Ratchet tensioner with backlash

Details Ratchet tensioner with backlash Ratchet tensioner with backlash

2001-02-02

2002-11-12

Bucci, David A. (Department: 3682)

Endless belt power transmission systems or components

Means for adjusting belt tension or for shifting belt,...

C474S110000, C474S109000

Reexamination Certificate

active

06478703

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ratchet tensioner with backlash used to provide an appropriate tension to a timing chain of a vehicle engine and the like.
2. Description of the Related Art
In a chain drive for transmitting a rotating force between a crankshaft and a camshaft of a car engine, a ratchet tensioner having backlash as shown in
FIG. 3
has been widely used for applying an appropriate tension to the slack side of a timing chain, thereby suppressing vibrations generated during travel of the timing chain.
FIG. 3
shows in cross section an example of use of the conventional ratchet tensioner having backlash. In
FIG. 3
, the ratchet tensioner
1
with backlash (hereinafter, simply referred to as “tensioner”) is attached to an engine body on the slack side of a timing chain
6
stretching over a driving sprocket
3
rotated with the crankshaft
2
and a driven sprocket
5
fixed to the camshaft
4
.
The tensioner
1
has a plunger
8
retractably projecting from a front surface of a tensioner housing
7
. The plunger
8
has a front end held in pressure contact with a back surface in the vicinity of the distal or free end of a tensioner lever
10
pivotally mounted on the engine body by means of a support shaft
9
, so that a shoe surface
11
of the tensioner lever
10
is held in slide contact with the slack side of the timing chain
6
to thereby provide a tension to the timing chain
6
.
The tensioner housing
7
has a plunger-accommodating hole
12
formed therein for slidably receiving therein the plunger
8
. The plunger
8
has a hollow section
13
formed therein, and the hollow section
13
one end opening to an end face of the plunger
8
opposite to the bottom of the plunger-accommodating hole
12
. A plunger spring
14
comprised of a compression coil spring is disposed, in a loaded or otherwise compressed state, between the bottom of the plunger-accommodating hole
12
and the bottom of the hollow section
13
, so that the plunger spring
14
normally urges the plunger
8
in a direction to project outward from the housing
7
, this direction being hereinafter referred to as “projecting direction”.
FIG. 4
is a partially enlarged cross-sectional view of the tensioner
1
shown in FIG.
3
. As shown in
FIG. 4
, a series of rack teeth
15
are formed on a circumferential surface of the plunger
8
along one side of the plunger
8
. The rack teeth
15
are spaced at equal pitch, and the total length of the rack teeth
15
is set in accordance with an expected slack of the timing chain
6
. The rack teeth
15
are formed such that the rack angle &agr; at the forward side of the plunger
8
and the rack angle &bgr; at the backward side of the plunger
8
are both 45. Also, as shown in
FIG. 4
, Y represents the height of the rack teeth
15
, and X represents half pitch of the rack teeth
15
. The term “rack angle &agr; at the forward side of the plunger
8
” used herein is intended to refer to an angle formed at the bottom of each tooth space between a line perpendicular to the axis of the plunger
8
and a flank of the rack tooth
15
located on the forward side of the perpendicular line. Similarly, the term “rack angle &bgr; at the backward side of the plunger
8
” used herein is intended to refer to an angle formed at bottom of the same tooth space between the perpendicular line and a flank of the rack tooth
15
located on the backward side of the perpendicular line.
As shown in
FIG. 4
, a ratchet pawl
17
is pivoted by a shaft
16
to the tensioner housing
7
at a position opposite to the rack tooth
15
. The ratchet pawl
17
is always urged in the clockwise direction by the force of a ratchet spring
18
acting between the ratchet pawl
17
and the tensioner housing
7
. The ratchet pawl
17
has a first prong
17
a
formed on a lower edge thereof for meshing engagement with the rack teeth
15
to prevent the plunger
8
from moving in the backward direction, and a second prong
17
b
formed on the lower edge of the ratchet pawl
17
and spaced backward from the first prong
17
a
by a predetermined distance for releasing engagement of the first prong
17
a
and the rack teeth
15
when the plunger
8
moves in projecting direction by the force of the plunger spring
14
. first prong
17
a
has a triangle shape corresponding to the tooth shape of the rack
15
at the front end of the lower edge of the ratchet pawl
17
, so that the first prong
17
a
can be closely fit with a pair of rack teeth
15
without a gap. The second prong
17
b
is smaller in height than the first prong
17
a
and is spaced backward from the first prong
17
a
by a distance of three rack tooth pitches, for example. When the first prong
17
a
is closely fit in a tooth space of the rack teeth
15
, the second prong
17
b
is in the non-contact or separated state at a position spaced from the first prong
17
a
by a predetermined distance. The second prong
18
b
, unlike the first prong
17
a
, is not designed to mesh with the rack teeth
15
for preventing movement of the plunger
8
, but operative to engage a rack tooth
15
when the plunger
8
starts to move.
In
FIG. 4
, &thgr; represents a angle of rotation of the second prong
17
b
achieved about the center O of rotation when the second prong
17
b
moves from a first position (indicated by the solid line in
FIG. 4
) in which the first prong
17
a
closely fits in a tooth space of the rack teeth
15
of the plunger
8
, and a second position (indicated by the phantom lines in
FIG. 4
) in which the second prong
17
b
is released from engagement with a rack tooth
15
after having been urged in the counterclockwise direction by the forward movement of the plunger
8
.
FIG. 4A
shows a vector diagram of the force acting on the tip of the first prong
17
a
when the first prong
17
a
re-engages a rack tooth
15
to prevent backward motion of the plunger which would otherwise occur when the plunger
8
is subjected to a force F acting in the backward direction of the plunger
8
.
The amount of backlash of the tensioner
1
shown in
FIG. 4
is the amount of backward movement of the plunger
8
occurring when the plunger
8
, which has been fully projected by the force of the plunger spring
14
due to a decrease in the tension on the timing chain
6
immediately before the disengagement of the second prong
17
b
from the rack tooth
15
, moves backward by the action of a backward force F, thereby allowing the first prong
17
a
to come back into engagement with a rack tooth. The backlash amount is determined by the rotating angle &thgr; of the second prong
17
b.
Tensioners for use with a timing chain of an engine are required to have a small pitch of the rack teeth and a large amount of backlash. The small rack teeth pitch enables continuous tensioning of the timing chain, and the large backlash amount is able to prevent undue tensioning of the chain which may occur due to the difference in thermal expansion coefficient between the chain and the engine when the chain is made of steel and the engine is made of aluminum alloy.
To deal with this problem, the rack teeth pitch shown in
FIG. 4
is made small, as shown in FIG.
5
. In a ratchet tensioner
1
′ shown in
FIG. 5
, the pitch of the rack teeth
15
′ is reduced, and the rack angle &agr; at the forward side of the plunger
8
and the rack angle &bgr; at the backward side of the plunger
8
are both 45. The tensioner
1
′ shown in
FIG. 5
is the same as the tensioner
1
shown in
FIG. 4
, except that the rack teeth pitch is reduced. Accordingly, the like or corresponding parts are designated by the same reference characters, and a further description will be omitted.
However, because in the tensioner
1
′ shown in
FIG. 5
, the pitch of the rack teeth
15
′ is reduced while the rack angle &agr; at the forward side of the plunger
8
and the rack angle &bgr; at the backward side of the plunger
8
are both kept equal to 45, the height Y of the rack teeth
15
′ is relativ

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