Key-Way for Gates Sprockets

The Gates sprockets we have are smooth bore. We’d like to cut a key-way in them, but don’t know how to do it. Is there a special tool to do this? Where can I get it?

Yes, you will need a broaching tool, the right broach key, and an arbor press.

Look at the “single keyway broach” on this page:

I would assume a tool and die shop/supply store would stock a set.

We use this one to great success, but you have to be careful not to break the broaches with an offset press. Bending = bad!

The good news is that overall they’re quite easy to use. This set goes up to 1/2" diameter holes, which is as big as we’ve ever needed. We were given a set for even larger holes and have never used it!

You can also try drilling into the “hub” part of the sprocket and using a set screw. Once the sprocket is in place on the shaft, drill into the shaft, about 1-2 mm, through the hole for the setscrew to give the setscrew somewhere to “bite”. It is also possible to use two setscrews for added security. Make sure the sprocket is clamped securely when drilling. This is probably best done on a drill press.

In my experience, there is no reason to use a set screw when you can use a spring pin. We replace all of our set screws with spring pins after a wonky set screw almost cost us an FLR 2010 blue banner!

Otherwise, yes, you can use a drill press to drill straight through the two items you want to couple and then press in a spring pin… I caution, though, that while spring pins are strong they are not invincible, and a keyed shaft is much stronger.

We’ve used this method before on sprockets with varying degrees of success. It’s not the ‘right’ way to do the job, but it will work - assuming you can keep the set screws tight at all times and that they don’t slip. We’ve dimpled the shaft (Drill a small blind hole into the shaft for the set screw to fit into as stated above) and also done ‘D’ and ‘Double D’ shafts where a flat side or two is added to the round shaft for the set screw to ride on. Again, it’s not the right way to transmit torque but if you’re in a jam it will ‘work’.

Broaching as stated above would be the ideal method, but requires semi-specialized tooling and a bit of skill/patience. Depending on your location, you may be able to find a local machine shop that’s willing to broach the sprockets for you for little to no cost, from the few times I’ve seen it done, it doesn’t take long at all.

You could also buy a hex broach (my preferred method of power transmission) and run hex shafts etc. Running hex does present it’s own set of issues though, you’ll need to find hex bearings, hex stock, etc, etc - and I’m not sure what the availability of those items looks like over your way.

Hex is nice, and some of those issues can be worked around. Hex stock from Mcmaster is cheap enough that I dont cringe about buying from them in a pinch. And for hex bearings, we’ve often used the “poor mans” hex bearing instead of buying specific hex ones for 3/8" hex shafts. We take a standard 1/2" FR8ZZ roller bearing and a bronze bushing. Broach the bronze bushing with a hex, and insert the bushing into the bearing.


Friendly Reminder: In many cases when using hex shafts, it is possible to use a lathe to turn down the two ends of the shaft to a round to use standard bearings.

To the teams that have an arbor press what rating do you recommend. Can a 1/2 ton arbor press get you by or do you need a 1 ton or better? Is there certain brands to stay away from?

The first year we broached keyways, we tapped them through with a rubber mallet. It worked great as far as creating the keyway, though it’s hard on the broach and is not something I would recommend. (We were both time-desperate and had no idea what we were doing.)

So that said, I think a small press would be fine. The 12-ton bottle jack press we now have is massive overkill.

It seems like most places have a 1/2 ton for around $50 and a 1 ton for around $100 I am wondering if the extra $50 in necessary when mostly we would want to make 1/8" keyways. Or am i not even in the ball park when it comes to the press? I figure maybe to extra $50 is better spent on a good quality broach.

Yesterday we took apart something on our 2009 robot, a steel sprocket that sat on a keyed shaft. The keyway in the sprocket looked funny…and slowly I remembered why…we made it using the saber saw! Just clamp the pulley in a vise, and saw the slot in it.

Scary, but it worked well enough on that one to win the AZ regional and it survived Championships.

I would recommend a 2-ton press, or if you wish to do hex somewhere down the line, a 3-ton press.

Saber saw keyways are scary, but not as scary as relying on setscrews for power transmission. I’m a firm believer that setscrews are evil for anything other than potentiometer knobs, sensor couplings, and holding keys firmly in their slots. You may think using a setscrew for power transmission will be quick and easy, but I guarantee you will pay the price down the line.

If you are going to use an arbor press make sure the press is large enough so that the broach/sprocket length distance will fit into the press. A 3 ton Grizzly arbor press has a working height of 11 1/8 inches. While a 1 ton only has a working height of 5 inches. It really doesn’t take a lot of force to broach a small keyway slot but the equipment must be able to open up enough for your work piece to fit.

While not the correct way, we have been using the drill press to broach 1/8 and 3/16 key ways. This year we have a press. Note hardened sprockets and gears are nearly impossible to broach. Annealing the hub with a torch thru the bore worked last year.