Another servo safety issue?

[gvgoff99]

On the TruTrak servos the motor is held to the servo gear housing by four small Allen screws. I have heard from a good source that these Allen screws have been known to loosen causing another potential failure of the servo. A premium builder in the Houston area, Doug Knab, came up with this fix and also added the AN3H-3A safety wired bolts to hold the servo on the mounting bracket. The safety wired screws to hold the gear housing to the servo motor are AN502-10-6 fillister head screws. If you do this "fix" make sure that the screw and bolt do not bottom out on each other (putting a washer on each should prevent them from touching (they both go into the same threaded hole in the servo but from opposite sides as you probably figured out).

 

[niblettda]

I'd think another option would be to just use a longer bolt with a castleated nut on the back. Then you can safety wire or pin the nut.

Having not reached this stage yet, I don't know if there is enough clearance to support the idea, but I think that would be easier than using 2 bolts per hole and safety wiring both.

 

[gvgoff99]

Longer bolt?

David, A longer bolt could be used but the hole through which it passes is threaded. Your way would work though. The screws that are safety wired need to only be done once and should last a lifetime. The Bolt that holds the servo to the mounting bracket would need to be redone if the servo was removed.
I wrote this because someone is alleged to have been doing an annual and the Allen screws had backed out completely and the gearbox had separated from the servomotor. I say "alleged" because I did not see it with my own eyes but my source is good.

 

[niblettda]

Good to know George. Now I have plenty of time to think about other creative ways to bolt this down.

Interestingly, I'm just about to start on my quick build wings, so I'll be at this point sooner than later.

Thanks!

 

[TrutrakTech]

To address this issue

This was an issue on servos that were manufactured several years ago. We now apply threadlocker to those gearbox screws so that the vibrations cannot work them loose. We have had no trouble (that we know of) with the servos that have the threadlocker on them.

If anyone is having this issue on a recently (within the last 2 years or so) servo, please let me know.

 

[gvgoff99]

This was an issue on servos that were manufactured several years ago. We now apply threadlocker to those gearbox screws so that the vibrations cannot work them loose. We have had no trouble (that we know of) with the servos that have the threadlocker on them.

If anyone is having this issue on a recently (within the last 2 years or so) servo, please let me know.

Lucas, Thanks for the update. This is probably a touchy subject but your input is excellent, honest and will be appreciated by the members of this forum. Would you recommend those with servos older than 2 years or so doing something to keep the Allen screws from backing out? Peen them or the "fix" David or I mentioned, applying a threadlocker or another approach?

 

[TrutrakTech]

Lucas, Thanks for the update. This is probably a touchy subject but your input is excellent, honest and will be appreciated by the members of this forum. Would you recommend those with servos older than 2 years or so doing something to keep the Allen screws from backing out? Peen them or the "fix" David or I mentioned, applying a threadlocker or another approach?

The easiest method (and one we've had zero problems with) is to just use some low strength threadlocker (purple). This leaves them completely servicable. Medium strength would also be ok. Since servos older than 2-3 years didn't have this done initially, it's ok to check these screws. If they're loose, go ahead and pull them out to put the threadlocker on them.