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Curved-top tappets

For those familiar with the design of overhead cam engines, the most common type of cam follower used in bespoke race engines of this type is the solid 'bucket' tappet. These are akin to the flat-faced followers used in pushrod engines, except that the load path is directly from the cam lobe to the lash-cap. They have much to recommend them - they are a geometrically simple part, and lots of people understand how to design and make them. So few people outside Formula One use finger-follower valvetrains that not many companies offer finger followers on a commercial basis.

A fundamental limitation of the inverted bucket tappet is the opening and closing velocity of the valve. The valve spacing dictates the spacing of the tappet axes and the maximum diameter of the tappet. This maximum diameter in turn limits the valve opening velocity. If we assume that the follower's axis intersects the camshaft axis (most do) then it can be proved that the instantaneous distance from the follower axis to the cam-to-follower contact is equal to the lift velocity, when the velocity is measured in mm (or any other unit of length) per radian of cam rotation.

If we had a 30 mm diameter tappet, the maximum velocity - assuming we were happy to sweep the cam profile contact point right to the edge of the tappet - would be 15 mm per radian. In practice, the limit is lower than this, owing to edge detailing on the top of the tappet and a desire to keep a certain finite width of lobe sitting flat on the face of the tappet. If we wish to use a symmetrical lift profile, it is unlikely that we will want there to be an offset between the camshaft axis and the follower axis, lest we limit velocities further.

If the valvetrain designer cannot increase the diameter of the follower then he can avail himself of extra velocity by using a tappet with a curved top. This has the same geometric effect as changing from a flat-faced lifter to a roller-lifter on a pushrod engine. While maintaining the same valve lift profile, the point of contact remains closer to the follower axis for any value of pressure angle and follower curvature. For a given pressure angle, a smaller cam follower curvature decreases the distance from the follower axis to the point of contact. This can be very handy if your new valve lift profile developed with your engine simulation software overhangs the edge of a flat tappet and you are prevented by engine geometry from increasing the tappet size.

Followers with curved contact surfaces have disadvantages though. They are taller and heavier than their flat counterparts, and the increase in mass can require a different valve spring. Such followers also require there to be an anti-rotation feature on the follower which locates with a corresponding feature in the follower bore. In order to maintain the correct alignment of the follower with the cam lobe, we therefore need to increase the complexity and cost of the follower and the machining in the cylinder head or cam carrier.

Written by Wayne Ward

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