Fastener Selection

Design issues and fastener selection.

• Reduce the stress at the bearing surface by one or more of the following:
  • increase the fastener head diameter
  • add a flat washer
  • reduce the clearance hole diameter
  • reduce initial clamp load at assembly
• Add a spring element to the joint such as a helical and flat washer combination
• Add a metal sleeve in the clamped component to carry some of the clamp load
• Use a shoulder bolt to transfer the load to the nut member
• Increase stiffness of plastic by adding a filler or changing the base resin

1. Gather enough sample applications to generate a statistically significant sampling – a minimum of 30 per specific joint condition. These samples must include all components in the joint stack-up such as clamping components, nut members and fasteners.
2. Use a drive gun that can torque the fasteners to failure and has the same RPM as the drive gun that will be used in production.
3. Use a torque measuring device and a strip chart or graph and capture the peak drive torque and ultimate torque values. Drive the fasteners to failure. Calculate the average of the peak drive values and of the ultimate torque values.
4. Next calculate the average of these two resulting values. This is the target seating torque. Factor a value of ±10% for gun accuracy. Compare this torque window to +3 σ of the average drive torque value and -3 σ of the average ultimate torque value. If there is no overlap of the two regions, the torque window will allow the fasteners to be seated without failure at assembly. If there is overlap in either of the two regions, a redesign of the joint is warranted.