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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 repeatedly 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?

Thanks!

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A conventional car work with high energy density fuel. But the drive works efficient in some revolutions only. That's way there is a gear box.

Working with electric drives is not new. It was invented at the same time (to be precise a little before) the Benz developed his car. But there are weak points too. Accumulators are heavy and the energy density is lower then of fuel. one has to change the accu often or it is necessary to rebuild the main road infrastructure and to build contactless recharge lines under the roads.

With a robot arm it is easier. The electric infrastructure does exist and a high speed drives and some elctronic, soften together with a high torque gear box (harmonic drive) allows to use electricity.

The combination of a small, but high dynamic electric device with a high step-up gear box is the effectivest solution. Direct drives are more heavy and less dynamic.

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