# How to account for pistons inside a free body diagram? [closed]

If you have taken basic statics classes you'll remember the free body diagrams used to calculate the forces inside a mechanical system; such as a beam structure consisting of pivots, rollers, fulcrums and fixed points.

One area of study that is not covered in great detail however is the addition of lateral moving parts in these systems, such as pistons.

In the system that I am calculating a piston is conected to a secondary beam at a pivot, the secondary beam is connected at the opposite end to a primary beam by a pivot. and the primary beam is connected to a fixed position by a pivot again. The purpose of this system is work as a mechanical gripper pulling in and pushing out on the primary beam.

My question is how to calculate the forces acting in the system giving the changing shape of the system given a known force value of the piston.

Note: I apologise for not uploading an image to provide clarity, but I am unforetunately unable to do so at work. Additionally I apologise for being vague, I am just looking for a starting point to determine the rest, researching the topic so far has been a futile effort.

• Without a diagram I can't visualise what your describing; however, as you mention in the question, a free body diagram would typically be used for a static problem - if you are interested in a dynamic system then you require a slightly more involved solution, perhaps a simulation or solving some simultaneous equations?
– welf
Commented Apr 7, 2015 at 12:47
• If the piston is moving slowly, and accelerating slowly. Then the system can be modeled a static system at each position. If centripetal forces (from the velocity) or inertial forces (from the acceleration) are significant, then I would recommend looking into Lagrangian Dynamics to solve the problem.
– Rick
Commented May 6, 2015 at 17:32
• Sam - any chance that you would have time to upload an image in order to clarify your question? I believe you have a good question here, but I think the lack of answers demonstrates a need for more detail or an image.
– user16
Commented Nov 25, 2015 at 19:22