"Slop" in the HEX hubs/shafts

We are using hex hubs and shafts for our arm; however, there is a ton of slop to the point where the arms can swing several degrees before actually moving the shaft/motor. What are some ways you have removed that?

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If you’re dead set on hex shaft, I know teams have put some shim stock in between hubs and hex shaft to fix the typically loose fit. In general, for big arms, I would recommend using a large round shaft as a dead axle pivot and bolting whatever sprocket/gear/pulley is rotating your arm directly to the arm.


You can get some traditional hex stock from McMaster or some such place that might be a little larger.


We have used .001 shim stock from McMaster Carr on our FTC Robots in the past. Works great.

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In addition to the shims mentioned above, Loctite 638 is a retaining compound that can reduce slop, but it also bonds your gear to the shaft which can cause some serviceability headaches.

I’m definitely seconding @Knufire’s advice that you should re-think using a hex shaft to transfer torque to an arm. Using a large OD sprocket on a dead axle is the tried-and-true method for driving a high load arm. It keeps the high torque out of the relatively thin axle, and also removes the slop (assuming your chain is properly tensioned).

Old school 330 is the go-to example for making big arms.


I’m still waiting for someone to sell rounded hex with oversize, press-fit hex. Sell it in like 4" - 6" lengths. Ez money.

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How are you driving this hex?

Is there a chance the is a sprocket drive for the hex near the first stage of the arm?

If so you can put a bolt thru the sprocket (and standoff spacer) and into the arm this will greatly protect (or bolt thru gear, but sprocket are better in this application due to spreading load over multiple teeth vs single teeth on a gear).

We put set screws in the hubs to help lock them in.
It can be done with a hand drill and a tap.

One way this is commonly handled in Industry (Think the kind of motors you would find in an MFG plant) is a “Taper Lock” Bushing.

To make something like this work in FRC, you’d need to be able to make a (2) Part assembly - (1) Part that looks like a conventional bearing bore hub, with some bolt pattern and a slightly tapered bore - Part (2) would be a “Tapered Bushing”, basically some tapered shape, split some amount (you can either split through in one spot, or split in multiple spots without breaking through) and with some method of Tightening the Bushing into the Hub.

The taper would then cause the bushing to “Grip” the hex shaft as it’s installed, removing most, if not all of the Backlash.

With that being said - this method works great for removing a single stage of backlash, or for fixing something axially and radially to a shaft. If you’ve got something like 2-3-4 stages of reduction, it gets very impractical (though not impossible) to implement something like this, because you’re going to gain backlash with each stage of power transmission.

The inverse version of this also works, and is probably a bit easer to make - Basically take a standard hex hub with flange - cut the “Male” (Snout) of the hub in half, or thirds (thirds are better) to about 1/2 - 3/4 of the length of the snout (do not cut the flange entirely in half) - then throw a clamp over the area you cut. This will get most of the backlash out, though a single split (so split into halves) runs the risk of not being perfectly concentric, so there’s that.

Alternative is as many people above have said, you can put set screws in a hub (This is nice because you can use more than one which’ll keep the shaft centered), or move away from hex as you get further from the power source. (Ala, 330, who proved to all of us that a single jointed arm is generally always the correct answer.)


We are driving it with a chain and sprocket. We have the sprocket bolted to a hub which then bolts to the arm. We are using 2 arm sides spaced roughly 10 inches apart on the first stage.

So are you transferring torque through the hex? Or just through the hub to the arm? To phrase a different way: is the hex primarily there just to be an axle, not to transfer power?

The hex is just an axle with bearings on each end. The power is being transmitted through the sprocket bolted to the arm.

Since there is no power going through the hex I would go the shim route. I am a little confused by the statement

If the motor and hub are linked with a presumably well tensioned chain then this hex slop should not matter to the point of “several degrees”

When the chain is on the arm the slop is less, but is still present.

Since this hex shaft is just holding position as a pivot I don’t really see a problem with the slop (at least how I am imagining it in my head), keeping the chain at the right tension is the bigger thing imo. Easy to over tension in the pursuit to remove backlash.

How are you tensioning the chain?

Currently we are just moving our motor mount that is holding the driver sprocket to tighten the chain. We may add a spring tensioner at some point though