Solve the problem of stainless steel bolt and nut locking

Update:29 Nov 2019

Causes and solutions of stainless steel fasteners:   Fi […]

Causes and solutions of stainless steel fasteners:
  First, the reasons for stainless steel fasteners locking up
1 The deflection angle of the thread fit and the soft characteristics of stainless steel are likely to cause lockup
1.1 The fit between the screw and the nut is a clearance fit, and has its own international standard tolerance range. Therefore, the central axis of the internal and external threads is basically not on the same straight line when mating, and the contact surface of the internal and external threads is reduced due to tilt.
1.2 The force is uneven or tilted during the locking process, which results in the maximum tilt of the central axis of the screw and the nut, so that the screw does not fully contact the active surface of the screw thread of the nut. The force on each unit becomes larger.
1.3 The larger the deflection angle, the smaller the force-bearing area of the thread fit, and the more easily the tooth pattern is damaged.
1.4 Stainless steel is softer than carbon steel. If iron scraps are rubbed off during the locking process, they will stick without falling, which interferes with the smooth entry and exit of the thread, causing the bottom of the tooth and the top of the tooth to die.
2 Excessive locking force and low thermal conductivity of stainless steel can easily lead to thread locking
2.1 When tightening the nut, the torque (locking force) must be greater than the frictional force of the screw and the nut before the nut can be rotated. When the nut rotates and descends along the thread, the reaction force will cause the screw to stretch. When the torque (locking force) exceeds the drop point of the screw (that is, the safe torque), the screw will have residual strain and remain in an extended state . When the screw is stretched beyond its elastic range, permanent deformation will occur and the thread will lock up.
2.2 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 generated pressure and heat destroy the chromium oxide layer (the reason why stainless steel is not easily rusted) As a result, the metal teeth are directly blocked / sheared. In addition, the stainless steel is relatively soft and sticks. The greater the tightening force, the greater the amount of heat generated, and the more easily the stainless steel in the screw mountain is likely to stick.
The above principle is also true during the loosening (screw nut, withdrawal) process.
 Second, how to reduce the lockup ratio
In view of the above-mentioned reasons that are prone to locking, the use of stainless steel screws needs to pay attention to:
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 barrel wrench as much as possible to avoid excessive force and control the torque within the safety stock torque range.
3. Try to use a manual wrench to slow down the locking speed and keep the threads clean. Use lubricating oil to reduce the coefficient of friction, reduce thermal energy, and thereby reduce adhesion.
4, choose a relatively high hardness products to reduce thread shear.