Patent ID: 7100549

Claim:
A mechanism comprising: a piston-and-cylinder assembly including a piston housed in a cylinder; a pin member passing through the piston and a guide member having a guide recess accommodating an end of the pin member; the guide recess being so shaped and orientated in relation to the piston-and-cylinder assembly that a common axis exists between the guide recess and the piston-and-cylinder assembly, the guide member and the piston-and-cylinder assembly being so mounted as to be rotatable relative to each other about the common axis; the guide recess including an inner periphery and an outer periphery, the inner and outer peripheries including respective mid-portions where the periphery is narrowest lying between two end lobe portions, for guiding the pin member continuously in a path which includes two end lobe portions separated by a narrower mid-portion; rotation of the guide member and the piston-and-cylinder assembly relative to each other causing the piston to sweep up and down the cylinder; and the radius of curvature along the narrowest part of the narrower mid-portion of the inner periphery of the guide recess being always larger than the radius of curvature along the end lobe portion of the outer periphery of the guide recess.