# Calculate torque for gearbox

1. Apr 14, 2015

### fishinhrox12

I am designing a gearbox to take a human input with a crank 12 in. long attached to a shaft that drives the first gear which goes through a gearbox and eventually turns a motor shaft in case of electrical failure. This is for a class concept design. The motor has 5ft-lb of torsional inertia and I need to calculate the operator force.
the gear box is set up with 4 spur gears, A,B,C, and D.
A is a 30t gear, 3in diameter
B is a gear with a big tooth and smaller gear attached to the top of it so that they both spin in the same direction at the same speed(see attached picture).
B will have a 60t large gear attached to a 10t small gear with respective diameters, 6 and 1 inches.
C is the same idea, the large is a 60t 6in. gear and the smaller is 30t, 3in.
D will be a single spur gear with 20t, 2in.

How do I calculate the torque or force that the operator must apply to the input shaft(the 30t gear, top left)

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• ###### Screen Shot 2015-04-14 at 10.01.08 AM.png
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2. Apr 14, 2015

### jack action

3. Apr 14, 2015

### OldEngr63

What is 5ft-lb of torsional inertia about? The units do not make sense. Mass moment of inertia should be in slug-ft^2 if you are working in USC units with feet for length.