Our team has been using the new VEXPro two stage ball shifters for about 2 weeks now on our practice drivetrain. Yesterday while driving, one of the hex output shafts slid out of the gearbox. After some investigation both with the CAD file and disassmbled gearbox we realized the output shaft is just press fit into the gearbox. After reassembly by tapping the shaft in with a hammer and what seemed like a solid fit, we put everything away for the night.
During today’s meeting we drove the DT around for a few hours before the shaft came out a second time. We are wondering if anyone else has experienced a similar problem, it seems like our pit crew might have one area to keep a very close eye on this season.
Hi Brendan,
These are a press-fit, but are also cured with loctite. We haven’t seen the failure you’re describing. You can try using a strong loctite on your shaft, or contact our support department and we’ll ship you a replacement. ([email protected] or 903.453.0802)
Please make sure to use loctite 609. We had a few shafts in early production that were not properly cleaned prior to bonding. This may be one of those shafts.
In any case, we will replace it if you can’t fix it yourselves.
We experienced the same thing today when testing our drive base on blocks. One shaft came out and the chains flew off…the other started coming out. Chains were in alignment. The back pair of chains were a little too tight, perhaps that’s the reason.
We are thinking about machining a collar to grip the shaft to the shaft housing so our drivetrain will be worry free.
We experienced the same thing yesterday, we finally got our drive train together and we were just driving around in our schools hallway playing around with the two speed and with in 15 minutes of driving it broke. Our plan is to support it on the end of the shaft to it is supported on both ends while containing it so it can slide out either
We also expereinced one of the shafts coming out. For us we noticed before we powered the system and upon investigation noticed that the shaft appeared to have some sort of adhesive on it. So we used the Scotch Adhesive CA8 that came in the KOP. We have not noticed the shaft coming out on this or the other gearboxes we have.
We had the same problem of the press fit between the output shaft and the encoder gear not being tight enough. We cleaned up the shaft, bearing, and encoder, then added a dab of super glue in the gap between the shaft and the inner bearing race before we pushed the encoder into the bearing. It seems to hold well enough now.
I am not familiar with these gearboxes but could you remove the components from the box, assemble the two pieces and pin them together with a 1/16" roll pin or hardened dowel? It appears you wouldn’t need anything too substantial since all the torque is carried by the hex but a pinned connection would guarantee it wouldn’t release. If that stub shaft is driving anything with a side load, the slightest play or flex in the material will almost guarantee that it will eventually wiggle out of its hole. Make sure you use the “green” locktite retaining compound 680 or similar, not the threadlocker types. JB weld could also be a wise choice. I would imagine the brittle nature of superglues (cyanoacrylates) would crack and debond.
Fox,
The encoder gear presses into a counter bore in one end of the shaft. The gear is delrin or some type of plastic. The counter bore is located in the end of the shaft supported by a bearing. I am inclined to use super glue but I fear the long term effect on the gear as I do not know the exact material it is made from.
After a second look at that picture, I’ve noticed if you have that drive sprocket in the picture on the shaft and that nylon spacer is too long, when you tighten the bolt in the end of the shaft it will put a compressive force on that sprocket. The result will be that the bolt in the end of the shaft will be pulling the hex out of the transmission almost like a gear puller. This could certainly break any adhesive bonds and pull the hex stub shaft out of its press fit. Make sure you have the appropriate sized spacers on the end of that hex shaft and leave some space - like 0.010" for the drive gear to slide back and forth to ensure you do not put any tension on the stub shaft and pull it out of the bore.
Anyone in this thread that’s had a hex shaft come out: Are you retaining your sprocket with a bolt at the end of the shaft? Is it possible that you’re tightening the bolt so much you’re literally pulling the shaft out of that coupler?
I will check tonight when I can get a close look at the transmission. It is possible but IIRC after inspecting the completed drivetrains those hex sprockets had a little bit of play in them.
We are using the timing belt pulley’s which were far enough from the end of the shaft that the bolt tightened into the end of the shaft with some slop left on the pulley.
It is very easy to remove. The anodization, left over schmutz and grease makes it a week bond. Clamp the shaft in a vise and tap it off with a brass drift (or that screw driver you don’t like).
When you make the new shaft you need to drill out a hole at the end so that the spool is not blocked. Measure the hole on the old shaft.
Use Loctite 603 and be very careful to clean everything before bonding. You are best off lubricating after assembly.
That is what we did after the 4 we had failed. The new ones that were shipped were drilled and pinned before ever seeing the robot and drive time. We have not had any issues since.
