pulley should be limited; 2) The angle α1 of the driving wheel affects the maximum effective tension of the belt. The smaller α1 is, the smaller the maximum effective tension of the belt is. In order to increase the maximum effective tension of the belt drive and prevent slipping, α1 is generally ≥ 120°; 3) If the belt speed is too small, it means that the diameter of the small pulley is too small, which will make the required effective tension Fe too large, resulting in too many roots z of the belt, making the belt drive structure larger; if the belt speed is too large, the centrifugal force Fc is too large, so the belt speed should be (5~25) m/s.
19. Advantages and disadvantages of rolling spiral.
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Answer: Advantages - 1) The wear is very small, and the adjustment method can be used to eliminate the gap and produce a certain pre-deformation to increase the rigidity, so its transmission accuracy is very high, 2) It is not self-locking and can change linear motion into rotational motion. Disadvantages - 1) The structure is complex and difficult to manufacture, 2) Some mechanisms need to add a self-locking mechanism to prevent reversal.
20. What are the principles for selecting keys?
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Answer: There are two aspects: type selection and size selection. Type selection should be based on the structural characteristics of the key connection, the use requirements and working conditions. Size selection should be determined in accordance with standard specifications and strength requirements. The size of the key is the cross-sectional size (key width b*key height h) and length L. The cross-sectional size b*h is selected from the standard by the diameter d of the shaft. The length L of the key can generally be determined according to the length of the hub, that is, the key length L≦hub length, while the guide flat key is determined according to the length of the hub and the sliding distance. Generally, the hub length L'≈(1.5-2)*d
