About
The clamp load, also called preload, of a cap screw is created when a torque is applied, and is generally a percentage of the cap screw’s proof strength. Cap screws are manufactured to various standards that define, among other things, their strength and clamp load. Torque charts are available that identify the required torque for cap screws based on their property class.
When a cap screw is tightened it is stretched, and the parts that are captured are compressed. The result is a spring-like assembly. External forces are designed to act on the parts that have been compressed, and not on the cap screw.
The result is a non-intuitive distribution of strain; in this engineering model, as long as the forces acting on the compressed parts do not exceed the clamp load, the cap screw doesn’t see any increased load. This model is only valid when the members under compression are much stiffer than the capscrew.
This is a simplified model. In reality the bolt will see a small fraction of the external load prior to it exceeding the clamp load, depending on the compressed parts’ stiffness with respect to the hardware’s stiffness.
The results of this type of joint design are:
* Greater preloads in bolted joints reduce the fatigue loading of the hardware.
* For cyclic loads, the bolt does not see the full amplitude of the load. As a result, fatigue life can be increased or, if the material exhibits an endurance limit, extended indefinitely.
* As long as the external loads on a joint don’t exceed the clamp load, the hardware doesn’t see any motion and will not come loose (no locking mechanisms are required).
In the case of the compressed member being less stiff than the hardware (soft, compressed gaskets for example) this analogy doesn’t hold true. The load seen by the hardware is the preload plus the external load.