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Mechanism that only rotates when driven from one side

  1. May 19, 2015 #1
    I have a geared DC motor taken from a hand drill. When I manually turn the internal motor, the drill bit rotates. This can be done in both directions. However If I try and turn the drill bit it does not rotate.

    What is the mechanism called that achieves this?

    My friend says it is a sprag clutch. But from what I have read a sprag clutch is used to only allow either clockwise or anticlockwise rotation, which is not what's happening here.

    But is he correct?
  2. jcsd
  3. May 19, 2015 #2
    Try applying more force to the drill bit.....maybe with a pipe wrench?
  4. May 19, 2015 #3


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    What about when you turn the chuck by hand?

  5. May 19, 2015 #4


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    It is often difficult to reverse the direction of energy flow through a reduction gear train.

    If it has a permanent magnet motor then it will require some torque threshold before it starts to rotate. The torque that you must apply to the output shaft is scaled by the gear ratio. Use more torque.

    Sometimes the friction of the motor or pinion bearings will be sufficient to prevent rotation of the motor from the output shaft of the gear train. That is called a “self locking” gear train. Remove any dry grease and use a more fluid lubricant on the motor and pinion bearings. Does that make a difference?
  6. May 21, 2015 #5
    I don't have a wrench on hand at the moment. But I did accidentally apply a lot of force during an experiment today. We were running the motor with a fly wheel attached, ramping the speed up and down in both directions. We put it up to full speed, and then suddenly turned the power off. The locking mechanism kicked in and the drill and flywheel stopped dead, and almost broke my friends hand in the process.

    Because of this I doubt that it is because I'm not applying enough force. There must be some sort of locking mechanism, I just need to know the name.

    Unfortunately I can't try this. We've taken the drill apart. When I said I "turned the drill bit", what I really meant is that I tried to turn the bit that rotates when power is applied. It did not turn though.

    As I said in my reply to William Turrey, I doubt it's because I'm not applying enough force. I also doubt it is just friction, because that wouldn't be enough to bring the fly wheel to a sudden hault when the power was removed.
  7. May 21, 2015 #6


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    A brake-motor has a friction brake that is released only when power is applied to the motor.
    What is the make and model of the drill your motor comes from?
    What is the approximate gear ratio?
  8. May 21, 2015 #7
    The problem with this theory is that when I stick my fingers through the vent in the back of the motor, and turn the fan looking thing, (like the black plastic you can see through the gap in this motor), I can turn the other end of the drill even when power isn't applied.

    It is the Black and Decker hp148f3. Im not sure what the gear ratio is, but when its in the high gear it has 36Nm torque.

    I'll upload a video of me trying to turn the drill from both sides in a min. You can see in the video, in low gear, I can turn it from one end very easily, however I cannot turn it from the other end. I only use hand and not a wrench because I do not have one here, however I use quite a bit of force. Like I said earlier, the flywheel experiment applied lots of force and it did not turn.
  9. May 21, 2015 #8


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    There is no room for doubt in the analysis. You must verify the torque by using a wrench of some sort.

    It is possible that the electronics stop the motor by applying a short across it's terminals when you release the trigger.

    I do not have video bandwidth. A video will not demonstrate anything not already communicated.
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