Lock nut, also known as self-locking nut, currently mainly includes all-metal lock nut, non-metallic insert lock nut, and a lock nut of metal clip. The all-metal lock nut is divided into 3 points of end face pressure, which has a certain impact on the pitch of the thread and forms the locking characteristics; There is also a kind of flattening on the side, so that the end thread changes from round to oval to form a locking characteristic.
The coefficient of friction will affect the final preload, which has been recognized and valued by everyone, but for the self-locking nut, how do you design this tightening torque?
I believe you still have a lot of doubts about this issue. Today, Screw Jun is here to talk to you.
Description of torque in VDI2230 for self-locking nut parts
In VDI2230, the tightening torque of a self-locking nut is described as follows:
This description shows that for parts such as self-locking nuts, in addition to the normal thread tightening torque mg and face tightening torque mk, there are also the effects of the threaded screwing torque mu (self-locking nut) and the mkzu of the resistance torque of the end attachment (e.g., bolt tightening with teeth) when determining the tightening torque or the calculation of the tightening torque.
However, the VDI 2230 standard has another note that for high preload couplings, the threaded screwing moment mu is negligible. The meaning of this sentence is that if the bolt is tightened to a high preload, the thread screwing torque mu does not need to be added to the total torque. In the end, how much does it take to tighten to a high preload, so that it does not need to be added, and how to measure this high preload?
Measured coefficient of friction of the lock nut
In the case of a nylon insert lock nut, for example, only the nut was tightened in the test to illustrate the problem. The torque-angle and axial force-angle curves of the lock nut are shown in the following figure, and it can be seen from the test results in the figure that the lock nut has a certain screw-in torque, that is, from the bolt to the nut, and the bolt reaches the locking part of the nut, a certain screw-in torque or locking torque will be generated; After the bolt is fully extended from the nut, the screw-in torque will reach a stable stage, and the torque will not continue to rise at this stage; When the nut is fully attached to the connected part, the torque will increase proportionally.
In the screw-in torque stage, the axial force of the bolt is basically zero, and it is basically in a horizontal straight line, which means that even if a certain tightening torque is displayed, the axial force of the bolt is basically zero at this time.
The thread friction coefficient - rotation angle, total friction coefficient - rotation angle curve of the lock nut is shown in the following figure, it can be seen from the thread friction coefficient curve, the friction coefficient changes with the change of the tightening angle, after the nut is completely attached to the connected parts, the thread friction coefficient and the total friction coefficient are reduced with the increase of the axial force (or the increase of the rotation angle), which also illustrates: if the tightening torque of the lock nut is relatively low, the setting or calculation of the tightening torque can not be calculated according to the normal torque axial force, It needs to be calculated according to the actual coefficient of friction, or a certain amount of screw-in torque needs to be considered to be more in line with the actual situation.
The friction coefficient of the supporting surface of the lock nut changes little relatively speaking, after the nut is attached to the connected piece, the friction coefficient of the supporting surface of the nut is basically the same as the friction coefficient curve of the normal non-lock nut, and the friction coefficient of the supporting surface will not change greatly with the increase of the preload (bolt axial force).
If the lock nut is tightened normally, when the lock nut is developed according to the required friction coefficient, it can be tightened according to the normal tightening torque, and there is no need to consider the problem of screwing torque.
Therefore, when the lock nut is developed, the friction coefficient test is also carried out according to the guaranteed load of 75%, that is to say, the friction coefficient meets the friction coefficient required at the time of development when the normal tightening torque is tightened.
From the above actual test results, it can be seen that the thread friction coefficient of the lock nut remains basically unchanged when tightened to 1600°, that is to say, when the lock nut is tightened to 1600°, about 50% of the final preload, the thread friction coefficient basically does not change greatly, and it can be considered that the friction coefficient is basically unchanged from the final at this time, and it can be consistent.
Therefore, from this test result, it can be determined that if the actual design preload of the lock nut can reach more than 40% of the guaranteed load, it is basically possible to no longer consider the screw-in torque. In other words, the high preload mentioned in the VDI 2230 standard can be considered to be at least 40% or more of the guaranteed load. If the torque of the design is too low, the screw-in torque of the self-locking nut needs to be considered.
In addition, it should be noted that the VDI 2230 standard does not specify the circumstances in which the additional torque under the head should be considered in the case of bolt heads, bolts or nuts with teeth on the support surface of the nut.
It can be understood in this way: for bolts and nuts with teeth, the friction coefficient (or equivalent friction coefficient) will gradually increase during the tightening process, especially when the preload is relatively large, the equivalent friction coefficient will be very large, which is equivalent to the bolt head will cut the surface of the connected part.
Which parts (situations) need to consider the locknut screw-in torque
For example, the piston rod in the shock absorber with the mount nut is mounted. Due to factors such as piston rod in order to reduce weight, the outer diameter of the piston rod will not be designed to be very large, often the effective contact surface is only about 3mm, and even some designs will be smaller, therefore, under the premise of meeting the requirements, the tightening torque of the mounting nut can not be formulated too high, otherwise, it will be easy to make the mount crush or permanent plastic deformation, resulting in the attenuation of tightening torque.
Depending on the force requirements, a large clamping force is not required here to meet the external load requirements, so shock absorber top nuts often do not need to be designed with large tightening torques. The nut design is M14x15 thread specifications, but the tightening torque is often about 60Nm. In this case, due to the fact that the self-locking nut itself has a certain screw-in torque, for example, M14X1The standard maximum screwing torque of 5-10 all-metal self-locking nuts is 31 Nm, if the design screwing torque is close to 31 Nm, will the clamping force be reduced if the tightening torque is set to 60 Nm? In this regard, it is very important to determine the coefficient of friction of the self-locking nut.
Screw Jun's experience and summary
For self-locking nuts, if the tightening torque is normal torque, there is no need to add the screw-in torque of the threaded part when calculating the tightening torque, because the normal tightening torque preload is relatively high at this time.
For some parts that adopt self-locking nuts, the tightening torque is far lower than the normal tightening torque, such as 50% of the normal tightening torque, at this time, it is necessary to consider the screw-in torque of the self-locking nut, which is very important, otherwise, the actual clamping force will not reach the required clamping force requirements, resulting in the loosening of the parts.
High preload, which can be considered as 40% of the bolt guaranteed load.
In the case of toothed bolts and nuts, this additional torque needs to be taken into account in all cases.
Today's topic is shared here, and criticism and correction are welcome if it is inappropriate; If you have any questions or suggestions, or need to communicate with your partners, you can pay attention to WeChat***gaf-luosijun