20 Fundamental Concepts: How Many Do You Remember as a Mechanical Engineer?
1. What are the failure modes of mechanical parts?
Answer: Fracture of the entire part, excessive residual deformation, surface damage, and failure caused by the disruption of normal operating conditions.
2. Why do threaded connections often require locking mechanisms? What is the essence of locking? What are the common locking measures?
Answer: While standard threaded connections generally satisfy self-locking conditions and do not loosen on their own, the nut may gradually loosen under vibration, impact loads, or significant temperature fluctuations. Thread loosening is primarily caused by relative rotation between the mating threads; therefore, locking measures must be implemented in practical design. Common measures include: 1. Friction-based locking-maintaining friction between the mating threads (e.g., using spring washers or jam nuts); 2. Mechanical locking-using locking components (e.g., slotted nuts and cotter pins); 3. Permanent locking (altering the thread interface)-disrupting or changing the relationship between the mating threads (e.g., the impact method).
3. What is the purpose of tightening a threaded connection? Name a few methods for controlling the tightening force.
Answer: The purpose of tightening is to generate a preload in the bolt. Preloading enhances the reliability and tightness of the connection, preventing gaps or relative sliding between the connected parts under load. Effective methods for controlling tightening force include using torque-measuring wrenches or preset torque wrenches (locking the fastener once the required torque is reached) or controlling preload by measuring bolt elongation.
4. What is the difference between elastic slip and gross slip (skidding) in belt drives? Why is there a minimum diameter limit ($d_{min}$) for the small pulley when designing a V-belt drive?
Answer: Elastic slip is an inherent characteristic of belt drives and is unavoidable; it occurs because the belt is an elastic body and there is a difference in tension between the tight and slack sides. Gross slip (skidding), on the other hand, is caused by overloading; it is a failure mode that can-and must-be avoided. Reason: Slippage occurs on the smaller pulley. The greater the external load, the larger the difference in tension between the two sides, which increases the elastic slip zone; when elastic slip occurs across the entire arc of contact, actual slippage (gross sliding) ensues. Elastic slip is a quantitative change, whereas slippage is a qualitative change. Small pulleys have small diameters, small arcs of contact, and small frictional contact areas, making them prone to slippage.
5. Why are the allowable contact stresses for gray cast iron and aluminum-iron bronze worm gears dependent on the sliding velocity of the tooth surfaces?
Answer: Because the primary failure mode for gray cast iron and aluminum-iron bronze worm gears is tooth surface scuffing (scoring), and the occurrence of scuffing is related to sliding velocity; therefore, their allowable contact stress depends on the sliding velocity. In contrast, the primary failure mode for cast tin bronze worm gears is tooth surface pitting, which is caused by contact stress; thus, their allowable contact stress is independent of sliding velocity.
6. List the common motion laws for cam mechanism followers, their impact characteristics, and their application scenarios.
Answer: Constant velocity motion, constant acceleration/deceleration motion, and simple harmonic motion (cosine acceleration motion). Constant velocity motion involves rigid impact and is used for low-speed, light-load applications. Constant acceleration/deceleration motion involves soft impact and is used for medium-to-low-speed applications. Simple harmonic motion (cosine acceleration motion) involves soft impact when there is a dwell period (used for medium-to-low-speed applications) and no soft impact when there is no dwell period (used for high-speed applications).
7. Briefly state the fundamental law of gear tooth meshing.
Regardless of the position where the tooth profiles make contact, the common normal passing through the contact point must always intersect the line connecting the centers of rotation at a fixed point to ensure a constant transmission ratio.
8. What are the methods for the circumferential fixation of components on a shaft? (List at least four methods.)
Circumferential fixation methods include: key connections, spline connections, interference fit connections, set screws, pin connections, and expansion sleeve connections. 9. What are the main methods for the axial fixation of parts mounted on a shaft? What are the characteristics of each? (List at least four types.)
Axial fixation methods include: shaft shoulders, shaft collars, sleeves, shaft-end plates, and elastic retaining rings. Fixation via shaft shoulders, shaft collars, and sleeves is reliable and capable of withstanding significant axial forces; elastic retaining rings can withstand smaller axial forces; shaft-end plates are used to secure parts at the end of the shaft.
10. Why must a thermal balance calculation be performed for enclosed worm gear drives?
Worm gear drives involve relative sliding and high friction; furthermore, enclosed worm gear drives have poor heat dissipation and are prone to scuffing (galling), necessitating a thermal balance calculation.
11. What are the two theories regarding gear strength calculation? Which failure modes do they address respectively? What is the design criterion for an enclosed gear drive with soft tooth surfaces?
Answer: The two calculations are for tooth surface contact fatigue strength and tooth root bending fatigue strength. Contact fatigue strength addresses surface fatigue pitting, while bending fatigue strength addresses root fatigue breakage. For an enclosed gear drive with soft tooth surfaces, the design criterion is to design based on tooth surface contact fatigue strength and then verify the tooth root bending fatigue strength.
