Firstly, this is not a homework etc question, but am trying to make a proof of concept for something. I have a mechanical force acting on a shaft (threaded bar) moving it in the direction indicated in the pic below:
Movement is roughly 10mm and force roughly 50kg/mm or 0.48 Nm. I an using this to drive a motor via a rudimentary crank. The movement is going from right to left in the pic above and is a one time thing, so results in about 10 to 20 degrees of rotation.
The first motors I tried were both brushed DC geared motors, 30:1 and 120:1, so this converted my small movements into a few turns of the actual motor. I also tried a stepper motor, I posted the results here.
My problem is, I'm not getting nearly the amount of power from the motor as I would like to see. I understand the power in watts is a factor of the time taken for the 10mm movement, and this is, in my setup, affected by the load, but by looking at the outputs on an oscilloscope I can view the duration of the 10mm movement.
The best result I got was 0.6W out for a theoretical mechanical input of nearly 5w (operation in roughly 100ms) which is ~10% efficiency. I think I'm loosing a lot in the gearbox, firstly having to accelerate it and the motor from a standstill (it's very hard to turn [get started] by hand with no load), then the frictional losses, and I think the fact I'm putting power in the "wrong way", into the output shaft is not helping.
Also, the speed of the motor is low, well below where it is designed to run as a motor.
The stepper motor seems to max out at ~0.6W even when driven by a mains drill @2,700 RPM - I salvaged it from a printer so no specs for it.
So my question is, what, if anything, might yield more power in my setup, where I can only turn the motor relatively slowly and for just a few turns at best. What will generate good power (in the 2-8 W range, so very small) at low speeds and has low frictional/inertial losses?
Maybe another stepper motor is worth a try? Or, I have not yet looked at BLDC. Would it be possible to make a linear generator of sorts?
UPDATE RE COMMENTS
3 March - Just to clarify some things from the comments so far (the first 4)
I think I confused things by saying I drove the stepper with a drill. This was just to test the stepper in isolation - I am not trying to generate electricity by using a motor to turn a generator.
Yes, there is a force acting on the rod. I should have used the word work in my figures. If you allow me to be a little lax with terms for a second, there is 5gk pulling on the rod and it moved 10 mm which is 50kg/mm of work, or 0.05kg/m *9.8 =0.49Nm of work. I measured this with a load cell.
If I can summarise my question, the DC motors I have are designed to run at roughly 4,500 rpm, even with the gearbox I'm not getting anywhere near that speed, and I believe the gearbox is causing huge losses. I have no specs on the stepper but it seems to top out at 0.6W regardless of how fast I run it.
So, what might be my best bet to generate 2W to 8W while running at a low speed (< 1,000 RPM)