Vex Versaplanetary gearbox repair

[Dominick Ferone]

While at FLR we had mid match one of our lead screws stop running. When we went on field we spun the lead screw and it worked but we were worried we stripped all the gears. To our surprise when we opened up the gearbox the gears were fine. It wasn't until someone pointed out the gear had something on it we noticed the snap ring got pulled down and blocked the gear. We had this issue appear 3 or 4 more times during the day, the last being in between semis matches where we were quickly able to repair it. When we took it apart to work on a more permenant fix we noticed the groove for the snapping isn't deep enough to told it.

We are now going to try and deepen the groove and maybe switch to an eclip and run some stress test. Does anyone have other ideas to avoid this issue in the past or have run into this problem?

[marshall]

Can you post pictures? I'm curious about this.

[tim-tim]

I am interested in seeing the pictures of this as well. Do you have more than one gearbox, to compare the groove in each?

I have never seen this, will inspect a few of our VersaPlanetary gearboxes tonight. Not knowing how VEX handles QC (random inspection of x% of lot, etc.) I would guess this a rare part that wouldn't pass inspection if it was selected.

[Joe G.]

We have had a shaft fail the snap ring and pull the shaft out of the carrier plate. We believe that the initial failure was user error on our part, with the snap ring not being fully seated, combined with a pretty signifigant tension load on the shaft pulling it outwards. This same kind of loading would likely be present on a versaplanetary direct driving a leadscrew. We found that once the failure happened once, it happened again to the same shaft a number of times, as the initial failure rounded out the snap ring groove. It's easier to do this on a versaplanetary than a solid shaft of the same size, due to the spline grooves.

We deepened the groove and used a slightly smaller snap ring as a stopgap measure, replaced the shaft entirely as soon as possible, and haven't seen it reoccur since. I imagine an e-clip would actually be more likely to strip out the groove, due to the smaller contact patch.

My guess is that your issue also can be tracked to an incorrectly installed snap ring, rather than an out of spec shaft. Properly specced snap ring grooves are much shallower than it seems like they should be, and they look even shallower when on a splined region as they are in the versaplanetary.

[Bryce Paputa]

We have had far too many (factory installed, I think) snap rings fall out of ours over the past two years (not going to blame vex pro, we seem to like to put huge combined loads on the shafts), so we made plates that bolt on to the front of the gearbox and provide extra support for the shaft with a bearing. Haven't had any problems since and they're pretty easy to make with a cnc mil.

[Jared]

We had a little pin that transmits torque between the two gears fail on us at our second competition. We attempted to fix it by drilling/tapping for a 4-40, and putting a set screw in, but we broke off the tap in the gear, and used it as a pin for one match.

[Chris is me]

Were you re-using the same failed snap ring over and over again? If so, that's your problem. They're dirt cheap, bring spares. If the shaft groove has worn out from multiple failures, consider replacing the output shaft, which also isn't too expensive.

Putting tension on a VP gearbox output shaft isn't the best idea. Even something like a 1:1 reduction onto a shaft better equipped to handle this loading would be better. But I think it should still work.

[billbo911]

Originally Posted by Bryce Paputa

We have had far too many (factory installed, I think) snap rings fall out of ours over the past two years (not going to blame vex pro, we seem to like to put huge combined loads on the shafts),...

We have experienced a similar issue unrelated to the manufacturing of these gearboxes. Cantilevering the "El Toro" intakes last year put a LOT of forces on the output shafts that they were never intended to have. In fact, I am amazed they held together as well as they did! Sure, it is simple to replace the output shaft and snap ring. The problem is, they seem to know when the worst possible time to fail is.

Originally Posted by Bryce Paputa

... so we made plates that bolt on to the front of the gearbox and provide extra support for the shaft with a bearing. Haven't had any problems since and they're pretty easy to make with a cnc mil.

I would love to see these plates. You never know when they may be needed again. A CAD file would be even better!!

[Dominick Ferone]

What happened was our old lead screws had some back drive in the nuts. When we switched out the nuts, it then made the gearbox and coupler grouped together tighter and when driving the nut down it pulled up on the snap ring. We never had the issue occur until we switched the nuts. I believe we switched the snapring once and we didn't have any extra shafts with us to switch.

Tomorrow we will be repairing it and I will post before and afters and document the results. This does work as a learning experience.

[zinthorne]

This happens to our team all the time. We do not know why. I can look for some pictures. We check all of our snap rings are put on very well because of this problem, but they still occasionally come loose. We have had it happen 3-4 times this year on our competition bot and more in build season. A way to combat it is always test your gearboxes after every match. They will still usually run ok when the snap ring comes out, but will start only running part of the time(When the gears come out of contact) Also the shafts tend to fall out on us so that is a good indicator...

[JesseK]

I'm very curious as well. Could be a rare QA issue, could be something overlooked - hard to tell.

1. What motor was attached to the VersaPlanetary?
2. When was the VersaPlanetary purchased?
3. What orientation was the motor/gearbox in?
4. What was attached to the output shaft and how was it attached?
5. What sort of shock/vibration loads was the gearbox experiencing?
6. Do you remember having to fidget with the snap ring when assembling the gearbox?

