Practical Mechanical Design Knowledge Points
Q1
What are the causes and measures for the prevention of thread loosening in threaded connections?
A
Causes: Threaded connections may experience a momentary loss of preload under the action of impact, vibration, and variable loads, which could lead to the loosening of the connection. High-temperature threaded connections may also loosen due to differences in thermal deformation. Therefore, anti-loosening measures must be considered in the design.
Measures: Utilize additional friction forces to prevent loosening, such as using slotted nuts with cotter pins, lock washers, etc. Other methods include point peening for loosening prevention and adhesive methods.
Q2
Measures to improve the strength of bolted connections
A
(1) Reduce the variation range of the total tensile load Fa on the bolt:
a. To reduce the bolt stiffness, the diameter of the bolt shank can be reduced, hollow screws can be used, or the length of the screw can be increased.
b. The connected parts themselves have high rigidity, but the rigidity will be reduced if a soft gasket is used on the joint surface that needs to be sealed. Using a metal thin gasket or an O-ring as a sealing element can maintain the original rigidity value of the connected parts.
(2) Improve the load distribution between the螺纹 threads.
(3) Reduce stress concentration.
(4) Avoid or reduce additional stresses.
Q3
Failure modes of gear teeth
A
(1) Gear tooth breakage, which generally occurs at the root of the tooth because the bending stress is the greatest at the tooth root when the gear is under load, and there is stress concentration, which can be divided into overload breakage and fatigue breakage.
(2) Pitting on the tooth surface.
(3) Scoring on the tooth surface.
(4) Tooth surface wear.
(5) Plastic deformation of the tooth surface.
Q4
Lubrication of gear transmission
A
Open gear drives are usually lubricated by manual periodic oiling, using either lubricating oil or grease. The lubrication method for closed gear drives is determined by the circumferential speed V of the gear. When V ≤ 12, oil bath lubrication is commonly used. When V > 12, oil bath lubrication is not suitable because:
(1) The circumferential speed is too high, and most of the oil on the gear is thrown off and does not reach the meshing area.
(2) Excessive agitation causes the oil temperature to rise, reducing the lubrication performance.
(3) It can stir up the sediment at the bottom of the box, accelerating the wear of the gear. Spray oil lubrication is often used instead.
Q5
Why is it necessary to calculate the heat balance and cooling measures for worm gear transmission?
A
Due to the low efficiency and high heat generation of worm gear transmission, if the heat is not dissipated in time, it will cause the oil temperature in the box to rise, leading to lubrication failure, increased gear wear, and even scoring. Therefore, heat balance calculations are required for continuous operation of closed worm gear transmissions.
Measures:
Increase the heat dissipation area and design the box structure reasonably, casting or welding on heat dissipation fins.
Improve the surface heat transfer coefficient, install a fan on the worm shaft, or install a serpentine cooling water pipe in the box oil pool.
