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I am looking to better understand why this scissor mechanism so easily moves by directly pulling on the lift...yet, seems incredibly difficult/impossible to move via the attached point in the slot?

Ideally I would be able to pull the bar in the slot down to extend the lift. Thought it would be ... apparently not lol

I have tried many methods to pull the slotted rod down and no luck, hoping for some ideas

Video demonstration: https://youtu.be/9kB9OfOV67I

Here are two videos of someone else, that has it working. I did notice that the very first cross joint in the scissor lift closest to the slot seems disconnected but perhaps is still rigid enough to work?

https://youtu.be/sQCJVU6hxH0 https://youtu.be/IuZV_84EnG

afreeland
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    That's not any different from why a worm gear behaves the way it does. The force applied at the pin is in the wrong direction relative to the direction of motion of the lift so it locks up. Especially when the scissor lift is compressed, the force applied to the pin is almost directly parallel to the scissor left members and has very little leverage. You're not meant to drive the lift from the pin anyways; The pin is just there to guide the mechanism. – DKNguyen Oct 11 '22 at 04:12
  • hoping for some ideas ... about what? – jsotola Oct 12 '22 at 05:58
  • @jsotola some ideas on how to overcome this disadvantage and be able to extend/retract via the slotted pin – afreeland Oct 12 '22 at 12:31
  • @afreeland Disconnecting that first cross joint would help since you would not be pushing directly against the other first member. It removes that first rigid triangle. – DKNguyen Oct 12 '22 at 14:07

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Notice the bottom part movement speed, when you pull on the top. If the mechanism was manufactured very precisely and the friction was independent of the force, the force to move it would be proportional to inverse of the speed of the bottom movement. So you would need higher force when pulling on the bottom part. In reality, this higher force leads to higher friction forces and may also deform the mechanism. All of these factors further increase the necessary force for the movement.

Tomáš Létal
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  • Thanks for the breakdown, definitely helps picture it a little easier. I edited the question to include some extra videos of someone that has it workingnand why I think theirs might work. Be curious to get your thoughts on the updates question – afreeland Oct 12 '22 at 12:41
  • @afreeland In the first video, the mechanism is almost horizontal, which might help a lot (you could try that with yours), the second video is not available anymore. – Tomáš Létal Oct 12 '22 at 14:40