## Equations for two stage scissor jack

## Equations for two stage scissor jack

(OP)

Hello, I have a two stage scissor jack likes the one shown in the photo but with the addition that a DC motor is connected to the knob. Could anybody please show me how to obtain the dynamics equations for this system? Thank you

https://en.wikipedia.org/wiki/Laboratory_scissor_j...

https://en.wikipedia.org/wiki/Laboratory_scissor_j...

## RE: Equations for two stage scissor jack

## RE: Equations for two stage scissor jack

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Equations for two stage scissor jack

Power required to lift is weight times speed (yes you need to know how fast). This tells you the ideal (frictionless) motor

powerrequired. FYI power is torque * rotation speed.Determining friction... that's something you need to measure, most likely. If you can't do that then start with double your ideal power requirement for the motor.

## RE: Equations for two stage scissor jack

As posted above, the equations should be easy to derive ... single input (turning the knob), single output (raising the platform). I'd start with a small rotation of the knob produces what changes to the scissor jack ? Then you can determine the work done by that small rotation and so the work required to turn the knob.

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

## RE: Equations for two stage scissor jack

## RE: Equations for two stage scissor jack

I'd've thought your configuration (with a screw drive through the scissor joints) is easier to analyze ...

turn the knob, change the distance between the two fittings mounted to the thread, change the height of the platform (and also change the height of the CG of the scissors).

I think you've got two things to worry about with your motor ... the work required of it (ie the torque) and the rate of work (ie Power).

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

## RE: Equations for two stage scissor jack

## RE: Equations for two stage scissor jack

With systems like these, I usually looked the system in a best and worst positions, and determined mechanical advantage from small movements (e.g. 1/4 turn of shaft or 1mm vertical movement). Simple enough to get this from a 2D CAD sketch. Comparing resulting torque or force to performance curve of an actuator or motor provides results that are as dependable as the said performance curves (here's where you find out how honest your motor manufacturer was).

## RE: Equations for two stage scissor jack

## RE: Equations for two stage scissor jack

I have no issue with using virtual work to figure out Fout vs Tin for a given geometry, or platform speed vs rpm, but that ain't dynamics.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Equations for two stage scissor jack

Done it all: kinematics, simulation, test fixture, product, etc. Had a job where I had to figure this stuff out for linear actuators on hospital beds. Classic kinematic stuff.

Nothing wrong with doing the kinematics. I can do it. It's just that it's usually the long way, and prone to errors and doubt. For a case like this I would do one of two things:

## RE: Equations for two stage scissor jack

https://www.theseus.fi/bitstream/handle/10024/1131...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein