Cause of chain ratcheting issues

[Chris is me]

It looks like the sprocket on the bottom is cantilevered almost an inch away from the gearbox face. This is a big part of your problem - there is a lot of flex in the VP output shaft when you have an unsupported load that far from the output bearing.

No real way around it; you have to support the other end of the shaft or move the gearbox closer. If you didn't cut a big gash in the 1x1 tube right by the output shaft, you could have popped a half inch bushing in there and turned the end of the VP output shaft to round in order to ride in the bushing. A bit more friction than is ideal, but it'll work.

[Monochron]

Originally Posted by Chris is me

No real way around it; you have to support the other end of the shaft or move the gearbox closer. If you didn't cut a big gash in the 1x1 tube right by the output shaft, you could have popped a half inch bushing in there and turned the end of the VP output shaft to round in order to ride in the bushing. A bit more friction than is ideal, but it'll work.

The VP output shaft actually doesn't quite reach far enough for that.

I can run it through some tests tomorrow but I guess I'm still a little skeptical that this issue would only crop up now, after our other changes. How much deflection in the end of the shaft are you expecting? All the mounts for the motor are still as solid as ever.

[Thad House]

Originally Posted by Monochron

The VP output shaft actually doesn't quite reach far enough for that.

I can run it through some tests tomorrow but I guess I'm still a little skeptical that this issue would only crop up now, after our other changes. How much deflection in the end of the shaft are you expecting? All the mounts for the motor are still as solid as ever.

Actually theres another trick you can do to extend a VP shaft. If you get some 1/2 round stock with a 1/4 inch hole, like the vex round tubing, you can screw that into the end of the VP shaft with a 1/4-20 bolt, and then put the round stock through some tubing and a bearing. As long as you get the bolt tight, it will have plenty of support.

[GDG 2337]

On our 2011 Bot arm, we experienced chain skipping using a long run of properly tensioned dual #25 chains with 16 tooth sprockets. We eliminated the issue by adding a roller that increased chain wrap around the sprockets. We made different diameter rollers to account for chain stretch and sprocket wear throughout the season.

[FrankJ]

Looking at your pictures.

If you are ratcheting, either the CD is changing or the chain is elastically stretching. CD changing seems more likely to me.

You definitely have some drive sprocket wear. Whether or not it is enough to be an issue, hard to say. The wear changes the pressure angle which can increase chain tension for a given torque. Maybe trade the idler sprocket for the drive sprocket?

Make provision to take up slack in the chain?

The motor support & motor sprocket has a lot of cantilever. Run a piece of flat bar to the frame rails or wheel gear boxes to add a bearing to help support the gear box shaft?

[Monochron]

We have mostly fixed the issue by doing two main things.

1. Sliding the drive sprocket further back on the drive shaft. Looking at it more closely (and very hard to see in the pictures) revealed that the point in the chain that was mounted to the elevator was directly above the middle of the drive shaft, not directly above the end of the drive shaft. I'm guessing that over time the shaft collar holding the sprocket in place loosened and inched forward.
2. We also removed some of the slack in the steel cable that drives the second stage of the elevator. I don't think this had a big influence, but it definitely reduced some of the shock loading that was going on when the cable was pulled taught.

After these two changes the ratcheting is gone. However, now when the elevator is under a lot of load I definitely see the motor mount bending upwards, thus changing the c-c distance. It doesn't ratchet anymore because the chain is better aligned with the drive sprocket, but it shows us the next thing to be fixed.
Thanks for the help everyone. All of your suggestions have been great.

[BJT]

It looks like your whole motor mount is sticking out quite a ways from where it attaches to the robot. As others have said, you maybe should have used that piece you cut out to support the motor mount and/or the shaft. Two sprockets need to be as solidly tied together as possible to eliminate that change in c-c distance you are experiencing.

[Monochron]

Originally Posted by BJT

It looks like your whole motor mount is sticking out quite a ways from where it attaches to the robot. As others have said, you maybe should have used that piece you cut out to support the motor mount and/or the shaft. Two sprockets need to be as solidly tied together as possible to eliminate that change in c-c distance you are experiencing.

This isn't quite the problem, the position of the motor in relation to where it mounts to the robot hasn't caused skipping at all, rather it was the perpendicular alignment of the chain and the recent extra load that caused the skipping. As I said before, this setup ran without issue since February until now. I don't think we could have used the cut away piece of 2x1 because the shaft is hex, the shaft extended past the part we cut away, we had no way to turn the shaft down, and no bearings we found could fit in that 1" aluminum face.

What you say about tieing the sprockets tightly and supporting the motor shaft is all absolutely true, but it's important to know that none of that caused our ratcheting. I know this to be the case because we haven't changed any of that and our ratcheting is completely gone.