Higher gear ratio but less torque?

[kolleamm]

TL;DR Summary

Comparing two motors

There's a micro motor with a gear ratio of 298:1 and there's a servo with a ratio of 188:1.
The first runs at 12v and the other at 7.4v.
How is it that the motor with the bigger gear ratio has only a 70 oz-in torque, while the second motor has a 560 oz-in torque?

First motor :
https://www.sparkfun.com/products/12285
Second:
https://www.pololu.com/product/2375/resources

 

[andrewkirk]

I must be missing something here. Less torque is what we expect when we increase the gear ratio. Think of what happens on a bicycle when you change to a lower gear. The torque delivered to the rear wheel for a given force applied to the pedals will increase. That's why track cyclists start so slowly in the 4000m pursuit or 1000m time trial - because they only have one gear and need it to be a big one in order to be able to reach speeds of 50-70kph, so the torque they can apply when starting from rest is very low.

 

[Baluncore]

The product of torque and RPM is power.
Maximum torque occurs at zero RPM.
Maximum RPM occurs at zero torque.
The information is not complete because the figures given are never for the same situation.
Neither specifies the motor current, the winding resistance, or the strength of the magnetic fields.

The first is specified as 70 oz-in, and 90 RPM. But not at the same time?
The second is specified as 560 oz-in, and 0.18 sec/60° = 1 RPM.

 

[jrmichler]

It takes some digging into the datasheets, then some calculations:

Motor 1 Motor 2
90 55.6 Output RPM at zero load
298 188 Gear ratio
26,800 10,400 Motor RPM

495 3920 Output torque - N-mm
1.66 20.8 Motor torque - N-mm

12 7.4 Volts
1.6 9.0 Amps at stall
19.2 66.6 Watts at stall
17 165 Weight - grams

Gearmotor 2 has a slower speed motor with much higher torque. If you have the full design information of the motor, the torque can be calculated from the stator pole area, the magnetic flux density, and the armature diameter. More pole area requires more back iron, more copper, and a larger armature, so a heavier motor of similar design will have more torque.

 

[kolleamm]

Ah ok this makes more sense, so I'm guessing the key thing was that a higher gear ratio means it has less torque, and that the size of the motor used also affects the torque.
I remember I did disassemble one of those servos and it did have a larger motor than the micromotor with the 90 RPM.

 

[kolleamm]

Looking at these linear actuators however it would seem a higher gear ratio results in more torque?
If you click on Technical Specs you can see the force increases with the higher ratios.

https://www.actuonix.com/L12-R-Linear-Servo-For-Radio-Control-p/l12-r.htm

 

[Baluncore]

... you can see the force increases with the higher ratios

As the force increases, the speed reduces, by the same ratio.
If they all use the same motor, then that is exactly what you should expect.