Wed May 5 2010

Contemplating TruTrak servo arm attachment screw

Well, today I didn't really get much of anything done on the plane, but I spent several hours contemplating how to resolve the issue with the shaft screw coming out. Probably the most fullproof solution would be to remake the shaft with a male threaded end that's cross drilled so a castle nut and cotter pin could be used to retain the arm. That, however, would be a costly fix for TruTrak since they have tens of thousands of these servos out there. It's also likely that they would need to have the servos returned to exchange the shaft at their facility rather than trusting the owners to perform the job. Someone on the list suggested another idea which nearly accomplishes the same thing. They screwed a set screw into the end of the shaft and secured it with red Locktite. Then they crossdrilled the setscrew and used a castle nut and cotter pin. Red Locktite is really strong when applied correctly, so this is nearly as good as a replacement shaft. Caution would need to be taken, however to ensure no locktite gets on the outside of the shaft or the arm, effective locking the arm to the shaft. The arm is supposed to be able to rotate on the shaft if the shear pin is sheared.

After doing a bunch of reading on the net, I can see how TruTrak likely arrived at this design. I believe their first iteration lacked the shear pin (not sure about that), and the arm was held on to the shaft collar with three nylon screws that provided the shear interface. However, this would result in a loose, floppy linkage similar to what caused that pilot's controls to lock up, so I can guess they changed the design so that the arm always stays attached to the servo by eliminating the nylon screws. The shear point may have been moved to the shaft collar at this time. Upon studying this further, I was thinking that another resolution would be to put a relief groove around the shaft near the end, then use another roll pin that passes through that relief groove. If the shear pin sheared, this pin would retain the arm regardless of whether the shaft screw fell out or not. Errr...wait a second...that's not right...the shaft screw is really the only thing holding the arm to the shaft collar. The other screw you see there just provides alignment of the arm to the shaft collar. The hole that screw is in is a plain through hole and the head of the screw is a close fit with that hole. Thus, if the center screw fell out, the arm would still fall off even though a second roll pin retained the shaft collar. I suppose changing the arm design to use three screws to hold the arm to the shaft collar would cure that problem, but now the change is getting too complex. OK, I give up for now. Let's see what TruTrak comes up with. If all else fails, the setscrew with red Locktite idea is a pretty good backup plan.