Internal-combustion engines – Poppet valve operating mechanism – Rocker
Reexamination Certificate
2002-01-22
2003-09-02
Denion, Thomas (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
Rocker
C123S090440, C123S090430, C123S090460, C123S090360, C123S1960CP
Reexamination Certificate
active
06612276
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority, under 35 U.S.C. 119, of earlier-filed Italian Application TO2001A000133, filed Feb. 15, 2001.
BACKGROUND OF THE DISCLOSURE
The present invention relates to a rocker arm for valve trains of internal-combustion engines, and more particularly, to rocker arms for use in valve gear trains of the “end-pivot” rocker arm type.
For a better understanding of the state of the art regarding the subject in question and the problems relating thereto, firstly a rocker arm of known design will be described, with reference to
FIGS. 6
,
7
A and
7
B of the accompanying drawings.
FIG. 6
is a view, partially sectioned longitudinally, of a valve train, generally designated
1
, which is able to cause the alternating rectilinear movement of an engine poppet valve (only a stem
20
of the valve being shown in
FIG. 6
) in accordance with a predetermined opening sequence. The valve train comprises a rocker arm
2
, a hydraulic tappet
3
and a cam-type actuating member
4
.
The mutual arrangement of the above-described components may vary depending on the type of engine and the type of distribution chosen. In particular the present invention relates to rocker arms of the type comprising end portions
5
and
6
able to engage the tappet
3
and the valve, respectively, and an intermediate portion
7
intended to receive a roller
8
co-operating with the cam-type actuating member
4
. An example of an embodiment of a rocker arm of this type is illustrated in detail in
FIGS. 7A and 7B
which show longitudinally sectioned and cross sectional views thereof, respectively.
The operating principle of a valve train of the above-mentioned type is well-known to a person skilled in the art: the rotational movement of a cam shaft (which is not shown, but which rotates the cam-type actuating member
4
) is converted into the alternating rectilinear movement of the valve. Such rectilinear movement is the result of the interaction between the cam member
4
, having a base circle portion
9
and an eccentric profile (lift portion)
10
, and the roller
8
of the rocker arm, said interaction acting so as to cause oscillation of the rocker arm in its own longitudinal plane of symmetry (coinciding with the plane of the sheet showing FIG.
6
), about a fulcrum point located in the zone of contact between the rocker arm
2
and the tappet
3
.
At present this type of rocker arm is advantageously produced by means of the operations of:
(a) shearing of a shaped element
30
(see FIG.
8
), from a sheet of steel with a low carbon content, the element
30
having a form symmetrical with respect to a longitudinal axis
35
and being provided with an opening
36
, in a substantially intermediate position, and with two holes
21
and
22
situated laterally with respect to the opening
36
;
(b) pressing the above-mentioned shaped element
30
in order to perform bending upwards (or downwards) of lateral portions
31
and
32
, along bending lines
33
and
34
, respectively, so as to provide the part with a substantially U-shaped cross section (see FIG.
7
B), having a horizontal plate portion
11
which connects two vertical side walls
12
and
13
;
(c) forming the horizontal plate portion
11
, at the end
5
of the rocker arm, so as to produce a partly spherical portion
14
having a concave surface of revolution
15
with an essentially ogive-shaped section able to engage with an essentially hemispherical convex outer surface
16
at the top of the hydraulic tappet
3
;
(d) forming the above-mentioned horizontal plate portion
11
, at the end
6
of the rocker arm, so as to produce a shoe element
17
having a surface
18
with its concavity directed downwards and an arched cross section (in the plane of oscillation of the rocker arm), able to interact with the top (tip)
19
of the stem
20
of the engine poppet valve; and
(e) inserting and locking a cylindrical pin
23
in the two seats defined by the above-mentioned holes
21
and
22
, the roller
8
being rotatably mounted on the pin
23
by means of rolling elements
24
so as to project partially from the opening
36
in order to engage with the cam member
4
.
The shape and dimensions of the rocker arm
2
are dictated by the design requirements of the engine manufacturer and must therefore be able to satisfy precise geometrical constraints associated with predetermined positions, in the engine cylinder head, of the other valve train elements with which the rocker arm
2
must co-operate. The geometrical constraints determine the arrangement, in the plane of longitudinal symmetry of the rocker arm (coinciding with its plane of oscillation), of three significant points A, B and C, indicated in
FIG. 6
, as follows:
A is a center of the theoretical circumference (or hemisphere) of contact between the engaging surfaces
15
and
16
of the rocker arm
3
and the tappet
4
, respectively;
B is a center of the pin
23
of the roller
8
; and
C is a theoretical point of contact between the contact surface
18
of the rocker arm
2
and the contact surface on the valve tip
19
of the valve stem
20
.
If the design requirements of the engine cylinder head result in the positioning of the above-mentioned point B at a sufficiently large lateral distance from the straight line passing through the other two points A and C at the opposite end zones
5
and
6
of the rocker arm
2
, respectively, the rocker arm may be manufactured by means of simple shearing and bending operations, with low production costs.
In order to clarify this point, it should be noted, with reference to
FIG. 8
, how the central opening
36
of the semi-finished product
30
has an elongated shape in the longitudinal direction, with an intermediate section
37
having a transverse dimension, or width, which is smaller than that of two longitudinal end sections
38
and
39
and how the two holes
21
and
22
are positioned opposite the above-mentioned intermediate section
37
. The width of the intermediate section
37
of the opening
36
cannot be less than a certain minimum value imposed by the technological constraints associated with the feasibility of the shearing operation. Consequently, the width of internal flanges
41
and
42
located between the intermediate section
37
and the holes
21
and
22
, respectively, has an upper limit value, once the dimensions of the above-mentioned holes and their distance from the axis
35
have been fixed.
In the design situation where the center B of the pin
23
is located underneath the straight line joining the end points A and C, the operation of bending of the lateral portions
31
and
32
of the semi-finished product
30
into a “U” is performed downwards, along the bending lines
33
and
34
. These bending lines, viewed in the longitudinal plane of symmetry of the rocker arm, are substantially parallel to the straight line passing through the points A and C. Observing, in
FIG. 8
, the geometry of the shaped element
30
, it can be easily understood that, if the distance of the point B from the straight line passing through the points A and C is fairly large, then the maximum width of the flanges
41
and
42
is sufficient to perform the function of laterally containing the rolling elements
24
(usually rollers) of the roller
8
.
When, on the other hand, the center of the pin
23
(point B) must be located above the straight line joining the end points A and C, the above-mentioned operation of bending into a “U” shape is performed upwards, again along the lines
33
and
34
. The two internal flanges
41
and
42
are thus positioned, at the end of bending, underneath the holes
21
and
22
of the pin and therefore must no longer perform the function of laterally containing the rolling elements of the roller, but must ensure the necessary flexural stiffness of the rocker-arm body. If, therefore, the distance of the point B from the straight line A-C, i.e. the distance of the centers of the holes
21
and
22
from the bending lines
33
and
34
, respectively, is
Denion Thomas
Eaton Corporation
Kasper L. J.
Riddle Kyle
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