Note: Descriptions are shown in the official language in which they were submitted.
CA 02773016 2012-03-28
TENSIONER AND ENDLESS DRIVE ARRANGEMENT
Field of Invention
[0001] The invention relates generally to the field of tensioners for
an endless
drive, and more particularly to a belt drive arrangement for a starter-
generator unit
which uses a Y tensioner.
Background of Invention
[0002] An ever increasing number of engines having a starter-generator
unit
have been developed since the 1990s in order to improve fuel mileage. In such
engines,
the combustion process is stopped when the vehicle comes to rest, for example,
at a
stoplight. In this condition the starter-generator unit is operated as a
starter motor to
restart the engine. Once the engine is started, the starter-generator unit can
be
selectively operated as a generator to recharge the batteries.
[0003] The starter-generator unit is mechanically connected to the
engine via an
endless drive such as a belt or chain. The endless drive is subject to tension
fluctuations, particularly as the starter-generator unit shifts its function
between starter
and generator, in which case the tight side and slack side of the endless
drive reverses.
The endless drive tensioning system must handle this and other tension
fluctuations that
occur whilst the engine is operating.
10004] Various dual arm tensioners are known in the art, example of
which are
found in publication numbers DE 102 53 450 Al; EP 1 464 871 Al; US
2004/0171448
Al; EP 1 122 464 Al; and DE 42 43 451 Al. However, the invention seeks to
provide
a more robust solution to effectively compensating for longitudinal shifts
occurring in
portions of the endless drive as a result of a changeover between the tight
side and the
slack side.
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=
Summary of Invention
[0005] According to one aspect of the invention an endless
drive arrangement
for an internal combustion engine is provided. The arrangement includes an
endless
drive guided around an endless driving wheel of the endless drive arrangement.
A
starter-generator unit is connected to the endless driving wheel. A tensioner
with a first
tensioning arm and a second tensioning arm is provided. The first and the
second
tensioning arms are pivotable about a common first pivot axis, wherein the
second
tensioning arm is articulated to the first tensioning arm so as to be spring-
loaded and
pivotable about a second pivot axis located at a distance from the first pivot
axis. A
first tensioning pulley is rotationally connected to the first tensioning arm
about a first
axis of rotation, the first tensioning pulley resting against a first strand
of the endless
drive so as to tension the same. A second tensioning pulley is rotationally
connected to
the second tensioning arm about a second axis of rotation, the second
tensioning pulley
resting against a second strand of the endless drive so as to tension the
same. The first
strand becomes a tight side of the endless drive when the starter-generator
unit operates
as a starter and the second strand becomes the tight side when the starter-
generator unit
operates as a generator.
[0006] According to another aspect of the invention an endless drive
arrangement for an internal combustion engine is provided. The arrangement
includes
an endless drive guided around an endless driving wheel of the endless drive
arrangement. A starter-generator unit connected to the endless driving wheel.
A
tensioner with a first arm and a second arm is provided. The first and the
second arms
are pivotable about a common first pivot axis, wherein the second arm is
articulated to
the first arm and pivotable about a second pivot axis located at a distance
from the first
pivot axis. A coil spring is connected between the first and second arms so as
to bias
the arms towards each other. A first pulley is rotationally connected to the
first arm
about a first axis of rotation, the first pulley resting against a first
strand of the endless
drive so as to tension the endless drive. A second pulley is rotationally
connected to the
second arm about a second axis of rotation, the second pulley resting against
a second
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strand of the endless drive so as to tension the endless drive. The first
strand becomes a
tight side of the endless drive when the starter-generator unit operates as a
starter and
the second strand becomes the tight side when the starter-generator unit
operates as a
generator. The first pivot axis is fixed relative to the engine at a position
that is
substantially in line with a hub force vector experienced by the endless drive
wheel
when the starter generator unit is in a quasi-static mode of operation.
[0007] According to a third aspect of the invention an endless drive
arrangement
for an internal combustion engine is provided. The arrangement includes an
endless
drive guided around an endless driving wheel of the endless drive arrangement.
