Timing Belt Pulleys with Machine Key Slots?

Hello all.

We’d like to use timing belt pulleys on our motors however aside from the pulley provided in the kitbot (which is for 2 belts and is gigantic) there doesn’t seem to be any pulleys (to my knowledge) with machine key slots.

Is there a place to get this? Should we carefully carve out our own machine key slots into the pulleys?


Sorry, don’t know of any suppliers (off the top of my head). If you have access to a broach, you could put keyways in the pulley. Most machine shops should be able to do it for you (might not be free).

Do you really need keys? There are alternatives (lock-ring bushings, set screws, pins) and those can do quite a good job.

I recently milled my own GT2 Pulleys with a .5 hex bore… Completely doable and possible, you knock out the edges of the hex with the radius of the tool and you are g2g.

If you want a key way the best/really only way to do it is with a broach.

From what I’ve been able to dig up, HTD pulleys with keyed bores aren’t too common, or at least not in FRC sizing.

To get around this, you could do one of a few things, none of which are that difficult.

-Buy unfinished bore pulleys from McMaster, SDP-SI, Gates, etc and finish the bore. This would consist of boring out the hub to 1/2" ID and then running a 1/8 keyway broach though it. If you’ve got a lathe, opening up the bore is pretty straightforward and if you don’t have a lathe it’s possible to open them up with a drill press or mill but a little more tricky. As far as the broaching operation, you’d need a press with sufficient travel (about ~6"-8" depending on the broach), a key broach (similar to mcm #315316) and the appropriate bushing for your bore size (mcm #3151A109 or similar).

-Take your existing pulleys from AM and modify them to be smaller. Depending on what tooling you have access to, the process changes a bit, but basically you’d remove the outer flange (IIRC, it’s pressed on?) and then cut the pulley down ~15mm or so to make it a single wide pulley. If you’re cutting on a lathe, you should be able to cut in the features required to reuse the flange without too much hassle - but if not, you can always rig up some sort of flange.

-Buy additional 42T AM Pulleys (http://www.andymark.com/product-p/am-2234.htm) and bolt them up to a hub like this one: http://www.andymark.com/product-p/am-0976.htm or this one http://www.andymark.com/product-p/am-0077.htm. Using a 42T Pulley at the output shaft (or any size other than 39T) is going to change the ratio from what was supplied in the kit, which depending on your application, this may or may not be an issue. (Switching to a 42:42 vs the 39:42 would result in a ~7% increase in wheel speed with a ~7% decrease in power)

Personally, I’d probably go with broaching my own pulleys from blanks as it seems like the most straightforward way to make exactly what the application calls for, but I also have access to everything required to do so. Also, it’s worth noting that if you’re going to be doing some broaching anyway, it might be a good time to look into swapping to hex outputs on your transmissions (AM has them for most/all of their transmissions) and broaching your pulleys with a 1/2" hex instead of a key. You’ll never have to worry about losing a key again. :smiley:

It’s worth noting that in the grand scheme of the expense of first, broaches and an arbor press are relatively cheap, and will see near-infinite use over the years.

Thanks for the help guys.

I’ve never used or even heard of a broach - what kind of equipment is necessary and how is the process done?

IIRC Gates makes keyed pulleys. I know they used to, that is.

I remember the nightmare of having to turn some down on a lathe. Not easy when the sidewalls are pressed on (and the body of the pulley spins).

This is a good intro to broaching and keyways: https://www.youtube.com/watch?v=8tAz5YDFtAs

Basically, a broach is a multitoothed cutter that cuts a specific profile in a hole. To use it, you simply (well, not that simply) push it through the hole. You generally need a press to do this, and a specific one (if you want a good job) at that.

A typical broach used for cutting 1/8 keyways in a 1/2" bore is as follows

from www.mcmaster.com

p/n 3153A16 broach
p/n 3151A109 type A collared bushing

don’t let the broach fall and hit the floor or it may shatter. very strong, but brittle.

We have the G9771 broach set. It’s not too bad for the amount of work we need to do with it.


And it should go without saying, but don’t use a hammer instead of a press!