9 Versaplanetaries in 3 seasons, 2 with BB550's, 1 with BB775, 6 with BAG (including 1 in a non-standard axial-load)- 0 with problems. I don't remember doing anything special with them, except making sure to read all of the little notes in the instructions and also manually reverse-running them after assembly to make sure they were assembled correctly.

[Rob Stehlik]

We had a shaft pop out of a Vex planetary gearbox several times last year. This was mainly because we had rather large rollers cantilevered off the gearbox, and I don't think they are designed to handle much in the way of axial loads. The retaining ring groove in the splined portion of the shaft is very shallow. After seeing this failure mode about 4 times, I decided to fix it by cutting the groove on the shaft slightly deeper for a 9/16" retaining ring. It hasn't been a problem since.

[Kevin Sevcik]

We did this a lot last year thanks to a poorly thought out design. You can see the pertinent stuff in the student's reveal video here: Leopards 2015 Reveal
Gory details below:

That giant roller arm is supported by a leadscrew in tension on the back side of the pivot. If I recall correctly, it was a 1/2-10 2:1 screw. I would estimate a static load of about 40-lbs. Dynamic loads much higher if someone hit the arm and forced it down. We originally coupled it by turning the screw down to 8mm and using a ruland rigid coupling to a CIM output shaft. The litany of failure reported to me from out first regional:

1. Coupling failed by slipping axially on CIM output shaft. Fixed by set screwing coupling to pin it axially.
2. Retaining ring failed. Partially due to improper installation (it's HARD to seat it correctly). Even after seating properly, they could still fail it running into something. Fixed by screwing a retaining plate to the front of the VP. Hole was just big enough to let the 8mm shaft through.
3. CIM output shaft was completely pulled out of the retained part of the output shaft assembly. I had assumed the output shafts were atomic parts, so I had no idea what the team was were saying over the phone until I pulled up the cad and realized the CIM shaft was press fit into the other half of the assembly. I believe they had someone hit the tail end with a bit of weld for this one.

The ultimate solution was to conclude that VPs simply aren't meant to support axial loads. As Chris said, the only actual solution is a 1:1 torque transfer to a shaft that's actually designed to support an axial load. I ended up spending a weekend with a lathe and CNC mill doing a full-on leadscrew end prep with a seat for an angular contact bearing, threads for a lock nut, and keyed shaft. The bearing and shaft went into a block that bolted to the front of the VP. The keyed 8mm shaft slid into a hole that was conveniently left in one of the shaft assemblies a CIM output was forcibly removed from.

That whole solution is one step further than Bryce's. His solution would work fine until you started separating the two parts of the output assembly. I think I moved the failure point to the 10-32 screws holding the block to the VP.

TL;DR. VPs aren't designed for significant axial loading. Especially in tension. Design for a shaft that can support the axial load and a 1:1 torque transfer to said shaft, or your VP is going to disappoint you at some point.

[Dominick Ferone]

This was a before to show how the gearbox was resting.


Now after pulling apart the gearbox we made a few modifications.
This was the output shaft beforehand.


We made the grove deeper manually with a mini file and widened it slightly to make the snap ring want to glide back down if it were to pull up.

Along with it to try and prevent it from pulling up we tried having the lead screw be pushed down a little and add in some spacers, and switch out the lexan plate with aluminum in the hopes of preventing the lead screw from pulling the shaft in the first place.

If this plan doesn't work the next idea is to add in a spring system so it compresses a spring and bring the lead screw down an 1/8th of an inch instead of pulling the shaft up numerous times and forcing the snap ring down. Also we noticed the cim adapter wasn't flush with the motor so we shortened the output shaft a bit more to sit flatter in hopes of reducing any future failures.

We are open to other ideas to try, as we have a demo this weekend and if the snap ring fails again we will try other ideas next week.

[Kevin Sevcik]

Originally Posted by Dominick Ferone

We made the grove deeper manually with a mini file and widened it slightly to make the snap ring want to glide back down if it were to pull up.

Along with it to try and prevent it from pulling up we tried having the lead screw be pushed down a little and add in some spacers, and switch out the lexan plate with aluminum in the hopes of preventing the lead screw from pulling the shaft in the first place.

If this plan doesn't work the next idea is to add in a spring system so it compresses a spring and bring the lead screw down an 1/8th of an inch instead of pulling the shaft up numerous times and forcing the snap ring down. Also we noticed the cim adapter wasn't flush with the motor so we shortened the output shaft a bit more to sit flatter in hopes of reducing any future failures.

We are open to other ideas to try, as we have a demo this weekend and if the snap ring fails again we will try other ideas next week.

I wouldn't have done that with a file. The depth of the groove matters some, but the squareness of the shoulder matters a lot too. If the shoulder you filed isn't square, it'd basically like its already failed, and it's probably not much better than the original groove.

Honestly, that big bearing block you have bolted to the top should be what you use to stop this. Replace whatever that white disk is with shims to take up all the space between the coupling and the inner race of the bearing. Think lock washers, only without the twist. If you do that, then the leadscrew will only be able to pull up on the coupling and through the coupling the bearing block. Worst case then is you fail at the coupling and the leadscrew pulls out of the coupling, which you should be able to fix with a set screw into a nice sized divot in the leadscrew. If the lexan plate is flexing, then yeah, swapping that for aluminum is a good idea.