Why do fasteners fail, and how to eliminate fastener failure.

The factors that cause a fastener to loosen.

  • The impact load causes the assembly to resonate.

  • Focused on the first engaged thread of the fastener, cyclic plastic deformation occurs.

  • The torsional energy is released.

  • The bearing surface slips.

  • Once the loosening starts at a given pre-load the pre-load will progressively reduce. Once the pre-load is reduced the rate of loosening will increase until the failure occurs. Therefore, it is prudent in the design to eliminate the possibility of any loosening. Unalock eliminates this problem.

The initial pre-load torque value for the fastener is too low.
The pre load torque value is greater than the yield stress of the fastener.

Choosing the right material for the fastener is critical.

One of the reasons fasteners fail is when the yield stress of the fastener material is the wrong choice for the application.
Selecting the proper grade, strength material for the fastener is critical.


Installation Pre-Load is critical to preventing failures.
Clamping Force should be appropriate for the fastener material and diameter.

Suggestions for eliminating fastener failure (clampforce).

  • Determine the needed clamping load for the fastener.
  • Achieve the proper clamp-load in the assembled joint.
  • Eliminate uneven surfaces in the assembled joint.
  • Minimize the number of joint elements and materials in the assembled joint.
  • Minimize the hole clearances in the assembled joint.
  • Use Fine threads in assembled joints that will experience vibration.
  • Make a tighter tolerance fit, by reducing thread clearance.

Friction Coefficient is a very important factor.

  • Torque values are strictly based on the friction present in the joint
    assembly. Variances in the friction will lead to variances in the amount of clampload/tension attained at a specified torque value.

  • Controlling the friction coefficients, in both the
    thread and under-head regions, will result in more consistent and predictable clamp load values.

  • Increasing the pitch torque by lowering the friction coefficients will cause an 
    increase in clamp load.

    Each variable listed below has an effect on clampforce.
  • Coefficient of friction-underhead
  • Coeffcient of friction-threads
  • hole clearance
  • head or nut diameter
  • thread pitch

Friction coefficient testing helps obtain maximum clampforce in assembled joints.

Know the environment your fasteners will be operating in,
Temperature, Pressure, Moisture, and Vibration.

Elevated Temperatures that cause weakness in the fastener.

UnaLock Fasteners can be manufactured to withstand extreme temperature.
-101 Celsius to +500 Celsius


Vibration causing the fastener to loosen.
High stress amplitudes above the endurance limit.

Vibration forces reduce the friction force on the threads, allowing the fastener to slide in a loosening direction.

Suggestions to eliminate fastener failure in assembled joints.

  • By using Unalock fasteners, you can sustain the initial level of bolt tension that is higher than the tension where self loosening occurs.

  • Unalock creates an extra spring in the joint which increases the overall strength of the assembled joint.

Using rolled threads instead of cut threads provides greater fatigue resistance.

Each joint assembly design should be individually tested and evaluated using specific guidelines, to reduce fastener failure.

Unalock fasteners minimize the amplitude of alternating stresses.

Unalock fasteners reduce stress concentrations, which reduces fatigue on the bolt and the entire assembly.

UnaLock fasteners increase the ability of the joint to capture, retain, and deliver energy back to the joint.

Contact us for direct consultation regarding your fastener application needs.


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