Lifting mechanism

[A13N007]

Hi! Our team is a bit ahead and were stuck and wondering if anyone has had there "Arm" part built yet. Our system (without letting the cat out of the bag) is were planning to pick up 3 totes at a time, or 2 totes and a recycle bin. We are assuming to use a 16:1 gear ratio, were thinking its kinda low on 1 cim motor. Anyone have any kind of expirence with a diffrent gear ratio on thier lifting mechanism? Any help would be appreciated, thanks !

[Amar Shah]

If you are using a linear mechanism for lifting, the gear ratio does not tell much. What ultimately determines the speed at which totes can be lifted is the relationship between turns of the motor and distance traveled, and is affected by both any gearing you do and by the radius of a winch spool or pinion gear meshing with the rack, or the thread density of a leadscrew.

Have you considered using more than one CIM motor? There are many COTS gearboxes which support two or three motors.

[A13N007]

Thank you fully understand what your saying were more considered with torque than speed, do you think this will be enough torque to lift and hold?

[pwnageNick]

Originally Posted by A13N007

Thank you fully understand what your saying were more considered with torque than speed, do you think this will be enough torque to lift and hold?

Speed and torque tend to directly relate to each other. It's hard for any outsider to judge if your ratios are okay if we don't know what kind of mechanism it is? Is this a linear lift (i.e. elevator) or is it an arm (i.e. 1-joint arm/link) or is it some kind of linkage (i.e. 4-bar, 6-bar, etc.).

I understand not wanting to give everything about your robot away, but it's tough for people to give advice about detailed design points without knowing the general concept of the mechanism.

[A13N007]

Sorry for the late reply, we have a linear lift with one motor.

[JamesCH95]

Draw a diagram, give us more detailed information, describe your mechanism in better detail. We want to help, but can't.

It's week 4 of build, I doubt many teams are going to try and copy your arm design. I bet many teams already have comparable designs. I can sort-of understand wanting to keep your design a secret, but, at bag day would you like to have a secret non-working mechanism, or a known and working mechanism?

Alternatively, use the JVN design calculator, the pull-down edition is best, to analyze your arm. You can find it with google.

[Mike Marandola]

Originally Posted by A13N007

Sorry for the late reply, we have a linear lift with one motor.

What size pulley/sprocket/drum?

Originally Posted by JamesCH95

Alternatively, use the JVN design calculator, the pull-down edition is best, to analyze your arm. You can find it with google.

This is a good idea. It's what we and many other teams use.

[StephenNutt]

You generally want to run a motor at 66-75% of its free speed, which will be 25-33% of it stall torque and stall current. If you look at Motor Information you can see what speed and torque this will be for a CIM.

It is good to avoid running the motor slower than 50% of its free running speed as you can get the same amount of power out of the motor by changing your gear ratio and running the motor faster, producing less heat and using less energy.

[GeeTwo]

For a CIM-based system, we usually figure this by looking at the 40A point on the performance curve, since we can maintain that speed for a time without tripping breakers. Then, look at the torque, multiply by your gear ratio, and divide by your moment arm (the radius of your pulley or half the pitch diameter of a sprocket). This will give you the theoretical lift force. You can probably expect a 10%-20% loss from there.

In your case, assuming a one-CIM, 16:1 gearbox driving a 15-tooth #35 sprocket (I have its pitch diameter of 1.8" memorized since we're using these this year in this application), that's 100 oz-in * 16 / .9in = 111 lb; plenty of lift, and very close to what 3946 is building. If your sprocket/pulley is larger, say 4" in diameter, it would only be about 50 lb, which (after losses) might not be enough to lift a stack of 6 totes.

[PaulEleven]

We got the building block 220 from andymark with first credits, so we're using a mini cim, 64:1 on a 15T#35 sprocket, gives you roughly 10in/s without load, stall torque of over 700lbs, and at maximum load during a competition somewhere between 4in/s-6in/s reliably.
If you think we're not safety nut enough it's a double sprocket....

[jimwick]

We've lifted four totes and a bin using a single CIM and a 12.75 tranny. We wind directly on the shaft of the tranny. Speed is acceptable for 1-3 totes, a little slow for four. Groans a little but goes. Backdrives a little but workable.

[asid61]

"Linear mechanism" as in a leadscrew?