"Five-prevention loosening" for threaded pair connection

The assembly direction of the disc spring gasket should be correct

Disc spring washers do not cause more scratches on the surface of machine parts, so their use in mechanical vehicles is increasing day by day. However, if they are not installed properly, they will not play a role in preventing the loosening of threads. For instance, the disc spring washer installed between the lock nut of the rear flange of the second shaft of the EQ140 heavy-duty truck transmission and the flange became loose due to improper installation, which caused the driving gear of the speedometer and odometer to slip, resulting in an error between the speed indication of the odometer and the mileage traveled. At the same time, due to the forward and backward movement of the second shaft, the third gear dropped. The correct installation direction should be to position the concave side of the disc spring washer towards the connecting piece and the convex side towards the nut. After the nut is tightened in this way, due to the deformation of the disc spring gasket under force, the supporting surface is subjected to a counterforce and thus plays a locking role.

The length of the cotter pin on the connecting rod bolt should be appropriate

When the connecting bolt of the big end of the engine connecting rod is loose, it will cause the connecting rod bearing to become loose, produce abnormal noises, and even damage the engine. When disassembling and inspecting some abnormal noise faults of connecting rod bearings, it is often found that the cotter pins of the lock nuts on some connecting bolts have disappeared. The main reason is that the cotter pins are too long and break when they collide with the crank during high-speed movement, eventually falling off. The method to determine the correct length of this cotter pin is to insert the cotter pin into the corresponding hole of the lock nut, strike it in place with a hand hammer, and the extended length of the cotter pin must not exceed the plane of the crank. Then, lock the cotter pin as required.

The tightening direction of the safety wire should be correct

Some parts of the vehicle chassis are tightened with fuses to prevent the threads from loosening. This method is often used to prevent the threads of bolt groups from loosening, such as the connecting bolts between the shift fork and the shift fork shaft. The safety wire is relatively reliable for preventing loosening, but when installing it, attention should be paid to the selection of the safety wire specification and the tightening direction after it is inserted into the screw hole. If the safety wire is too thick, it will be difficult to pass through. The diameter of the safety wire should be slightly smaller than the diameter to be pierced, and there should be a little resistance when threading. The criterion for distinguishing whether the direction in which the safety wire is inserted is correct or not is as follows: If a certain bolt is loosened, the safety wire should be in a taut state, that is, loosening one bolt tights the two adjacent bolts; otherwise, it is incorrect.

The lock plate should be locked on the side plane of the nut

For large mechanical parts, the locking of nuts often adopts the method of lock plates to prevent loosening, such as the lock nuts of the steering knuckle in automobiles and the lock nuts at the front end of the second shaft of the transmission. When using a lock plate for locking, after the nut is tightened to the specified torque, fold the lock plate onto one side of the nut's plane and strike it tightly with a hammer. Under no circumstances should the lock plate be placed at the corners of the nut, as this will not effectively prevent loosening.

The elasticity of the spring washer should not fail and its size should be appropriate

Spring washers are easy to install, have a low manufacturing cost and are widely used. It utilizes the elastic deformation of the washer to axially tension the threaded pair, and at the same time, the bevel tip of the washer also helps prevent loosening. However, during use, this type of gasket often loses its due elasticity due to heat or prolonged compression. Therefore, before installing the spring washer, it is necessary to check whether its elasticity has failed. The method of inspection is to see if the two ends of the gasket interface are offset by a certain Angle up and down. If the misalignment Angle is small or there is no misalignment Angle, then this gasket has failed. At the same time, the size of the washer should correspond to the size of the mating bolt. It is appropriate that the outer diameter of the washer is slightly smaller than that of the nut.

To sum up, although the parts used for preventing loosening in threaded connections are relatively small and insufficient, if the above issues are ignored during mechanical maintenance, it will not only affect the maintenance quality but also pose potential faults for the normal operation of the machinery.