In processing, we will inevitably make mistakes. In order to avoid unnecessary mistakes and improve production efficiency, I have prepared 20 mechanical design knowledge points for everyone, which can help reduce the chance of making mistakes in the machining processing.
1. What are the reasons and measures for preventing loosening of threaded connections in machining?
Reason: Under the action of impact, vibration, and variable load on the threaded connection, the pre-tightening force may disappear at a certain moment, and the connection may loosen. The threaded connection at high temperatures may also loosen due to the difference in temperature deformation and other reasons. Therefore, anti-looseness must be considered in the design.
Measures: Use additional friction to prevent loosening, such as grooved nuts, split pins, and stop washers.
2. Measures to improve the strength of bolt connection
1) Reduce the variation range of the total tensile load Fa of the bolt:
A: In order to reduce the stiffness of the bolt, the diameter of the polished rod of the bolt can be reduced or the hollow screw can be used, and the length of the screw can also be increased.
B: The rigidity of the connected part itself is relatively large, but the rigidity of the joint surface between the connected parts will be reduced when a soft gasket is used because it needs to be sealed. Using thin metal gaskets or O-ring seals as the sealing element can still maintain the original rigidity of the connected parts.
2) Improve the load distribution between the threads.
3) Reduce stress concentration.
4) Avoid or reduce additional stress.
3. Failure mode of gear teeth
1) Gear tooth fracture generally occurs in the tooth root part, because the tooth root bending stress is the largest when the tooth is stressed, and there is stress concentration, which can be divided into overload fracture and fatigue fracture.
2) Pitting corrosion on the tooth surface.
3) Glue the tooth surface.
4) Tooth surface wear.
5) Plastic deformation of the tooth surface.
4. Lubrication of gear transmission
Open gear transmission usually uses manual lubrication regularly, and lubricating oil or grease can be used. Generally, the lubrication method of closed gear transmission is determined according to the peripheral speed V of the gear. When V<=12, oil pool lubrication is often used. When V>12, oil pool lubrication is not suitable.
This is because:
1) The peripheral speed is too high, and most of the oil on the gear is thrown out and cannot reach the meshing area.
2) Excessive stirring of oil will increase the temperature rise of the oil and reduce the lubricating performance.
3) It will stir up the impurities deposited at the bottom of the box and accelerate the wear of the gears. Oil spray lubrication is often used.
5. Why do worm drives need to perform heat balance calculations and cooling measures?
Due to the low transmission efficiency of the worm and the large heat generation, if the heat is not released in time, it will cause the oil temperature in the box to rise, lubrication failure, resulting in increased gear wear, and even gluing. Therefore, the thermal balance calculation should be carried out for the continuously working closed worm drive.
Measures:
1) Increase the heat dissipation area, rationally design the box structure, cast or weld the heat sink.
2) Improve the surface heat transfer coefficient. Install a fan on the worm shaft, or install a serpentine cooling water pipe in the oil sump of the box.
6. Advantages and disadvantages of belt drive
Advantage:
(1) Suitable for transmission with large center distance.
(2) The belt has good flexibility, can alleviate impact and absorb vibration.
(3) Slippage occurs between the belt and the pulley when overloaded, which can prevent damage to other parts.
(4) Simple structure and low cost.
Disadvantages:
(1) The outer profile size of the transmission is relatively large.
(2) A tensioning device is required.
(3) Due to the sliding of the belt, a fixed transmission ratio cannot be guaranteed.
(4) The belt life is short.
(5) The transmission efficiency is low.
7. Definition of elastic sliding and slipping
Elastic sliding refers to the sliding caused by the elastic deformation of the material. Slippage refers to the overall sliding caused by overload. Elastic sliding is caused by the difference in tension. As long as the circumferential force is transmitted and tight edges and loose edges appear, elastic sliding must occur. Therefore, elastic sliding is inevitable, and V2 is always greater than V1.
8. Compared with belt drive and gear drive, the advantages and disadvantages of chain drive
Compared with belt transmission, chain transmission has no elastic sliding and slipping, and can maintain an accurate average transmission ratio. It requires small tension and pressure on the shaft. It can reduce the friction loss of the bearing. It has a compact structure and can Work under harsh environmental conditions such as high temperature and oil stains.
