Rounding out clutch square holes/Shorted out bumper switch

Two of my four groups are having trouble with the clutches on their square-bots. The clutch hole that is on the wheel side of unit is rounding out. After it did it on the first groups bot I did a “stand-down” so we could take a look and try and figure out what caused it.

The only thing I could see on their bot that might be causing it is that the wheels were pushed up against the chassis too tight after they had installed collars to keep the gears lined up. The groups fixed that problem on all their bots; however, it happened again to the second bot last night and I have no idea why.

Has anyone else experienced this? If so how did you stop it from happening again?

Now for one of the bumper switches. It appears to have a short in it somewhere. Whenever it is plugged in, the bot doesn’t work. But when we unplug it, the bot works just fine. I have tried the same switch on another bot and the symptom is repeated. How do I got about getting that part replaced? We received out kits in early October of this year.


I lead a group that has a dozen VEX kits. Over the last couple years we have had the same problem. There are two toothed (for lack of a better description) pressured plates inside the clutch that are supposed to slip (in a non-destructive manner), preventing permanent damage to the motor or servo. However, we often have problems where one of the square holes on the outside of the clutch will fail before the clutch will begin to slip. We have just started keeping a supply of extra clutches around, and in some cases, my teams will take the risk and not use a clutch. While that could lead to motor damage, we have only damaged one motor out of the 100, or so that we have, in the last couple years. That is not to say everyone should stop using the clutches, they are there for a reason. But sometimes weighing the risk against having a clutch fail in a competitive match is the type of design decision an engineer needs to make.

I have often found the clutch / motor to fail even before the clutch slips. Upon disassembling, the spring seems too compressed. The square hole in the clutches round out after some time, so it may be good to keep spares.

We’ve seen the problem. We think its from putting the axle into the clutch only part of the way. Then under load it starts to warp the plastic.

In most cases we don’t use clutches on driven wheels, but we do on arms or anything with high gear ratios. The wheels have a natural slip against the tiles or carpet. We’ve twisted a number of axles, hence the clutch rule for arms.

“I lead a group that has a dozen VEX kits.” We have about 8 kits and another $3.2 million in parts :slight_smile: So far we’ve destroyed around 3 of the clutches and one servo across the last 3 years.

This is the main reason for clutch failure. If the shaft is fully inserted into the clutch socket, I’ve never seen a failure.


my clutchs work just fine. i barely ever use them, though, so that way i can put in motor clutches. no damage to any motors just yet :slight_smile:

Let me add that the shaft collars have to be placed correctly, so that the shaft STAYS inside the clutch.

Thanks everyone for the replies!

Any help from the Vex folks about how to swap out the defective bumper switch?

you could probably just open it (you only need a philips screwdriver) and check what’s wrong. only the black and white wires should be leading to the switch, the red should not be connected because bumper switches don’t draw power.

bumper switches are pretty simple. You could very easily open it, find an appropriately sized switch online or at radioshack, and switch the two out.

so, yea, basically the same thing octane said.

in retrospect, that wasn’t very helpful. sorry.