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I have a fan-like setup where a 4 mm-thick rod is attached to a small motor-driven shaft (also 4 mm thick) such that the rod turns about its centre as the shaft turns:

It's a closing mechanism for a catflap. It normally can rotate in both directions, but when late at night, it should lock the outward direction but allow the inward direction.

The flap's right rail slides over the rod so when the rod rotates, the flap opens up. The flap's other rail slides over a similar rod, but is connected to a bearing instead. Here's a rough sketch:

How can I block the shaft from rotating further clockwise (or the rod, for that matter) regardless of the current position? I'm hoping to have the shaft/rod blocked (using another motor) so that it won't rotate by +5 degrees or so even under some 7 kg-cm torque.

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  • $\begingroup$ It would be helpful to indicate the normal direction of rotation. Also providing the reason you want to lock the shaft rotation may provide more helpful answers. $\endgroup$
    – ericnutsch
    Commented Jan 2, 2016 at 20:55

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One method to control a shaft this way would be to use an electric clutch to connect to a Sprag Clutch, sometimes called a "one-way bearing". However, this would be a little expensive for your task.

Another method that may be more appropriate for you application is to use a plain hinge at the top (a piano hinge would work well), and a solenoid spring latch near the middle or bottom (chose location based on toque requirements). There is a wide variety of different types. You could also construct your own from a plain solenoid if necessary. Just have your timer/control engage the solenoid when you want the one-way operation to occur.

There are lots more options if the design is changed to sense the inbound cat and control the system accordingly.

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  • $\begingroup$ The shaft should be able to rotate in both directions normally though. Only under certain conditions, it rotates in just one direction. I think a bearing would have to be fixed rather than being triggered - right? $\endgroup$
    – Kar
    Commented Jan 2, 2016 at 21:23
  • $\begingroup$ Thanks. With a spring latch, it seems it'd just lock the flap in place rather than allowing for only one-way traffic. Is my understanding correct? $\endgroup$
    – Kar
    Commented Jan 3, 2016 at 6:40
  • $\begingroup$ @Kar, It wont use a striker plate like a normal door, just blocking it on one side. Actually now that you mention it, you could just use a regular solenoid. Just put it so the pin extends into the path of the outward swinging door. When not engaged it swings both ways, when engaged it will swing inward but not outward. $\endgroup$
    – ericnutsch
    Commented Jan 4, 2016 at 1:13
  • $\begingroup$ I thought about using a solenoid, but most of the decent ones seem to use at least 12V and draws 1A. These things heat up fairly quickly too. To properly power them with pulses of power, I'd need a giant capacitor which can be very hard to find. $\endgroup$
    – Kar
    Commented Jan 4, 2016 at 6:15
  • $\begingroup$ As an alternative you can look at latching solenoids or rc servos. These do not require continuous power. The servo arm will need to be placed such that an arm sweeps in front of the door. This way the motor will not spin when a force from the door is applied to it. tlxtech.com/understanding-solenoids/theory-operation/… adafruit.com/products/1142 $\endgroup$
    – ericnutsch
    Commented Jan 5, 2016 at 1:33

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