It is an application of Hobson’s joint for air compressors.
Input: orange shaft.
Center of the conrod head moves along an ellipse (in green).
Advantages:
- Pressure angle on piston is small.
- Small crank radius (R) gives large displacement (L) of the piston.
L = 2.R.sin(A/2)
A: angle between crank rotary axes.
Here A = 60 deg., L = 4R
Recirculating ball linear bearings for round shaft should be used for the green angular bar.
The mechanism has unstable positions when the planes created by axes of two joints of each crank are vertical.