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Torque & Stall Check

Put a load on the drive and see where it settles on the motor's torque curve: how much speed you keep, and how close you are to stalling.

Inputs

Mecanum and flex wheels share a diameter with the plain ones, so pick by name rather than by size. Anything not listed can be typed in.

lbf

What is fighting you: another robot pushing back, a wall, a slope. The Pushing Force calculator gives you an upper bound.

Results

Of stall torque

—%

Does it stall?

—

Speed under this load

—ft/s

Free speed for comparison

—ft/s

Torque asked of each motor

—N·m

Force at stall

—lbf

Torque asked of each motor, against its stall—

At stall the motor holds and does not turn. The first mark is where speed starts falling away sharply; past the second it is heating up to stand still.

Lean on the robot

Lean on the robotmost usefulheatspeedpushing force
30% of stall

The straight line a motor follows: full speed with nothing to push, slower as the load grows, stopped dead at stall. The dot is your drive at this load. The shaded band in the middle is where a motor does its most useful work; to the right of it the motors mostly make heat, and past the end of the line they stop.

When you would use this

  1. Our lift gets halfway up and just stops.

    Put the weight it is holding and your gearing in. If the fraction of stall comes out over about 80%, the motors are not failing, they are being asked for more than they have. Gear down and try again before replacing anything.

  2. We want to push the other robot off the platform.

    Enter the force you want at the wheels. The tool tells you what fraction of stall that needs and what speed you keep at that load. Anything near 100% means you win the shove but cannot move while doing it.

  3. Our motors keep going hot and cutting out mid-match.

    Overheating is almost always a gearing problem rather than a broken motor. Work out the fraction of stall you are sitting at during normal driving. Anything above roughly 60% held for long stretches will cook.

  4. We are deciding between a 200 and a 600 RPM cartridge.

    Run it twice, once with each. The faster cartridge gives less torque, so the same load lands at a higher fraction of stall. If that fraction goes past about 80% the fast cartridge is not usable however much you want the speed.

How this is calculated

A motor trades speed for torque continuously, not in steps. At no load it spins at free speed and produces no torque; held still it produces stall torque and no power. Between those it falls along a line:

torque = stall torque × (1 − speed ÷ free speed)

Rearranged, that says the speed you keep is whatever fraction ofstall torque you are not using:

speed = free speed × (1 − torque used ÷ stall torque)

Ask for 25% of stall torque and you keep 75% of your speed. Ask for 90% and you crawl. Ask for more than 100% and nothing moves at all.

Getting from force to torque

torque per motor = force × wheel radius ÷ (motors × gear ratio)

Every term is a lever you can pull. Gearing down divides the torque each motor has to find, which is exactly why a geared-down drive holds its speed better under load, even though its top speed is lower.

Where power actually peaks

Power is torque times speed, so it is zero at both ends: no torque at free speed, no motion at stall. It peaks in the middle, at half of stall torque and half of free speed.

That is a useful design target. A drive spending most of its match near half of stall torque is extracting close to everything the motors have. A drive that is barely loaded is carrying motors it is not using, and one that lives near stall is converting the battery into heat.

Sources & assumptions

Stall torque is published for the 100 RPM cartridge (2.1 N·m) and derived for the 200 and 600 RPM cartridges by scaling inversely with speed. Free speeds are VEX published figures.

The linear torque-speed relationship is the standard model for a brushed DC motor and is a good approximation in the middle of the range. It ignores VEXos current limiting, thermal derating, gearbox friction and battery sag, all of which make real performance worse than shown, especially under heavy load.

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                                Gear Ratio Calculatorfills in your gear sizes herePushing Force & Traction Limitfills in your pushing force hereGear Ratio Solverfills in your cartridge hereMechanical AdvantageChain & Sprocket Ratio Calculator

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                                Torque & Stall Check

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                                Shaft & Structure Load Checkcheck the shaft can survive the torque you are asking forbrings your gear sizes with itMotor Current & Battery Budgetsee how much battery motors working that hard will useuses your motor count againMotor Diagnostics Dashboardcheck your motor logs for overheating from that much strainMechanical Advantagecarry the same motors and gears on, then pick a wheel as the outputbrings your gear sizes with it

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