FAQs

Controlling torque and preload for collar fasteners

• Can I reduce preload scatter by tightening more accurately?

  Improved tool accuracy and process control helps, but preload scatter cannot be eliminated. Critical joints should be validated through testing.

• If I specify a torque, will I always get the same preload?

  No. A given tightening torque will produce a range of preload values due to normal variations in friction and tool accuracy. This preload scatter is unavoidable and is influenced by surface condition, lubrication, materials, and the assembly method used.

• Is using 90% of fastener yield a safe rule for setting torque?

  This is an industry convention for metal-to-metal joints and is not sufficient for composite or bonded structures.

  Use alternative governing criteria whenever fastened materials, interfaces, or performance requirements impose additional limits.

• Should I control assembly using torque or preload?

  It depends on what you need to protect or control.

  • Use torque limits to protect the bigHead collar from thread strip, or the embedding/bonded interface from excessive torsional loading.
  • Use preload limits to manage clamp load and compressive forces in the fastened materials, or comply with the axial load limits of the fasteners.

  In some applications, both limits apply. The more restrictive limit should govern your assembly.

• Why can’t you just give me a torque limit?

  Because torque is only an indirect way of controlling clamp load, and clamp-force limits in composite and bonded structures are governed by the fastened materials and interfaces, not the bigHead fastener alone.