# RS775 Gearboxes

It seems strange that banebots wouldn’t have these available by now.

I’m looking to use four S775 to lift a ten pound arm thats about 60 inches long. Is this overkill, or would it allow me to maximize the speed of raising and lowering the arm?

Any suggestions for a gear ratio/gearbox and where I might be able to get it? The goal is to turn all four output shafts from the banebots motors into a single pivot to move the arm.

I conacted Banebots about this and they said they expect them to be up Monday or Tuesday.

I think 4 will be complete overkill. Our calculations allow us to lift an arm double the size and weight (i.e. 4x torque), running them at only 1/4 of their stall torque with just 2 motors.

4 is complete overkill. They’re roughly 240W motors if I recall correctly - think the equivalent of a 2010 FP motor.

I think we’re going to get away with one for our lift…

Then again, we might just use the FP.

Thanks for the fast responses. If I go for two RS775, what typ of gear ratios should I be looking for? It will be a parallel lift with a diagonal surgical tubing assist. I’m looking to making the arm raise and lower as quickly as possible without worrying about the motors not being able to handle it.

~390 if I did my math correctly; ~(voltage)(stall current)/4 = (12)(130)/4 = 390 (not factoring in motor inefficiency)

Also, the 2010 FPs had roughly 190w. This year’s have 290w. (Both figures from the FIRST motor curves)

The reduction we’re going with is a 512:1 (256:1 with banebots gearbox, and then 2:1 with sprocket). This gives us ~576 lb-ft of torque if you go with 65% gearbox efficiency, which is more than enough to lift it, while we still have 25RPM, which is 150deg/s.

I think you took the stall current at 18 volts instead of 12 volts.

DON’T DO THIS. That is, unless you’ve carefully considered the following.

Banebots suggests a maximum torque output of 85 ft-lbs for the P80 CIM gearbox. If your mechanism is actually going to require anything close to 576 ft-lbs of torque, you’d destroy a P80 gearbox rather quickly. And P80s are tougher than the P60, which is smaller with an aluminum ring gear. BB doesn’t list the max recommended torque for the P60s, but it’s going to be less. At least half, possibly a quarter. So I strongly urge you to look at the maximum safe output of your gearbox and make sure you plan your gear reductions accordingly. You may want to decrease the reduction in your gearbox and add an additional reduction stage of sprockets. 256:1 gear reductions provide surprisingly little torque once the gears have failed.

I was under the impression that the P60s were counterintuitively stronger than the P80s. If the P60s can output less than 85 ft-lbs of torque, they’re pretty much completely useless to me.

Remember - if you have a 15 lb end effector at 4 feet from your pivot, that’s 60 ft-lbs. If you then have a 2:1 sprocket and chain reduction leading into your P-60, then it will never seen more than 30 ft-lbs of torque statically. Dynamically is dependent on the speed you move it of course.

Yeah - this is what happens when you do math in inch pounds and are too tired to remember to convert. The gearboxes should be fine for me.

Thanks for letting us know. We will probably switch to a 128:1 planetary and 4:1 sprocket in that case. At most we would need 130 lb-ft of torque to turn the arm. With 4:1 that means the gearbox should only need to put out 32.5 lb-ft of torque. That should be fine I think, but I’ll ask Banebots if they have that info on the P60s.

Be sure not to just model for torque but for speed as well. An arm that rotates 180 degrees in 1 second is not very useful.

Spoke to Banebots Tech in the morning, he listened to my calculations and things but he couldn’t comment on the specific torque info on their gearboxes. They do not have that data available. Do your calculations and leave room for unexpected loss. The RS-550 has a Stall Torque of 4.4in-lbs., I am planning on 1.1in-lbs in my calculations, if free speed is 19300RPM, then you need to subtract from that # 25% because of the 1.1in-lbs 25% of the Stall Torque, then the motor will run at 15000RPM at 75%.,

Based on our prototyping, watching some old 2007 videos, and current consensus on design, we’ve found that speed close to optimal. Of course, it’s only week 2, and there’s a lot more testing to do, so our plans may change quickly.

Sanity check: Strap a five pound weight on the end of a 2x4 and swing that from a pivot point about five feet from the end 180 degrees in a second.

Then try to precisely place a tube on a rack with that rotation rate.

I haven’t done it, so maybe I’m talking out of my butt here… but that seems pretty darn fast! ::safety::

What’s wrong with 180 degrees in one second in this game (specifically)? You have half (or all) of the field to drive before you are in position to score, after all!

I guess I haven’t been clear enough. We probably will be going with something slower for human control (as I said, week 2 just started, lots of testing to do still), I don’t know of many people who would be able to control something that fast.

It’s far too fast to be easy to control, in my opinion. Most arms have about 90-100 degrees of travel. I"m thinking 45 degrees per second is much more reasonable.

With some degree of automation (PID, specifically) and the proper control systems, you could go with a much faster speed without losing much control.