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Trying to DIY IKEA Skarsta hand crank sit-stand table, I've come to a point when I seem to get all things except this one:

How does such mechanism hold the position of a desk? 

I get it if a crank handle never leaves a transmission gear - clip a handle after rotation and a holding bracket will do its job. But it seems to be just the opposite in one from IKEA - there are definitely push/pull actions on a handle that make me think it does leave transmission...

Mechanics reference image

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    What makes you think that the Ikea table uses the mechanism shown in the patent sketch you referenced? If the Ikea table uses a worm drive, then that is almost impossible to reverse drive - that is, turn the handle by pushing down on the table.
    – SteveSh
    Jun 29 '21 at 17:49
  • @SteveSh, video demonstration of table usage explicitly demonstrates push/pull actions on the crank handle - so I doubt there's a worm drive.
    – Denis
    Jul 1 '21 at 11:55
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The crank arm is like the longer arm of a lever. The threads of a screw are like a very low angle inclined plane. For every 1 cm vertical rise or fall in the table the end of the crank arm moves around and around through maybe 30 cm or more. A small force with a hand overcomes the friction in the threads and gears and the table moves easily.

But if you take away the hand, the table does not fall because the friction in the threads and gears locks the shaft in place. The weight of the table is insufficient to overcome friction. If you would increase the downforce by putting a weight on the table, the static and dynamic friction would inrease proportionately.

Some mechanisms might have a lock which is set once the height is adjusted with a light load. But I used to drive a Ford Focus which had a hand crank to raise and lower the driver seat and it did not have a lock.

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Look at that threaded rod (giant screw) in your reference image, which is what sets the position.

Pushing or pulling on a screw won't make it turn. Theoretically it could, but in reality, there is too much contact area, with too much friction, and too shallow of an angle.

It's the same as any other screw. Pushing or pulling it won't move it, no matter how hard you try (until it breaks). If you want it to move, you have to turn it.

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  • Thank you! I thought about friction, but it seemed too unreliable to me to hold 20-30 kg without movement. You're absolutely correct about screws in some base material, but not so correct about thread-thread contact, as it seems to me. Tread-base case has much more contact area - no arguing with that. In fact, I've witnessed some cases of loose threaded rod spinning in a coupling nut just my gravity and hand vibration. It's not entirely the case I'm asking about, but still makes me feel uncertain about friction usage only.
    – Denis
    Jul 1 '21 at 12:04
  • I must add it was a threaded rod with quite a pitch......
    – Denis
    Jul 1 '21 at 12:16
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I suppose you've never changed a tire on an automobile, at least with the included screw jack?

The mechanism is very similar (indeed, a couple of screw jacks from the auto junkyard would be a great way to get the needed parts) and those don't spin backwards with a load of hundreds of pounds (or slightly less than half as many hundreds of kilos) on them.

The same force multiplication that allows them to easily lift heavy loads makes it very hard for the heavy loads to move the screws.

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  • Ecnerwal, you suppose wrong :) Still, screw jack had never come up while I was thinking about my issue, thank you. So, friction it is - that's what you're implying? Smaller pitch, bigger depth, sharper corner - and no problem? Despite the fact that forces will be applied so much differently as opposed to screw jack design?
    – Denis
    Jul 1 '21 at 12:11
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The phrase you're looking for is worm drive or jackscrew.

It works a lot like a bolt and nut. Many revolutions on one results in small motion of the other. Because of the angles of forces involved, power can be transmitted only in one direction. But I think this is obvious.

enter image description here src

But...

Looking at that patent diagram, the jackscrew can barely be seen. The prominent feature is that 3-gear gear box. That's not actually part of it. That simply turns the crank's motion 90 degrees. It could be eliminated if the user didn't mind a vertical crank.

What isn't illustrated very well is the long jack screw. It isn't drawn with proper threads: that is a "drawing style" which draftsmen and machinists know means "that is a thread".

It really looks like the photo above.

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