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.
More on bolt stretch
More on rotation angle
More on torque
Example
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.
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.
Handy Excel spreadsheet for calculating load, torque, stretch (screenshot below) Download Spreadsheet Tool
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