# Motor/Gearbox Calculation

## Main Question or Discussion Point

Hey,

I have an engineering project where I need to be a able to lift a 1kg load with a balsa wood crane, but have it fail at 1.5kg so as to be not over engineered.

Various components need to fail (eg truss, which is easy) but I am trying to work out how to get the electro-mechanical component to fail.

One of my ideas was to utilise the friction in the gearbox to hold the weight of the 1kg load when the motor is not running, but when there is a 1.5kg load and the motor stops running, the friction in the gears will not be strong enough and the gears will slip into reverse and drop the load.

I have gauged that this is possible, but am getting a bit confused with Holding/Stall torque and if I should be calculating this for the motor or gearbox or both.

Any guidance?

Cheers! Sean.

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Baluncore
2019 Award
A gearbox is a mechanical torque transformer.
If you use a DC motor then the torque on the stalled output shaft will be proportional to the motor current.

Sorry could you explain further. Essentially (I Think) I want the friction in the gearbox to resist a certain load when the motor is not running. I am trying to work out how to calculate this?

Baluncore