Potentiometer

Hello, could anyone help me on how to mount a potentiometer on a rod shaft !!

We’re going to need a bit more information here if you want a complete answer. What kind of potentiometer? What kind of shaft? What machining capabilities do you have? What is the application, and are there any packaging requirements?

The “correct” way to connect two shafts is using a shaft coupler. Depending on the size of the shafts, you may need to turn one or both of them on a lathe to a common size to find a suitable shaft coupler.

There are also some “hacky” ways that you would never see in industry but work fine for the low loads and short life-spans of potentiometers on FRC robots. One way is to take a short piece of surgical tubing, stretch each end over the end of each shaft, and close zip ties over the surgical tubing ends. The negligible inertia of the potentiometer means the surgical tube acts as a direct coupler. Another common way is to bore a hole in the larger shaft at the same diameter as the smaller shaft, cut a slit in the shaft through the bored hole, then close a shaft collar around the larger shaft. This makes the larger shaft itself act like a collar and squeeze the smaller shaft, keeping it from slipping.

You are also likely to have to use a compliant mount for the potentiomer since it will be difficult to get the axis of the shaft you are wanting to sense and the axis of the potentiometer to line up exactly. Some of the coupling methods (the surgical tubing Ari mentions) will help make this misalignment more tolerable.

This will also depend on how the potentiometer is mounted. If it’s attached rigidly to the frame, then you will certainly need something to take up the misalignment. If it’s attached via a flexible mount, like bent polycarb, you can get away with a more solid connector.

I think one of the best ways is to use a beam coupler. They allow for misalignment of the shafts while still being rigid to get accurate rotation readings.

Before everyone jumps on the price of these… historically these have been in first choice for a significant discount to teams. Also it wont count against your BOM since you get it in FIRST choice.

Also you can use a standard shaft coupler. Turn the end of your shaft down to 1/4" and get a potentiometer with a 1/4" shaft. We usually use 2 standoffs and a small plate or 3d printed plate to screw the potentiometer into. I would disagree with anyone that says you “have” to use a compliant method as it is difficult to line up exactly. My suggestion if you are using standoffs is to tighten the shaft first and then tighten your standoffs.

I’m in the “allow for some modest misalignment” camp as few things ever align as well as you hope.

It’s easy to get things together in a mode where even a modest stress causes problems in the long run (and often times the “long” run is a lot shorter than we might like).

I feel surgical tubing is too too flexible and too easy to twist from even modest torques. I’ve used short lengths of rubber fuel line as couplers in the past (I’ve built road racers in my shop so there are a lot of those sorts of things around) or some clear tubing as well (Tygon? PVC? It was what I had around - probably more for a lawn mower than a race car).

Attached is a photo of a pot mount for a past robot. The design of the pivot was originally not done to accommodate a sensor so it might not be exactly what we might have done if this was planned from the outset but it turned out OK. The key was the “crazy” nut which coupled both locked the fastener in place and provided the protrusion to stick into the coupling line. One of the team members commented that he’d never seen a nut like that before and I told him it was probably because that specific instance didn’t exist until we made it :wink:

It was pretty solid and had relatively low amounts of slack and hysteresis so it tracked the arm angle pretty well.

  • Chris Herzog, Mentor, Joliet Cyborgs #4241
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Put the body of the pot on a plate, then attach the shaft with either surgical tubing or heatshrink. I’m personally a fan of heat shrink if it works for you, as it doesn’t have as much rotational compliance as surgical tubing.