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This invention relates to a hydraulic lash adjuster used
in valve gears or valve mechanisms for internal combustion
engines.
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A conventional hydraulic lash adjuster comprises a bottomed
cylinder fixed to a cylinder head and a plunger accommodated in
the cylinder so as to be vertically moved. The plunger has an
upper end protruding from the cylinder. A rocker arm is supported
on the upper end of the plunger. The interior of the plunger
serves as a low-pressure chamber. A lower interior of the cylinder
is divided by a bottom wall of the plunger, thereby serving as
a high-pressure chamber. The bottom wall of the plunger is formed
with a valve port of a check valve. The low-pressure chamber
is filled with a hydraulic fluid supplied from a fluid supply
passage via communication holes formed in the circumferential
walls of the respective cylinder and plunger. Further, the
high-pressure chamber is filled with the hydraulic fluid supplied
via the valve port of the check valve.
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A spherical valve element is accommodated in the
high-pressure chamber and is biased in such a direction that it
closes the valve port. The valve element and valve port constitute
a check valve. When the side of the rocker arm applies a downward
pressing force to the plunger, the valve port is closed by the
valve element such that the high-pressure chamber is tightly
closed, whereupon the hydraulic fluid filling the high-pressure
chamber prevents the plunger from moving downward. Further, when
the plunger is moved upward such that the volume of the
high-pressure chamber is increased and the pressure reduced, the
valve element is moved downward relative to the plunger, thereby
opening the valve port. As a result, the hydraulic fluid flows
from the low-pressure chamber into the high-pressure chamber,
so that the interior of the high-pressure chamber remains filled
with the hydraulic fluid. For example, JP-A-5-288020 discloses
one of hydraulic lash adjusters of the above-described type.
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In the above-noted lash adjuster provided with the check
valve, the valve element collides against a valve seat face of
the valve port every time the valve element opens or closes the
valve port. The valve element is made of a steel having a large
specific gravity, for example, SUJ2, in the conventional lash
adjuster. Accordingly, when the collision of the valve element
is reiterated many times during the operation of the engine, there
is a possibility that the valve seat face may be worn out or the
valve element may bite into the valve seat face thereby inhibiting
the free movement of the valve element.
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Therefore, an object of the present invention is to provide
a lash adjuster in which the wear of the valve seat face can be
reduced and the valve element can be prevented from being caught
by or adhered to the valve seat face.
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The present invention provides a hydraulic lash adjuster
for an internal combustion engine including a cylinder head and
a rocker arm. The hydraulic lash adjuster comprising a bottomed
cylinder fixed to the cylinder head, a plunger having a bottom
wall and an upper end supporting the rocker arm, the plunger being
vertically movable while being brought into sliding contact with
an inner circumferential face of the cylinder, a low-pressure
chamber defined in the plunger and filled with a hydraulic fluid,
a high-pressure chamber defined in a lower interior of the cylinder
and partitioned by the bottom wall of the plunger from the
low-pressure chamber, the high-pressure chamber also being filled
with hydraulic fluid, a valve port formed through the bottom wall
of the plunger so as to communicate with both the low-pressure
and high-pressure chambers therebetween. The valve port having
at the high-pressure chamber side an opening edge formed with
a valve seat face and a valve element provided in the high-pressure
chamber so as to abut and depart from the valve seat face, thereby
closing and opening the valve port. The hydraulic lash adjuster
characterized in that the valve element is made of a material
having a specific gravity smaller than steel.
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The valve element in the above-described construction has
a smaller specific gravity than the conventionally used valve
element of a steel ball. Accordingly, the inertial mass of the
valve element in a collision against the valve seat face is also
reduced. Consequently, the wear of the valve seat face due to
the collision of the valve element against the valve seat face
can be reduced, and the valve element can be prevented from biting
into the valve seat face thereby being caught by, stuck, or
otherwise adhered to the valve seat face, inhibiting the free
movement of the valve element.
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In a preferred form, the valve element is made of a ceramic
containing silicon nitride. The valve element thus has a higher
hardness as compared with the conventionally used valve element
and accordingly, the valve element can be prevented from being
broken or deformed when colliding against the valve seat face.
Consequently, the valve element can fulfil its functions
sufficiently and reliably over a relatively longer lifetime.
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The invention will be described, merely by way of example,
with reference to the accompanying drawings, in which:
- Fig. 1 is a longitudinal section of the lash adjuster in
accordance with one embodiment of the invention, showing the lash
adjuster mounted on the cylinder head;
- Fig. 2 is a longitudinal section of the lash adjuster;
- Fig. 3 is an enlarged section of the valve element closing
the valve port; and
- Fig. 4 is an enlarged section of the valve element opening
the valve port.
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One embodiment of the present invention will be described
with reference to the accompanying drawings. The hydraulic lash
adjuster A, in accordance with the embodiment, is applied to a
valve gear for an internal combustion engine. The valve gear
will firstly be described. The valve gear comprises a valve 41,
the lash adjuster A, a rocker arm 42 and a cam 43. With the rotation
of the cam 43, the rocker arm 42 vertically oscillates with an
upper end of the lash adjuster A serving as a fulcrum, thereby
vertically moving the valve 41, as is well known in the art.
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The lash adjuster will now be described. The lash adjuster
A comprises a cylinder 10 and a plunger 20. The cylinder 10 is
formed into the shape of a bottomed cylinder and includes a
generally circular bottom wall 11 and a generally cylindrical
circumferential wall 12 extending from a circumferential edge
of the bottom wall 11. The cylinder 10 is fixed in a mounting
hole 44, opening in an upper face of a cylinder head 40. The
circumferential wall 12 of the cylinder 10 has a communication
hole 13 extending therethrough. The communication hole 13
communicates with a hydraulic fluid supply passage 45 provided
in the cylinder head 40.
