1. What are the failure modes of mechanical parts?
(1) Overall fracture, (2) Excessive residual deformation, (3) Surface damage to parts, and (4) Failure caused by disruption of normal operating conditions.
2. Why do threaded connections often require anti-loosening measures? What is the essence of anti-loosening measures? What are the anti-loosening measures?
Answer: Generally, threaded connections are self-locking and will not loosen automatically. However, under vibration or impact loads, or when subjected to large temperature fluctuations, the connecting nut may gradually loosen. Thread loosening is primarily caused by relative rotation between the threaded pairs. Therefore, anti-loosening measures must be implemented in practical designs. Commonly used measures include the following: 1. Frictional anti-loosening: Maintaining friction between the threaded pairs to prevent loosening, such as adding spring washers or double nuts; 2. Mechanical anti-loosening: Using stoppers to prevent loosening, commonly used are slot nuts and cotter pins; 3. Destructive anti-loosening: Destroying and altering the threaded pair relationship, such as the impact method.
3. What is the purpose of tightening in a threaded connection? List several methods for controlling tightening force.
Answer: The purpose of tightening in a threaded connection is to create a preload in the bolt. Preload enhances the reliability and tightness of the connection, preventing gaps or relative slip between the connected parts when loaded. An effective method for controlling tightening force is to use a torque wrench or a fixed torque wrench. Tighten when the required torque is reached. Alternatively, the preload can be controlled by measuring bolt elongation.
4. What is the difference between elastic slip and slip in a belt drive? Why is the dmin limit imposed on the pulley when designing a V-belt drive?
Answer: Elastic slip is an inherent and unavoidable characteristic of belt drives. It occurs when there is a tension differential and the belt is elastic. Slip is caused by overload and is a form of failure that can and must be avoided. The reason: Slip occurs on the pulley. The greater the external load, the greater the tension differential between the two sides, which increases the elastic slip zone. When elastic slip occurs throughout the wrap angle, slip occurs. Elastic slip is a quantitative change, while slip is a qualitative change. The small diameter and wrap angle of the small wheel result in a small friction contact area, making it prone to slippage.
5. Why is the allowable contact stress of gray cast iron and aluminum-iron-bronze turbines related to the sliding speed of the tooth surfaces?
Answer: The primary failure mode of gray cast iron and aluminum-iron-bronze turbines is tooth scuffing, which is related to the sliding speed. Therefore, their allowable contact stress is related to the tooth sliding speed. The primary failure mode of cast tin bronze turbines is tooth pitting, which is caused by contact stress. Therefore, the allowable contact stress is unrelated to the sliding speed.
6. Describe the common motion patterns, impact characteristics, and application scenarios of cam mechanism followers.
Answer: The law of uniform velocity, the law of uniform acceleration and uniform deceleration, and the law of simple harmonic motion (cosine acceleration).
The law of uniform velocity has a rigid impact and is used in low-speed and light-load applications.
The law of uniform acceleration and uniform deceleration has a flexible impact and is used in medium- and low-speed applications. The law of simple harmonic motion (cosine acceleration) has a flexible impact when there is a dwell period and is used in medium- and low-speed applications, but no flexible impact when there is no dwell period and is used in high-speed applications.
7. Briefly describe the basic laws of tooth meshing.
Regardless of where the tooth profiles contact, the common normal through the contact point must pass through a certain point on the centerline to ensure a constant transmission ratio.
8. What are the various methods for circumferentially securing parts on shafts? (Name at least four methods.)
Circumferential securing: key connection, spline connection, interference fit connection, set screw, pin connection, and expansion joint.
9. What are the main methods for axially securing parts on shafts? What are their characteristics? (Name at least four)
Axial securing: Shaft shoulder, shaft collar, shaft sleeve, shaft end plate, and elastic retaining ring. Shaft shoulder, shaft collar, and shaft sleeve provide reliable securing and can withstand large axial forces; elastic retaining rings can withstand smaller axial forces; shaft end plates are used to secure parts at the shaft end.
10. Why is a thermal balance calculation necessary for closed worm gear drives?
Worm gear drives involve relative sliding, resulting in high friction. Furthermore, closed worm gear drives have poor heat dissipation and are prone to galling, so thermal balance calculations are necessary.
11. In gear strength calculations, what are the two strength calculation theories? What failures do they address? If the gear transmission is a closed soft tooth surface transmission, what are the design criteria?
