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From my understanding, there are two uses of a gearing system: to change the speed of output rotation (trading it with torque), and to change the axis of rotation. Now, in a car, for example, it is necessary to have multiple available gear ratios, to allow for high torque and high acceleration when the car begins to move from stationary, and also high speed when the car is already on the move.

However, I know that some systems still have gears when they only used a fixed gear ratio. For example, in my lab, we work with a robot arm, and my colleagues often talk about the gearing system in the arm. But the arm does not repeatedly change its gear ratio like in a car. So what are these gears actually doing?

If they are to change the speed / torque of the output of the motor, and this is a fixed gear ratio, then why was the arm not designed with a different motor entirely -- one which provides the desired speed / torque properties? My intuition is perhaps that it is easier to mass produce motors that have high speed and low torque, and so it is more economical to buy one of these generic motors and attach it to a gearing system, rather than design a bespoke motor that has very high torque at its output by default....is this correct?

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    $\begingroup$ This question deserves a real answer, but in broad strokes your intuition is right. Most motors spin very fast and have relatively little torque, whereas many applications like your robot arm want high torque at low speed. There are lots of other design trade-offs too, but that's the biggest reason. $\endgroup$
    – Ethan48
    Aug 21, 2015 at 16:26
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    $\begingroup$ If your application requires specific values of speed and/or torque, then it might be difficult to find a motor that meets these requirements exactly. In that case the only work-around is to buy a motor that's close, and make fine adjustments using gears. $\endgroup$
    – Carlton
    Aug 21, 2015 at 17:20
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    $\begingroup$ "My intuition is perhaps that it is easier to mass produce motors that have high speed and low torque" - it is much easier to design a small motor to have high speed and low torque, rather than low speed and high torque. High torque needs large electrical currents, and/or many turns of wire in the motor coils. Also, if you use a worm gear the output shaft will remain in a fixed position (because of friction between the gears) even when no power is applied to the motor. That may or may not be relevant for your robot arm of course. See en.wikipedia.org/wiki/Worm_drive $\endgroup$
    – alephzero
    Aug 21, 2015 at 23:12

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There are various reasons why one might chose to use a geared motor with a fixed gear ratio for a robot arm over an ungeared motor:

  • As Carlton commented on your question, it may be difficult to find an ungeared motor that meets the specific speed/torque requirements for a given application and a motor that can provide enough torque ungeared may be too large/massive for the robot arm (and may be too power hungry as well)
  • From a controls perspective, assuming the motor is small and the gear ratio is used to increase torque/reduce speed, a large change in the angle of the motor would result in a small change in the angle of the robot arm; this would mean that you might be able to more accurately control the position of the arm by measuring and commanding the position of the motor (bigger angle = easier to measure)
  • It is also entirely possible, that they happened to have a motor of a certain size available and decided to use it instead of procuring a different, perhaps more expensive, motor for the arm

I'm sure there are many other factors to consider, but, at a high level, this should give you an idea as to why the robot arm may not have been designed to use an ungeared motor.

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Electric motors typically only operate well at high speeds, and compared to something like an internal combustion engine, relatively low torque. Fixed gear ratios are used to cope with this inherent limitation.

Additionally, some designs will use a gear train so that they can design a point of failure in their system that is not the motor. If the system, robot arm in your example, uses a very expensive motor, you might want a cheaper component to fail and protect your motor in the event of an overload.

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    $\begingroup$ An example, is that often in old water pumps, wooden gears were used, so if something jammed then the teeth would snap off. This means nothing else underground (i.e. expensive) is destroyed $\endgroup$
    – George
    Aug 21, 2015 at 18:50
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A given mass of copper and iron can only produce so much torque : increasing torque further would require more current which means (a) the increased magnetic flux saturates the iron and (b) the I^2R losses in the copper overheat the motor. So more torque requires more iron and copper; i.e. a bigger, heavier, more expensive motor.

Furthermore, because increased torque comes at the expense of increased I^2R losses, electric motors are most efficient when used at light torque loadings.

There is no equivalent fundamental limitation on speed. Brushed motors have limits from brush wear, friction, heating, and erosion from sparking but newer motors (BLDC etc) have no such considerations up to the speed limits of their bearings, and material strength limits under centrifugal force.

So to use a motor efficiently you will run it fast producing relatively little torque. Then it has torque in reserve when you need it for short term purposes (acceleration etc).

So, if you need high torque and low speed, gearing a high speed motor will usually be more efficient, lower cost, lighter, using less material and thus probably cheaper than a direct drive motor used at low speed.

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Gearing and belting etc. is used to change the speed and more importantly the torque of the input device (motor). Rotary loads are accelerated by torque to a commanded speed. You reduce the inertia of a moving load by deploying a gearbox or other speed reduction device. If for example you have a 5:1 reduction ratio, the motor torque is increased by a factor of 5 and the inertia is (1/5)**2 the non reduced value.

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