Custom Gearbox is very loud

So we just build our first ever custom gearbox, and it’s one of the “opened” once. As in, you can see the gears and all, and when testing it we feel like it’s very very loud compared to anything wev’e had before.
Now, all our previous gearboxes were enclosed once, like the ballshifter for example.

So what we want to know, if that loud noise is just how it is supposed to sound like with an opened gearbox, or is that means that something’s wrong.


Part of the reason it is louder is because of the exposed gears, but I’m guessing it’s not the whole reason. Have you put any kind of lubrication or grease on the gears? That usually helps. Also make sure your gears are not binding up, if they are meshed too closely together, they tend to bind up/be loud.

No it shouldn’t be that loud.

We have built several custom open gearboxes times in the past few years - One time it was being put together - it seemed a little hard but the students persevered - and then it screamed when first run! - well it was put together with 14 tooth pinions where the gear centers were set up for 12 tooth pinions - once fixed - it purred.
We also find that if using 2 parallel plates with spacers for the gearbox that before you tighten the bolts through the spacers, feel how the gears turn, get them right before socking it down. Ideally we want to control the center distances by a couple thousandths or so - but there is typically more slop in the bolting up process - things you can get away with in FIRST.
If it is too loud - screeching - then I would guess the center distance between the gears you are using is too tight, If too loose, it will not be as loud - just lots of backlash, and premature wear.

If you have a tacometer, measure output speed vs theoretical output speed. This can help estimate how much drag you have. If you are at 75% or lower free speed, you definitely have something too tight. If the gearbox turns very free, and free speed is high say 97% then you may have gear centers too far apart. There will also be a lot of lash compared to other gear boxes.

We are using the 12 pinion once, and we did grease the whole thing.
The plates were made in our CNC machine, and the CAD was done according to pitch diameters of each gear. The loud noise seems to come from the gear that attaches to the CIM pinions (2 motors go to one VEX 72 teeth gear).
We would like to avoid fabricating the whole thing again, any suggestions? How much should we separate the gears beyond the tangent distances of the 2 gears?

We had a different set up, but it sounds analogous. We were able to quiet a TB-mini simply by removing a CIM, turning it 180 degrees around its axis, and re-mounting. Shouldn’t make a difference, but it did. I would try mounting each motor separately and in both orientations to see where the binding is occurring. If you have binding all over, you may need to increase the spacing.

Edit: this happened to us twice with different gearboxes and plates in different years. I can’t prove it was the same CIM in both cases, but I wouldn’t be a bit surprised if they were.

Was it the exact pitch diameter sum, or did you add something to it? What is the actual center-to-center distance on the plate, as machined?

For drive in FRC applications (short runtime, friction bad), it’s conventional wisdom to add about .003" to soak up manufacturing variations and go a little loose and free.

We used the exact pitch diameters.
We will try checking each motor separately on Sunday, and than if that doesnt work we’ll probably remake the plates.
I’ll keep the results posted here.
Thanks everyone!

If you didn’t add the center distance, and you’re using aluminum gears, one trick is to run the gearbox on a bench for about an hour or so, ungreased, changing direction frequently. This will wear down the parts somewhat, to sort of “break in” the gears. Once this is done, clean the gearbox out thoroughly, heavily grease it, and you’re good to go.

The .003 center adder trick is a way around doing this, at the cost of a bit more backlash in your system.

Have you done this before? The 1 hour of running part i mean. It sounds like the wear might be too much. What speeds would you run the gear at for that?


I haven’t personally done this in a long time, I should have specified. I’ve heard of it done before from a few friends who swear by it, and the times I have done these things have been with gearboxes with center distance added already and it didn’t change much.

The wear is to some extent controlled by the system not being under load. This would be on a bench, with maybe a wheel or something on the output shaft just for a smidge more inertia.

Yes, high speed, no load. We have used a “suicide cable” for this, so we were running at free speed. With the amount of resistance it sounds like you have, you should have proper fuses/breakers at least.

What CNC did you use? For the abused tooth profile of Vex gears (assuming you used those), no C-C add should still work fine, unless you hold poor tolerances. Use a caliper to measure the distance between centers to check the exact distances and see if that’s really your problem.
Is there excess friction?

It’s worth noting that if you are using Vex gears, running them to wear them down might not be advisable as you would be wearing down the protective ceramic coating and possibly exposing the 7075 aluminum (which is more brittle when exposed compared to standard 6061). The ceramic coating would likely also take longer to wear down than a standard aluminum gear using this process.

If time and cost aren’t an issue, I would recommend either using AndyMark gears if you aren’t already or remaking the gearbox plates with the 0.002" - 0.003" extra center distance between the gears as others have already suggested (we have always just added the center distance and it works great for us, hardly any noise).

We tried the exact same thing too, it’s weird that it worked. It may have been the mounting method with the TB mini, but it’s worth a try on any gearbox.

Going of off the advice already offered in this thread, the first thing to do is check the actual center distances between your gears. The other thing is to make sure your shafts are perfectly parallel. Gears can take a few thou of play in the center distances without too much issue. They can not take a few thou of axial misalignment without protesting.
If your center distances are at exact or anything below that, you can add a .003 to .005 to loosen the meshes up a little. If this still doesn’t fix it, or your actual center distances already had a bit of space, you can try the methods GeeTwo and Type suggested, and you can try wearing the gears in by running the gearbox with no load. I’ve heard the run times for this need to be anywhere from 30 minutes to an hour. If you take this route, remember to clean your gears after the run-in period. Wipe all the grease off, use a bit of acetone to really clean the teeth, and then re-grease. Hopefully, one of those things fixes your problem.

EDIT - All the dimensions I mentioned are in inches.

Thanks everyone.
I’m leaning towards trying the wear down methods just to learn more about it and see how it goes. If that won’t go we’ll probably remake the whole thing with the suggested distance adjustments.

If I had a gearbox constructed like the one that you have, I would do the same thing. Good experiment to run! If you can, post a picture of your setup and findings if you do it.

I’ll echo what everyone else said: we add 0.003" to our center-to-center spacing between gears. Using hex shafts and gears with hex bores, it’s really hard to keep everything within a tight tolerance. The extra spacing keeps gears from binding.

How are you locating the CIMs? Since you say it seems that that is where the sound is coming from it is possible that the pinion to first gear spacing is the problem and all the others are fine. I would recommend loosening the CIM mounting bolts, then pulling the CIM away from the other gear (increasing the C-C distance by a hair) while someone else tightens the CIM mounting bolts back.

Are the cim motors square to the gearbox? If the cims aren’t mounted securely they can sag and cause quite a bit of misalignment. Have someone run the gearbox while lifting the motors up to check.