A
starter-generator unit connected to the endless driving wheel. A tensioner
having a first
arm and a second arm is provided. The first and the second arms are pivotable
about a
common first pivot axis, wherein the second arm is articulated to the first
arm so as to
be pivotable about a second pivot axis located at a distance from the first
pivot axis. A
coil spring is connected between the first and second arms so as to bias the
arms
towards each other. A first pulley is rotationally connected to the first arm
about a first
axis of rotation, the first pulley resting against a first strand of the
endless drive so as to
tension the endless drive. A second pulley is rotationally connected to the
second arm
about a second axis of rotation, the second pulley resting against a second
strand of the
endless drive so as to tension the endless drive. The first strand becomes a
tight side of
the endless drive when the starter-generator unit operates as a starter and
the second
strand becomes the tight side when the starter-generator unit operates as a
generator.
The first pivot axis is fixed relative to the engine and the second pivot axis
floats
relative to the engine, the second pivot axis being eccentrically positioned
away from a
line between the first pivot axis and the first axis of rotation. The distance
between the
first pivot axis and the second pivot axis is at least a third of the distance
between the
second pivot axis and one of the first axis of rotation and the second axis of
rotation.
[0008] According to a fourth aspect of the invention an endless drive
arrangement for an internal combustion engine is provided. The arrangement
includes
an endless drive guided around an endless driving wheel of the endless drive
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arrangement. A starter-generator unit connected to the endless driving wheel.
A
tensioner having a first arm and a second arm is provided. The first and the
second
arms are pivotable about a common first pivot axis, wherein the second arm is
articulated to the first arm so as to be pivotable about a second pivot axis
located at a
distance from the first pivot axis. A coil spring is connected between the
first and
second arms so as to bias the arms towards each other. A first pulley is
rotationally
connected to the first arm about a first axis of rotation, the first pulley
resting against a
first strand of the endless drive so as to tension the endless drive. A second
pulley os
rotationally connected to the second arm about a second axis of rotation, the
second
pulley resting against a second strand of the endless drive so as to tension
the endless
drive. The first strand becomes a tight side of the endless drive when the
starter-
generator unit operates as a starter and the second strand becomes the tight
side when
the starter-generator unit operates as a generator. The first pivot axis is
fixed relative to
the engine and the second pivot axis floats relative to the engine, the first
pivot axis
being situated at a position that is substantially in line with a hub force
vector
experienced by the endless drive wheel when the starter generator unit is in a
quasi-
static mode of operation, the second pivot axis being positioned away from a
line
between the first pivot axis and the first axis of rotation, and wherein the
distance
between the first pivot axis and the second pivot axis is at least a third of
the distance
between the second pivot axis and one of the first axis of rotation and the
second axis of
rotation.
[0009] The tensioner of the foregoing aspects of the invention pivots
about the
common first. This axis enables the first or second tensioning arm to better
align with
the dominant force on the first or second pulley as the starter-generator unit
shifts
between the starter and generator modes. This minimizes the torque about the
second
pivot axis following which the size of the spring needed to tension the two
arms
together can be reduced.
:30
Brief Description of Drawings
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[00010] The foregoing and other aspects of the invention will be more
readily
appreciated having regard to the accompanying drawings, wherein:
[00011] Fig. 1 is a top view of a tensioner according to a preferred
embodiment
of the invention;
[00012] Fig. 2 is a perspective view of the tensioner shown in Fig. 1;
[00013] Fig. 3 is a cross-sectional view of the tensioner taken along a
line 111-111
shown in Fig. 2;
[00014] Fig. 4 shows a model of the tensioner shown in Fig. 1 in a
starter-
generator belt drive arrangement in an initial, quasi-static, position;
[00015] Fig. 5 shows a model of the tensioner in the belt drive
arrangement of
Fig. 4 in a first position where the starter-generator operates as a starter;
[00016] Fig. 6 shows the model of the tensioner in the belt drive
arrangement of
Fig. 4 in a second position where the starter-generator operates as a
generator; and
[000171 Fig. 7 shows the torque characteristics of a starter-generator
unit.
