Turntable sideways force

How much sideways torque can a large turntable take before friction makes it hard to spin? I mean sideways torque like what a robotic arm would provide: an unbalanced weight hanging off of one side.

In Skyrise some teams used the turntable to rotate claws around. Ours took the weight of a small one, probably just a pound, but it will most likely be able to take around 3 pounds.

Edit: Sorry, not much experience with these things. Take the others’ word over mine.

I’ve seen the smaller one hold Skyrise cubes around 8" from the pivot and still be able to rotate fine so…

This is what I am looking for: if I hang a weight off of a c-channel so the turntable wants to tip over, how much torque like that can it take (big one)

Going off of what TurboTech said, that’s a minimum of 9 lb-in (1 N-m) for the smaller one, but I am hoping that the big one can take over 6 N-m.

Is this all the weight that the big turntable can take?? For just weight that’s centered on the table, it’s got to be more than that…

Is your turntable horizontal or vertical?

You would have a torque contribution with more push on the bottom and pull on the top with a larger weight in the turntable in a vertical configuration. That leads to skips more than grinding as weight increases.

If the turntable is horizontal, then you get a pure thrust force on the turntable making it harder to turn. The thrust force is the force going into the hole of the turntable. Something gears don’t like much.

Which set up are you doing?

I would estimate about:
D=(WeightCoefficient of friction) Distance of outermost contact=Usable torque needed=Don’t use stall torque

The weight of the system is decided by you, the coefficient of friction is probably going to be somewhere between .40 to 0.05 (I would just use 0.40 to be safe). and distance of outermost contact is the distance from the center of rotation (axle) to wherever the end of the bearing is

So reduced form is about

W0.40Distance=Usable torque needed

Keep in mind, as you reduce distance, you’re not cheating the system. If you reduce distance, you need to consider 3 dimensional torque, and may lead to some/tons of wobble you don’t want. Also keep in mind that usable torque is not stall torque. Usable torque in our case, is dependent on current draw and temperature

I would suggest putting some kind of counterweight on the opposite side of your arm to mitigate the sideways force.

I am using a horizontal setup (see the overdue picture for clarification). There is a torque on the table that is trying to tip it over: how much torque can the table take like that?
turntable setup.png

How tipping over works is that if the center of mass of a structure goes beyond the vertical boundaries of the support base, the structure will tip over. I say just find out the maximum weight experimentally.

It will be a bit less than you want. The mass at the end of the arm makes a nice torque arm on the bearing turntable inside piece. This gets a nice twist about the axis coming out of the paper and pulls up the far side and pushes down the near side of the turntable gear in its housing. You may be able to hear it scrape or detect more power to get the motor started.

These slop about a bit easier than you might want before it wants to move. So 2 ft-lb of torque may be where it starts to cause the turntable to start getting friction and take more to get started. The 2 ft-lb (24 in-lb) is just my initial guess. A 5:1 ratio on a 7 in-lb motor should give you 35 in-lb before the motor is in trouble, but you have a second item here on the thrust force. Not sure what the final “can’t really move well” weight will be.

I would do some experiments with different weights at your desired distance and hook an IME on the motor attached to the turntable. Try and have the turntable allow the arm to move all 360 degrees to get a good full speed on the motor. Plot the power and rotational velocity of the IME vs time. You can see the initial delay before the motor starts as well as the max velocity achieved by the motor for the different weights.

If you used the smart motor library or attached a current sensor, you could see the amps needed to get that arm rotating too in order to not trip the IME.

You can also repeat the experiment with some lithium grease on the metal gear attached to the motor and see what effect that has.

I think it could be a good idea to put a small amount of grease on the areas where the inner part of the turntable contacts the outer part.