Is resisting DC motor bad for it?

If I keep the shaft of a DC motor in place while it is running does this harm it in any way? Is it bad for it at all?

Answer

In general, yes, because the stall current for a motor can greatly exceed the rated current, and exceed the continuous current rating of the motor's windings, brushes and commutator, and burn out the motor.

In some motors it won't instantly kill the motor but heat it - the motor will survive short overloads but can't dissipate the heat from a continuous overload. If you've been making heavy cuts with a saw or a drill it's often good practice to run the motor unloaded for a minute afterwards so the built-in fan blows cool air through it.

This excessive current under heavy load is a necessary consequence of keeping the winding resistance down to keep the motor's efficiency high under normal (high speed) operation.

Cheap motors tend to have incomplete data but for a reasonably complete specification see this datasheet and note (the first column)

"Maximum continuous current" 6A. (The nominal rating for 100% duty cycle)
"Starting current" 105A. (This is also the stall current).

Typical motors like the Mabuchi RS550 are designed for lower efficiency so the stall current may only be 6-10x the rated current (here 83A vs 10.8A at max efficiency, max continuous current is not specified).

In neither case should you mistake the stall current for the rated current : the RS550 surely cannot survive 83A at 9.6V (about 800W) for very long!

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However, in your case (a motorized fader) the motor is small, low powered, probably has quite a high winding resistance and low efficiency, and may be able to survive a fairly prolonged stall. This is a deliberate design choice to limit its stall torque rather than injure a sound engineer's fingers! Alternatively its drive current may be deliberately limited. Over and above that, its controller apparently detects its drive current to detect stall or manual override, and cuts off the power before any damage can be done. It is completely safe to stall this motor by hand.