What is preventing the scissor jacks from collapsing? 4

[mousecheese]

Hello, I have some questions about the scissor jacks. Is it the friction between the lead screw thread and the two nuts at the front and back sides of the jack? What type of friction is helping the jack to uphold the load to a specified weight without collapsing? Is there a formula? Thanks

 

[IFRs]

Free body diagrams are your friend

 

[BrianPetersen]

Worm gears are non-backdriving if the helix angle is shallow, which it normally is.

 

[TheTick]

Free body diagrams do not answer this question. You will have to pay for the premium body diagrams.

 

[mousecheese]

Thank you

 

[kjoiner]

Hello,

Lead screws have a lower efficiency than a ball screw which can be used to an advantage to hold a load. Check out some of the technical information by a lead screw company such as Nook. Manual machine tools use lead screws vs. ball screws for the same reason.

Kyle

 

[moon161]

Self locking screws:

https://en.wikipedia.org/wiki/Screw_(simple_machine)#Self-locking_property

Self locking of a wedge:

https://en.wikipedia.org/wiki/Screw_(simple_machine)#Self-locking_property

Remember that the angle of a thread face to the axial load is formed both by the included angle of the thread, and also the pitch.

 

[LittleInch]

Also the heavier duty ones have counter threaded bar so that the rotational effect on the rod is balanced out, i.e. one nut is CW, the other CCW.

This means they both have bar sticking out at either end.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

LittleInch, I think the point you are trying to make is incorrect. It is still only friction that prevents the screw from turning under load. All that the left/right threads do is keep the screw centered in the jack.

 

[LittleInch]

You're probably right now I think about it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rb1957]

in the picture shown, doesn't rotation of the threaded rod change the height of the lift ?

so if the rod can't rotate ('cause, say, a pin is engaged in some structure) then the lift position is restrained.

no?

another day in paradise, or is paradise one day closer ?

 

[hydtools]

See this link:

http://www-mdp.eng.cam.ac.uk/web/li...dvd_only/DAN/threads/mechanics/mechanics.html

Locking depends on both friction and thread helix angle. The critical coefficient of friction equals the tangent of the helix angle. If the coefficient of friction is greater than the critical coefficient, the thread locks the load.

Ted