REFLECTOR: Lord Mounts

davedent at comcast.net davedent at comcast.net
Tue Sep 9 11:35:22 CDT 2008


Terry 
the torque on any bolt or nut is the same. Castle or otherwise.  You are stretching the fastener to the point of yield or equilibrium causing  or preventing motion and this is given as the best for that particular application matching hardware.  

So let it be known that the hardware should be matched.  Such as, don't use a grade eight bolt with a grade five nut. Way to many times I have found that there is a mismatch of hardware in aircraft.  Like putting a grade eight bolt into a grade five AN nut.  Most of your AN hardware is grade five or equivalent.  Most of the AN standard hardware was developed during world war ll.

I could go into fine thread and course threads as well as pitch and wet or dry torque of fasteners but it's not needed here now.   

Don't use a self locking nut on a bolt designed for a cotter pin.  It can be done on greater then 5/16" bolts but if the fastener is torque to the value it was designed for, the bolt is weakest at the drilled point.  I have always made it a practice to never use a drilled bolt with a self locking nut. Nylon or steel type.  It makes no difference.  Also it is not a good practice to use fiber lock nuts in areas of high temperature, like in the engine compartment.  If and when the nylon can flow, by temperature, the torque is now lost.  DO NOT use fiber lock nuts on engine mounts, castle or steel lock nuts are OK. 

Also never use a self locking nut on a pivot point that does not include a bearing.  Such as a bushing or sleeve.  Always use a castle nut or pivotal fastener at this point.  For instant I used a castle nut with a drilled grade eight bolt and nut on my main landing gear pivot point.  If the bolt was to get bent ever so slight, it can then prevent rotation and the nut could turn. If and when this happens the torque on that nut has now been compromised.  Boy you don't want to know how that feels when you loose this.  Mine did  four times before I found out what was going on.  A prime example of this is in the rudder and brake system.  Many moving parts with no bearings.  These should all be castle nut type fasteners.   I think you are getting my drift here by now. All that I have talked about can found in your AC 43.13 practices. It makes good reading for those just starting out in building an aircraft or doing there own maintenance. 
Dave

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