Internal-combustion engines – Poppet valve operating mechanism – Tappet
Reexamination Certificate
2002-12-24
2004-05-04
Denion, Thomas (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
Tappet
C074S569000
Reexamination Certificate
active
06729288
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a lash adjuster for automatically adjusting the valve clearance of a valve gear in an internal combustion engine.
In a valve gear for opening and closing an intake valve or an exhaust valve (hereinafter simply referred to as a valve) by rotating a cam, a valve clearance is automatically adjusted by means of a lash adjuster.
As a lash adjuster of this type, the present applicant has already proposed a thread type lash adjuster which can reduce the axial length of the valve gear, is small in the number of parts and easy to assemble (JP patent publication 11-62519).
With the lash adjuster of this publication, as shown in
FIG. 14
, a nut member
8
is fixed e.g. by brazing to the bottom surface of an end plate
2
of a lifter body
1
. A threaded hole
3
is formed in the nut member
8
. An adjust screw
4
threadedly engaging in the threaded hole
3
is axially biased by an elastic member
5
mounted in a closed end of the threaded hole
3
. Female threads
3
a
of the threaded hole
3
and male threads
4
a
formed on the adjust screw
4
are serration-shaped so that the flank angle of pressure flanks
6
, which bear push-in loads applied to the adjust screw
4
, is larger than the flank angle of clearance flanks
7
. The serrated threads have such a lead angle that under the bias of the elastic member
5
, the adjust screw
4
will move in the axial direction while turning.
The lash adjuster having such a structure is, as shown in
FIG. 14
, mounted between a cam
10
and a valve stem
12
provided on a valve
11
to press the end face of the valve stem
12
against the end face of the adjust screw
4
by the bias of a valve spring
13
for biasing the valve stem
12
toward the cam
10
and serves to transmit the pressing force to the cam
10
through the lifter body
1
so that the valve
11
will be opened and closed as the cam
10
rotates.
In mounting such a lash adjuster, if a valve clearance is produced between the valve stem
12
and the adjust screw
4
, by the biasing force of the elastic member
5
, the adjust screw
4
will move in the axial direction along the clearance flanks
7
while turning, thus absorbing the valve clearance.
Also, when the adjust screw
4
receives a push-in force from the valve stem
12
, it will retract until axial clearances formed between the female threads
3
a
and the male threads
4
a
disappear. When a further push-in force is applied, it will be borne by the pressure flanks
6
, which prevent the adjust screw
4
from retracting while turning.
If the push-in load applied from the valve stem
12
to the adjust screw
4
is a continuously acting varying load, the adjust screw
4
will retract axially while turning to a position where the minimum value of the varying load becomes zero, thereby keeping uniform the valve clearance.
With the already proposed lash adjuster, when the push-in force is applied to the adjust screw
4
, the circumferential component of the vertical resistance produced between the thread surfaces gives the adjust screw
4
a turning force.
At this time, if the frictional force working between the thread surfaces is large enough to give a sufficient turning resistance, the adjust screw
4
is prevented from retracting. But if the frictional force is insufficient, the adjust screw
4
will retract while turning, so that the valve lifting amount decreases. If the retracting amount exceeds the ramp of the cam
10
, the valve
11
will get impulsively seated on the valve seat
18
, producing an abnormal sound.
The surface roughness of the pressure flanks
6
of the female threads
3
a
and the male threads
4
a
greatly influences the frictional force between the thread surfaces. Generally, for oil-lubricated frictional surfaces, the larger the surface roughness, the higher the friction coefficient. Thus, with a lash adjuster as described above, it is preferable that the surface roughness of the female threads
3
a
and the male threads
4
a
is large.
Mass-produced screws are typically manufactured by rolling in view of lower cost and high strength. The surface roughness of flanks (thread surfaces) of screws formed by rolling is relatively small.
The inventors of the present invention formed the male threads
4
a
of the adjust screw
4
of the lash adjuster by rolling, and measured the valve lifting amount in a revolving number sweeping test for a lash adjuster in which is mounted this adjust screw
4
. The test results are shown in FIG.
5
. The surface roughness Ra of the flanks
6
and
7
of the male threads
4
a
of the adjust screw
4
was 0.1.
In the graph of
FIG. 5
, the bent line A
1
at the lower part of the graph shows the number of revolutions of the crankshaft. It linearly increases from the idling revolution 800 (r/min) to MAX 6000 (r/min) and linearly decreases back to 800 (r/min).
The upper part of the graph shows a lifting curve B
1
of the valve
11
. In the graph, only one lifting curve is shown enlarged. But actually, such lifting curves appear continuously in the direction of the horizontal axis (that is, time axis) of the graph. The density of lifting curves are coarse in a region where the number of revolutions of the crankshaft is low and it increases as the number of revolutions of the crankshaft increases. Displaying such lifting curves is difficult. Thus, they are shown with the valve closed positions and the valve open positions connected by lines. In the graph of
FIG. 5
, the upper line C shows the valve closed position and the lower line D shows the valve open position.
As is apparent from the test results, if the surface roughness of the threaded surfaces of the male threads
4
a
of the adjust screw
4
is relatively smooth, under some conditions of the lubricating oil, a sufficient frictional force would not be obtained between the flank surfaces, so that stoppage upon loading is delayed momentarily, thus reducing the valve lifting amount.
An object of this invention is to provide a lash adjuster in which the valve clearance is automatically adjusted by axial movement of the adjust screw having serrated male threads, and the valve lifting amount is kept constant by providing a sufficient frictional resistance between the thread surfaces during loading for the adjust screw to stop.
With the lash adjuster mounted in a direct type valve gear shown in
FIG. 11
, due to displacement of the relative position of the cam
10
and the lifter body
1
the lifter body tends to turn under a turning force by contact with the cam
10
. Also, the valve
11
, too, tends to turn according to operating conditions. These rotations are not constant in the rotating direction and rotating speed. When such a rotation occurs, the thread engagement portions between the threaded hole
3
a
and the adjust screw
4
are acted by torsional moment produced due to a relative turning between the lifter body
1
and the valve
11
.
If such a torsional moment is relatively small, even if it is a turning force in such a direction as to push in the adjust screw
4
, frictional force between the pressure flanks
6
will prevent the adjust screw from slip-turning.
On the other hand, the torsional moment applied to the thread engagement portions between the adjust screw
4
and the threaded hole
3
a
depends on the relative turning speed of the lifter body
1
and the valve
11
and the frictional force between the end faces of the valve stem
12
and the adjust screw
4
, so that its intensity and direction vary according to conditions such as the kind of engine, the number of cylinders or the number of revolutions.
If the torsional moment should be inputted at such an intensity as to overcome the frictional force between the pressure flanks
6
in such a direction as to push in the adjust screw
4
, the adjust screw would retract, thereby varying the valve lifting amount. If the retracting amount exceeds the ramp height of the cam
10
, the valve
11
may be impulsively seated on the valve seat
18
, thus producing abnormal sound.
The second object of this inve
Maeno Eiji
Yamamoto Ken
Denion Thomas
Eshete Zelalem
Wenderoth , Lind & Ponack, L.L.P.
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