775 vs CIM

So we as a team are trying to look into the benefits of different motors and also the drawbacks of them.
What are the differences between the 775 pro motors and the CIM motors? Advantages? Disadvantages? Feel free to open the discussion to other motors as well!

I would suggest looking through motors.vex.com for more information about all the different legal FRC motors.

The short story is that both have about the same peak power output, but the free speed of the 775pro is over 3 times as fast compared to that of the CIM. Because the 775pro is fan cooled, it cannot handle heat buildup as well. Also you are limited to 6 CIMs on a robot, but unlimited 775pro motors. Well 16 because of the PDP… :stuck_out_tongue:

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So you’re saying that they have the same output power but the 775 spins faster but also heats up faster too? And is that about it as far as differences go?

775pro will overheat if stalled. This is why you don’t see teams using them on drive systems, as they will smoke if your robot gets into a pushing match. They won’t survive more than a few stalls.

775pros actually have a bit more power than CIMs but require high reduction gearboxes to get any useful torque out of them. This usually means usage of a planetary gearbox, which is also subject to gear stripping when exposed to quick directional changes or very large loads (depending on the gearbox used, of course).

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There were definitely a lot of successful 775 drives this year, 2767, 16, 2451 all come to mind just to name a few.

If not used correctly.

At 12V stall, they’ll smoke in about 3.5 seconds.
At 12V peak power, they’ll smoke in about 90 seconds.

Design your mechanisms CAREFULLY with the 775pro. Try to keep it spinning on the faster (more efficient) side of the max power curve.

CIMs are much more forgiving. You can run them at peak power for > 3 minutes, or stall them for about 35 seconds without smoking them.

edit -

Teams using 775pros in their drive-train will usually design the drive-train so that the wheels will slip before the motors stall (traction limited). But that takes careful planning…

Everyone seems to be leaving out the factor that makes 775s most appealing to me: the weight.

775pro https://www.vexrobotics.com/217-4347.html:

0.8 lbs, 347 watts, 18K rpm, 45mm dia 66mm long, $17.99

CIM https://www.vexrobotics.com/217-2000.html

2.8 lbs, 337 watts, 5.3K rpm, 64mm dia x 115mm long, $27.99

Also, cost saving ( $10) looks good but will be cancelled by extra gearing to get to floor speed I think.

For about 9 seconds. At that point, the 775 pro peak power curve falls below the CIM peak power curve.

This assumes the same internal temperature vs. time for both motors. In FRC matches, that won’t usually be the case.

775pro motors heat up more quickly, as the referenced data shows. However, CIMs take longer to cool down after flogging. Fans in the 775pro allow them to recover sooner.

With the extra gearing, the 775Pro max power (delivered to the load) is probably no longer greater than the CIM’s.

Didn’t your team try using hollow bolts to cool your CIM motors this year? How well did that work?

Drive systems should certainly be traction-limited, making stalling not a concern.

If by ‘a bit’ you mean 10w out of ~340w, then sure! For practical purposes they have the same power output.

What makes a planetary gearbox more prone to stripping as compared to a spur gearbox as you imply? Why do you assume the usage of a planetary gearbox when products like the CIM-ile exist?

Difficult to say.

We did get extra practice time by shortening motor cool-down breaks, but eventually practice has to stop when you run out of batteries.

On the competition robot, we think forcing some air through the motors between matches helped keep motor power high when matches were close together, as often happens in district play. But the evidence is anecdotal. Maybe we can get some controlled test data in the off season.

It seems like the main focus with using 775s has to be something done very well to prevent smoking or overheating. Also, if not used in the drive train how useful are they for other applications that require a major load but not being used as often such as with a drive train?

775 pros are very useful in any situation the requires a motor. We used them this year for our climber and they worked flawlessly. As long as you gear them to the speed you need they should provide the most power possible ( assuming the same amount of inefficiency).

How useful do you think they would be for something like a turret that doesn’t necessarily need to move much but it would have a decent load. I know it would be doable with CIMs but we are trying to experiment to get experience so in a case like with a turret do you think we could even go down to a lesser motor than the CIM or 775 and be able to fine tune it more?

JVN’s Mechanical Design Calculator. Use it, know it, love it.

+1 to JVN calc, it’s an amazing tool. It doesn’t do all that much for turrets however.
Turrets don’t have a ton of load on them. A 775pro could be good just to standardize, but even a smaller motor like a 550 would work fine. Just make sure you gear it down to a low rpm (200-500) so you can control it.

Does anyone have any sort of data about stalling 775s; i.e. how long can you stall one safely before it blows? Would be cool to be able to design to that rather than cross fingers and hope your mechanism doesn’t stall too long.

More generally:

(click on the motor you want)

Edit: also, yay FSAE!