Multi level scissor lift calc

[Simon]

Hello everyone,

I have been trying to do some hand calc for a scissor lift mechanism.
There is a post on the forum (http://www.mechanicaldesignforum.com...ft-calculation) describing the calc for a on stage mechanism which I agree with and found the same result separately. It is also refering to this webpage (http://www.engineersedge.com/mechani...issor-lift.htm, which has been corrected since). This website also states that for a n-level sicssorlifts stacked up, the required force is multiply by n. Somehow I can believe that is right (the base of a one stage sicssor has to travel twice the distance of a two stage scissor with hlaf the arm length); but I can not put a freebody diagram on it! Can somehow explain to me with a free body diagram in the static case why that is?

Thanks a lot in advance.

[Ramana Rao]

I cannot post a diagram but my thinking is this.
Pay load, scissors arm and lifting height remaining the same, the base of a two stage scissor has to travel half the distance as a single stage one. Since the work done (force x distance) is the same for both, the required force for the two stage scissors will be twice the single stage one.

[Simon]

Hi Ramana,

I totally agree with you. This is how I believe it is right. It is a proof and is enough. But as a mechanical engineer, I always like to understand things with forces and torques, but I cannot put my finger on this one. Any help with a free body diagram and force vectors on it?

thanks a lot for your reply.

Regards,

Simon

[Ramana Rao]

Hi Simon,
It is embarrassing to admit my inability to post a diagram not knowing how to do it. My proficiency at the computer is limited to Autocad and MSWord.