Comment Re:It's not "dragging" the sixth wheel (Score 1) 272
I'm not aware of any type of motor that simply locks up when no power is applied. However, there are multiple scenarios which could cause the wheel to lock, or at least to become so difficult to turn that it drags rather than freewheels.
1. Bearing failure. Considering the atmosphere on Mars, cold and dusty, this is a distinct possibility. However, if this were the case, the motor should be drawing a lot of current, and the motor is reported to be drawing none. Perhaps there's a failsafe circuit that shuts the motor off if it draws excess current, though.
2. Gears. If the motor is geared down by a significant amount before driving the wheel, it would require more force to turn the wheel manually. If there was a worm gear in the drive train, it would not be possible to turn the wheel manually, since worm gears only work one way.
3. Short circuit. If you short-circuit the terminals of most motors, they become more difficult to turn by hand. The reason for this is that in this situation, the motor is effectively a generator attempting to provide power into a short circuit. A short circuit can sink a lot of energy, and generators are more difficult to turn the more energy they are asked to supply. Again this would mean that the motor should be drawing a great deal of current, but there may be a failsafe circuit which shuts it off.
4. Internal Jam. Certain types of permanent magnet motors have the armature surrounded by a relatively brittle permanent magnet stator. Mechanical shocks to the motor can cause pieces of the stator to break off and become wedged between the stator and the rotor, causing the motor to jam. This is most common in cheap motors that are used in toys and cassette recorders. I should hope that the rovers use something of higher quality!
1. Bearing failure. Considering the atmosphere on Mars, cold and dusty, this is a distinct possibility. However, if this were the case, the motor should be drawing a lot of current, and the motor is reported to be drawing none. Perhaps there's a failsafe circuit that shuts the motor off if it draws excess current, though.
2. Gears. If the motor is geared down by a significant amount before driving the wheel, it would require more force to turn the wheel manually. If there was a worm gear in the drive train, it would not be possible to turn the wheel manually, since worm gears only work one way.
3. Short circuit. If you short-circuit the terminals of most motors, they become more difficult to turn by hand. The reason for this is that in this situation, the motor is effectively a generator attempting to provide power into a short circuit. A short circuit can sink a lot of energy, and generators are more difficult to turn the more energy they are asked to supply. Again this would mean that the motor should be drawing a great deal of current, but there may be a failsafe circuit which shuts it off.
4. Internal Jam. Certain types of permanent magnet motors have the armature surrounded by a relatively brittle permanent magnet stator. Mechanical shocks to the motor can cause pieces of the stator to break off and become wedged between the stator and the rotor, causing the motor to jam. This is most common in cheap motors that are used in toys and cassette recorders. I should hope that the rovers use something of higher quality!
