https://imgur.com/a/2THq9K3
I understand how a scissor jack can lift a heavy object, but what I don’t get is how the lock mechanism keeps it in place while letting it move whenever the crank is spun. A visual illustration would be greatly appreciated as well.
In: 2
To raise the jack, you’re turning a screw. In order for the jack to be lowered by force from the top, it would have to “unscrew” the screw. This takes a LOT of force.
Here’s an analogy. Say you’re screwing a bolt into a nut. It’s easy to turn the bolt with a screw driver. But if you try to screw it in by “pushing” on it, it won’t budge. The same thing is happening here.
TLDR, that screw can’t be back-driven.
In order for the lift to go up or down, the screw has to turn either clockwise or counterclockwise. That reduces or increases the length between the two points that the screw’s attached to, which translates to moving the horizontal pads either closer or further.
If you try to move the pads, it will put the screw under tension or compression, but the friction in the screw will keep the tension/compression from actually turning the screw. Keep in mind that the friction force in the screw scales with how much tension/compression the screw sees, so no amount of tension/compression is going to make the screw turn. Turning the screw shaft itself is the only way motion along the screw is going to happen (ignoring slop). Just like a standard bolt and nut. This is a non-backdriving screw.
Other applications for threaded shafts can be back-driven; some printer heads are this way. For a backdriving screw, you can move something along the screw and it will turn the shaft. During design, the threaded rod/screw is almost always chosen to do one or the other. So for applications where you want something to stay put, you’ll design for a screw that can’t be back driven
The pitch of the screw is fine enough that weight on the jack increases the friction force holding it in place as much or more than it increases the force trying to unscrew the screw.
Think about putting a cardboard box on a gently sloped driveway. Making the box heavier won’t make it more likely to slide, but making the driveway icy or steeper might. A screw is just a ramp wrapped around a cylinder.
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