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Detailed Description of Preferred Embodiments
[00018] Fig. 1 is a top view of a tensioner 1 according to a preferred
embodiment
of the invention. The tensioner comprises a first tensioning arm 2 and a
second
tensioning arm 3. The first tensioning arm 2 is pivotable about a first pivot
axis 4. The
second tensioning arm 3 is articulated to the first tensioning arm 2 so as to
be spring-
loaded and so as to be pivotable about a second pivot axis 5. The first
tensioning arm 2
supports a first tensioning pulley 6 that is rotatable about a first axis of
rotation 7 and
the second tensioning arm 3 supports a second tensioning pulley that is
rotatable about a
second axis of rotation 9.
[00019] The second pivot axis 5 is located at a distance from the
first pivot axis
4, that is, the second pivot axis is eccentric relative to the first pivot
axis. More
particularly, the location of the second pivot axis 5 is preferably offset
from a line AA
between the first pivot axis 4 and the first axis of rotation 7 and from a
line BB between
the second pivot axis 5 and the second axis of rotation 9.
[00020] In the illustrated embodiment the tensioning pulleys 6, 8 are
in the form
of belt pulleys. However, it is also possible to design the tensioner as a
chain tensioner
comprising chain sprockets and the tensioner then tensions a chain in the form
of an
endless drive.
[00021] The distance D1 between the first pivot axis 4 and the second
pivot axis
5 is at least a quarter of the distance D2 between the second pivot axis 5 and
the first
axis of rotation 7 and/or the distance D3 to the second axis of rotation 9.
Preferably, the
distance D1 between the first pivot axis 4 and the second pivot axis 5 is at
least a third,
more preferably at least half of the distance D2 between the second pivot axis
5 and the
first axis of rotation 7 and/or the distance D3 to second axis of rotation 9.
Advantageously, the distance D1 between the first pivot axis 4 and the second
pivot axis
5 can also be selected to be approximately as large as the distance D2 between
the
second pivot axis 5 and the first axis of rotation 7 and/or the distance D3 to
second axis
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of rotation 9. In the present exemplary embodiment, the second pivot axis 5 is
disposed
at an approximately equal distance from the first pivot axis 4, the first axis
of rotation 7
and the second axis of rotation 9, i.e., D1, D2 and D3 are approximately the
same.
[00022] The greater the distance D1 between the first pivot axis 4 and the
second
pivot axis 5, the smaller can the distances D2 and D3 be between the second
pivot axis
5 and the first axis of rotation 7 and the second axis of rotation 9. An
opening angle 10
between a first line 11 connecting the second pivot axis 5 to the first axis
of rotation 7
and a line 12 connecting the second pivot axis 5 to the second axis of
rotation 9 can be
selected to be appropriately larger, particularly when the tensioning pulleys
6, 8 are
otherwise in the same position.
[00023] The opening angle 10 can be maintained in the range of 60 and
90 , for
example. The larger the opening angle, the smaller are a first angle 13a (see
Fig. 4)
between a first hub load force introduced by means of the first tensioning
pulley 6 and
first line 11 (or angle a to line AA) as well as a second angle 15 (see Fig.
4) between a
second hub load force introduced by means of the second tensioning pulley 8
and the
second line 12 (or angle 13 to line BB). The smaller the first and the second
angles 13
and 15 (or a and 13), the higher is the respective resulting force component
that is
absorbed as tensile force by the tensioning arm in question. As a result, the
spring force
required for tensioning the tensioning arms 2, 3 becomes smaller. In the case
of an
opening angle 10 in the range of 60 to 90 , the tensioning arms do not open
that
markedly even when the belt tension increases sharply as a result of the
operation of the
belt drive arrangement. That is, the wrap angle of a belt pulley, of which the
strand is
tensioned by the tensioner 1, reduces less sharply.
