Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to connecting rods. More
particularly, it relates to connecting rods used in
internal combustion engines and compressors to connect a
crankshaft to a piston.
In internal combustion engines and other machines,
connecting rods transmit the reciprocating motion of the
pistons to the crankshaft and thereby convert
reciprocating motion to rotary motion. Typically,
connecting rods have been made by a die-casting process
which facilitates providing the shank of the rod with an
indented or "I-beam" shape in order to reduce weight
without sacrificing strength. To reduce manufacturing
costs and achieve further reduction in weight, efforts
have been made to produce connecting rod parts by an
extrusion process, often from a light weight material
such as aluminum and even plastic. The extrusion process
produces a connecting rod whose shank is unindented,
i.e., it lacks the typical "I-beam" cross-sectional
shape.
It has been found that the I-beam shape of the
conventional connecting rod diffuses the high stresses to
which the shank is subjected at the bottom of the
aperture which accepts the wrist pin of the piston (the
"wrist pin holen). Because an extruded rod lacks this
indented I-beam shape, a severe hoop and tangential
stress condition can occur at that location. This stress
condition produces high cycle fatigue which causes cracks
to propagate down the center line of the shank from the
bottom of the wrist pin hole.
Machining the extruded rod to create the I-beam
shape would be expensive and could also weaken the rod
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excessively if it were made out of a light weight
material such as aluminum.
Accordingly, there is a need for a way to prevent
the excessive stress condition which is inexpensive and
which does not unduly weaken the rod.
The present invention provides a connecting rod for
connecting a piston to a crankshaft, comprising a beam
made of one or more components, the beam having
lengthwise an elongated intermediate portion between a
first end portion and a second end portion, wherein the
first end portion includes a first aperture located on a
first axis and adapted for connecting the beam to the
piston, wherein the second end portion includes a second
aperture located on a second axis and adapted for
connecting the beam to the crankshaft, wherein the beam
includes a third aperture which is located on a third
axis nearer to the first aperture than to the second
aperture.
In one embodiment of the invention, the intermediate
portion of the beam, which lies along a longitudinal
axis, has an unindented cross-sectional shape in a plane
perpendicular to the longitudinal axis. In another
embodiment, the third aperture is a cylindrical through-
bore which is located nearer to the first aperture than
to the second aperture. Other aspects of the invention
relate to sizing and locating the third aperture.
While the invention necessitates little additional
manufacturing cost and substantially preserves the
strength of the connecting rod, it diffuses the stress to
which the rod is subjected at the bottom of the wrist pin
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hole and thereby reduces cracks and failures which result
from high cycle fatigue.
These and other features and advantages of the
invention will be apparent from the description which
follows. The preferred embodiments will be described in
reference to the accompanying drawings. These
embodiments do not represent the full scope of the
invention. Rather, the invention may be employed in
other embodiments.
In the drawings:
Figures 1 and 2 are perspective and front
elevational views respectively of a connecting rod
embodying the present invention;
Figure 3 is an enlarged partial cross-sectional view
taken along line 3-3 of Figure 2;
Figure 4 is an enlarged cross-sectional view taken
on line 4-4 of Figure 2; and
Figure 5 illustrates the indented or I-beam cross-
sectional shape of the shank of a typical die cast
connecting rod.
Figures 1 and 2 illustrate a connecting rod 10 made
of two parts, a forked member 12 and an arcuate cap 14,
fastened together by two bolts 16. The assembled
connecting rod comprises a beam having a shank 17
("intermediate portion") between a piston end 18 ("first
end portion"), which is adapted for connecting the rod to
the wrist pin of a piston (not shown), and a crankshaft
end 20 (nsecond end portionn), which is adapted for
connecting the beam to the crankshaft. The boundaries of
the portions are identified by lines 19. The shank 17
lies along a longitudinal axis 21.
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The piston end 18 includes a knuckle 22 of enlarged
width which has an aperture ("first aperture") in the
form of a cylindrical through bore 24 ("wrist pin hole"
24) oriented on a first axis 26. The wrist pin hole 24
is designed to accept the wrist pin of a piston (not
shown). In some embodiments, a bearing could be inserted
in the wrist pin hole 24, but no bearing is included in
the embodiment of the drawings herein.
