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Unread 01-08-2018, 02:35 PM
Oblarg Oblarg is offline
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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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FRC Drivetrain Characterization

Last edited by Oblarg : 01-08-2018 at 02:37 PM.
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Unread 01-08-2018, 04:07 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 04:11 PM.
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Unread 01-08-2018, 05:16 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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Unread 01-08-2018, 07:15 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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Unread 02-23-2018, 11:13 PM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

So, I thought I'd update this thread as prior to bag day we characterized our main robot, which was built using the same model gearboxes with the same spread (but a different third stage). It showed the same pattern of large losses of efficiency in low gear.

We have a third frame that we might get around to characterizing this weekend; I'll report whether we see the same thing there, too. The fact that we've seen this twice now, though, makes me suspect that it is a real attribute of the gearboxes.

I'm still mostly at a loss for potential causes. The only one that comes to mind that makes any sense to me is that the "plunger" in the shifter shaft might be axially loading the shaft in low gear but not in high gear, if the tolerances of the "pocket" that it is in are not quite right. If our third pair of gearboxes show this same behavior, I'll investigate it further.
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Unread 02-24-2018, 09:42 AM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

I am just going to put one on the dyno this week to see what it shows.

If you could share your data and math behind the characterization maybe I can see the anomaly.
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Unread 02-24-2018, 10:23 AM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

Quote:
Originally Posted by Paul Copioli View Post
If you could share your data and math behind the characterization maybe I can see the anomaly.
Data: https://drive.google.com/open?id=1Xp...9WWnc0AIxIj0XA

Math: https://github.com/blair-robot-proje...acterization.R

The quasistatic test used a voltage ramp of 0.15 volts/sec and the step voltage (fwdAccel/revAccel) used a step voltage of 6 volts. The maxAccel tests aren't used for characterization. The logs we actually used for the characterization are the ones with the latest timestamp for their name (because we kept trying until we got it right). The gearboxes were constructed with the wave washer. Characterization is versus voltage as measured by the talon, so battery sag doesn't matter. Results are here and the procedure is the exact one detailed in the data sharing thread.
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Unread 02-24-2018, 12:49 PM
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Re: VexPro Ball Shifter Frictional Losses in Low Gear

Note: We re-characterized our test bed today with the compressor off (the shifters were simply held in place by zip ties, with little/no axial force), and the low gear efficiency was unchanged, so the "axial loading" hypothesis seems to be busted.
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