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What’s the Deal with Castle Nuts?

Castellated nuts, often called a castle nuts, employ cotter pins as a locking mechanism. We will walk through two key points that render many castle nut bolted joints ineffective and unreliable.

1. The castle nut must be in a certain orientation to allow the cotter pin through.

2. The difference between full tension and almost no tension on bolted joints can be only a slight turn of the nut.

With a hex castle nut, the nut can only be positioned at 60° intervals. The nut will not be rotated enough to create proper bolt tension or it will be rotated too far. Either of these conditions can result in premature failure of the joint.


Below is an example of a 1” diameter nut at a clamp distance of 6”. As you can see, the final 51.8 degrees of rotation take it from the elastic origin to the proof point. 

Thread Diameter 1.0″
Pitch 8.0 threads/in
Clamp Distance 6.0″
Clamp Load54,500 lbs
Assembly Torque908 ft-lbs
Bolt Stretch Distance0.018″
Clamping Angle (near zero to full clamp)51.8°

Applying 908 ft lbs to turn the final 51.8° stretches the bolt 0.018” to achieve 54,500 lbs of clamp.

A castle nut might not be able to be installed to the correct position due to the cotter pin alignment with the slots. Installed to an improper torque value, a castle nut is subject to loosen. Even if the cotter pin prevents the nut from falling off the bolt, the effects of losing preload on a bolted joint are disastrous:

  • bolt breakage
  • bolted joint failure
  • replacement/maintenance costs
  • safety concerns

Additionally, the loose nut will also move and eventually wear through the cotter pin.

Security Locknut

On top of providing actual vibration resistance, the Security Locknut can be installed properly at any angle, not relying on windows every 60° like castle nuts.

Having problems with bolted joints? Give us a call. We can help.

(847) 970-4050