Retaining Things?


In recently thinking about designs for gearboxes and drives, I am led to wonder how gears are held in place in these designs. I know that for a cylindrical shaft, keys can be used to keep gears/hubs from rotating, and that they have enough friction to prevent it from sliding on the rod. Alternatively, retaining rings can be used, as on CIM motors (is that how it’s done on the banebots RS motors, too?)

The obvious advantage of hex shafts is that you don’t need to use a key, but that leaves the question of retaining things on the shaft. I’m pretty sure retaining clips can be used if there’s a slot machined into the shaft, but I’m curious if there’s any other ways of doing it. I know people also use shaft collars and set screws, but we’ve had bad experiences with each of those things falling off …


Inside of the gearboxes, things are usually spaced out using spacers to keep them in place. Then retaining rings just go on the ends. Flanges on bearings are usually strategically placed as well, so when a shaft gets pulled against a retaining ring, that pushes against the flange, which usually pushes against something solid. If doing this on both sides of the shaft, it makes it impossible for the shaft to move axially.

How about the use of a roll pin? These can serve double duty. They transmit the torque and locate items on the shaft axially. Be sure to check the torque capability of the pin if you torque requirements are pretty high.

Typically (not always, but typically) we have shafts for most things supported by structural members on both ends. You need some way to ensure the shaft doesn’t slide out the end, but that’s a different problem… to keep gears/sprockets/wheels in place, we use a combination of set screws, shaft collars, and spacers (usually just cut PVC tubing), depending on the application.

Tapped holes on the ends of shafts + fender washers work reasonably well. Shaft collars are pretty bulletproof as well as long as you get the clamp-on variety rather than those that use set screws. These are very nice, but pricey. For weight and size, nothing beats retaining rings or E-clips (the snap-on variety, not the toothed ones that come with CIM pinions), but you need to machine the groove yourself. Not too hard to do with a lathe and the right tool though, in either hex or round stock.

A side note, keys really shouldn’t be used to retain items axially.

From my experience, shaft collars work great for 1/2" hex or round. We make sure to put thread lock on the clamping screw and I haven’t had one fall off. Vex 's hex spacers also work well for spacing wheels and pulleys.

Center drill and tap. Screw with a fender washer.

Snap rings are great as long as they are not located in portions of shafts supporting torsional loads. That is a recipe for having a bad day. :slight_smile:

Plenty of ways to do this. The hassle of retaining on hex is way worth it compared to the hassle of dealing with keys.

Easiest way to do this is to design parts such that the distance between your bearings is exactly as much space as you need (or a hair more). You see this on things like AM / Vex gearboxes. Hex spacers work on the same principle for longer spans.

You can also use shaft collars which are a nice no machining solution. The Vex hex collars are nice, small, easy to use, and convenient. Honestly in a lot of places I don’t even bother designing in spacers / snap rings anymore; the collars are that cool.

Snap rings are awesome, definitely the lightest way to retain stuff, but don’t put them between loads. You’re just weakening your shaft significantly when you do that. On the ends it’s more than fine.

You can drill and tap the ends out to use a screw and washer to retain stuff. You may need a spacer to press against the washer.

Something else to keep in mind is that you’ll also want to retain the hex shaft itself in place. Lots of these methods are more tailored for that purpose. One good way to retain the hex shaft is to turn the ends round and support the shaft with round bearings. Again, check out the Toughbox for a good example of this. You can even buy 3/8 hex with round ends from AndyMark, ready to drop in exactly two Vex or AM gears.

Turning the ends of the shaft down to 1/2" or 3/8" is pretty popular. To retaing stuff axially, we use e-clips (not quite circlips) which can be put on from the side of the shaft instead of from the front/back.
Hex spacers are also really useful. Vex sells spacers (buy a ton of 1/16" and save yourself some time), and you can 3D print them in a pinch.

If you’re using hex shaft, you can use .500 round bearings and turn down the shaft to .500. Then you can just put spacers between everything. If you’re doing it this way, it’s helpful to put a wave washer on the shaft so that you don’t have to be dead accurate on your spacer lengths. This is probably the lightest way to do this, and it also doesn’t weaken the shaft at all.

When using hex shaft, it’s a little difficult to slide the snap rings designed for 1/2" round shafts over the 1/2" hex shafts. It might be worth using a metric sized snap ring/groove.

Instead of snap rings, you can use e-clips. They mount from the side, so you don’t have to slide them.

There is goop called retaining compound and allot of it is green not red or blue as like thread locker. We use it to glue a miter gear to a 3/8" drive shaft on our swerve module.

Retain things? Zip-ties of course! … just kidding.

In a gearbox I usually see bearings on the ends and spacers in between gears when appropriate. For more spaced out options hex collars are good as long as the load isn’t too large. We have had he collars slip on our drivetrain due to the forces when strafing. Fortunately the distance was not very far, so there wasn’t a problem.