# a nice video about mechanical mechanisms

Hi everybody! Just a simple post to share a nice video I found on youtube … ðŸ™‚

Last week I was surfing the web looking for potentially awesome ideas for my PhD project… well, actually I just needed a list of mechanical mechanisms and configurations in order to evaluate new design proposals for the knee prosthesis I am currently working on. As most of you will know, for sure, the step google research -> silly video is more or less immediate ðŸ˜‰

Most of the times the silly video you find has nothing to do with what you were looking for. Most of the times you watch it anyway, since “it only takes 2:35 minutes …”Â  ðŸ™‚ Â and in this magic way, each day, dozens and dozens of hours are wasted by PhD students all over the world.

This timeÂ I was lucky! I found a very simple video, but actually it’s a really nice one! The user says it’s part of a “work by Ralph Steiner” … I don’t know anything about that, but the mechanisms that it shows are really amazing. Some of them have such strange shapes (I mean, the pieces that move are shaped in a bizarre way!) that you would bet the whole system won’t ever be able to move and next… the result is simply wonderful!

So simple + so strange = an amazing precise and regular motion!

Ah, by the way… finally the structure I adopted for my knee prosthesis is a particular screw-nut configuration ðŸ˜‰

# Scissor Lift Jack Equations

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 ðŸ™‚