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Unread 01-08-2018, 03:35 PM
Oblarg Oblarg is online now
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VexPro Ball Shifter Frictional Losses in Low Gear

We recently built a test bed using a pair of VexPro 3-CIM ball shifters, with a 2.16 spread and a 34/50 third stage reduction.

We weighted the drive to 135lbs and characterized it according to the methodology described in our whitepaper.

Upon comparing the measured values for Ka to those theoretically-calculated from the motor specs (see whitepaper for details, linked in my signature), we found that, in terms of torque, high gear was 93% efficient (!), but low-gear was 65% efficient.

We're somewhat at a loss for what could cause such a discrepancy. Perhaps someone with more detailed knowledge of these gearboxes could offer a possible explanation?
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Last edited by Oblarg : 01-08-2018 at 03:37 PM.
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Unread 01-08-2018, 05:07 PM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

Quote:
Originally Posted by Oblarg View Post
We recently built a test bed using a pair of VexPro 3-CIM ball shifters, with a 2.16 spread and a 34/50 third stage reduction.

We weighted the drive to 135lbs and characterized it according to the methodology described in our whitepaper.

Upon comparing the measured values for Ka to those theoretically-calculated from the motor specs (see whitepaper for details, linked in my signature), we found that, in terms of torque, high gear was 93% efficient (!), but low-gear was 65% efficient.

We're somewhat at a loss for what could cause such a discrepancy. Perhaps someone with more detailed knowledge of these gearboxes could offer a possible explanation?
Eli,

That doesn't sound right. I assume you are talking about mechanical efficiency. They should be close to the same since the number of gear reductions is the same. There are three stages of reduction no matter if you are in high or low. Our gears are about 95% efficient (depending on the loading condition) so they both should be around 95% efficient mechanically.

I'll need to dig in to your testing parameters to see why those numbers don't jive with our dyno testing.

Paul
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Last edited by Paul Copioli : 01-08-2018 at 05:11 PM.
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Unread 01-08-2018, 06:16 PM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

Quote:
Originally Posted by Paul Copioli View Post
Eli,

That doesn't sound right. I assume you are talking about mechanical efficiency. They should be close to the same since the number of gear reductions is the same. There are three stages of reduction no matter if you are in high or low. Our gears are about 95% efficient (depending on the loading condition) so they both should be around 95% efficient mechanically.

I'll need to dig in to your testing parameters to see why those numbers don't jive with our dyno testing.

Paul
Yeah, this is why we're so confused; there's no obvious reason from the gearbox design that low-gear should have greater frictional losses. Regardless, I'm pretty sure the result is "real," at least for this pair of gearboxes; the wheels are significantly harder to turn in low gear, much moreso than in any other shifting gearbox I've used (and much moreso than can be explained by the 2.16 shifting spread).

We may want to take the gearboxes off and inspect them internally, but we also do need to run things on the test bed and so I'd like to do as much debugging as I can now. Is there any common error in assembly that would cause this behavior? It's present in both gearboxes.
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FRC Drivetrain Characterization
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Unread 01-08-2018, 08:15 PM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

I'd look inside. If things weren't greased evenly (all the lube squirted on one pair of gears) that would do it.

Barring that I'd start looking to make sure all the shafts are parallel and true. Might need broken in?
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