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The plunger 20 is also formed into the shape of a bottomed
cylinder and includes a generally circular bottom wall 21 and
a generally cylindrical circumferential wall 22 extending from
a circumferential edge of the bottom wall 21. An interior of
the plunger 20 serves as a low-pressure chamber 23. The bottom
wall 21 of the plunger 20 has a centrally located circular valve
port 24 vertically extending therethrough. The valve port 24
has a valve seat face 25 formed on a lower (or high-pressure chamber
31 side, as will be described later) opening edge. The valve
seat face 25 comprises a reverse tapered face or an arcuate face
(a rounded face). The circumferential wall 22 of the plunger
20 has a reduced portion 26 formed in an outer circumference thereof.
The reduced portion 26 has an inner communication hole 27 extending
through the circumferential wall 22. The plunger 20 has an upper
end formed with a generally semispherical (or dome-like) support
28. The rocker arm 42 abuts an outer face of the support 28 from
above so that an oscillation fulcrum of the rocker arm 42 is
supported on the support 28. The support 28 has a circular vent
hole 29 which is formed in a central top thereof so as to extend
vertically therethrough.
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The plunger 20 is fitted in the cylinder 10 and vertically
moved while the circumferential face of the plunger 20 is in sliding
contact with the circumferential face of the cylinder 10. Further,
a circumferential communication passage 30 is defined between
the reduced portion 26 of the plunger 20 and the inner
circumferential face of the cylinder 10. The communication
passage 30 communicates with both of the outer and inner
communication holes 13 and 27.
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A high-pressure chamber 31 is defined in the lower interior
portion of the cylinder 10 and partitioned from the low-pressure
chamber 23 by the bottom wall 21 of the plunger 20. A spherical
valve element 33 is provided in the high-pressure chamber 31.
The valve port 24 and the valve element 33 constitute a check
valve 32. The valve element 33 is biased by a first spring 34
in such a direction that it closes the valve port 24 (upward).
The valve element 33 is enclosed in a ball-cage 35, and the first
spring 34 is disposedbetween the ball-cage 35 and the valve element
33. Further, a second spring 36 is provided between the ball-cage
35 and the bottom of the cylinder 10. The second spring 36 urges
both of the plunger 20 and ball-cage 35 upward. A hydraulic fluid
is supplied sequentially through the hydraulic fluid supply
passage 45 of cylinder head 40, the outer communication hole 13,
the communication passage 30, and the inner communication hole
27, being then stored in the low-pressure chamber 23. The
hydraulic fluid stored in the low-pressure chamber 23 is further
supplied through the valve port 24 into the high-pressure chamber
31.
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When the side of the rocker arm 42 applies a downward pressing
force to the plunger 20, the valve port 24 is closed by the valve
element 33 such that the high-pressure chamber 31 is tightly closed,
whereupon the hydraulic fluid filling the high-pressure chamber
31 prevents the plunger 20 from moving downward. See Fig. 3.
Further, when the plunger 20 is moved upward such that the volume
of the high-pressure chamber 31 is increased and the pressure
is reduced, the valve element 33 is moved downward relative to
the plunger 20 to depart from the valve seat face 25, thereby
opening the valve port 24. See Fig. 4. As a result, the hydraulic
fluid flows from the low-pressure chamber 23 into the
high-pressure chamber 31, so that the interior of the
high-pressure chamber 31 remains filled with the hydraulic fluid.
Upon the stopping of the upward movement of the plunger 20, the
valve element 33 is urged by the first spring 34to abut the valve
seat face 25, whereby the valve port 24 is closed. As a result,
the high-pressure chamber 31 is filled with the hydraulic fluid
and tightly closed.
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In the check valve 32, the valve element 33 collides against
the valve seat face 25 of the valve port 24 each time during the
opening or closing of the valve port 24. In the prior art, the
valve element is made of a steel having a large specific gravity,
for example, SUJ2. Accordingly, when the collision of the valve
element is reiteratedmany times during the operation of the engine,
there is a possibility that the valve seat face may be worn out
or the valve element may bite into the valve seat face thereby
inhibiting the free movement of the valve element.
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In view of the above-noted problem, the valve element 33
is made of a ceramic containing silicon nitride in the embodiment,
instead of the steel ball. The valve element 33 made of the silicon
nitride containing ceramic has a higher hardness as compared with
the conventionally used valve element and accordingly, the valve
element 33 can be prevented from being broken or deformed when
colliding against the valve seat face 25. Consequently, the valve
element 33 can fulfil its functions sufficiently and reliably
over a relatively longer lifetime.
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Furthermore, the ceramic valve element 33 has a smaller
specific gravity than the conventional valve element of steel
ball and accordingly, the inertial mass of the valve element 33
in the collision against the valve seat face 25 is also reduced.
Consequently, the wear of the valve seat face 25 due to the collision
of the valve element 33 against the valve seat face 25 can be
reduced, and the valve element 33 can be prevented from biting
into the valve seat face 25 thereby being caught by or adhered
to the valve seat face 25.
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Mechanical characteristics of the silicon nitride
containing ceramic made into the valve element 33 of the lash
adjuster A are as follows. The silicon nitride ceramic has a
specific gravity of 3.2, a hardness of 1500 [HV], a linear expansion
coefficient of 3.2×10-6 [1/□], and a heat-resistant temperature
of 800 [□]. For the sake of comparison, the conventional steel
ball (SUJ2) has a specific gravity of 7.8, a hardness of 750 [HV],
a linear expansion coefficient of 12.5×10-6 [1/□], and a
heat-resistant temperature of 180 [□].
