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To change speeds on many drill presses and lathes, the user needs to turn off the machine, and manually move the belts between a variety of pulleys.

On a machine with variable speed control, the user can simply adjust a lever to change the speed. On at least certain models, the speed can only be changed while the machine is running.

How does this mechanism work? Why does it require the machine to be running?

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The machines which need to be running in order to adjust the variable speed often use a Reeves drive, which consists of two variable-diameter pulleys linked via a V-belt. The two sheaves of each pulley are adjusted closer or further from each other to increase or decrease the diameter of the pulley, respectively. In order to preserve the proper belt tension, one pulley's diameter must increase while the other pulley's diameter decreases, and the machine must be running so the belt can track these adjustments.

Animation illustrating the concept behind a Reeves drive

(image source)

Some other types of machines with variable speed control use electronic speed controls to directly adjust the speed of the motor.

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Power tools with infinitely variable speed work by changing the power input to the motor; originally this was done using a rheostat, these days the control will often be electronic.

Why does it require the machine to be running?

Not every power tool does require the machine to be running. Some have dial-set power controls, which can be adjusted to the required speed before the machine is turned on, or during operation.

Most modern power drills, both corded and uncorded, have speed control built into the trigger so for those the speed is directly tied to trigger pressure and therefore power must be flowing.

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