Gear failure

In a practice we were running our robot and a gear came flying off. We inspected the robot and found that half of a twelve tooth gear came flying off the axle is there a good way to stop this from happening?

Either use the metal High-Strength pinions, or sand down one side of 2 pinions and double them up. This might also be a pointer that you should be using two motors to better distribute the load.

We are using two motors. You can mix plastic and metal gears.

Yes. It’s not ideal, but its fine.

We have talked abut spreading the load across a second set of gears for each motor but since our state tournament is this weekend and we are a first year team this is unlikely to qualify for anything we are just going to leave it and replace the gears when they do break.

Of course you can mix metal and plastic. The only metal gears are the 12T ones. You must have misunderstood me when I said pinion. Pinion refers to the gear mounted to the motor, which tends to have fewer teeth than the spur.

I’ve broken two gears.

  1. 12T plastic gear. The teeth chipped off.
  2. 60T plastic gear. The center broke out.

And, I’m not sure what you’re talking about. First of all you have to do metal to plastic if you want to change any gear ratios, and second of all, the plastic has give that the metal doesn’t have. If you have 2 metal gears and they can’t move, then you’ll burn out the motor. If you have a plastic gear, then the plastic gear will strip, but so what, plastic gears are expendable. They’re a lot cheaper to replace than motors.

If you don’t believe me, look here. Many RC trucks like this have large plastic spurs and brass pinions. This is so that if the wheels are suddenly stopped on a hard landing, the motor does not get ruined.

And, I’m not sure what you are talking about.

I specifically said you can use metal and plastic gears together. In VEX that is the design intention. My statement about it “not being ideal” was a general statement. You won’t find many examples in real industry where metal gears are meshed with plastic.

Hence…it is fine for VEX and our applications here…but generally speaking it is not ideal.

Your RC car use is a good example. It is used in some places, but not many. And I happen to be an RC car/quadcopter fan and have personally destroyed many plastic gears in high powered RC car setups…specifically because some cars mesh metal with plastic. You can be assured, that if there is a plastic pinion gear meshed with a metal one in an RC car, there is an aftermarket metal pinion gear for sale…which works much better.

Huh? Why would you design it so they can’t move? Even if they couldn’t move…it would not burn out the motor. There is PTC protection not only in the motor but in the Cortex as well.

I care. I would rather not break anything.

Well at this point, you have your answers, nevermind the side discussions. The situations described were worse case scenarios. Of course you wouldn’t design it so it couldn’t move, I was referring to accidents/unexpected situations, e.g. driver tries to keep lifting scissor lift past maximum, or robot gets wheel caught and stuck, etc. Ideally, you’d use metal gears like 4149G said, with some sort of clutch/slipper. But the Vex clutches slip to easily and aren’t very good. Thus, in the Vex localized environment it is best to use plastic for the larger gears because of their light weight, lower cost, and they have a slight give. However, it doesn’t really matter, because as I said earlier, you don’t really have any options beside plastic for the other gears. Of course we could keep arguing, but there’s no point, so let’s just drop it.

In essence, you have two options for the 12t gear. Use metal gears, or use file and sand down one side of a 12t plastic gear, which makes it as wider and distributes the load better and reduces slipping.

For the other gears, there are not metal options, just the standard gears and the high strength ones. Either use the high strength ones, or double them up. the 84t gears are really easy, as one side is already flat and they have many bolting holes.

Our team used doubled plastic 12t gears and doubled 84t gears, we were fine. The most important think is to keep gears close to mounting points and always try to sandwich the gears in between two mounting points.

It is also important to remember that quality varies greatly with Vex. Some gears are just crappy, some may turn out to be really good.