12. What are the functions of couplings and clutches? What is the difference between them?
Answer: The function of both couplings and clutches is to connect two shafts so they rotate together and transmit torque. The difference is that shafts connected by a coupling cannot be separated during operation-separation requires stopping the machine and dismantling parts-whereas a clutch allows the two shafts to be engaged or disengaged at any time while the machine is running.
13. What are the necessary conditions for an oil film to support a load?
Answer: 1. 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 velocity, with a direction that ensures lubricating oil enters the wide end and exits the narrow end; 3. The lubricating oil must have appropriate viscosity, and the oil supply must be sufficient. 14. Briefly describe the meaning of the designation, characteristics, and application scenarios for the bearing model 7310.
Answer: Designation meaning: 7 - angular contact ball bearing; (0) - normal width (can be omitted); 3 - medium diameter series; 10 - bearing inner diameter is 50 mm.
Characteristics and applications: Capable of simultaneously withstanding radial loads and axial loads in a single direction; features a high limiting speed; generally used in pairs.
15. In a transmission system comprising gear, belt, and chain drives, which type of drive should generally be placed at the highest-speed stage, and which at the lowest-speed stage? What is the reason for this arrangement?
Answer: Generally, the belt drive is placed at the highest-speed stage, and the chain drive at the lowest-speed stage. Belt drives offer smooth operation and the ability to cushion shocks and absorb vibrations, making them beneficial for the motor when placed at the high-speed stage. Chain drives generate noise during operation and are suitable for lower-speed applications, so they are typically placed at the low-speed stage.
16. What causes non-uniform speed in chain drives? What are the main influencing factors? Under what conditions can the instantaneous transmission ratio remain constant?
Answer: 1) The primary cause of non-uniform speed in chain drives is the "polygon effect." 2) The main influencing factors are: chain speed, chain pitch, and the number of sprocket teeth. 3) The instantaneous transmission ratio remains constant (specifically, constant at 1) only when the number of teeth on the driving and driven sprockets is equal ($z_1 = z_2$, i.e., $R_1 = R_2$) and the center distance of the drive is an integer multiple of the pitch ($p$).
17. In a cylindrical gear reducer, why is the face width of the pinion ($b_1$) made slightly larger than that of the large gear ($b_2$)? When calculating strength, is the face width coefficient ($\psi_d$) based on $b_1$ or $b_2$? Why? Image
Answer: 1) To prevent a reduction in meshing face width-and a consequent increase in working load-caused by axial misalignment during assembly, the face width of the pinion ($b_1$) is made slightly larger than that of the gear ($b_2$); 2) The face width coefficient ($\psi_d$) is calculated based on the gear's face width ($b_2$), as $b_2$ represents the actual contact width when the pair of cylindrical gears mesh.
18. In a belt drive, why must the small pulley diameter $d_1 \ge d_{\min}$ and the wrap angle of the driving pulley $\alpha_1 \ge 120^\circ$? The recommended belt speed is usually between 5 and 25 m/s; what are the consequences if the speed falls outside this range?
Answer: 1) A smaller pulley diameter results in higher bending stress in the belt; therefore, a minimum diameter is specified to avoid excessive bending stress; 2) The wrap angle of the driving pulley ($\alpha_1$) affects the maximum effective pulling force; a smaller $\alpha_1$ reduces this force. To increase the maximum effective pulling force and prevent slippage, $\alpha_1$ is generally required to be $\ge 120^\circ$; 3) If the belt speed is too low, it implies the small pulley diameter is too small, requiring a higher effective pulling force ($F_e$) and a greater number of belts ($z$), which increases the size of the drive assembly. Conversely, if the speed is too high, centrifugal force ($F_c$) becomes excessive. Thus, the belt speed should be maintained within the 5–25 m/s range.
19. Advantages and disadvantages of rolling-contact screw mechanisms.
Answer: Advantages: 1) Minimal wear; backlash can be eliminated and stiffness increased through adjustment methods (creating a certain amount of pre-deformation), resulting in high transmission precision; 2) Non-self-locking, allowing for the conversion of linear motion into rotary motion. Disadvantages: 1) Complex structure and difficult manufacturing; 2) In some mechanisms, an additional self-locking device is required to prevent reverse rotation.
20. Principles for selecting a key?
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Answer: Selection involves two aspects: type and dimensions. The type should be selected based on the structural characteristics of the key connection, operational requirements, and working conditions. Dimensions should be determined in accordance with standard specifications and strength requirements; the key's dimensions consist of the cross-section (width *b* × height *h*) and the length *L*. The cross-sectional dimensions (*b* × *h*) are selected from standards based on the shaft diameter *d*. The key length *L* is generally determined by the length of the hub (i.e., *L* ≤ hub length), whereas for a guiding parallel key, it is determined by both the hub length and the sliding distance; typically, the hub length *L'* ≈ (1.5–2) × *d*.
