Gear Clicking


I built a very complicated gearbox for our allied-robot lifter, and when we tested it lifting some weight, the gear that is attached to the arm of our 4-bar lift arm and the gear touching that click a lot whenever we try to lift anything over 5 pounds. Clearly, we need to be able to lift more.

TL;DR: I am wondering if anyone has tips for making gears stop clicking in high torque situations.

As far as I can tell, this video indicates that axles bending and spacing between the support arms are the two leading cause of skipping. Our supports are as close together as I can get them, so that cannot change but, would high-strength axles make it work better?

High strength axles would probably make things better. If you want better information, it would be a good idea to say what your gear ratio is and show us a photo.

I don’t have a photo, but the gearing roughly 1:45.

I drew a diagram of how our gearing is set up. The clicking seems to be between the gears attached to the arm and the adjacent gear.

Gear Ratio Diagram

I drew this somewhat quickly, so I hope it makes sense.

Ok, that helps a lot.

1:45 is a massively strong gear ratio. I don’t think I’ve ever heard of a ratio that strong before :slight_smile: . So if none of the gears skip and the motor is putting out full torque, then the torque on the arm will be pretty huge.

As you said, the most likely set of gears to skip is the very last set in the chain (5:3 on your robot).

Technical explanation starts here - For anyone who’s reading, if my poor drawing skills mean this doesn’t make sense then that’s ok. You can just read the rest of the post and skip the explanation

What makes the gears skip isn’t actually the torque on the arm, but the tangential force at the gear interface. Here’s a diagram (possibly confusing, but hopefully not):

A is a tooth on the driving gear, and B is a tooth on the driven gear. A pushes B with a tangential force Ft. Torque = force * distance, so we know that Ft = (torque of gear A) / (radius of gear A).

But that force doesn’t convert directly into a tangential force on gear B. The force between the gears can only be transferred across the point where their surfaces touch, and since the surfaces are designed to have minimal friction the force can only be transferred perpendicular to the surface.

This is what is represented by Fab (the force of tooth A on tooth B) and Fba (the force of tooth B on tooth A). These forces can each be represented as two perpendicular components: a tangential component equal to Ft and a radial component that we can call Fr. The radial component Fr pushes the gears away from each other. The gears will meet at the same angles whatever amount of force is applied to them, so increasing Ft will increase Fr.

Technical explanation ends here. tl;dr: increasing the force where the gears meet increases the force pushing them apart.

Of course if there is more force pushing the gears apart they are more likely to skip, so we want to reduce the tangential force between the gears. Force = torque / radius, so we need to decrease the torque and/or increase the radius.

A simple way to increase the radius is just to use a larger gear on your arm. Use an 84-tooth gear attached directly to the arm, and move the 60-tooth gear that’s currently on your arm to an earlier point in the chain. That will reduce the force pushing the gears apart by about 30%.

Reducing torque isn’t quite as simple, but the general idea is that you want to bring the load closer to the “shoulder” of your arm and increase the angle that your arm has to travel through. If lifting a robot to 12" requires your arm to rotate by 45 degrees, then the arm needs more torque than if it was rotating through 120 degrees.

I came across some other Ideas, and I was wondering what you thought about them. I found the idea for an idler gear to go above the last gear in the series (source) . I was also thinking that using rubber bands to pull the axles together might not be so bad if I make sure not to bend the axles with that force.

Do not use idler gears and rubberbands to fix the issue, it’s the same as greasing a bent axle so that it will spin properly, it’s only covering up an existing problem.

Your axles are not as close together as possible. In your diagram you showed that you had 3 sets of gears sandwiched between two pieces of metal. When building high torque gearboxes, htere should only be at most 3 square holes of space between the pieces of metal sandwiching the gears.
This measn that you still need to support your axles properly.

If you minimize the space between gears as much as possible and there is still skipping, use a smaller gear ratio of say 25:1, which sould be more than enough to lift 15 pounds provided that you adequately assist the lift with rubber bands.
Heck, there are lifters out there that are raising 18 pound robots with only rubber bands. Piston locks hold down the arm for the beggining of the match and release the spring loaded lift to raise the ally robot.

Today, we spent a while completely rebuilding the gearboxes, and it was well worth it.

We switched from our old 1:45 gear ratio to a 1: 27 gear ratio (1:5, 3:7, 3:7), and we switched the support arms to be closer to the axles. We also switched all but the first axle to high strength.

The clicking is gone, and we got it to lift 10 pounds (4.5 kilograms) with just one of the arms.

We took some pictures, and we hope you like them!

side view
top view
isometric view
assembled gearbox
with lift arms

Thanks to everyone for helping us solve the issue.

I am not too advanced with the use of high strength shafts. But I believe that it is possible that the holes that were made to fit the larger shafts through could be slightly off centered. In this event, it may keep the gears ever so slightly apart which wont effect much unless a large load is placed on them. If this is not the problem, then good luck.

I am not sure if this is your problem, but typically you want your gears in a linear format (all of the axles are in a row).

They said they solved the problem.

You can see from the pictures that the spacing between the axles is four holes up and three across, meaning that the axles are exactly 5 holes apart. There’s nothing wrong with doing it this way.

The only remaining feedback I have is that you could get away with a lot more rubber bands than you currently have :slight_smile:

They said they solved the problem, lol.

The position of the high strength shaft is determined by the bearing, and the position of the bearing is determined by the holes it’s screwed into rather than the hole it sits on top of. As long as the hole you drill is a bit bigger than the radius of the axle, it doesn’t matter if the hole is a little bit off centre.

We needed to use the non-linear design because the arm was being lowered into a gear. Fortunately, we found that we could move it and it would still work.