Endless belt power transmission systems or components – Positive drive belt – Belt formed of rigid links
Reexamination Certificate
2000-12-28
2002-10-08
Bucci, David A. (Department: 3682)
Endless belt power transmission systems or components
Positive drive belt
Belt formed of rigid links
Reexamination Certificate
active
06461263
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a silent chain drive mechanism for transmitting power between sprockets with different numbers of teeth as achieved when driving camshafts from a crankshaft of a four-stroke engine.
In a four-stroke engine shown in
FIG. 6
hereof, a silent chain drive mechanism used for driving camshafts from a crankshaft has two sprockets
11
a,
11
a
on the camshafts side for operating intake valves and exhaust valves, respectively, a sprocket
11
b
on the crankshaft side, a silent chain S for transmitting power between the crankshaft sprocket
11
b
and the camshaft sprockets
11
a,
a tensioner T for applying tension to the silent chain S, a tensioner lever L, and a chain guide G for suppressing run-out of the silent chain S.
The intake and exhaust valves are designed to complete a single cycle of operation while the crankshaft sprocket
11
b
makes two revolutions. Thus, the rotational speed of the crankshaft sprocket
11
b
must be reduced to half at the camshafts. To this end, the number of teeth of each of the camshaft sprockets
11
a
is twice the number of teeth of the crankshaft sprocket
11
b.
The silent chain S used in such silent chain drive mechanism is exemplified in Japanese Patent Laid-open Publication No. Hei-8-184348, which includes, as shown in
FIGS. 7 and 8
hereof, a series of longitudinally overlapping link plates
13
(three,
13
a,
13
b
and
13
c,
being shown) having teeth adapted to mesh with a sprocket (not shown). When the chain is stretched substantially straight, rectilinearly profiled outside flanks
15
of the respective link plates
13
a-
13
c
form a series of teeth having a trapezoidal profile.
In one specific form of the exemplified silent chain, when the silent chain is stretched substantially straight as shown in
FIG. 7
, inside flanks
16
of one link plate
13
b
are retracted from the outside flanks
15
of the adjacent link plates
13
a,
13
c.
In another specific form of the exemplified silent chain as shown in
FIG. 8
, when the silent chain is stretched substantially straight, barreled or convexly arcuate inside flanks
16
a
of one link plate
13
b
project outward of the outside flanks
15
of the adjacent link plates
13
a,
13
c.
The crankshaft sprocket
11
b
and the camshaft sprockets
11
a
are formed by a sprocket tooth hobbing process, such as shown in
FIG. 9
hereof and described in Japanese Patent Application No. Hei-1-037745. In the hobbing process, a hob cutter with teeth having a profile similar to that of a series of trapezoidal teeth formed by the outside flanks
15
of the longitudinally overlapping link plates
13
described previously is used. The hob cutter cuts the periphery of a cylindrical workpiece with an amount of addendum modification set in such a manner that the silent chain, as it is wrapped around the sprocket, forms an equilateral polygon with one side equal in length to a pitch P of the silent chain. Thus, an involute tooth form is produced.
In the case where sprockets with different numbers of teeth are to be formed to achieve a reduction gear ratio of 1:2, the crankshaft sprocket
11
b
and the camshaft sprocket
11
a
are both cut or hobbed by using a hob cutter profiled to have a pressure angle Ah equal to the pressure angle Af defined by the outside flanks of the silent chain. As an alternative, in order to avoid undercut or root thinning caused by the number of teeth of the crankshaft sprocket
11
b
which is half the number of teeth of the camshaft sprocket
11
a,
teeth of the crankshaft sprocket
11
b
are cut by using a hob cutter having a pressure angle larger than the pressure angle Af of the outside flanks
15
of the silent chain.
Thus, the tooth flank configurations of the camshaft sprocket
11
a
and crankshaft sprocket
11
b
are formed by involute curves arranged to satisfy the expression
A
2
≧
A
1
=
Af
where A
1
is the pressure angle of the camshaft sprocket
11
a,
A
2
is the pressure angle of the crankshaft sprocket
11
b,
and Af is the pressure angle defined by the outside flanks
15
of the link plates.
