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The Schlage FBE365 electronic deadbolt uses a DC motor attached to a helical screw to raise and lower the C-shaped piece shown. The way the rest of the mechanism works is not important for this question - all that matters is that the C-shaped piece needs to go up and then down and it does that by rotating a motor and spring.

Additional detail: The C-shaped piece has a tang on its back that inserts between coils. When the rotation happens, the portion of the spring below the tang becomes compressed and the portion of the spring above the tang becomes elongated, which causes the C-shaped piece to be pulled up. Its held there for a couple seconds, then the motor rotates back to its original position, causing the C-shaped piece to be pushed down. Figure 1 shows the C-shaped piece in the down position. Figure 2 shows the C-shaped piece in the up position. Figure 3 shows the C-shaped piece removed - the spring and tang are visible. Figure 4 shows a close up of the tang. See here for a video of it operating: https://www.youtube.com/watch?v=OqjcM-eb8ZY

Prior to seeing the insides, I would have guessed there is a solenoid moving parts around as is common in many locks. It seems like that could definitely work here - have a solenoid pull up on the C-shaped piece and then have a spring return it to its original position. Any ideas why they chose to use a motor rather than a solenoid? One idea I had is that the solenoid plunger would have to be ferromagnetic, leaving the lock susceptible to someone using an external magnet to lift up the C-shaped piece and open the deadbolt. Any other possible reasons? Has anyone else ever seen a spring used as a screw drive in a design?

Figure 1: Figure 1

Figure 2: Figure 2

Figure 3: Figure 3

Figure 4: Figure 4

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The reason drive screws are used is because they give mechanical advantage.

Solenoids are linear devices and don't have inherent use leverage for mechanical advantage. A rotary motor with a small lever arm produces more force and somewhat decouples the actuator length from the motor size. A solenoid gets only one stroke to do the same amount of work where the rotary motor gets to spin many many times. That lets you use a stronger spring or a smaller actuator.

I've never seen a spring used as a drive screw before though. Not sure if they used that because they wanted the compliancy or because a hollow threaded would require custom manufacture and be very expensive by comparison. It certainly neat how it reduces size since you can shove the motor inside the spring.

Seems to me you don't strictly need the compliancy of the spring. To me it seems like the tang just runs inside the spiral space of the spring and acts like a normal lead screw. A hollow, rigid lead screw could also do the job.

Where a spring would help though is at the end points. I don't see anything that looks like limit switches to indicate motor endpoints for stopping. The spring's compliance would make it be more forgiving on the motor with regards to slamming into the end points and abruptly, and immediately stalling. With the spring, you could more effectively get away with using a safer, more gradual onset of motor overcurrent to know when to shut the motor off. Also less backlash.

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  • $\begingroup$ You're right, since there's no stop at the bottom, there won't be an compression in the spring, just elongation at the top. $\endgroup$
    – Ryan
    Mar 25 at 22:39
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    $\begingroup$ @Ryan I think I I see where you think there might be compression or extension. In a traditional rigid lead screw when you travel too far it would jam, but with a spring it could stretch or compress to comply. I am not certain whether the lock actually travels into such a range though. But none of that changes the essence of my answer. $\endgroup$
    – DKNguyen
    Mar 25 at 22:43
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    $\begingroup$ @Ryan I don't know what limit switches the motor uses, if any, but the spring's compliance would make it be more forgiving on the motor with regards to slamming into the end points and stalling. You might not need limit switches in that case and just be able to rely on motor overcurrent to know when to stop. $\endgroup$
    – DKNguyen
    Mar 25 at 22:46

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