[000241 In spite of that, longitudinal shifts occurring in the belt,
for example,
during a changeover between the tight side and the slack side, can be
compensated
effectively by the tensioner 1 by means of the distance D1 between the first
pivot axis 4
and the second pivot axis 5 as selected according to the present invention.
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[00025] In the present exemplary embodiment, the first line 11 and a
third line 17
connecting the first pivot axis 4 to the second pivot axis 5 form an obtuse
angle 18 (see
Fig. 1) that is preferably in the range of 140 to 175 . As a result, it is
possible to
reduce the length of the second tensioning arm 3 as compared to a stretched
form of the
first tensioning arm 2 and consequently, a larger opening of the opening angle
10 is
possible, particularly when the tensioning pulleys 6, 8 are in an otherwise
same
position. This additionally favors the maintenance of a good wrap angle and
allows
further reduction in the force required for tensioning the two tensioning arms
2, 3.
[00026] However, it is also possible to provide an angle 18 that is greater
than
175 or even an angle of 180 between the first and the third lines 11, 17. In
other
embodiments, it would also be possible to provide an angle of 140 to 175 or
greater
between the third and the first lines 17, 11 on both sides of the second
tensioning pulley
8. Generally speaking, the angle 18 between the first and the third lines 11,
17 can be in
the range of 180 +1- 40 .
[00027] Fig. 2 is a perspective view of the tensioner 1 of the
invention. It can be
seen clearly that a spring that tensions the tensioning arms and that is
accommodated in
the region of the second pivot axis 5 occupies less installation space. Thus
the lesser the
spring force required for tensioning purposes and thus the weaker the
necessary spring
itself, the smaller the required installation space. That is, the design of
the tensioner 1
suggested by the preferred embodiment and the resulting reduction in the
necessary
tensioning force also leads to a reduction in construction volume.
[00028] Fig. 3 is a cross-sectional view of the tensioner 1 taken along a
line III-
III marked in Fig. 2. The tensioner 1 can be mounted, for example, on an
internal
combustion engine by means of a mounting screw 19 extending through a bearing
bolt
20 on which a base plate 21 is provided integrally. The bearing bolt 20
extends through
a bearing eye 22 of the first tensioning arm 2. The bearing bolt 20 extends
further on
the side of a head 23 of the mounting screw 19 through a front plate 24. On
the
opposite side, the bearing bolt 20 extends additionally through a laminated
disk spring
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25 resting against the base plate 21 and through a pressing disk 26 resting
against the
laminated disk spring 25. Between the bearing eye 22 and the bearing bolt 20
there is
provided a bearing bush 27 that has radially outwardly extending flanges 28,
29 at its
opposing ends. The bearing bush 27 is a one-part component in this embodiment
of the
invention, but it can also be bipartite.
[00029] The bearing bush 27 has a dual function. First, it supports
the first
tensioning arm 2 so as to be free to rotate. Second, it damps its rotational
movement by
means of friction damping. More particularly, the friction damping is produced
with the
1 0 help of the two flanges 28, 29, the laminated disk spring 25 pressing
the friction
partners of the flanges 28, 29 against the same, namely the front plate 24 and
the
bearing eye 22 on the one hand and the pressing disk 26 and the bearing eye 22
on the
other.
[00030] Instead of the flanges 28, 29, provision can also be made for
separate
damping disks for the bearing bush 27 that can be in the form of Teflon-coated
steel
disks, for example. See, for example, U.S. Publication No. 2008/0280713, the
contents
of which are incorporated herein by reference in their entirety.
[00031] The first tensioning arm is freely rotatable about the first pivot
axis 4,
that is, without being spring-loaded.
[00032] The first tensioning arm 2 comprises an approximately cup-
shaped
spring housing 30. A second bearing bolt 32 extends integrally from a base 31
of the
spring housing. The second bearing bolt 32 extends through a bearing eye 33 of
the
second tensioning arm 3. Between the bearing eye 33 and the second bearing
bolt 32
there is provided a second bearing bush 34 by means of which the second
tensioning
arm 3 is mounted on the second bearing bolt 32 so as to be free to rotate. A
spring
cover 35 comprising a collar 36 that protrudes axially toward the base 31
extends
radially outwards from the bearing eye 33 of the second tensioning arm 3. The
bearing
eye 33 and the spring cover 35 formed integrally therewith are secured by a
second
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front plate 37 axially on the second bearing bolt 32 against an axial force of
a coil
spring 38.
