Patent ID: 7210397

Claim:
A scotch yoke fluid device which includes: a crank including a big end having an axis which orbits about a main axis for the crank; connecting means mounted on the big end axis; at least one piston mounted for reciprocal motion in a cylinder along a piston axis, the piston having a cross-sectional area perpendicular to the piston axis, the piston having guide means including a linear surface transverse to the piston axis, the guide means engaging engagement means on the connecting means; and at least one restraining means for constraining the piston to move along the piston axis; wherein the piston guide means bisects the piston cross-sectional area and at least part of each restraining means is located within a volume defined by the piston cross-sectional area projected along the piston axis, but is not located along the center line of the bisection formed by the piston guide means, characterised in that at least one of: i) the or at least one of the restraining means; and ii) a constraining means for the or at least one of the restraining means always extends into a volume defined by the swept area of the crank projected along the main axis of the crank projected along the main axis of the crank; wherein the constraining means comprises one or more rollers.