I'm looking at a teardown of a Masterlock. The guts look like this

enter image description here

What's strange to me is the black, plastic pinion that appears to be smooth and a strangely shaped cam:

enter image description here

Here's the cam magnified:

enter image description here

The motor doesn't seem to be encoded in anyway to prevent stall and the PCB only has one microswitch, but it's for the spring-loaded, shuttle-like metal piece rather than the cam. So I wonder how the motor knows when to stop without stalling. There doesn't seem to be a current shunt on the PCB either.

My take is that the square groove of the cam (top facing) faces down in the housing. The round groove on the right of the square groove is where the plastic pinion contacts. That leaves the two not-so-visible grooves facing the mat (ie those facing away from the housing). Does anyone know how the cam is turned? The bottom groove of the cam appears round, so it could probably grip on the black plastic "pinion". But then, if cam shaft is the axis of rotation, that round face can't be the contact face with the pinion.

Does anyone know how how the shaft is rotated or how the motor is encoded such that it won't stall?

I'll also magnify the black plastic pinion here:

enter image description here

  • $\begingroup$ The pinion+cam is probably part of a eight of a geneva drive. $\endgroup$ – ratchet freak Jan 15 '16 at 14:21
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    $\begingroup$ What makes you think the motor doesn't stall? They may be just using it as a cheap rotary solenoid. $\endgroup$ – Dave Tweed Jan 15 '16 at 14:27
  • $\begingroup$ If the motor stalls, it'll overheat and burn down, right? Plus, I wonder how the pinion grips on the cam. $\endgroup$ – John M. Jan 15 '16 at 16:07
  • $\begingroup$ @ratchetfreak I'm not sure if it's like a Geneva drive. The drive wheel and the driven wheel rotate in reverse to each other. The first picture shows the lock state where the pin groove faces up. So for the cam to rotate the motor shaft needs to rotate anti clockwise. However, to unlock, the cam needs to rotate anti clockwise. $\endgroup$ – John M. Jan 15 '16 at 18:26
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    $\begingroup$ it's easy enough to add a current limiting resistor in series to avoid the burn up. $\endgroup$ – ratchet freak Jan 16 '16 at 0:14

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