Pinions, set screws, and glue

One of my intermediate goals for building new actuators for SMMB is to get them robust enough to jump continuously for some duration of time.  Progress is slow, as things break, new parts are ordered, repairs are made, and jumping resumes.  The most recent failure is at least interesting enough to me that it is worth writing up.

To recap, I’m building a brushless servo based around a Turnigy Elite 3508 brushless motor and a custom 5x planetary gearbox.  The 3508 is intended for quadcopter applications, so to install a spur gear I first extracted the original shaft, then pressed in a new shaft with two flats on it.  One flat for the set screw attaching the rotor to the shaft (which had a press fit), and a second for the set screw attaching the spur gear to the output of the shaft.

While jumping, the set screw holding the spur gear kept freeing itself, no matter whether thread lock was applied or not.  In retrospect, the reason should have been obvious.  In all my previous RC motor experience, pinions were attached with set screws and a slip fit.  However, all those previous applications also used a motor that spun fast with a high ratio gearbox, so the actual torque applied to the motor shaft through the spur gear was relatively low.  In this application, with just a 5x gear reduction and relatively large amounts of torque, the spur gear was seeing torque on the order of 1 N*m.

As the internet will tell you, set screws are not an appropriate attachment mechanism for anything but the smallest of loads.  Even with a shaft with a flat, the entire torque load ends up being concentrated in just one tiny bit of the set screw until it elastically deforms and eventually either destroys or frees itself.

Ben Katz, who built the inspiration for this work, used a shrink fit to attach the spur gear to the motor.  I knew that, but didn’t have sufficient machining tolerances easily available to me to make that happen.  Other simple mechanical options, like a dowel pin didn’t seem like they would be that much more effective than the set screw.

What seemed like more of an option after a little calculation, was glue!  Or rather, LOCTITE 680 brand retaining compound .  The shaft is 4mm in diameter, and the spur gear covers about 8mm of the shaft’s length.  That means there is \pi * 4mm * 8mm =100mm^2 of contact surface area.  A 1 N*m torque at 2mm radius thus results in 1 N*m * 2mm / 100mm^2 ~= 5 MPa of shear strength required.  LOCTITE 680 has a shear strength when cured of approximately 25 MPa, which gives a 5x safety margin.

Retaining compound was applied, jumping recommenced, at least until the next thing broke…