Figure 3-48.Hydraulic tappets.
mushroom tappets. Another variation is the roller
tappet. It has a roller contacting the camshaft and is
used mostly in heavy-duty applications.
HYDRAULIC tappets are very popular in
overhead valve engines. They use oil under pressure to
maintain zero clearance in the valve mechanism
automatically. The lifter body, which contacts the
camshaft lobe, is hollow. Inside the lifter body, there is a
plunger that operates the valve mechanism. Injecting
oil into the cavity under the plunger regulates its height,
thereby adjusting valve mechanism clearance. The
hydraulic lifter operates as follows (fig. 3-48): oil,
supplied by the engine lubrication system, reaches the
lifter body and enters it through passage (1). The oil
then passes through passage (2) to fill the plunger. The
oil then passes through passage (3) where it pushes the
check valve off its seat to enter the cavity under the
plunger. As oil fills the cavity, it pushes the plunger up
to where it contacts the valve mechanism. When the
camshaft pushes the lifter body up, the oil is trapped in
the cavity and cannot escape because the check ball
seals the opening. This trapped oil then becomes a solid
link between the lifter body and the plunger. The
constant pressurized supply of oil will maintain zero
clearance in the valve mechanism.
The face of the tappet and the lobe of the camshaft
are designed so the tappet rotates during operation (fig.
3-49). The cam lobe is machined with a slight taper that
Figure 3-49.Tappet-to-can lobe relationship.