The crankshaft end 20 is generally a rectangular
base having an aperture ("second aperture") in the form
of a through bore 28 (ncrankpin hole" 28) oriented on
second axis 30. The crankpin hole 28 is designed to
accommodate a crankpin on the crankshaft of the engine.
As illustrated particularly by Figure 3, the shank
17 has a simple rectangular cross-sectional shape--i.e.,
it does not have an I-beam cross-sectional shape (such as
shown in Figure 5) or any other indented cross-sectional
shape which is typical of connecting rods made from a die
casting process. As noted above, the unindented cross-
sectional shape of shank 17 results in excess stress
conditions on the shank 17 at the bottom 32 of the
crankpin hole 24. The excessive stress condition is
believed to result from the relative lack of compliance
of the shank near the bottom 32 of the wrist pin hole 24.
This lack of compliance causes load to accumulate on the
surface of crankpin hole 24 near the bottom 32. This
loading accumulates both circumferentially and in the
direction of first axis 26. It is believed that in
conventional connecting rods the indentation (33a in
Figure 5) of the shank results in the distribution of
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these loads downward along the unindented edges of the
shank (33b in Figure 5).
As shown in Figures 1, 2 and 4, forked member 12
includes an aperture ("the third aperture") in the form
of a stress relief hole 34 which extends through forked
member 12 along the third axis 36 and which is located
near wrist pin hole 24. The stress relief hole 34 is
sized and placed to relieve the local stiffness in the
forked member 12, which helps to produce a more uniform
stress field at the bottom 32 of the wrist pin hole 24.
The third aperture need not extend completely through the
forked member 12. There could be more than one third
aperture.
A cylindrical stress relief hole 34 can be
manufactured very economically by drilling. A circular
shape also appears to be most effective in achieving the
desired diffusion of stress. Some other shapes, such as
an arched-shaped hole, may increase rather than reduce
stress concentration. Centering the stress relief hole
widthwise (i.e., along the lengthwise centerline of
forked member 12) appears to be more effective than
offsetting one or more holes. The appropriate size and
location of the stress relief hole 34 depends on the
geometry of the piston end 18, particularly on the
relative size of the wrist pin hole 24. The stress
relief hole 34 needs to be located far enough away from
the wrist pin hole 24 to prevent undue variation in the
shape of wrist pin hole 24 in operation, which could
cause excessive wear. However, the stress relief hole 34
should be near enough to wrist pin hole 24 so that the
local flexibility in forked member 12 resulting from
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stress relief hole 34 can effectively distribute of the
stress resulting from the load at the bottom 32 of wrist
pin hole 24.
Finite element analysis has been used successfully
to size and locate the stress relief hole 34. In an
extruded aluminum rod, best results have been obtained
when a cylindrical stress relief hole 34 is centered
widthwise on the shank and one more of the following
relationships pertain: the ratio of the cross-sectional
area of the wrist pin hole 24 to the cross-sectional area
of the stress relief hole 34 is in the range of 5 to 10;
the ratio of the diameter of the stress relief hole 34 to
the width of the shank at the location of stress relief
hole 34 is in the range of 0.3 to 0.4; the ratio of (a)
the shortest distance from the circumference of stress
relief hole 34 to the bottom 32 of wrist pin hole 24 to
(b) the shortest distance from the circumference of
stress relief hole 34 to the edge of the shank 17 is in
the range of 1.0 to 1.6.
In one particular application, the forked member 12
and the cap 14 were extruded from an aluminum alloy 6061.
The overall length of the connecting rod 10 was 160 mm
and its thickness (i.e., the dimension along an axis
parallel to first axis 26) was 26 mm. The width of the
knuckle 22 was 29 mm and the width of shank 17 at its
narrowest point was 11 mm. The wrist pin hole 24 had a
diameter of 19 mm. It was found that effective stress
relief occurred when the stress relief hole 34 had a
diameter of 7 mm and its center was located 20 mm from
the center of the wrist pin hole 24. The axes 26, 30 and
36 were parallel. The invention is not limited to a
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connecting rod with the foregoing specifications.
Rather, they are given as an example of an embodiment of
the invention.
