VALVE ROTATORS

VALVE ROTATOR

A valve rotator turns the valve on its movement, due to this service life of valve increase, it prevents the carbon deposition on the seat and valve guide, and also it helps to maintain the valve at uniform temperature.and prevent pitting due to hotspots  As the valve rotates, the seating surfaces are constantly changing. This helps prevent forming deposits and keeps the valve from developing hot spots.

TYPES OF VALVE ROTATOR

There are two types of valve rotators

1.non- positive type or release type

2. positive type

NON- POSITIVE TYPE

Non positive type rotators release the spring tension from the valve while open. this allows the valve to rotate from engine vibration

There are two types of non positive valve rotators

1.split-collet valve rotators

2. thimble valve rotators

SPLIT-COLLET VALVE ROTATORS

 The stem of the valve has three annular grooves of semi-circular section and the inner surface of each collet has three semicircular ribs. These press against each other providing a small radial clearance between the valve and the collects. The vibration of the valve gear rotates the valve at between 15 and 25 rpm at engine speeds above 1500 rpm.

 THIMBLE VALVE-ROTATOR.

In the thimble rotator design, a steel cap fits over the end of the valve stem and rests on two semicircular collets which fit in the valve-stem groove below. The valve spring maintains a pressure on the retainer against these collets and so keeps the valve closed. When the valve is required to be opened, the rocker-arm presses down the cap which in turn bears against the two collets, and then moves the valve spring and retainer downwards. The spring pressure is now taken by the cap, and hence the valve is freed from spring pressure. It is still moved
downwards, because the closed end of the cap then abuts the valve-stem end, but it is free to turn.

POSITIVE TYPE VALVE-ROTATORS ALSO KNOWN AS ROTOCAP

 

This incorporates a ball-retaining plate with six ramped grooves for balls to roll along. A small spring pushes each of these balls to one side. A Belleville-type dished spring washer fits over these balls to form an upper race, which is supported on its outer edge by a spring-seat retainer. This retainer holds the whole assembly together and also provides a seat for the helical-coil valve springs. In the closed position of the valve, the dished washer is suspended between the spring-seat retainer and the ball-retainer, so that the balls move freely to the top of the ramp and abut against the end of the groove. During opening of the valve, the dished spring washer deflects with the increase in the compression load on the valve spring. The outer edge of the dished washer bears against the spring-seat retainer as before, but the inner part of the washer now bears against the six balls and hence pushes them down their ramps. The ramps are so shaped that, as contact with the washer is maintained, the spring-seat retainer is rotated and hence the poppet-valve is rotated by the same amount. As the valve closes, the washer comes back to its original position between the spring-seat retainer and the ball-retainer. This releases the load on the balls due to which the small bias springs now push the balls up their ramps and thereby bring the spring-seat retainer and the valve assembly back to its starting position.

 

WINGED VALVE ROTATOR

In two stroke engines valve  operated by hydraulically uses winged valve rotators in this type valve spindle is fitted with wings (vanes) the kinetic energy of the exhaust gas rotates the valve at small amount as it passes