• What happens as a bolt is tightened?
  • Should I tighten the bolt head or the nut?
  • What is bolt preload and why is it important?
  • Why do bolts come loose?
  • What is proof load and how is it different from yield strength and ultimate strength?

What happens as a bolt is tightened?

When using a traditional wrench to tighten a bolt, the torque applied to the nut causes it to slide up the inclined plane of the threads. This relative motion between the nut and the bolt attempts to reduce the distance between the bearing surfaces of the bolt and nut. This dimension is the grip length of the bolted joint. When the joint members within the grip resist, the bolt begins to stretch like a stiff spring, developing tension and simultaneously compressing the components together creating the all-important clamp force.

Should I tighten the bolt head or the nut?

Either is acceptable, however a torque value defined for tightening the head does not necessarily apply to tightening the nut. Tightening the head vs. the nut can result in different nut factors and therefore change the torque required to achieve proper preload.

What is bolt preload and why is it important?

Preload is the tension created in a fastener when it is tightened. This tensile force in the bolt creates a compressive force in the bolted joint known as clamp force. For practical purposes, the clamp force in an unloaded bolted joint is assumed to be equal and opposite of the preload. If proper preload, and thus clamp force, is not developed or maintained, the likelihood of a variety of problems such as fatigue failure, joint separation, and self-loosening from vibration can plague the bolted joint leading to joint failure.

Why do bolts come loose?

There can be many possible causes for bolts to loosen in service. When we say “loosen” here we mean lose their tension, or preload. Here are five major causes:

  • Vibration which can create relative transverse movement of the bolted materials leading to self-loosening of the nut.
  • Relaxation of the bolted joint after tightening due to embedment or gasket creep.
  • Elastic interactions occur when multiple bolts are present in a bolted joint. The additional force applied to the joint members by tightening a bolt can affect the amount of tension on the other previously tightened bolts. Elastic interactions can either increase or decrease bolt preload making it even more difficult to predict.
  • Temperature fluctuation of the components.
  • Insufficient initial preload developed at installation.

The design of the bolted joint can minimize relaxation and embedment, and ensuring sufficient preload at installation can reduce the effects of vibration and likelihood of relative transverse movement. In other words, properly designed bolted joints that a properly preloaded should not self-loosen!

What is proof load and how is it different from yield strength and ultimate strength?

Each of these are basic mechanical properties that help define the expected tensile strength performance of a specific fastener and can be measured in units of force. In USCS and SI systems, force is measured in pound-force (lbf) and Newtons (N), respectively. Since the strength of fasteners is generally quite large, it is also common to see these forces listed in kilopound-force (klbf) and kilonewton (kN).

Proof load is defined as the maximum tensile force that can be applied to a bolt that will not result in plastic deformation. In other words, the material must remain in its elastic region when loaded up to its proof load. Proof load is typically between 85-95% of the yield strength.

Yield strength can be defined as the tensile force that will provide a specified amount of permanent deformation (most commonly 0.2%) within a specific fastener.

Ultimate tensile strength can be defined as the maximum force a specific fastener must withstand before fracture.