What gear ratio is needed to lift an object

So this morning I decided to do some physics reading on VEX mechanisms and found this old beauty of a VEX EDR learning curriculum: https://curriculum.vexrobotics.com/curriculum/mechanical-power-transmission/applying-gear-ratios-to-dc-motor-systems.html

So it seems that this curriculum uses DC motors instead of the speed, torque, and regular motors I use in my school robotics team.
The link I have provided leads to an article that explains how to calculate a nescessary Gear reduction for a mechanism that can lift a 30-newton object.

I know V5 and DC motors are vastly different, but out of curiosity, how do I calculate the nescessary gear reduction/ratio if I am lifting an object that is of a certain weight(in kg), without going beyond the motor’s limit?

Additionally, If the article states that the mechanism must lift a “30 newton” object, what are “newtons” and is this something that weight is measured in?

All help is appreciated, and also I haven’t learnt much physics yet in school. Thanks Y’all.

Newtons is a unit of weight. There should be a formula to convert from pounds to newtons.

Also how many balls are you trying to pick up?

Newtons is a unit of force, its unit being \frac{ kg\space m}{s^2}

Yes, weight is a force.

The maximum torque of a V5 motor with a 100RPM cartridge is 2.1 Nm (18.59 lb in) according to the information on VEX’s website.

Torque is the product of perpendicular force * radius, so we need to know how far from the center of the motor your object is. If we assume your object is 18" away (.4572m), then we get that the motor itself is exerting about 4.59N (1.03lbs) of force on any object with its center of mass 18" away.

Because of the constraint of using VEX parts, the smallest gear ratio you would need to be able to lift a 30N object would be 7:1, which would give you an output force of about 32.13N (7.22 lbs).

This does not account for the weight of the lift and intake, the force provided by the rubber bands you use on it, or friction, however. For lifts, I usually opt for a 5:1 or 7:1 external ratio with either a 100RPM or 200RPM cartridge, depending on how many motors the lift uses and how much weight it has to lift up (since 30N is a lot for VEX, I’d spring for a higher torque output to start). You usually want the output force of the lift to be much higher than the weight you have to lift to minimize the chances of your motor(s) burning out and to make it easier for the lift to hold itself up with the object.

Edit: @technik3k has brought it to my attention that the 2.1Nm is the torque output of a motor with the 100RPM cartridge, not the raw output of the motor independent of the cartridge. I corrected the numbers accordingly.


Another thing to point out, the rubber bands on a lift should ideally cancel out the weight of the lift. Note, this is only for the start of the movement, but then you have to factor in momentum throughout the movement among other various factors. The rubber bands won’t cancel out the entire weight of the lift throughout the entire movement because of a change in geometry of the rubberbands. To put it short, it is difficult to mathematically model a lift. Especially more complex lifts, not just a simple arm connected to a gear.

Overcompensating is generally a good strategy, and tune as the season goes. It is very hard to hit an ideal ratio on the first build. There are so many unpredicatable (or unforeseen) factors.


Correct me if I’m wrong, but isnt the v5 motor just a DC motor inside a casing?

yes. theres gears in there somewhere too. I opened one up once

The classification “DC Motor” encompasses many more specific types of motors. The VEX V5 motors are a type of DC motor, a brushed DC motor, with a built-in gear box. (The 600 RPM, 200RPM, or 100RPM planetary gearbox you choose for your motor). Generally a DC motor is anything you can drive with direct current. This includes the single-phase brushed DC motors such as in VEX 393 motors or V5 motors, as well as other types such as multiple-phase brushless motors you may find in drones and RC cars.


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