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I have heard a lot of claims that manually turning an NXT motor by hand can potentially damage it. I was wondering whether this was at least partially true, and whether there is any evidence to confirm or refute this idea.

I know that some projects (e.g. etch-a-sketch) use the built-in rotation sensor to measure how much the motor has turned, so I was thinking that perhaps whether the motor is idle or set on break is an important distinction, or perhaps there is even a special 'rotation sensor' mode that needs to be switched on in order to prevent damage.

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In general, no, it will not hurt the motor directly, BUT, the motor is geared down considerably. This means that when you turn the exterior part that you can physically touch by 1/4 turn - the motor might have actually completed 5 full rotations. The problem can happen when you turn the servo too fast, it may cause gear breakage simply due to excess speed internally.

It does not hurt the motor, but it may break a gear. Simple solution is to just not turn it fast.

If the motor is set on 'brake', it will apply opposite power to counter your turns, but you're not going to really hurt anything by over powering it, slowly! It is easier to turn if the motor is not in brake mode.

For example, in RobotC, this code releases the 'brake'

// release motors
    nSyncedMotors = synchNone;
    bFloatDuringInactiveMotorPWM = true;
    nMotorPIDSpeedCtrl[LEFT] = mtrNoReg;
    nMotorPIDSpeedCtrl[RIGHT] = mtrNoReg;
    motor[LEFT] = 0;
    motor[RIGHT] = 0;
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    $\begingroup$ I'm sure @Spiked3 knows this, but for completeness... one small caveat to "you're not going to really hurt anything by over powering it." When the motor is actively resisting you are drawing large currents through the motor wiring and power path components. This causes heating (power dissipation) and can often exceed specification for these components since the motor isn't designed to drive into a mechanical fault. If at all possible, you should always avoid manual cranking of an energized (braked) motor. $\endgroup$
    – DrFriedParts
    Dec 7, 2012 at 20:40
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    $\begingroup$ @DrFriedParts you are right to some extent, but I can say I've never seen any permanent damage done - and remember an experiment distinctly that pushed it a few times, for several days :) What happens for example is a motor tries to hold '5' - and you push it, it eventually gives up and tries to hold '6'. I'm not sure exactly how this happens in the firmware, but it is what I observed. $\endgroup$
    – Spiked3
    Dec 7, 2012 at 22:12

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