# Lift problem (gear ratio)

So right now one of our schools robots runs off of 2 motors on one axel on each side of the lift. The motors run a metal 12 tooth gear rotating the arm that is on a 60 Tooth high strength gear. From our past failures we know not to put 2 motors on 1 axel because they aren’t the same speed and will hurt the lift. They want to know if they add another metal 12 tooth gear to the bottom of the first 12 tooth gear and put a motor on each axel with it give them more power to lift there robot or not? We say yes it will but what do you guys think? We told them it would be smarter to put a 60 tooth gear on all 4 bars of the lift and have one motor per arm but they don’t think they have enough time! Thanks for all the help!

Putting two motors on the same axle won’t “hurt” the motors. Even if they spin at slightly different rpm it will only combine their torque and raise the rpm under load. The only downside to putting the motors like that is it makes it more difficult to replace motors or work on the gearing.

Adding another driving 12 tooth gear below the first wouldn’t hurt. I agree with aaronlucas, though. Two motors on one axle should not have any detrimental effect, unless one of them is VERY different from the other. The old 3-wire motors from 2009 might not have been able to handle it, but since Vex changed the internal gears to metal instead of plastic, they’re much more robust now.

Some basic math might help with your lift. Basic physics tells us that Torque=ForceDisplacement (it’s actually more complicated than this, but for a basic ballpark proof of concept, ForceDisplacement works).

So, let’s consider the moment where your arm stalls out. Theoretically, this will occur where the arm is straight out, parallel to the ground (this is because the force due to weight acts tangentially to the circle created by the arm’s motion. Basically, this is the point where it’s hardest for the motors to lift, and thus where the torque from the motors equals the torque from the arm’s weight).

At this point, [NumberOfMotorsStandardMotorTorqueGearRatio = ArmWeight*ArmLength]. Here, the gear ratio just multiplies the torque provided by the motors by an integer. When using this equation consider the ratio of ROTATIONS, not the ratio of TEETH. A 12 tooth gear on a motor attached to a 60 tooth driven gear IS NOT a 1:5 ratio. It’s a 5:1 ratio for the purposes of this equation.

So, we arrange the equation as [ArmWeight=(NumberOfMotorsStandardMotorTorqueGearRatio/ArmLength)]

Assuming you have four 393 motors, and they perform to specification (outputting 14.76 inch-pounds), and you have a 17.5 inch arm, the equation would be [ArmWeight = 414.765/17.5 = 16.869 lbs. or about 7.65 kg].

This should give you a ballpark number, but know that motors don’t always perform to specifications and this doesn’t account for non-conservative forces (forces that cause loss of energy) like friction in the gears or joints. It also considers the weight of the arm itself to be focused completely at a point at the arm’s end, which is not true. Hope this helps, though. Good luck with your lift!

EDIT: This doesn’t work for scissor lifts, and it becomes increasingly more inaccurate with the number of bars used in an n-bar lift. For example, this is going to be more accurate for a 4-bar vs. an 8-bar.

First off running two motors on the same axle is not bad, in fact it increases efficiency by reducing the number of moving parts.

Secondly, you’re lift will not have enough power even with 6 motors 12 tooth to 60 tooth because that is a 1:5 ratio. 6 motor 1:7 is as low as you can go with gear ratios on skyrise scissor lifts.

Third, meshing 12 tooth gears to other 12 tooth gears is bad and should always be avoided because they don’t mesh very well and thus adds a lot of friction.

I recommend replacing the 60 tooth gear with a 84 tooth gear and instead of adding another 12 tooth to the bottom of the first 12 tooth, and it to the other side of the 84 tooth gear.

They were going to do that but we don’t own any high stregth 84 tooth gear at our school, we only have plain 84 gears, if the swith to those would they work or should they just so a compound gear system like 1:15 by putting the motors on 12t spinning at 36t with a 12t on the same axel spinning the 60t or is that to much torch and not enough speed for a lift? Thanks for all the help!

If you have enough, I would double up the 84 tooth gears and cut/grind off the little nub to make them as wide as a high strength gear. I would avoid compound gearing.