Center to Center with belts

Our team is prototyping a drivetrain in the offseason and we wanted to try out building it with exact center to center spaced wheels with timing belts

Last year we used bearing blocks and tensioned them but it would be a lot simpler if we could do away with the tensioning

I have a few questions about this
Once we calculate the correct distance and build it, how do you get the belts onto the pulleys. Our previous experience shows that the belts have to be pretty tight so it seems that it would be very hard to put the belts over the flanges on the pulleys

Also, how precise does it have to be built? All of our sheet metal work is done using a hand operated sheet metal punch and sometimes holes can be off by as much as 1/32 of an inch (less than 1/16) Would it be better to always err on the side of farther apart rather than having the belts be loose?

This will be a completely sheet metal drivetrain by the way

Any other advice about doing C-C belts would be appreciated

When building exact c-to-c drivetrains with chains, we put the chain over the sprocket, then line the sprocket up with the bearing block, then skewer the whole thing with the shaft. When using belts I’d think the approach would be similar, though a bit more difficult because you need more tension in the belts.

We did GT2 5mm x 9mm belts this year with C-C + .005 in our west coast drive train and loved them.

However we used tube and CNC milled them, down to probably +/- .003 tolerances. We never had an single issue after 2 regional.

This thread has a lot of useful information on how to do exact c-c spacing with belts.

Very nice arrangement, we’re working on something similar for an offseason project. Two questions:

  1. Do you mean you milled the GT2 pulleys? If so, can you provide some details on what is required to do such a thing. Are there specific CAD tools, special cutting tools, etc that make life easier?
  2. It sounds like you didn’t have any issues with belts skipping/ratcheting, even though 15mm belt sounds like a “safer choice” for a drivetrain. Using the 9mm did you drive it hard, go from full forward to full reverse, push much, etc without issues?

Take a look at the Andy Mark 2013 KOP drive train.

In the past we’ve built a tensioned belt setup and then last year we used the Andy Mark Setup. It is a excellent drive setup. Easy to assemble and maintain and we had no belt ratcheting whatsoever despite playing mostly defense. We were geared to 10’/sec , 6 wheel dropped with 4 CIMs. 6" kit wheels

The width of the belts, the size of the pulleys and the dead axle make assembly easy.

I was surprised to see so many 2013 kit bots in the Curie Finals. The Kit bot on Steroids is a excellent drive train.

Thanks :slight_smile: Really happy with this drive.

What I meant in my post was I CNC milled the bearing holes in the rails so that the C-C distance was a really tight tolerance. (I would think 1/32 off would be to much).

However I did machine my own pulleys, more info:
In short just a tiny (3/32th) square endmill + 3/8 alum plate + cnc. The pulley cad was downloaded from SDPSI and I used just my regular CAM software to come up with the tool path.

We did not have any issues with skipping and ratcheting. And the people that have seen me drive can vouch we pushed the drive as hard as we could, with doing all the above. Also to test I pretty much rammed into a test wall and stalled the motors and the belts still didn’t ratchet.

I used XL timing belts and pulleys from McMaster and the SDP-SI calculator and I would call it a very loose fit, but it never skipped. I would have liked the fit to have much less slop though.

The industrial answer is to make the perimeter that encompasses the OD of each pulley and the tangents to those edges equal to the inner perimeter of the belt, so that it fits over all pulleys at the same time. Then, either cam one shaft into location (ie - slide) or use a cam-style idler roller for belt tensioning (cam followers work great for this).

Of course, if you don’t want to have a separate tensioner and you want the shafts to be fixed in place, you can have one of the pulleys have either no flange or a removable flange. After all, you typically only need flanges on every other pulley.

A really good resource for timing belt information is
It has center-to-center charts, rules of thumb, engineering calculations, and design criteria.

We had fixed flanges on all of our pulleys this year and it worked fine. The trick is to put the belt on the pulleys before putting the pulleys on the shafts. Keep the pulleys in the same plane as you slide them onto the shafts and it will work fine.

-Mihir Iyer

What were your robot dimensions(in case we go back to 28x38 next year)

We use a worksheet to calculate C-C distances for axles driving chains or belts. This works really well without requiring tensioners for short runs (<=6"). Longer runs require tensioners or empirical determination. Link to worksheet below:

Just to clarify for my own knowledge, there should be little to no performance issues using 9mm wide belts compared to using 15mm wide belts (like those in this year’s KoP) assuming they both have the same tooth profile? I’m currently working on a practice sheet metal chassis using belts and I’m trying to make the wheel wells as thin as possible but I don’t want to compromise performance for space. Thanks,

Not necessarily true. The maximum torque that can be put on a belt varies proportionally to its width. If you’re right on the edge in terms of how much torque that can be on the belt, the switch from 15mm to 9mm definitely makes a difference in whether it will ratchet or not. If you want to be sure, check the Gates website. They should have some nice tables that give all of the relevant torque and speed information.

Just keep in mind that for FRC, your FoS or duty rating will probably be much lower than 1 (i.e., it can take more torque than what’s on the table). Those belts are designed for hundreds of thousands of cycles, something they’ll never see in FRC, so we can probably get away with more torque than they were designed for. Adam Heard probably has a more exact number, as he’s built a lot of belts drivetrains before.

OK thanks, I’ll check out the Gates website to try to get some more exact numbers.

We’ve always had perfect results by simply laying out the belt in a CAD sketch. Draw the pitch diameter circles of your pulleys, connect them with tangent lines, trim the inner portion of the circles, then measure the loop length. Adjust the C to C until the loop length matches the available belt pitch length.

We haven’t tried to do the C-C in our CAD until this year, but I am going to use this calculator to do it. It should get you pretty close, if not dead on a good tension. If it isn’t exactly how you want it, you should be able to make small tension changes if you are using COTS gearboxes.

Look at the Gates spec, though know that their numbers are very conservative. In my team’s experience, if you are using a smaller pulley (such as to fit in a small space height wise) you’ll want 15mm GT2 and if you’re using a larger pulley then 9mm is just fine. Since you’re doing sheet metal, you probably have enough room to use 2.5"+ diameter pulleys and can step down to 9mm.

The design I’m working on would be using the 42 tooth HTD pulleys from AndyMark for convenience sake so I wanted to see if I could get away with the 9mm belts so that I could fit two belts on a single pulley, which would save a significant amount of space.