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Originally Posted by HPA_Robotics_13
My AP physics teacher (along with my school's freshman physics teacher) just explained to me that gear reduction does not do anything to change the motors torque. Thus, after the 1:180 reduction of the gearbox and then another 1:10 to get it down to 12 rps, your system still has only .324 Nm at 6V.
We decided that a system using the fisher price motors should have a worm gear to give it the torque that the motor lacks to lift a 1.5 meter 100+ newton arm.
Am I just completely ignorant to have assumed that you would be driving the arm from a direct chain link to the FP gearbox shaft or is my teacher wrong?
I must be missing something becaue many people here, such as jgannon, have explicitly stated that the torque is greater after reduction.
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Your teacher is right when they way that adding gear reduction does nothing to change the motor's torque. A motor with .324 Nm of torque will always put out that much torque, AT THE MOTOR SHAFT.
However adding a gearbox to the system changes the system and therefore the output will change. Power = Torque x omega, where omega = rotational speed in radians/sec. Since the motor is putting out a given amount of power at a certain speed, and we can assume that power in to the gearbox is equal to power out (we'll neglect losses for the moment), then reducing the RPMs will result in a proportional increase in torque.
As I've said numerous times before, design your systems so the motor is operating at no more than 25% of stall torque and at least 75% of free speed and you should be fine. Electric motors like to run fast and this is especially true of the FP motors. By setting up you system this way, the motor actually starts producing more power when it bogs down and is more likely to pull through should it meet a higher than expected load.
Running the motor that fast also means that there is lots of gear reduction. The gear reduction helps reduce the holding torque required to help the arm hold position. If you go far enough with the reduction, then the inherent braking of the speed controller will be enough. If you can't go that far, then you are at least reducing the amount of voltage and current your motor requires to hold position. Either way you are helping your motor to "live long and prosper".
Keep 'em cool, run it fast!