# how many 775s would i need.

hello, i am working on a class project that takes the first powerup game and makes the concepts inside of it useful. my idea is to take the milk crates and turn it into an organization system where the robot knows where the crates are on a shelf and can retrieve them and put them back. i estimate the weight of the crates to be about 9 lbs. what kind of a motor setup am i looking at to operate that with a linear lift?

All depends on your gear reduction, drum size, etc. What kind of speed do you want the lift to run at? You could do it with one 775 but the needed gear reduction might be too slow for your liking.

i don,t need it to go very fast as i don,t want to jostle the items inside. i am using 16t sprokets with a chain drive. could i get away with using 2? and approximate what ratio should i use.

my quick plug into JVN shows that for a 775pro at 9 pounds load and a 0.5 inch pully diameter, with an overall ratio of 1:100, you would get approximately 4.9 inches/s under load

Good place to start is JVN’s Mechanical Design Calculator

You’re missing half the design constraints and only asking half the questions. A full system isn’t defined just by its motors.

Ask yourself what you are trying to optimize. Figure out the specifications for your project. For example, when we were building our lift this year, we were working under the constraints of sub-1 second ground to full extension, lifting 30 lbs, and able to maintain that at stall under 2V. If you don’t need to optimize speed, you could get away with fewer motors and a higher gear ratio. If you have a brake or don’t need to stall for very long, you can have a higher current draw, etc.

Knowing what you want to optimize and what your limits are, now you can start asking questions. A really good place to start is the JVN Design Calculator, which is best used with a healthy dose of intuition and a smattering of prototyping. Use the linear mechanism tab, put in your motors and weight constraints (keep in mind you’re lifting the box, plus the lift structure, plus your manipulator), then find a gear ratio and spool size that works for you.

Figure out how much power (Watts) it takes to lift your load at the desired speed. Then divide that power by 347. That will tell you the absolute minimum number of motors that will do the job. Multiply it by 2 to be sure. Then design your gear ratio to stay on the correct side of the power curve.

^^ THIS. And in case you didn’t know, the “high speed, low torque” side of the peak is the correct side of the power curve; that’s the side where a larger portion of your electric energy is being turned into useful mechanical energy. On the slow side, most of your electric energy is turned directly into heat.

Where do you get the 347 number? Wouldn’t different motors be able to put out varying amounts of power?

This thread is specifically asking about 775 motors, and that is the maximum power output of a 775 motor in watts. Though the motor will draw about 67 amps if you are using it at maximum power, which is a bit more than our 40 amp breakers and speed controllers want to spit out.

The 40A breakers, and some (all?) of the FRC speed controllers, will tolerate 67 amps for several seconds.

But that’s beside the point. The 2x safety factor means you’ll not need to worry about the breakers or the speed controllers (if you design the gearing properly).

We did try to put 80A each through two talon SRXs this year (for an overgeared climbing mechanism) and found that they cut power in a fraction of a second. We assumed that to be a safety mechanism in the speed controller, but didn’t investigate further. They seem perfectly happy around 40-50A.

But as you mentioned, that’s beside the point.

If what you say above is indeed true, then you probably were using Talons that had been programmed by your team to do so… unless CTRE has now made this the default.

Note that the 40A breakers can trip as quickly as 1.5 seconds at 80A… but you wrote “a fraction of a second” so I am dismissing that as an explanation.

It’s possible that our talons had an awry setting, or some other part of the control loop was interfering. I’ll blame the programmers whenever I can.

The test we did, for reference:

• 110 lb robot including battery/bumper
• 2 CIM gearbox, 12:50 reduction to spool, 2" spool
• Upon lifting, power cut within about 0.2-0.5 seconds (assumed to be talon cutting out).

Remedy: added 3rd CIM on it’s own speed controller

Ok I just wasn’t sure if this was a 775 specific number. Thank you