[dtengineering]

Don't panic about the gearboxes... be aware, but don't panic. You are in the same boat as probably close to 1,000 other teams and there is absolutely no way this boat is going to sink.

We have had some experiences with the 56mm gearboxes and have identified two problems, both with assistance from this forum.

1) The "Double-D" bowtie problem, which this thread is primarily concerned about, and which... as previous posts indicate, can apparently be solved by remanufacturing the final stage carrier plate. Given the sheer number of these gearboxes in circulation, you can expect that FIRST and Banebots will be involved in this solution. As for now, you can drive and test your robot. You may (will?) experience increasing backlash as the bowtie forms, but that is a ductile failure that should cause no permanent harm to the transmission. When (if?) a new final stage carrier plate becomes available it will not take long to insert. These transmissions can be rebuilt very quickly and easily with one 2.5mm hex key.

2) The "mounting plate" (aka spacer) problem. There is some variation in the thickness of the mounting plates for the trannies. This is more likely the cause of the metal and grinding that you have experienced. We found that three (of our four) BB 56mm gearboxes experienced this problem. The cause seems to be that the tranny is "too tight" when all the screws are done up, and the spacer that fits on the CIM shaft rubs against the gearbox end of the motor... since the motor appears to be aluminum in this place, and the spacer steel... well, that is probably most of the metal dust. Check to see if the end of the CIM surrounding the motor shaft appears to be wearing away and this will confirm this problem. We found that putting the spacer (that goes between the CIM and the first gear) on the lathe and turning about .010 off one end, then slightly grinding the motor shaft down to match has cleared up this problem and resulted in four nice smooth running gearboxes. In retrospect, we should probably have done this before "breaking in" the CIM... as the breaking in process seems to consist mostly of wearing down the end of the CIM. If you have access to a lathe (or even a vise, a file, and a steady hand) you should be able to solve this problem fairly quickly.

I would like to point out, that these are not solutions or observations original to our team, but rather are based on helpful posts by many other CD posters.

We have also noticed one other thing to look out for if you are using the Banebots encoders... some of the transmissions seem to have a bit of end play in the driveshaft. We have observed this on one of our four trannies in particular. The end play was sufficent that the encoder disk would move in and out enough that it would at times rub against the sensors on the encoder. A serrated steel disc rubbing against plastic optical sensors means it didn't take long before we were working with one less encoder.... we are considering a few solutions for this problem, but have not tried one yet.

Any suggestions CD community?

Thanks to everyone who has been posting their experiences and the teams who are leading the way in testing re-manufactured carrier plates. It has been about a week since the bowtie problem was first noted here, I believe, and it sounds like a solution is well in the works. That is an amazingly fast response, and just one more reason why FIRST is first.

Jason