Self-Locking Castellated Nuts

[David Paule]

These are useful for controls. Basically, they're exactly what the name says. You use them with cotter pins. If you forget the cotter pins they'll probably stay on, just like the AN 365 nuts.

The part number is MS17825 -3 or -4 or whatever.

Wick's Aircraft has them (that's their photo - thanks!), and they aren't exactly cheap. But if you don't feel comfortable with the use of the self-locking nuts that the plans recommends for controls, then these are worth considering.

Dave

 

[rockwoodrv9]

Dave,
I think I am going to do just what you said. I had to take a bunch of bolts out of the spar when I changed from tail wheel to nose gear. I got new lock nuts for those bolts and ordered quite a few extra in case I needed them. After the issue with the 10 last week, I am going to use these type of nuts for all my control connections. I am sure most builders can get an accurate torque on the nylon lock nuts, but I am not confident I can. With the danger in the control nuts being if they fall off, these castle nuts with cotter pins are a much better idea. Thanks.

We missed you Sat night at the airport.

 

[David Paule]

I just couldn't make the airport, too busy that day, sorry.

With the standard nuts (or these), simply add the friction or free-running torque to the specified installation torque and use that number. It'll be fine. I ordered a 1/4" low-range beam torque wrench specifically to check the free-running torque, but as I'm still making the left elevator and trim tab, haven't needed it yet.

As long as the bolt is completely through the elastic part of the nut, an elastic stop nut will be secure.

I have seen some interesting bolts and nuts. The bolts had a spring C-ring at the base of the threads which kept the bolt from falling out if the nut fell off. The nuts were similar to these, but with a relief groove for the C-ring. The installation process was to push the bolt through until it snapped in place, then the washer and then the nut, torqued, and then the cotter pin. Each step had to be witnessed by an inspector and signed off.

These were on a large Sikorsky helicopter back around 1970 or so. I guess there might have been some incidents of control fastener failures back then in the Vietnam war. I sure liked this solution - redundant to two failures. Or you might say, with the inspector right there, no chance for a single mechanic to cause a failure. Still, a nice technical solution to making a bolted joint multiply redundant. Note that it's only effective on a pure shear joint, though.

Dave

 

[snopercod]

These are useful for controls. Basically, they're exactly what the name says. You use them with cotter pins. If you forget the cotter pins they'll probably stay on, just like the AN 365 nuts. The part number is MS17825 -3 or -4 or whatever.

I'm late to the party but I just bought some of these today from ACS. It really makes sense to have a backup if a cotter pin lets go (or is forgotten, not that I would ever do that). I'll be using them where my rudder cables connect to the control horns.

 

[eaglen92ce]

I might suggest that this "belt and suspenders" approach is a bit overkill. The hardware called out in the plans works great and is completely safe when installed correctly.

I recently finished replacing a bunch of castellated nuts and cotter pins that were used on the elevator and aileron bearing points instead of the plans hardware nylock nuts on an RV6 a friend purchased. The castellated nuts would have been fine if the bolts would have been torqued properly, but the builder did not seem to understand how the bearings are supposed to work. He basically hand tightened the nuts and threw the cotter pin in. This allowed the control horn brackets to rotate on the bolt shank rather than clamping them up to the bearing and letting the rotation happen at the bearing as intended. This caused the bracket holes and bolt shanks to wear some and caused some excess play in the ailerons. This airplane had been flying like that for about 1300 hrs. Removing the cotter pins was not fun and I bet they were less to install during the original build. The were also not necessary.

Just my thoughts...

 

[dreed]

Awesome! Thank you-

Now I need to go out and figure out how many of each I need to order before final assembly. RV-7A in case anyone knows off the top of their head

 

[snopercod]

I recently finished replacing a bunch of castellated nuts and cotter pins...

I agree. At rod ends, I have abandoned the castellated nuts and cotter pins for elastic stop nuts. The rotation should occur in the rod end, not the bolt. On my Lancair, however, there are a number of places where the bolt is the bearing and needs to rotate. This is where I am using the self-locking castellated nuts.

 

[davidwv]

When complying with SB 96-12-1 (RV-4: nuts, bolts and cotter pins securing the rudder cables to the rudder pedal), these nuts may be a good belt and suspenders approach. In the SB, Van's calls for adding extra security to the castellated nut beyond the cotter pin. This includes applying a structural epoxy (such as JB Weld) over the nut/cotter pin and/or flattening the threads on the bolts "so that if all else fails, the nut won't easily screw off." Using these nuts probably accomplishes the same thing.

 

[rightrudder]

I agree. At rod ends, I have abandoned the castellated nuts and cotter pins for elastic stop nuts. The rotation should occur in the rod end, not the bolt. On my Lancair, however, there are a number of places where the bolt is the bearing and needs to rotate. This is where I am using the self-locking castellated nuts.

I've seen others use castellated nuts/cotter pins for rod ends on RVs, even on empennage control surfaces, but my -9A plans call for the elastic stop nuts here.

Are castle nuts/pins an accepted practice in this application for some RV models?

 

[JRichichi]

14 CFR 25.607

Take a look at 14 CFR 25.607. "Locking" castellated nuts such as these are required on things like flight controls for part 25 transport category aircraft (read: airliners). The reason behind this is you have to fail two, separate locking systems before the nuts can loosen and fall off.