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Why some castle nuts, and some plastic lock nuts?

claycookiemonster

Well Known Member
Inquiring minds want to know. I suspect there is some underlying rule or reason, but in aligning my ailerons (-8) I found some nuts in the linkage were castles while others were plastic lock nuts, but I couldn't figure out why? If they're all critical parts of a control linkage, why not all castle nuts and cotter pins? On the other hand, if some parts of the linkage can use plastic lock nuts, why not put them everywhere and do away with cotter pins and bolts with holes?
I know this isn't random, but I can't see the reasoning.
 
Inquiring minds want to know. I suspect there is some underlying rule or reason, but in aligning my ailerons (-8) I found some nuts in the linkage were castles while others were plastic lock nuts, but I couldn't figure out why? If they're all critical parts of a control linkage, why not all castle nuts and cotter pins? On the other hand, if some parts of the linkage can use plastic lock nuts, why not put them everywhere and do away with cotter pins and bolts with holes?
I know this isn't random, but I can't see the reasoning.

If the bolt is subject to rotation a castle nut with cotter pin is required. If the nut can be torqued solidly, such as on a rod end bearing, then a locknut may be used.
A castle nut/cotter pin can always be substituted for a locknut if properly torqued. (ref..AC 43.13-1B)
 
Nylock nuts can be used where the linkage has a bearing. Castle nuts with a cotter pin need to be used where the bolt can rotate and possibly cause the nut to back out.
 
The deciding issue is application. If:
- The nut and bolt hold an assembly that rotates together, but the rotation is via a captured bearing, then a lock nut (nylon or metal) is the right choice. Here the bolt and nut are fixed, not a rotation bearing (as in the bolt holds the bearing fixed - no relative motion between the bearing and the bolt). Examples are the aileron bell cranks and journal bearings. You could use a castle nut if you added just the right washers to get it to torque spec where it aligns with the bolt cotter pin hole. This was standard back in the day before decent lock nuts. My buddy at the Shannon Aircraft Museum pulls his hair out on these as they restore aircraft.
- If the rotation is on the bolt, this must have a castle nut to prevent the nut from coming loose because you cannot torque it tight. Example are the flaps on the RV-10 and RV-14, the nose gear nut that provides tension to set break out force and the main gear nuts that you tighten to get the right bearing pre-load.

There are exceptions. For example the VS bolt on the leading edge to the fuselage. This is a castle nut - Van’s decided that the vibration here could loosen a regular lock nut.

The other rule for lock nuts is where located. For the engine side of the firewall use metal lock nuts.

Carl
 
If the bolt is subject to rotation a castle nut with cotter pin is required. If the nut can be torqued solidly, such as on a rod end bearing, then a locknut may be used.
A castle nut/cotter pin can always be substituted for a locknut if properly torqued. (ref..AC 43.13-1B)

I've often wondered why Van's doesn't call out MS17825 nuts on the control system. It's the best of both options combined.
 
Clay,

Where do you see castle nuts in the control linkage? The only ones I see are through the control stick for forward/aft movement. I believe all the rest for the controls are AN-365s.
 
While everyone is correct in their response, I believe in the engine control parts castel nuts are used regardless of bearing present or not.
 
Clay,

Where do you see castle nuts in the control linkage? The only ones I see are through the control stick for forward/aft movement. I believe all the rest for the controls are AN-365s.
Break pedals are a good example.
 
While everyone is correct in their response, I believe in the engine control parts castel nuts are used regardless of bearing present or not.

No, this is not true. There are plenty of nylock nuts on the engine *as it comes from the Lycoming factory*.
 
No, this is not true. There are plenty of nylock nuts on the engine *as it comes from the Lycoming factory*.

Are you referring to engine compartment or engine control parts (throttle, mixture and prop)?

I am referring to the engine control parts.
 
Are you referring to engine compartment or engine control parts (throttle, mixture and prop)?

I am referring to the engine control parts.

So am I, in part. They're not all nylocks, I mis-wrote, but rather in some cases (throttle, prop, mixture controls) they are all-metal locknuts. (ref Van's drawings, e.g., OP-26). There are nylocks on the Adel clamps holding the fuel injection lines from the spider to the injectors. I'm sure there are others.

But they are not "[castellated] nuts regardless of [whether a] bearing [is] present or not", as stated.
 
A lot of the certified world still calls out castellated nuts on their control linkage rod end bearings; a recurring cause for debate. Probably overkill in most cases; however, it makes the case for proper bearing lube/operation. They can freeze in the socket which can then subject the fastener to rotation. Most have probably witnessed such a case. That said, the rod ends exposed to the elements should probably get a little extra attention during pre-flights.
 
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