Spirograph

Spirograph is a geometric drawing toy that produces mathematical curves of the variety technically known as hypotrochoids and epitrochoids. The term has also been used to describe a variety of software applications that display similar curves, and applied to the class of curves that can be produced with the drawing equipment (so in this sense it may be regarded as a synonym of hypotrochoid). The name is a registered trademark of Hasbro, Inc.

The following figure shows the concept and geometric paraeters

The following are some curves drawn by the spirograph code I wrote in Scilab

Piston sealing modes for Stirling engine

Here you are the figure that explains what I want

Force meter with mechanical amplification

The amplification depends on the ratio between areas of pistons. Small force can produce large pointer displacement.

This force meter can work with both compression and tension forces. Also it can work as comparator

The spring here (in this design) will be compressed, but we can modify the design to make it elongating.

Low friction sealing is required for this device