A silent chain drive mechanism comprised of the above silent chain S, camshaft sprockets
11
a
and crankshaft sprocket
11
b
for driving the camshafts exhibits a meshing condition shown in
FIGS. 10 and 11
.
First, consider a silent chain S having link plates whose inside flanks are retracted from the outside flanks of the adjacent link plates when the silent chain is stretched substantially straight as shown in FIG.
7
. When such silent chain S is wrapped around the sprocket
11
(
11
a,
11
b
), the outside flanks
15
of the link plates
13
are seated on the tooth flanks of every other sprocket tooth to assume meshing with the sprocket
11
in a polygonal fashion, while the inside flanks
16
of the link plates
13
do not interfere with the tooth flanks of either of the camshaft sprockets
11
a
and the crankshaft sprocket
11
b
because they are retracted from the outside flanks.
However, since the link plates
13
of the above silent chain S mesh at their outside flanks
15
alone with the teeth of each sprocket
11
a,
11
b,
the silent chain S makes, upon commencement of meshing, a polygonal motion involving up and down movements in a radial direction referring to the sprocket.
A quantitative analysis was made on the polygonal motion. As can be appreciated from
FIG. 10
, since a pitch angle &thgr; of the teeth of the sprocket
11
and the number of teeth Z of the sprocket
11
have relations expressed by &thgr;=(360°/Z), the following may be established.
First, when the chain pitch is P and a free span of the chain following the link plate
13
b
is positioned at a right angle to a line passing over the centers of the sprocket
11
and a connector pin
12
a,
as shown in
FIG. 10
, the distance from the center of the sprocket
11
to the connector pin
12
a
of the free span chain is determined by the height of the connector pin
12
a
from the center of sprocket
11
because of the outside flanks of the link plate
13
a
being seated on the tooth flanks of the sprocket
11
, and thus expressed by U=P/2 sin(&thgr;/2).
Next, as shown in
FIG. 11
, when the sprocket
11
is rotated a half pitch angle &thgr;/2 from the position of FIG.
10
and the free span chain is positioned at a right angle to a line passing over the centers of the sprocket
11
and the link plate
13
b,
the distance from the center of the sprocket
11
to the chain pitch line of the free span chain is determined by the height of the connector pin
12
a
from the center of sprocket
11
because of the outside flanks of the link plate
13
a
being seated on the tooth flanks of the sprocket, and thus expressed by V=P/2 tan(&thgr;/2). In
FIGS. 10 and 11
, the arrowhead indicates the direction of rotation of the sprocket
11
, and the profiled arrow head indicates the direction of travel of the silent chain.
As can be readily appreciated from the above discussion, the silent chain S just started meshing engagement with the sprocket
11
makes a polygonal motion of amplitude Hs=U−V involving up and down movements during half pitch angle (&thgr;/2) rotation of the sprocket
11
. Thus, the silent chain S displays a so-called “chordal action”.
Consequently, when the thus-arranged silent chain is used, the chordal action (polygonal motion) of amplitude Hs=U−V and associated up and down movements cause the outside flanks
15
of the link plates
13
a,
13
b ,
13
c
to hit the tooth flanks of the sprocket
1
, resulting in an undesired beating motion. In addition, when the silent chain drive mechanism is operated at a high speed, undesired vibratory and impact sounds are produced, which form a main source of vibrations and noises generated in the cam drive mechanism of a four-stroke engine.
Furthermore, the chordal action produced by the polygonal motion causes the tension of the chain to change, thereby deteriorating the durability of the silent chain.
Next, consider a silent chain S havin
Horie Hiroshi
Sonoda Masatoshi
Suzuki Kenshi
Bucci David A.
Howson & Howson
Johnson Vicky A.
Tsubakimoto Chain Co.
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