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Context: I am building an add-on to a door handle to create a "smart door handle" that I can move remotely. Working in IT, the software part is not a problem, but the mechanical one is. I am considering several solutions and one of them involves gears. I apologize in advance for my technical wording, not being a native speaker of English (and neither a mechanical engineer).

The generic idea is to have one gear mounted directly on the door handle axle

enter image description here
source: my first attempts to use the fantastic FreeCAD

It will be powered by a stepper motor via either a smaller round cogwheel or a worm gear (I am not sure which yet).

The problem: I need to be able to manually press on the handle to open the door (with my hand). With the simple solution I have so far it is not possible: pressing on the handle would force the movement of the wheels and this is not good either mechanically (the torque will be too big and probably break/damage the wheels) or electronically (the motor is not intended to work as a generator, and even if it was the torque is not correct).

The sought-for solution: a mechanism that would make it so that a movement triggered by the motor moves the handle, but when the handle is pressed and it is the source of the movement - it would not be transferred to the motor.

I thought that this was a showstopper but after watching 20 Mechanical Principles combined in a Useless Lego Machine I realized that I have no idea about what is possible in mechanical engineering.

My question: is there a standard solution for such a problem?

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  • $\begingroup$ It would seem that there are 3 steps to opening a door: unlock, move handle, and push/pull on door - so, without the 3rd step the door won't actually be "open". --- From what I've seen door openers use the 3rd step to avoid the 2nd step, and the 1st step is in addition to the 3rd (locking) at night; otherwise the 3rd step is essentially "locking" assuming someone won't break the door during the day. --- Clarify why or what your idea / question is better than the standard method please. $\endgroup$
    – Rob
    Oct 9, 2022 at 10:25
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    $\begingroup$ @Rob: I just want to automate the act of pressing the handle from inside the house, so that it can be done automatically/remotely. Everything else (unlocking, pushing the door) is not in scope $\endgroup$
    – WoJ
    Oct 9, 2022 at 10:29
  • $\begingroup$ are you sure that you want the handle to move by remote control? ... maybe all you need is a way to engage the outside handle to the bolt ... an electrically operated clutch of some kind $\endgroup$
    – jsotola
    Oct 9, 2022 at 17:24
  • $\begingroup$ @jsolota: could you please expand a bit? My ultimate goal is to be able to open the door from outside when it is slammed shut. It is a door that has a handle only inside (from the outside you need to use the lock to pull back the latch bolt. (any ideas are welcome, but I do not want to change the lock - this is an ultimate move) $\endgroup$
    – WoJ
    Oct 9, 2022 at 18:23
  • $\begingroup$ You haven't mentioned how the handle is going to go up again. Normally, that would be by a spring when the user releases it. But what about when it's driven by the motor? $\endgroup$
    – Simon B
    Oct 11, 2022 at 13:48

2 Answers 2

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Have a look at Freewheel and think of the freewheel mechanism on your bike. Here's the problem: when you press the handle the freewheel will work but when you let go the handle it can't come back unless the motor rotates.

Other search terms are "overrunning" or "over-running" clutch. You can think of your bike wheel over-running the speed of the chain sprocket when pedaling more slowly than required to drive the bike.

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Figure 1. Two-way overrunning clutch by thang010146.

You would connect your motor to the blue part and the handle to the pink part.

Do a web image search for "two-direction freewheel", "bi-directional freewheel" and repeat with "overrunning clutch". You might find other ideas.

One other solution is to add an electro-magnetic clutch to the motor. That just disconnects the motor drive from the handle drive when not in use. It will consume a little extra power.

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A sprag clutch will do it. When I was familiar with automatic transmissions they were common in them. It looks like a roller bearing except the rollers are oval.

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  • $\begingroup$ Are you sure? Both handle and motor have to run both ways. $\endgroup$
    – Transistor
    Oct 10, 2022 at 18:00
  • $\begingroup$ Sprags permit free rotation in one direction and lock up in the other direction. $\endgroup$
    – blacksmith37
    Feb 7 at 16:03

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