CIM Pinion issues in Toughbox

Today we were driving around testing our manipulator when a loud screeching noise came from the chassis. We checked everything out, and found that one motor on one side was very hot compared to the rest. We took the gearbox off of that side (we are using 2 CIMS in each of the Toughboxes, with one stage of gear reduction removed). When we disassembled the transmission, we found that the key in one of the pinions was sticking out at an odd angle and the pinion could rotate on its shaft (hence the screeching). The pinions were mounted as shown in the Toughbox assembly guide.

Upon further inspection, we deduced that the key slipped out from the between the teeth of the retaining ring, allowing it to remove itself from the keyway on the shaft and the pinion. Our solution to this problem was to cut the key so it is flush with the pinion and place a 1/4" washer at the end of the pinion before installing the retaining ring.

If you are using the Toughbox or any other gearbox that has the pinions mounted in a similar fashion, I strongly urge you to inspect your transmissions to ensure this won’t be a problem.

Thank you for posting this. In fact, thank you to 1726 for posting your results of various tests and efforts this season, I’m sure you have inspired several teams to try new things as a result.

However unless I misunderstand your description of the problem, I get the sense that this may be a real fluke, rather than an inherent flaw in the gearbox… I’m not saying it didn’t happen… obviously it did, however teams have been using similar methods of attaching the pinion and key for years without any problems. So, just to make sure I understand…

Referring to the tough box user manual you had part number 18 (the 2mm x 2mm x 14mm key) slip out past item 20 (the 5/16" clip)?

When I rough out an estimate of the spacing between the “teeth” of the clip, it seems they should be about 1.6mm apart where they contact the CIM shaft.

Theory says this shouldn’t happen… practice and observation says it did. (At least if I understand this correctly.) Perhaps it was something specific to your team’s modification of the gearbox, or how the retaining clip was installed?


This could very well be something specific to our setup. I’m not entirely sure if the keys we used were the ones that came in the kit (we have a bag lying around with about ten each of pinions, keys, and retaining clips), the key we used may have been longer. Aside from that, everything is set up the same. Our modifications didn’t change anything on the first stage of gearing, so I’m fairly certain it had nothing to do with that.
This is definitely not an inherent flaw in the gearbox (I am very impressed by the simplicity and reliability of the Toughboxes), and I was very surprised that it happened.

That makes sense… if the key was longer then the clip could have been pushed down over it, thus allowing it to sneak out as you drove.

The fix you have posted sounds pretty much foolproof, though, and fortunately it happened at a time when you had time to investigate and repair.

About our only “discovery” so far has been that unless the compressor is very well supported and restrained, that those little rubber vibration dampers shear off quite easily in a high-speed impact with a cement wall… even with bumpers on.

Okay… after SEVERAL high-speed impacts with a cement wall.


We likewise had a small problem with the key slipping out of one of our motors in the toughbox. Fortunately it didn’t cause too much of a problem at the time and the robot just starting going in circles when one side stopped working. We put a couple of spacers in and it solved the problem. The key just seemed a little bit short. We have had no problems since and the spacers worked great. Other than that, we have really liked the setup.

The key and clip were installed just as shown in the picture in the Toughbox User’s Guide, shown here.

This is probably a rare problem, but it is something to keep in mind if either you have the transmissions apart to work on something else (it’s a relatively quick and easy mod to shorten the key and install the washers) or if you encounter weird driving behavior or noise. With a single CIM per side, you’ll have the robot driving in circles, with dual CIMS you’ll get one hot motor and one cold motor and drifting or erratic driving.



We noticed the key being short and had the possibility to come out. After the key and gear were on, we used a center punch to make a dent in the key way preventing the key from coming out.

Well… looking at that picture, then, and hearing another report of this having happened, this may well be a design issue that AndyMark may wish to address in future toughbox/gearbox designs. Like you say, it is easy to fix… when you have the motors out.

Fortunately the biggest design advantage of the toughbox over previous KOP gearboxes is how easily the motors come out. In most cases teams can pull their motor without pulling the gearbox.

We’ll keep our eyes open for this… we may even be able to check it out through the lexan panel.


We had the same problem, with the keyways being to small to work and that they would fall out. Infact we had the same thing happening during our testing, thus, we need to open up the toughbox and rebuild the gear box, which promply fell on the floor and was covered with hair and metal shavings…:o
Also the CIM came lose and needed to be reattached.

Well guess what, the bot did break down when we tested it and i need to rebuild it, and your keyway trick worked thnxs.