Can anyone validate my motor calculations?

[LoneWo1f1998]

My team is debating using the andymark am-2971 planetary gear box with motor and encoder to lift up the stack of crates. We would have it attached to a three inch diameter sprocket with chain running to the bottom of our lift mechanism and the top in a triangle. I am relatively inexperienced in calculating torque and lift capacity so I tried to use a spreadsheet application, but would like to check my numbers since I wasn't sure on some of the values. We would have one or two of the motor-gearbox combinations, one for each side of the lifter so I assume they would each need to lift 30-40 pounds or one motor lifting 60 to 80 lbs. When I put in all of the numbers, the app told me the design could lift the whole stack 17 in in 2.32 seconds and would have a stall load of about 106 lbs. I wanted to make sure these numbers were right before we committed so if anyone who understands this stuff more could check them that would be great. I would also appreciate suggestions for a better system, but please keep in mind I am rather new.

I'm assuming that a 3" sprocket translates to a 1.5" Load Torque Lever arm. and input a 1:1 gear reduction because we put in the specs from AM on the gear motor itself.

Performance Specs:
Gearbox Reduction: 71:1
Total Counts per revolution: 497 (7 Counts per motor rotation multiplied by gearbox reduction, 71)
Voltage: 12 volt DC
No Load Free Speed: 75 rpm
No Load Current: 0.6 amps
Tested Gearbox Output Power: 44 Watts*
Stall Torque: 16.6 ft-lbf*
Stall Current: 22 amps*
Minimum torque needed to back drive a non-powered PG71: 1.3ft-lb
Maximum torque applied at shaft which breaks gearbox: 39 ft-lbs (breakage point is shearing of shaft)

[Ideal_Nerd]

I haven't checked your math, but this is what we use to calculate all of our driven mechanisms.

http://www.chiefdelphi.com/media/papers/2059

It is a great tool to have.

[LoneWo1f1998]

That is the excel app I tried to use

[Ideal_Nerd]

those numbers sound right. We are using the same calculator for our lifter. we are using a mini CIM with a custom gear box with a 26.7 to 1 reduction. we are getting it to lift 60in in 1.6 sec with 150lbs of force.

If you have the resources to make your own gearboxes i would do that, but your setup sounds good as long as you keep friction in your lifter to a minimum.

[GeeTwo]

Originally Posted by LoneWo1f1998

I'm assuming that a 3" sprocket translates to a 1.5" Load Torque Lever arm.

Provided that 3" is the pitch diameter of the sprocket, this is correct. If 3" is the outer diameter of the sprocket, the lever arm will be a bit shorter.

Assuming that's right, let's sanity check the numbers...

Stall current is 22A, so you shouldn't be breaker-limited unless you decide to be. That means you get an initial torque (I'm assuming a spike, or that you move quickly from zero to full on a speed controller) of 16.6 ft-lb. Applied through a 1.5" (.125 ft) lever arm, this is just shy of 133 lb at stall or startup, a bit more than the spreadsheet gave. This is probably due to assumed inefficiencies in the spreadsheet. Since you're already past the gearbox for your measurements, your losses are probably far less as long as your chains are straight and nothing is binding.

I'm not sure what you mean by "the whole stack", so I'll assume six totes. A tote weighs 7.8#, so six of them are 46.8#. Assuming 125# of lift, you'd get an initial acceleration of (125 - 46.8)/46.8 gravities, or 53 fps/sec. If you could keep up that acceleration, you'd reach 6 feet of elevation in under half a second, and be going up at 25fps, which corresponds to 200radians per second. So it sounds like you would very quickly approach equilibrium speed, which is about 75rpm * (125-46.3) / 125 or about 45 rpm. At this speed, the chain would move 45 * 2 * pi * 1.5 / 60 or about 7" per second. That roughly agrees with your 2.32 seconds if you were contemplating a 17" lift.

BTW, If you counterbalance your load with about 20-25 lb, you'd nearly double your top speed with a full load. At a price - unless you had a large chain loop or deep cable run so that your counterweight was at the back of the robot, this would move your center of gravity even farther "forward".

[BBray_T1296]

Originally Posted by GeeTwo

BTW, If you counterbalance your load with about 20-25 lb, you'd nearly double your top speed with a full load. At a price - unless you had a large chain loop or deep cable run so that your counterweight was at the back of the robot, this would move your center of gravity even farther "forward".

Counterweights don't have to be sheer mass, they can be things such as constant force springs, or surgical tubing. The vertical requirement of the lift is a determining factor in these options though

[MrForbes]

What percent of stall torque are you running the motor at, with the maximum load on it? We like to shoot for 20%, if possible. You want the mechanism to be about five times more powerful than it needs to be.