The deflection angle of the thread fit and the softer c […]
The deflection angle of the thread fit and the softer characteristics of stainless steel are easy to cause locking
The fit between the screw and the nut is a clearance fit, which has its own implementation of the international standard tolerance range. Therefore, the center axis of the internal and external threads is basically not on the same straight line during injection screw barrel the mating, and the contact surface of the internal and external threads is reduced because of the inclination.
The uneven force or tilt during the locking process causes the central axis of the screw and nut to be tilted to the maximum, so that the acting surface of the screw and the nut thread is not completely in contact, and the surface force is changed to point force or every The force on each unit becomes larger.
The greater the deflection angle, the smaller the force bearing area of the thread fit, and the more easily the tooth pattern is damaged.
Stainless steel is softer than carbon steel. If the iron scraps are rubbed off during the locking process, they will stick and not fall, thereby interfering with the smooth entry and exit of the thread, causing the bottom of the tooth and the top of the tooth to die.
Excessive locking force and low thermal conductivity of stainless steel can easily lead to thread locking
When locking the nut, the torque (locking force) must be greater than the friction force of the screw and the nut itself before the nut can be turned. When the nut is rotated and descended along the thread line, the screw will be stretched due to the reaction force. When the torque (locking force) exceeds the screw's yield point (that is, the safe torque), the screw will produce residual strain and maintain the extended state . When the screw stretches beyond its elastic range, it will permanently deform and cause the thread to lock.
During the rotation of the screw nut, heat is generated due to friction. The thermal conductivity of stainless steel is relatively low (basically in the range of 10-30w/mc°, about 1/3 of carbon steel), when the pressure and heat generated destroy the chromium oxide layer (the stainless steel is not easy to rust oxide layer) At this time, the metal tooth pattern is directly blocked/sheared, and the stainless steel has a soft characteristic, and then the adhesion phenomenon occurs. The greater the locking force, the greater the heat generated, and the more likely the stainless steel in the screw mountain is to stick.
The above principle is also true during the process of loosening (screw nut, exit).
Second, how to reduce the lock-up ratio
In view of the above-mentioned reasons that are prone to locking, use stainless steel screws as much as possible:
1. Keep the central axis of the screw and nut perpendicular to the locked surface to reduce the deflection angle.
2. Use a torque wrench or a barrel wrench as much as possible to avoid excessive force and control the torque within the safety stock torque range.
3. Use a manual wrench as much as possible to slow down the locking speed and keep the threads clean. Use lubricating oil to reduce the friction coefficient and reduce heat energy, thereby reducing adhesion.
4. Choose relatively high hardness products to reduce thread shear.