Theory of Keeping it Together

Monday, 1 April 2024
Clamp load, also called preload is designed in a bolt when a torque is applied to the Head or Nut.  The result is a spring-like assembly.  In a well designed joint, external forces are designed to act on the parts that have been compressed, and not on the fastener.
As long as the forces acting on the compressed parts do not exceed the clamp load applied, the Fastener is impervious to the forces.
This can be shown it be true with the aid of the model shown below. 

In figure 1, a Load of say 100kg is applied, deflecting the spring – equivalent to tightening a bolt.
In figure 2, a “block” is inserted, and the Load is removed.  The spring scale is unaffected, as the “block” is sized to maintain the spring extension.   
In Figure 3 is valid for any load up to and including 100kg, however if that external load exceeds that already in the spring, the spring will move and the Block will fall out.
Figure 4 shows the result of exceeding the Load initially set in the spring.
This is admittedly a simplified model.  In reality the bolt will see a small fraction of the external load prior to it exceeding the clamp load, depending in the compressed parts’ stiffness with respect to the Fastener’s stiffness.
The results of this type of joint design are;
  • Greater preloads in bolted joints reduce the fatigue loading of the Fastener.
  • Where cyclic loads exist, the Fastener does not see the full amplitude of the load .  As a result, fatigue can be reduced or, if the material exhibits an endurance limit, extended indefinitely.
As long as the external loads on a joint don’t exceed the Fastener clamp load, the Fastener doesn’t see any motion and will not come loose (no locking mechanisms are required).  In case of the compressed member being less stiff than the Fastener (soft, compressed gaskets for example) this analogy doesn’t hold true.   The load seen by the Fastener in that case is the preload plus the external load.
The accuracy of a Torque Wrench in applying a load to a Bolt is plus or minus 25%, due to the variable friction under the nut and in the mating threads.  Therefore most “experts” suggest tightening to a theoretical 65% of the Proof Load.   Up to Proof a bolt acts as a spring but permanently deforms above it.
As the model demonstrates, provided the sum of the preloads in the bolts in a joint exceed the load applied, the joint will maintain its integrity, without resorting to thread lockers or mechanical locking elements.
To assist our Customers, the Boltmasters torque tables are available in or technical data section.