Views: 0 Author: Site Editor Publish Time: 2022-05-25 Origin: Site
Some simple mechanics
1. What are the failure forms of mechanical parts?
(1) Overall fracture, plastic deformation
(2) Excessive residual deformation
(3) Excessive wear or damage of the working surface
(4) Occurrence of strong vibration, relaxation of the connection, friction drive skid, etc.
2. Why is thread connection often required to prevent loosening? What is the essence of anti-loose? What kinds of anti-loose measures are there?
Answer: Generally threaded connections can meet the self-locking conditions and will not automatically loosen, but under vibration or shock loads, or large temperature changes, the nut may gradually loosen. The main reason of thread loosening is the relative rotation between thread pairs, so in practical design, anti-loosening measures must be adopted.
Often used measures are mainly the following points:
(1) Friction anti-loose -- to keep the friction between the thread pair to prevent loose, such as adding spring washers, double nuts on the top;
(2) Mechanical anti loose -- the use of stop parts to ensure the anti-loose, often used is grooved nuts and cotter pin;
(3) Destroy thread pair anti loose -- destroy and change thread pair relationship, such as impact method.
3. What is the purpose of tightening in thread connection? Several methods to control tightening force are given.
Answer: the purpose of tightening in the thread connection is to make the bolt produce pretightening force. The purpose of pretightening is to enhance the reliability and tightness of the connection, so as to prevent the gap or relative sliding between the connected parts after being loaded. The effective method to control tightening force is measuring torque wrench or constant torque wrench. When the required torque is reached, it can be locked. Or the preload can be controlled by measuring the amount of bolt elongation.
4. What is the difference between elastic sliding and skid of belt drive? Why limit the dmin of the pulley when designing v-belt drives?
Answer: Elastic sliding is an inherent characteristic of belt drive and is inevitable. Elastic sliding occurs when there is a tension difference and the band is elastomer. Skidding is caused by overloading and is a form of failure that can and must be avoided. Reason: Skidding occurs on the small belt pulley. The larger the external load is, the greater the tension difference between the two sides is which leads to the increase of the elastic sliding zone. When the elastic sliding phenomenon occurs inside the envelope Angle, the skidding phenomenon occurs. Elastic sliding is a quantitative change, skid is a qualitative change. Small wheel diameter, small bag Angle, friction contact area is small, easy to slip.
5. Why is the permissible contact stress of grey cast iron and Alfa-bronze turbine related to the sliding speed of tooth surface?
Answer: Because: the main failure form of grey cast iron and aluminum-iron bronze turbine is tooth surface gluing, and the gluing 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 turbine is pitting corrosion on the tooth surface, which is caused by contact stress, so contact stress is allowed to be independent of sliding speed.
6. Say the CAM mechanism follower commonly used movement law, impact characteristics and application occasions.
Answer: constant velocity motion law, equal acceleration and deceleration motion law, harmonic motion law (cosine acceleration motion law); The law of constant velocity movement has rigid impact, used for low speed and light load occasions; The motion law of equal acceleration and equal deceleration has flexible impact, which is used for medium and low speed occasions; Simple harmonic motion law (residual 4-string acceleration motion law) when there is a stop interval, there is flexible impact, for medium-low speed occasions, when there is no stop interval, there is no flexible impact, for high-speed occasions.
7. Briefly describe the basic law of tooth profile engagement.
No matter the tooth profile is in contact at any position, the common normal made by the contact point must pass through a fixed point on the center line to ensure that the transmission ratio is constant.
8. What are the methods of circumferential fixing of shaft parts? (Indicate 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 fixing methods for shaft parts? What are their characteristics? (Indicate more than four)
Axial fixation: axle shoulder, axle ring, axle sleeve, axle end baffle, elastic gear ring, axle shoulder, axle ring, axle sleeve fixed reliably, can withstand larger axial force; The elastic baffle ring can withstand small axial force; Shaft end baffle is used to fix shaft end parts.
10. Why must heat balance calculation be carried out for closed worm drive?
Worm drive has relative sliding, high friction, and because of the poor heat dissipation of closed worm drive, it is easy to produce glue, so it needs to calculate the heat balance.
11. What are the two strength calculation theories in gear strength calculation? Which failures are targeted? If the gear transmission is closed soft tooth surface transmission, what are the design criteria?
Answer: The contact fatigue strength of tooth surface and the bending fatigue strength of tooth root are calculated. The contact fatigue strength of tooth surface refers to the pitting failure of tooth surface and the bending fatigue strength of tooth root refers to the fatigue fracture of tooth root. Gear transmission is closed soft tooth surface transmission; its design criterion is according to the contact fatigue strength of tooth surface design, check the bending fatigue strength of tooth root.