# URDF Gazebo mechanicalReduction

In lesson 3, section 2 step 1 you say “As for the reduction, it’s self-explanatory.”
Well, actually it’s not. I can think of several possible meanings. An actual explanation, even a brief one, would be helpful.
/K

Ha ha ha, Keith you are right! It is not self-explanatory!

The reduction tag is used to model a reduction in the joint. What is a reduction? You can find a full definition here. But basically it is a mechanical way to reduce (or increase) the rotation of a joint from the original rotation of the motor. Since the motors rotate with a very small cylinder, that makes that the speed of the motor is too fast for most applications. Hence a reduction is placed between the motor and the actual join to reduce the amount of movement to a proper value for the joint.

For instance, it may happen that the motor rotates at a speed of 3 complete turns per second. But you may need that the joint that the motor is attached to rotates at 0.10 turns per second. Hence you would put a reduction there of 3/0.10 = 30.

In most cases in Gazebo simulation that value is set to 1 because you don’t need to simulate a reduction (you usually don’t nd such a complex setup for your simulation) since the full system is already simulated by the controller.

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Thanks very much. I’m very familiar with gear reduction - just wasn’t clear that that was what was being referred to. Your explanation is extremely helpful. Thanks,
/K