Joint Relaxation in Fastened Joints
Joint relaxation occurs to some extent in all fastened joints and is caused by the surfaces of the part embedding or by “soft parts”, such as gaskets, collapsing under the clamping force. For correctly designed joints, relaxation is small and can be virtually ignored. However, relaxation is a particular problem on joints where gaskets, or parts such as spring washers, are present. On these joints it can take a long time before the joint settles and results in a reduction in clamping until the condition has stabilized.
The physical phenomenon for the collapse of material is called “creep.” All material creep to some extent. For example, glass is a liquid and creeps over time. That is why an old glass window may show “waves” at the bottom of the window.
The creep is most obvious and dramatic right after the force has been applied. In many tightening applications the majority of the creep, reducing the clamp load (and sometimes static torque), appears within the first 10-50 milliseconds.
The techniques that can be used to reduce the effect of creep are:
1. Torque the fastener, undo the joint and retighten.
2. Redesigning of the joint (for example, replace soft gaskets with sealing compound).
3. Torque the fastener, wait briefly and then apply again (can be repeated in several steps).
4. Low RPM of the tool at final clamp down.
With a given design, action 3 above is quite commonly used. It is especially easy to achieve if the application is tightened with a fixture spindle in a station but with the introduction of sophisticated pneumatic and electric power toolset becomes more and more common even for those. However, the negative ergonomic ramifications should be considered if the tool is not suspended by a tool balancer or supported using a torque arm.
Source by: Mountz