Our team is using the AndyMark PG71 (am-2971) through a further 3 to 1 gear reduction to direct drive the angle of our ball handle. The ball handler is about 15" long and is also used to lift the portcullis. We use spring tension to compensate for the weight of the arm.
Originally, we used a PG188 motor with a direct arm drive (like a servo) to control the angle, but we shattered the gears in two motors before moving to the system we have now. The PG71 seemed to be more robust and provided the same performance with the 3 to 1 gear reduction. Unfortunately we experienced a gear failure at competition.
Under the normal operation (ball handling and lifting portcullis) my calculations show that the combination PG77 and 3 to 1 gear reduction gives a good balance of speed vs power and does not come close to the failing torque of the motor (39ft-lbs). I think the issue is that in competition the arm can be hit and impart forces in excess of 39ft-lbs.
Has anyone else had this issue and how did you deal with it? The only thing I can think doing is going with a more robust gear box, but that will be hard to find and retrofit at this point. Given we had only one failure in two competitions we may have to live with it, but I though I would ask in case there is something we can do to improve the situation.
Pictures of the gear failure will help the CD community understand what happened. Those same pictures should also be sent to AndyMark, so their experts can analyze and make recommendations.
We were looking at a similar problem with our pickup arm for Aerial Assist. One of our interim solutions used a belt reduction rather than gears. When the arm was struck, the belt jumped teeth rather than shattering anything.
However, we switched to pneumatics a few days later for concerns of consistency; this was long before bag-and-tag.
Another route might be not to use spur gears, but a lead screw/jack screw for the final reduction.
We used this gearbox on our intake in 2014 as well, and we experienced similar problems. If I recall correctly, we burned out a few motors and had issues with the “D” coupling on the transmission output shaft. We had to disassemble and reassemble it numerous times at our first district event, and we ultimately switched to pneumatics. We have avoided using it ever since (No offense Andymark).
If you are interested in changing to a different gearbox, I would suggest a VexPro VersaPlanetary Transmission. They are extremely versatile, and they can face mount using 2x 10-32 screws. We have been using them for the past two years, and we haven’t experienced any problems with them thus far. This year we are using four of them on our robot, if that convinces you of the confidence that we have in them. If you are face mounting the PG71 right now, unfortunately the hole mounting patterns aren’t the same (1.378" bolt circle diameter vs. 2.000"), but you might be able to mount the VexPro transmission using the four threaded holes on the side, or you might consider line drilling two new holes to accommodate it.
I hope you find a good fix, and good luck at District Champs!
Thanks for the suggestions. I like the idea of a lead-screw or belt drive that skips. That should solve or help the gear shattering problem. I would totally go with a VEX PG motor, but they are out of stock until May (a little late for what we need). I’d definitely consider pnuematics too but we would have to add a whole pnuematic system to the BOT and it is a little late for that. Good thoughts for next year.
We had failures in our PG188 gearbox a few years ago that was used to direct drive an “arm”. We broke off just about all of the teeth in the first planetary set.
What we essentially learned is that the driver was putting the arm up or down and keeping on driving the motor to keep there. Thus, if we have arms now, we put limit switches at both ends. The code is then set up so that if a limit switch is hit, the driver can’t keep driving the arm against the hard stop.