Help: Torque to Linear force conversion

[apalrd]

You can use JVN's mechanical design calculator (posted earlier) to solve this.

Use the Linear Mechanism sheet. Set motor specs, drum diameter, etc. as appropriate and set load to 1/4 of the bungee force (since you have 4:1 mechanical advantage). Set Travel Distance to the length of pull in, which you can figure out from your design.

We used a gearbox design similar to that which you speak of (pulling the gears apart) in 2010 on our kicker. We used a window motor which did not backdrive (which will not be nearly enough power this year), and opened the gearbox to kick. We only engaged the gearbox again after the kicker hit the hard stop and was not moving. We needed quite a large piston to keep it together. We ha no reliability issues with the gearbox at all, but we did kill 1 window motor. If I were to do it again I would probably put a ratchet somewhere in the high speed end of the gearing, on the motor side of the opening gearbox, and use a larger motor (probably a miniCIM or CIM for this kind of load and pull in time).

[Ginger Power]

Originally Posted by apalrd

You can use JVN's mechanical design calculator (posted earlier) to solve this.

Use the Linear Mechanism sheet. Set motor specs, drum diameter, etc. as appropriate and set load to 1/4 of the bungee force (since you have 4:1 mechanical advantage). Set Travel Distance to the length of pull in, which you can figure out from your design.

Thanks!! I never thought to do that (In spite of the fact that it was very clearly pointed out to me earlier).

[Nathan Streeter]

While it will be helpful to use the JVN Mechanical Design Calculator, there are a couple important things to understand about designing a motor setup (# motors, motor selection, gearing, etc.)...

First, a few things to know about motors:
- @ Stall: no speed, max torque, no power, max current draw
- @ Max Power: half speed, half torque, max power, ~half current draw
- @ Free Speed: max speed, no torque, no power, min current draw

You never want to design a system to operate at motors' max torque, because this is only provided at stall (torque then decreases linearly until 0 @ free speed)... For a "heavy-lifting" operation, I'd recommend designing for a point a little faster than max power... about 55-60% of free speed (which is 45-40% max torque). Designing for this point means that unexpected losses will push the motor closer to max power rather than further.

Also, always take in to consideration some losses... probably about 10-20% loss. Remember to account for this in both speed and torque calculations.

You can also shrink the pulley radius to increase your mechanical advantage... I'd personally recommend a smaller pulley than 2" diam for this reason.

Also, are you sure you really need ~500 pounds of force in your surgical tubing?! That is a phenomenal force! How are you pulling that back currently on your prototype? Perhaps try using more travel in your surgical tubing to enable as much energy storage but with a lower max force... (U = 1/2 k * x^2).

I'm low on time right now, but perhaps I'll post again later with a worked out example...