Climbing with CIM's: Versaplanetary Vs. P80 Vs. Multistage Planrtary/Spur Combo

Technical Motors
1

Over the course of the build season, we went through three iterations of our climbing mechanism, the first two being pneumatic, and the third powered by a CIM. Rev. 2 was implemented in time for our Week 0 Scrimmage, but it had a fundamental flaw that required a quick redesign. We decided to start over on 3:00 Sunday afternoon. Our frantic “three days to go” design session was centered around using parts we had lying around in the shop. This led to us using a slightly scary solution for our gearbox: a CIM attached to a three stage Versaplanetary with a 75:1 reduction (Motor–>Output: 3:1, 5:1, 5:1). We were scared because we knew that this gearbox was never designed to take this kind of a load, but it was our only option.

54 hours later, we finally tested our new system an hour before bag and tag. Due to a few easily remedied binding issues (our linear bearings were actually 8 year old drawer slides with no ball bearings) we stalled the CIM multiple times, and the gearbox remained perfectly intact.

Since Tuesday, I have been doing some research on other options for planetary gearboxes for CIMs, as well as looking into the actual load ratings for versaplanetaries.

What I’ve found so far:

The Versaplanetary has two failure modes, the output shaft and the “10:1 Carrier Plate” Source
The 1/2" hex shaft (what we’re using, supported on both sides) is rated for 116 ft-lbs
We don’t have a 10:1 ratio, so I believe we can ignore the carrier plate rating (74 ft-lbs)

What I’m confused about:
Does this mean that the Versaplanetary will never fail in a 3:1 or 5:1 gear stage?
Are we in major red, considering that a stalled CIM would give 134 ft-lbs of torque at the output shaft?

Our first other consideration was to switch to a Banebots P80. However, I was surprised to read that it is rated for only 85 ft-lbs, about half of what we want for safety. Why is this gearbox so weak if it’s designed for CIM? Here on chief, I found a bunch of horror stories of P80’s self destructing.

Our final option is to use a versaplanetary or P80 as a first stage (something comfortable, 20:1?) leading into a spur gear or chain reduction.

Anybody have any thoughts or comments? Please let me know If I have made any errors in my reasoning, I’m still a bit new at this.

Thanks!

2

For what it’s worth, our climber is using a mini-toughbox, with an additional 15:25 chain reduction. It’s nice and fast, and plenty powerful enough - I mini-CIM could get us all the way to the top, but we added a second for a little more speed (we wanted the bar to hit the bumpers when we climb, not one of our vertical frame members) and to help keep the motors cool while running.

3

We are using a Versa 5:1 with a 775 motor. It drives a ball-screw with a .2" lead. Robot weighs 93lb nekid.

It looks like you are in danger of failing your shaft if you stall the gear box under full power for any length of time.

4

Lowering the amount of reduction on the VP and adding an additional stage of chain or spur gear reduction is going to be a safe bet.

IMHO, stay clear of the P80, it has a storied history of failure you do not want to encounter 30+ inches off the floor.

5

This is the way I would do it. Perhaps as simple as a VP (20:1 or 15:1) and a 1:5.33 VEXpro Single Speed Gearbox.

6

All,

First off, the load rating for the VP is very comservative so I think using the VP in this instance is fine.

Also, since the rating is based on the ultimate stress failure of the aluminum hex, going to a single stage gearbox with a 7075 aluminum shaft (or unhardened steel shaft) won’t be any better.

In any case, you should be fine with the VP direct to your output for the climber.

7

Wow!

That perspective is actually a little game changing. How would you feel about versas on a drivetrain?

8

Thanks Paul, I wasn’t sure if the failure was caused by the spline or some other part of the shaft specific to the VP.