Shafting size, keyways, and Dr. Joe's Drill Shaft

Last year, our team coupled a 1/2" shaft to the output of the drill transmission locked in low. From the 1/2" shaft, we used No. 35 sprockets and chains for a further reduction of 3:1 driving 8" wheels. This simple combination gave us a lot of torque which enabled us climb the center divider, push or pull a fully loaded stretcher across the bridge, and balance the goals with relative ease.

I’m far from an expert when it comes to the strength of materials, but do you guys think we could have used a 3/8" shaft, No. 25 chains and sprockets (with a keyway) without breaking or bending anything? In other words, can I stall the drill motors locked in low with a 3/8" shaft with sprockets mounted with keyways without breaking anything? What if I used a dual motor design increasing torque by 40%? Would it be recommended to stick with a 1/2" shaft?

If the bosch drill is available this year, we would like to consider using Dr. Joe’s drill shaft. On the plans, I noticed that the maximum shaft diameter is 12mm. Unfortunately 12mm bearings are not legally available through SPI. 12mm is .4724" which is too small to use with .5" bearings. The closest SAE bearing that would work is .375" (3/8"). In order to use these, I would have to turn the shaft down to .375" which is considerably smaller and weaker than 12mm. Worse yet, I imagine that a keyway would make the shaft even weaker.

Anyone have any thoughts? Am I missing something? Is there some practical way to support a 12mm shaft with bearings legally? Would home made bearings out of bronze be adequate if we squirted a little oil in them?

With the competition closing in, I’m starting to have bad dreams of sleepless nights and weekends.

Here we go again!


You can always wrap a ring around the the custom made shaft to increase the diameter to 1/2", and therefore able to put a 1/2" bearing around that. It will be the same thing for the sprocket, except you will have to cut a slot on the ring to allow the key fit through. Of course the ring will have to be as wide as the bearing/sprocket.

Several way to make this ring: using a 1/2" diameter shaft (Steel or Aluminum), you drill out the center with a drill bit close to 12mm, or even just use a 3/8" drill bit, and then get the tool bit inside that hollow shaft and increase the inside diameter until the custom made shaft fit through. Or, you can just sit down with a file and the ring in front of a good movie, and file the inside until the custom-made shaft will fit inside. Then, you can just cut the ring along the side for a slot to fit the key in between, so you can still do a key way on both the shaft and sprocket.

Worse come the worst, if you don’t have any machining capability, just go ahead and buy sheet metal at different thickness, and get thick enough for (1/2" - 12mm). Then just wrap around the shaft with those sheet metal and fit the sprocket/bearing around.


We built a 1/2" version of the Dr Joe’s Drill Shaft. We had to replace the internal bearings in the housing with a bushing. This allowed us to use 1/2" bearings and No. 35 sprockets in the drive train. We used this the last two years without any problems.


In 6 years of building FIRST robots, there has yet to be a keyway on any of our robot.

I know that a lot of teams use them successfully, but for the most part, I try to avoid them.

We have used a pin joint several times but most of the time we have done this, we regret it before the Nationals are over.

Our basic rule is that if we are not going to take it apart, we weld it. If we need to take it apart, Trantorques are our first choice.

For my money, a trantoque is pretty hard to beat.

A 12mm trantorque can handle the toque of the drills, even in low.

Joe J.

P.S. a compromise between welding and removability is to weld a flange onto a shaft that we can then bolt various sprockets to. This lets us tune in our ratio without having to deal with keyways, etc. JJ

Hi all, thanks for you replies.


Thanks for your suggestion as it shows some good thought. A 12mm to 1/2" bushing is pretty tough to make. That means I would need to have a wall thickness of only .0138" (about 3-1/2 sheets of bond paper) which would be difficult to make on a mill. I think your shim idea would probably work, but might present us problem if I want to be able to pull the shaft while trying to size our final drive ratio.


Great suggestion, Just last night it occured to me that it might be possible to replace the transmission bearing with a 1/2" bushing. If you don’t mind, what sort of bushing did you use. Did you use a 1/2" sintered bronze bearing? I think I’ll dissassemble our spare transmission tonight and take a look.


Thank you for your insight about keyways and trantorques. Sounds like the trantorque is a great device. The one thing I notice though, is that a 12mm trantorque has an O.D. of .886" which is fairly large. What is the minimum wall thickness that can be used with a trantorque? As an example, lets say I purchased a 12 tooth No.35 sprocket from SPI. This has a hub diameter of 1-7/32". with a 1/2" bore. If I bored it out to .886" to fit the trantorque, that would leave a wall thickness of .166" is that enough? How about a wall thickness of .12" which is what I would get with a 18 tooth No. 25 sprocket?

Our team has always used double set screws on a 1/2" shaft with milled flats. This has worked for us, and with a lot of loctite has held up, but I get the feeling we were pushing our luck last year, as without the loctite, they would get loose. Our robot had a ton of traction last year which abused those set screws. I was thinking that a key way would be the way to go, but I like the idea of the trantorque as it would allow us to dissassemble things more easily. Our team has no experience with them, but maybe this will be the year.

Thanks all!


I would be fine with a 1/8 inch wall. I am more concerned about the thickness.

Basically, the trantorques need “hoop stress” to transfer the torque. This is why you will have a hard time keeping them tight with a plastic sprocket (bad idea). But the hoop strength of steel is plenty.

One thing that I am worried about is the thickness of the sprocket. The trantorques like to have the entire thickness of their wedges bearing the load. If the hoop is too thin, the wedges can crack. I think that a #35 chain would be enough. A #25 chain would be pushing it without a hup of some kind left to spread the load out.

Joe J.

The 0.0138" thick bushing is extremely easy to make on a lathe… It took me less than half hour to get a 1/2" shaft, drill the center out, and run a tool bit straight inside the “tubing” and remove the extra thickness away.

You are right, it is a little difficult taking the shim/bushing out if you make it just a little too big and have to push the shaft inside. But you can always leave a little space in between, say a couple of 1/100". I believe this will work well for a key way without replacing the bearing inside transmission with a bushing… And the bearing inside the transmission should be more efficient than using a bushing.

I’ve always found key ways work really well if welding isn’t too big for your team.


Yes, we did use a 1/2" sinther bronze bushing which we machined the outer dimension to fit in the housing.