Compared with gear transmission, chain transmission requires lower manufacturing and installation accuracy, and its transmission structure is simple when the center distance is large. The shortcomings of chain transmission-the instantaneous chain speed and the instantaneous transmission ratio are not constant, the transmission stability is poor, and there is a certain impact and noise in the work.
9. The role of the shaft, the difference between the rotating shaft, the transmission shaft and the spindle
A shaft is a mechanical part used to support rotation. The rotating shaft not only transmits torque but also bears bending moment. The drive shaft only transmits torque and does not bear the bending moment or the bending moment is very small. The spindle only bears the bending moment and does not transmit the torque.
10. The main requirements for the structural design of the shaft:
(1) The shaft should be easy to process, and the parts on the shaft should be easy to assemble and disassemble.
(2) The shaft and the parts on the shaft must have an accurate machining position.
(3) The parts should be relatively fixed firmly and reliably.
(4) Improve the stress condition and reduce the stress concentration.
11. The necessary conditions for forming a dynamic pressure oil film:
(1) There must be a wedge-shaped gap between the two working surfaces.
(2) The two working surfaces must be continuously filled with lubricating oil or other viscous fluids.
(3) There must be a relative sliding speed between the two working faces. The direction of its movement must be such that the lubricating oil flows in from a large section and flows out from a small section. In addition, for a certain load, the speed, viscosity, and gap must be properly matched.
12. The connection and difference between coupling and clutch
Both are mainly used for the link between the shaft and the shaft, so that they can rotate together and transmit torque. The two shafts connected by a coupling can only be separated after the machine is stopped and after being disassembled. The two shafts connected by a clutch can be easily separated or joined during machine operation.
13. The characteristics of fatigue fracture of parts under variable stress:
(1) The maximum stress of fatigue fracture is much lower than the strength limit of the material under static stress, and even the yield limit is lower.
(2) Regardless of brittle materials or figurative materials, the fatigue fracture openings appear as brittle sudden fractures without obvious plastic deformation.
(3) Fatigue fracture is the accumulation of damage.
14. The main types of mechanical wear:
Abrasive wear, adhesive wear, fatigue wear, and corrosion wear.
15. The role of washers
Increase the supporting area of the connected part to reduce the pressure at the contact point and avoid scratching the surface of the connected part when tightening the nut.
16. Advantages and disadvantages of rolling spirals
Advantage:
(1) The wear is small, and the adjustment method can also be used to eliminate the gap and produce a certain pre-deformation to increase the rigidity, so the transmission accuracy is very high.
(2) It is not self-locking and can be changed from linear motion to rotary motion.
Disadvantages:
(1) The structure is complex and manufacturing is difficult.
(2) In some institutions, a self-locking mechanism is required to prevent reversal.
17. In gear transmission, the influence of error on transmission:
(1) Affect the accuracy of the transmission movement.
(2) The instantaneous transmission ratio cannot be kept constant, which affects the smoothness of the transmission.
(3) Affect the uniformity of load distribution.
18. The power loss of gear transmission includes friction loss in meshing, oil resistance loss of stirring lubricating oil, and friction loss in bearings.
19. Advantages and disadvantages of single arc gear
Advantage:
(1) The tooth surface has high contact strength.
(2) The tooth profile shape is beneficial to lubrication and high efficiency.
(3) The tooth surface is easily saturated.
(4) There is no undercutting, and the number of tooth surfaces can be less.
Disadvantages:
(1) The precision requirements of center distance and tooth cutting depth are high, and the error of the two makes the load-carrying capacity of the transmission significantly reduced.
(2) The noise is large, and its application in the high-speed transmission is restricted.
(3) Generally, the bending strength of gear teeth is low.
(4) Different hobs should be used to cut convex arc tooth profile and concave arc tooth profile of the same modulus.
20. Performance of bearing material:
(1) The friction coefficient is small.
(2) Good thermal conductivity and low thermal expansion coefficient.
(3) Abrasion resistance, corrosion resistance, and strong adhesion resistance.
(4) It must have sufficient mechanical strength and plasticity.
