Advantages of the
rotating piston are:
Each stroke of the
piston presents a newly wetted portion of the skirt to the
side of the liner absorbing the thrust, reducing wear.
The piston rings
rotate with the piston, thus any blow by through the ring
gap is not blowing onto the same portion of the liner,
avoiding localised overheating and burning of the LO film.
The load on the
spherical bearing is uniform, eliminating distortion of the
The piston is
symmetrical in shape, therefore expansion is uniform; The
piston can be manufactured with smaller clearances,
avoiding piston slap at low loads.
Rotational movement of
the piston is restricted by the ratchet teeth, which engage with
slightly offset pawls. There are an uneven number of teeth and
during each swing of the conrod the piston rotates by one half
of a tooth pitch, i.e. one tooth /engine revolution, the
movement being transmitted by an annular spring. The piston
revolves slowly (about 6rpm). This small movement ensures even
wear of the piston skirt, helping maintaining it's circular
Because of the smaller
clearances, if the piston was to stop revolving, then there is a
danger of seizure due to uneven expansion. These pistons have
proved very reliable in service, however, and maintenance
periods for the piston, rotating mechanism and spherical bearing
are set at 48 - 60 000 hours. Additional costs must be borne in
mind when overhauling the piston due to the cost of replacing
the worn ratchet ring and pawls.