What is causing the difference in mechanical resistance of pololu 3493 (195:1 gear box) and pololu 1592 with a 200:1 gear box transfered from pololu 1120. By mechanical resistance I mean the torque needed to move the axis with a wheel of the same size attached to both gear boxes’ shafts when there is no current flowing.
My lay gut instinct was that the more powerful motor would be tougher to turn.
Pololu 3493 has a stall torque of 10kg / cm while pololu 1120 has 1.5 o / inch at 6 volts which with a 200:1 gearbox attached gives 300 oz / inch = 21 kg / cm.
Also why does the lighter and cheaper plastic motor provide so much more output than a metal and more expensive motor? Why only some 25D motors can match the performance of pololu 1592 at 6V?
The mechanical resistance of an unpowered motor is primarily caused by its cogging torque. Cogging torque depends on the strength of the motor’s permanent magnets, and it is amplified by the gear ratio. So, we would generally expect a more powerful motor to have more powerful magnets and have greater cogging torque (if we are comparing motors with the same gear ratio.) However, we have not characterized the cogging toque for our gearmotors, and it is generally not great to back drive motors like these. Why are you interested in comparing that?
As we note on the #1592 product page, these motors are cheap, high performance motors and have notably low lifetimes, especially at higher voltages. At 6V, they draw more current than any of our 25D metal gearmotors which likely would allow them to produce more rotational power (assuming that the losses from the gearbox are approximately the same, which is a big assumption). However, they cannot operate that way for long; think of them like fireworks.
By the way, torque is a product of force and distance, not a force per distance, so it should be expressed as kg-cm or kg•cm and not kg/cm. (Remember, kg here refers to kilogram-force.)
Thanks for pointing out the torque denotation, will keep than in mind in future.
I am asking for a few reasons. Firstly there is pure interest, I was just surprised to see the smaller plastic engine delivering more torque than what something that looked like a more powerful metal unit. When ordering the #1592 I didn’t first calculate the torque it would deliver if paired with the 200:1 gearbox which I transferred from another motor with the same format. I just applied #1592 with a gearbox in the device I am working on and it was able to provide the torque I needed. Then I connected #3493 to the device and to my surprise it wasn’t torquey enough, so I did calculations for #1592 with the gearbox and realised that it can provide twice as much force as #3493. I did notice the description on the page about it being a “firework” motor but I was still surprised by its performance.
Then I noticed that it is much easier to manually turn #1592. This is of benefit as in my device I need to take into account a scenario when the motor is off and needs to be turned manually to the starting position using a lever (which in return the motor is turning when its on and working). Given that I work with 3d printed parts it has the additional benefit that they don’t snap when the lever is used to turn back the motor. On the #3493 the 3d printed parts crack especially that I slightly increase it’s gearing ratio within the device to help bridge the torque deficiency. I guess I will need to use higher filament density to the issue and consider the 25D form motors.
It sounds like you have some external 3D printed gearing or other attachments, so if possible I suggest adding some type of external clutch mechanism for you to turn your mechanism back without back driving the motor.
Thanks Patrick, that’s probably the best solution