Are 775pro on a Versaplanetary gearbox, a good idea on a elevator?

Are 775pro on a Versaplanetary gearbox, a good idea on a elevator?, provided that they have a tendency to die when stalled.

Used 2 775pros in versaplanetaries last year to climb… You just need to size your reduction properly. Try JVN’s calculator which you can download on CD or the Vexpro website.

Last year we used two 775 on 50:1 I believe and we climbed in about 3-4 secound so I would not discount versaplanetary as a motor for climbing

Hundreds of teams have used the combination of a 775pro and a VersaPlanetary gearbox to climb in 2016 and 2017. As long as you follow the Load Rating Guide, you should be in good shape.

My team used 2 775s to climb last year, we originally had one and had issues with tripping breakers before we had issues with stalling the motor (destroying the motor). Just use the proper gear reduction using JVN’s calculator and you should be fine.

I’m not sure from your question if you’re talking about an elevator for lifting the power cubes, or for climbing. So, I will assume for lifting the power cubes. You have answers already for climbing.
You are correct in assuming that stalling the 775 pro with power on it is a bad idea! Elevators that do not have brakes on them, like I disk brake, need to use the motor to hold the mechanism in position. Thus, you will be stalling the motor with power on. If that is your use case, then I would suggest using a different motor, like a CIM, or Mini CIM.
Now as for the versa planetary, I would be a little bit careful with them. We have found that reductions greater than 5:1 can be a bit fragile. This is especially true of the 10:1. I have seen the 10:1 self destruct multiple times. Other than that, the versa planetary are fairly decent transmissions and can be used in an elevator as long as you design carefully.

Going off of what Billbo said, I recommend nothing greater than a 7:1 ratio. Even with that, I’d still look to go below that if possible, and you can always use sprockets or belt reductions to increase your reduction and get the gearing you need. If you were looking to climb with a 50:1 ratio with 2 775 pros, I’d do a two stage versaplanetary with 5:1 stages, then on the output shaft use an additional 2:1 reduction to get what you need to climb

This is false, you can stall the 775pro almost indefinitely if you gear it properly. There are several great videos out there on sizing your gear box for your desired motor and load as well as the JVN design calculator (granted if you don’t understand the math behind his calculator you should watch the videos 1st).

According to the VexPro motor tests, a 775 pro will die in seconds above 6 volts. You could gear the motor so that it only needs 6v, but then you’re nearly halving its potential. If you need a motor you can stall, I’d look somewhere other than a 775 pro unless you really need the weight or packaging.

Because Adam can speak better than I can… watch his video.

Yes, if you use them properly, e.g. if you use them on an SRX and enable current limiting. See the data on Vex’s testing of the motorand estimate your duty cycle to come up with your limit. 40A each would be quite conservative; you could stall the motor at 40+A for longer than a match and not burn it up (see the 6V locked rotor test).

Regardless of which conditions you can stall a 775 Pro in without blowing it up, I highly suggest finding ways to hold your elevators position without putting significant (or any) load on your motors. Using stalled motors to hold position is almost never a suggested design practice for numerous reasons. Look into the possibility of adding counter-balances to your elevator (springs, surgical tubing, etc), braking mechanisms, or gearing that resists back driving (worm gears, lead screws, etc). Also remember that the load placed on your elevator changes based upon whether or not you’re holding a game piece (or robot if you’re attempting to climb with said elevator).

Your not halving it’s potential. Stall torque goes from 0.71 to 0.47. A bit better than half. But speed drops by about half as well. Yielding around 1/4 the peak power.

The issue is that the 775 Pro is cooled by an internal fan that turns with the motor. When the motor does not turn then there is no cooling.
Gear it so that it turns at more than 1/2 it’s free speed while working and it will have enough cooling for nearly all situations.

You’re already doing something wrong if your elevator needs 1/2 stall current to maintain position. That’s pulling 65 amps to hold position! Even a CIM is going to get grumpy and lose torque and power potential if you’re constantly stalling it at the peak power torque. Not to mention the drain on your batteries.

Gear the elevator so that holding torque is at 1/4 stall torque or less and you should be okay. A single 775Pro geared like this should be able to move a 15kg elevator system at 1.7 m/s. (Discounting friction, efficiency losses, etc.) If you want to be more conservative, go with dual 775 Pros and gear for 1/6 stall torque and lift 15 kg at 2.6 m/s. That puts you on the 2V stall plot which looks pretty benign, especially since you’ll be moving up and down before and after the stall to get a little cooling done.

In summary, dual 775s into a VP probably weigh the same or less than a CIM, and you’ll get better performance out of them.

We built an elevator for recycle rush, which at times was holding 50+ pounds of game pieces. We didnt have much problem with station keeping. Once you gear down the motor it takes a lot of force to backdrag it.

This is one of those situations where having a bit of extra drag can help you.

The tiny gears in the Versaplantary don’t like hard forward backward motion with the dual high powered motors. I have seen a couple of seized transmissions in elevators. The 775 motors get smoking hot in a elevator mechanism constantly going up and down. Look for ways to provide additional air flow for the motors.

IMHO a VexPro single reduction clamping gearbox with a CIM and hexed gears a is simple, cheap, reliable method to make a transmission for a Power Cube elevator.

https://www.vexrobotics.com/217-4156.html

The hard forward and back is a good point. I will make sure and use our Talon SRX on the elevator and set a ramp rate to keep it from hurting itself.

We plan on using the same motors for the elevator as climbing. This creates a conflict with fast cube placement and strong climbing. But my math so far has said it’s not that bad of a compromise.

I also like the advice to limit to 1/3 stall torque if doing a holding operation.

I also plan on looking into supplemental cooling. Should be pretty easy to point a fan at it. I think there was something called an icebox a few years back.

1/4 stall torque. 1/6 would be better.

If you’re using this mechanism for climbing as well, remember you’ll need some sort of engagable ratchet to make sure you don’t backdrive, or you’re likely to burn out the motors while waiting for the game to end.

Are you thinking of one or two 775s for driving this system? Gearing to lift an entire robot with just one 775 is going to make you somewhat slow with the elevator. Let’s say you’re geared for 1/3 stall at 50kg. That means you’re climbing at .6 m/s. Then you’re geared for 1/10 stall for a 15 kg elevator, and you’re hitting about .8 m/s for the elevator. 2 seconds up to the scale just seems kinda painful to me. And another 2 seconds down. But it’s obviously your call. Putting it all in one system does simplify some things.

In 2015, we used a CIM with a 45:1 Versaplanetary to raise and lower our forklift.

The carriers inside the ring gear stages did nothing but break. We lost our forklift and had to replace gear stages at least three times. We advise our mechanical teams never to use those gearboxes again. But if you feel you must, make sure you have limit switches to stop the motor at the ends of travel.

We have given the 775 its fair share of abuse. We have found them to be very reliable even after stalling them. That said, we also tend to design our own gearing for high duty cycle applications. Below is a link to our video showing a complete stall test on our 775 drivetrain gearbox.

https://www.youtube.com/watch?v=qVFurJZcXpM&feature=em-subs_digest