Hang with 4 HS Motors?

Hey, am contemplating a design change before our states championship, so here’s my question - provided that we have a very lightweight bot (around 10 lbs), would it be possible to high hang with 4 high speed motors on the lift? Has anyone accomplished this? Thanks.

It’s definitely possible, try to get that weight down even further as much as you can. It all depends on your gear ratio.

1:5 Gear Ratio on the Lift btw

Overall gear ratio is more important than “high speed” or “torque” motors. Assuming you’re using a 1:5 with the high speed arm, It would be difficult unless you use an outside source of power such as a transmission or winch or elastic configuration that would benefit your hang(we have something in the works for our robot that we haven’t used in competition yet). Another tip if you wanna test hanging would be making your arm tower shorter and closer to the middle. That way you have less load on the motors.

4 high speed motors on a 1:5 isn’t enough to lift more than like 3 stars, much less hang. Personally I doubt the hang considering you will also be fighting rubber bands.

We can high hang our robot with two low speed (default internal gearing), 1:1 ratio, 393 motors (at full speed). Our robot weighs somewhere around 10lbs give or take a couple pounds and it pulls up our robot like it was nothing. You can find a picture of our robot with the hanging mechanism here.

I believe what they were asking is if it was possible to hang with 4 lift motors, not linear sliders, which is harder to do.

Gotcha. My bad.

My robot weighs 14 pounds and we can do a high hang 929X-style with 2 torque-geared motors. Given your robot’s weight, it may be possible to do the hang with 2 HS-geared motors.

Wait, can someone confirm this? I was basing this off of other robots I’ve seen that have 4 motors on their lift with a 1:5 gear ratio, that could lift 5+ stars very quickly. Someone told me they are using speed motors, but do you think I was mistaken and they were actually using normal (out-of-the-box) torque motors?

Yes. 4 motors on a 1:5 using torque motors are fairly quick and can carry like 7 stars. However speed motors is too aggressive and burns out very quickly.

Haven’t tested extensively, so not going to leave a personal opinion, just some math.

Good rule of thumb for sustainable torque of 1 motor is ~half stall torque. Stall torque is 14.76 inlbs, so I’ll approximate as 7 inlbs.

4 motors * 7 inlbs * 5 (gear ratio) = 140 inlbs of torque, without bands.
EDIT: THIS NUMBER IS WRONG, see end of post.

So assuming you have no bands on your lift, and your robot weighs 10lbs like you said, you can lift if your lever arm is less than 14 inches. You might have some leeway with this number because you’re only going to run your lift motors to hang once, at the end of the match where tripping PTCs doesn’t matter, but you also need to consider that the lever arm isn’t the length of the arm on your lift, it’s the greatest horizontal distance from the pivot point of your lift arm to the center of gravity of your robot during your lift.

Assuming your robot has uniform density, the center of mass would obviously be in the center of your robot, leading to a greatest horizontal distance to it from the corner being 9sqrt(2), or ~13 inches, which would occur when your robot was lifted 45 degrees from the ground. This number would change depending on how high you mounted your towers and how you distributed weight, but some extremely naive calculations would suggest that, yes, without bands, you can hang.

EDIT: Forgot about high speed.

140 in*lbs 5/8 = 87.5 inlbs
Lever arm = 8.75 in

So no, you almost certainly can’t hang without some clever engineering, at least based on these calculations.

The stall torque of a high speed motor is actually ~9.2 inch pounds, which translates to about 90 inch pounds of torque. Dividing 13 based off the number above, and you get about 7 pounds of lifting power. So I think it’s pretty unlikely to hang using 4 high speed motors.

I’ve got an edit at the end of my post, I missed that the first time around.

I wrote my post before you got the edit in, sorry.

In the beginning session our 15-pound bot was able to hang using 6 speed motors with a 1:5 gear ratio against 6 doubled up rubber bands like it was nothing. Looking back at it I don’t see why 4 speed motors and no rubber bands wouldn’t be able to do the same (granted I don’t have the math to back it up). I do however foresee some ptc troubles with 4 motors and no rubber bands when constantly picking up a lot of game objects.

655/87 = 131.25 inlbs
131.25/15 = 8.75 in

This is the same required lever arm length of 4 HS motors, but with bands, which would suggest that it is possible to hang with the setup described by the OP.

So your CoG was ~9 inches away from the pivot point. This is possible with either a small robot, high CoG close to the towers, or low pivot point. Also, you may be able to stress the motor PTCs up to stall torque for a few seconds without them dying. I’m not qualified to speak on that, though. I just do the naive calculations.