A scissor lift jack mechanism is a 
mechanical device used to lift up or down an upper platform. The term “Scissor” comes from the mechanical configuration, which is based on linked, folding supports disposed as a crisscross ‘X’ pattern. The platform displacement motion is achieved applying a force to one of the supports, resulting in an extension of the crossing pattern.
The force applied to extend the scissor mechanism may be hydraulic, pneumatic or mechanical (via a lead screw or rack and pinion system). It can be applied at the bottom of the mechanical structure or at its center pin.
Design Equations for Scissor Lift, with force applied at Center Pin:
For a scissor lift that has equal-length arms, it is possible to compute the relationship between the actuation force F and the load force W acting on the mobile platform. This relationship depends on the geometrical parameters of the mechanical structure. Starting from the notes published on this website, I made my own computations trying to fix some sign errors and better explain some notations. Just click on the two images below to open them in their original size. I hope it could sound interesting to somebody facing with such kind of problem 🙂
andreacollo 
I am mistreatment a lot of similar product, i feel you for the careful data regarding this.
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If the actuator was positioned at the bottom and not the middle, do you agree that Fx=W/2tan(theta)? http://twitpic.com/3tmecf
Hi Phil,
No, I do not agree with you.
I took some time to compute the equations again, after a long time… your question let me find out a few more inaccuracies! Thank you very much 🙂
Other bloggers had found the same mistakes, and the webpage I indicated as source updated the equations on their websites.
You can find a detailed explanation, WITH CORRECT EQUATIONS AND COMPUTATIONS, here: https://vamfun.wordpress.com/2011/11/12/error-in-scissor-lift-equations-at-engineers-edge/
I’ll certainly use it to update my post as well 🙂
Thank you again, have a nice day
Andrea
Really very efficient stuff.