# A Equations for scissor jack

1. Mar 3, 2017

### bbq_build

Hello, I need to derive dynamics equations for a two-stage scissor jack. I have searched many books in the libraries and on the internet but there is no such example. Could anybody please help? Thank you

2. Mar 3, 2017

### Staff: Mentor

A scissors jack is just an elaborate lever?

What do you mean by dynamics equations?

3. Mar 3, 2017

### bbq_build

Equations that one could use to simulate the behavior of the jack under different applied forces and loads. One that involves rigid body dynamics.

4. Mar 3, 2017

### sophiecentaur

I don't think you are likely to find a 'scissor jack equation' anywhere because such a mechanism is just a part of an enormous field of lever systems. From your question, I get the impression that you are not too familiar with the basics of moments in mechanical systems. I think that you could benefit from an intermediate mechanics text book with the basics and a number of worked examples. Most of the stuff you find on the net about levers and moments involves the simple see-saw type balance where the forces all act parallel to each other; useful but not enough for what you need here. I found this YouTube video, which gives a way into the topic and there are a number of relevant links leading from it. It's A level physics that you need - GCSE doesn't do 'angles' for moments.
If you have a nearby second hand book shop (or website) you should be able to get hold of a cheap A level text book, which is exactly what you need and need not cost more than a very few Quid.
I don't know of one but I could imagine that there are a number of Mechanics Simulation packages that could take the pain out of the problem - but then you would not necessarily learn any physics from the exercise (this is fine if you just need a result).

5. Mar 3, 2017

### malemdk

I myself worked out the problem -(Mechanical Engineer) since no where I could find the solution to this scissor lift problem- I think Its hard to post here its a lengthy calculation ,

6. Mar 3, 2017

### sophiecentaur

For a symmetrical (rhomboid) arrangement of four spars and a balanced load,, the problem can be reduced to considering just one spar, with two forces acting at each end. (Assume the jack is massless etc. of course). The calculation should not be too hard with so few variables. Don't ask me to put my money where my mouth is but the relationship between the vertical and horizontal forces with angle is going to involve a tan function of the the angle .

7. Mar 4, 2017

### malemdk

Yes it's for simple configuration, but the calculations become somewhat complicated for other configurations

8. Mar 4, 2017

### Nidum

@bbq_build : Is this a normal scissors jack with hand operated screw and nut drive or is it a more complicated version ? Can you post a picture ?

9. Mar 4, 2017

### sophiecentaur

I wonder whether an asymmetrical jack would ever be used. After all, it wouldn't be 'foldable' as a normal jack is and the range of usable angles / heights would be less.

10. Mar 4, 2017

### Nidum

11. Mar 4, 2017

### sophiecentaur

Wow. Never seen one of those; I never thought of that form of asymmetry. Could be very handy in the right conditions.
OK - multiply the number of equations by two (I don't think four is necessary). But still, there's no Statistics or Integration involved. Just plain old Algebra and a couple of Trig functions.

12. Mar 4, 2017

### Nidum

Just so . Balance forces or equate incremental work done at the handle to incremental work done at the jacking point .

Biggest problem in design of a real jack is in finding a reasonably efficient screw and nut combination .

13. Mar 4, 2017

### bbq_build

14. Mar 4, 2017

### malemdk

It's hydraulic power cylinder version, do you design any scissor jack or is it course project?

15. Mar 4, 2017

### malemdk

Yes it looks somewhat like, but not exactly what you have shown is small machine, the machine which I designed was for industrial use, the rated capacity of each cylinder is 40 t, there were 2 cylinders