Answer: Calculation of the contact fatigue strength of the tooth surface and the bending fatigue strength of the tooth root. The contact fatigue strength of the tooth surface addresses fatigue pitting failure of the tooth surface, while the bending fatigue strength of the tooth root addresses fatigue fracture of the tooth root. Gear transmissions are closed, soft-toothed transmissions. Their design principle is based on the contact fatigue strength of the tooth surfaces and the bending fatigue strength of the tooth roots.
12. What are the functions of couplings and clutches? What is the difference between them?
Answer: The functions of couplings and clutches are to connect two shafts so that they rotate in unison and transmit torque. The difference between the two is that the two shafts connected by a coupling cannot be separated during operation and can only be separated by removing parts after the machine is shut down. In contrast, a clutch can be used to separate or connect the two shafts at any time while the machine is running.
13. What are the necessary conditions for oil film bearing?
Answer: A wedge-shaped gap must be formed between the two surfaces in relative motion. The two surfaces separated by the oil film must have a certain relative sliding speed, and the direction must ensure that the lubricant enters from the larger port and exits from the smaller port. The lubricant must have a certain viscosity, and the oil supply must be sufficient.
14. Briefly describe the meaning, characteristics, and applications of bearing model 7310.
Answer: Meaning of the code: 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 bear radial load and unidirectional axial load at the same time, has a high limit speed, and is generally used in pairs.
15. In a 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 like this?
Answer: Generally, the belt transmission is arranged at the highest level and the chain transmission is arranged at the lowest level; the belt transmission has the characteristics of stable transmission and buffering and vibration absorption, so it is placed at the high speed level, which is beneficial to the motor; the chain transmission makes noise when working and is suitable for working at lower speeds, so it is generally arranged at the low speed level.
16. What causes the uneven speed of the chain transmission? What are its main influencing factors? Under what circumstances can its instantaneous transmission ratio be constant?
Answer: 1) The main cause of uneven chain drive speed is the polygonal effect of the chain drive; 2) The main influencing factors are: chain speed, chain pitch, and the number of sprocket teeth; 3) When the number of teeth on the large and small sprockets is equal, z1 = z2 (i.e., R1 = R2), and the transmission center distance is an integer multiple of the pitch p, the instantaneous transmission ratio is constant, that is, always 1.
17. In a cylindrical gear reducer, why is the tooth width b1 of the small gear slightly larger than the tooth width b2 of the large gear? When calculating strength, is the tooth width coefficient ψd calculated based on b1 or b2? Why?
Answer: 1) To prevent axial misalignment between the large and small gears due to assembly errors, which would reduce the meshing tooth width and increase the working load, the tooth width b1 of the small gear should be slightly larger than the tooth width b2 of the large gear; 2) The tooth width coefficient ψd is calculated based on the tooth width b2 of the large gear, because the tooth width b2 of the large gear represents the actual contact width when a pair of cylindrical gears mesh.
18. Why should the small pulley diameter d1 ≥ dmin and the driving pulley wrap angle α1 ≥ 120° in a speed belt drive? The recommended belt speed is typically (5-25) m/s. What are the consequences if the belt speed exceeds this range?
Answer: 1) The smaller the small pulley diameter, the greater the belt bending stress. Therefore, to prevent excessive belt bending stress, the minimum small pulley diameter should be limited. 2) The driving pulley wrap angle α1 affects the maximum effective belt tension. The smaller α1, the lower the maximum effective belt tension. To increase the maximum effective belt tension and prevent slippage, α1 is generally ≥ 120°. 3) Too low a belt speed means too small a small pulley diameter, which will result in an excessively large effective tension Fe, leading to an increased number of belts z and a larger belt drive structure. Too high a belt speed will also result in an excessively large centrifugal force Fc. Therefore, the belt speed should be within (5-25) m/s.
19. Advantages and disadvantages of rolling screws.
Answer: Advantages: 1) Minimal wear, and adjustment can be used to eliminate clearance and create a certain amount of pre-deformation to increase rigidity, resulting in high transmission accuracy; 2) It lacks self-locking properties and can convert linear motion into rotational motion. Disadvantages: 1) Complex structure and difficult manufacturing; 2) Some mechanisms require a self-locking mechanism to prevent reverse rotation.
20. Key selection principles?
Answer: Two aspects: type and size. Type selection should be based on the structural characteristics of the key connection, usage requirements, and operating conditions. Size selection should be based on compliance with standard specifications and strength requirements. Key dimensions are cross-sectional dimensions (key width b * key height h) and length L. Cross-sectional dimensions b * h are determined by the shaft diameter d and are specified in the standard. Key length L is generally determined by hub length, i.e., key length L ≤ hub length. Guide flat keys are determined by hub length and sliding distance. Generally, hub length L' ≈ (1.5 - 2) * d.
