Wednesday, May 13, 2020

Rotating Piston

The rotating piston has an alloy steel crown to maintain strength and resist corrosion at high temperatures and a cast iron skirt to guide the piston and transmit the side thrust to the liner. Three chrome plated ring grooves are machined into the crown and an oil control ring groove is machined into the bottom of the skirt
Instead of a conventional piston pin arrangement the top end of the con rod is machined to a spherical shape. This locates in a spherical bearing, which comprises of an upper and lower half as shown.
A spring loaded ratchet ring is bolted to the piston skirt. The ratchet has an odd number of teeth. Two spring loaded pawls located in the top of the con rod engage alternately with the ratchet ring as the con rod swings. This imparts a rotary motion to the piston (about 9rpm).
Oil is supplied up the centre of the con rod to lubricate the spherical bearing and ratchet assembly, and provide cooling to the crown. Cooling is effected by the "cocktail shaker" method and the oil returned to the crankcase through a drain bores in the piston.

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 piston.
    •    The piston is symmetrical in shape, therefore expansion is uniform; The piston can be manufactured with  smaller clearances, avoiding piston slap at low loads.

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