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WORKING PRINCIPLE :
Oil enters the hydraulic tappets through a feed
hole in the body and flows into the plunger through
the plunger feed hole, filling the "low pressure
chamber" (E, in Figure)
The oil also flows around the check ball and
through the slots of the ball retainer to fill the cavity
below the plunger, called the "high pressure
chamber" (A, in Figure). Oil is forced down into
this area by momentary low pressure which
occurs once during each cam rotation.
As the hydrauilic tappet rises due to the cam
rotation, the full load of the valve train is applied
on the tappet. A predetermined and closely held
clearance of .0002/.0003 inch (.0051/.0076 mm)
between the outer diameter of the plunger and the
inner diameter of the tappet body allows a
controlled amount of oil to escape up from the high
pressure chamber.
This oil compresses the plunger spring and allows
a small relative movement of the plunger with
respect to the body.
As cam further rotates, the tappets returns to its
original position. At this time, the plunger spring
provides the force to maintain zero back lash in
the valve train mechanism and forces the plunger
back to its original position. This allows engine oil
to once again fill the high pressure chamber so
that the cycle can be repeated during the next cam
rotation.
The overhead oil supply is accurately metered from
the "low pressure chamber", which is at engine oil
pressure, by using a flat metal disk (metering valve
C, in Figure), which wobbles against a cylindrical
radius curved surface on the bottom of the push rod
seat (D, in Figure). Oil flows up through the hole in
the push rod seat to lubricate the overhead valve
train components (oil flow path B, in Figure).
The metering valve moves sufficiently to keep the
push rod seat hole free and unclogged. When the
engine is switched off, the valve seats on the
shoulder of the plunger to minimize drain back of
oil.
If the engine structure or valve train expands or
contracts with changes in engine temperature or
other differentials, the hydraulic tappets will
automatically adjust its own internal length to
compensate for these changes.
PRECAUTIONARY MEASURES :
The hydraulic tappets have minute holes for the
oil to circulate and to also act at the hydraulic
media, hence oil contamination must be at the
minumum. The oil and the filters in the engine,
must be changed periodically as specified.
Whenever the tappets are removed for service
OR stored in spares, they must be kept upright
(the push rod seating surface pointing upwards
and the rollers at the bottom pointing downwards)
to prevent the oil from draining off.
04-4
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