In the present embodiment, the second bearing bolt 32 is cylindrical in shape.
It is also
possible to provide a cone bearing instead that tapers in the direction
extending away
from the base 31. Instead of the cylindrically hollow second bearing bush 34,
a bearing
bush tapering in the direction extending away from the base 31 would then be
provided
and an internal peripheral surface of the bearing eye corresponding to the
external
peripheral surface of this bearing bush would likewise taper in the direction
extending
away from the base 31. An example of such structure is found in U.S. Patent
No.
4,698,049, the contents of which are incorporated by reference herein in their
entirety.
[00033] The coil spring 38 loads the tensioning arms 2, 3 toward each
other. The
stronger the tensioning arms 2, 3 are pushed apart by belt forces, the greater
is the
reduction in the diameter of the coil spring 38. As a result, the coil spring
strongly
wraps around a slotted damping bush 40 provided between the coil spring and an
axial
extension 39 of the bearing eye 33. That is, the coil spring 38 increases the
force with
which the damping bush 40 rubs against the bearing eye 33 of the second
tensioning
arm 3, more particularly against its axial extension 39, as a result of which
the damping
force increases. A bottom end 41 of the coil spring 38 is provided for a
radially
outwardly extending flange 42 of the damping bush 40 for rotation therewith.
[00034] Alternatively, the spring 38 could be provided such that it
widens
radially when the tensioning arms 2, 3 are pushed apart by belt forces. Then a
damping
bush can be provided between the coil spring and a cylinder wall 43 of the
spring
housing 30 and said damping bush can rotate relative to the cylinder wall 43
and rub
against the same when the tensioning arms pivot relative to each other. An
example of
such structure is found in U.S. Patent No. 8,142,314, the contents of which
are
incorporated by reference herein.
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[00035] Figs. 4 to 6 show a simulation model of an exemplary belt
drive
arrangement 50 and a simulation model of the preferred tensioner 1 in various
operating
states. By way of example, the belt drive arrangement comprises a crankshaft
belt
pulley 51 connected to a crankshaft of an internal combustion engine, a belt
pulley 52 of
a starter-generator unit and an additional belt pulley 53 that can be
connected to an air-
conditioning compressor, for example. In this example, the starter-generator
unit is an
electric generator for generating electricity and it can also operate as an
electric motor
for starting the engine. Fig. 7 schematically shows a representative torque
curve for a
conventional starter-generator unit from which it will be appreciated that
when the unit
operates as a motor the peak torque on belt pulley 52 is quite high and when
the unit
operates as a generator the peak torque on belt pulley 52 is relatively lower.
[00036] A belt 54 is guided around the belt pulleys in the form of an
endless
drive. The belt 54 is tensioned in that the first tensioning pulley 6 rests
against a first
belt portion 55 that extends between the crankshaft belt pulley 51 and the
belt pulley 52
of the starter-generator unit and in that the second tensioning pulley 8 rests
against a
second belt portion 56 that extends between the belt pulley 52 of the starter-
generator
unit and the belt pulley 53 of the air-conditioning compressor. The tensioning
pulleys 6,
8 press against the belt portions 55, 56 from the outside.
[00037] In Fig. 4, the tensioner 1 is in a tensioning initial
position. The engine is
running and the generator load of the starter-generator unit is zero, i.e.,
the system is in
a quasi-static state. Note that in this state the hub load force 58 (the load
on the shaft of
pulley 52) is directed substantially along a line that passes through the
first pivot axis 4.
[00038] When the starter-generator unit is employed in a boost
function in order
to additionally drive the crankshaft to start the engine, the starter-
generator unit must
drag the engine by means of the crankshaft belt pulley 51. The first belt
portion 55
becomes the tight side and it is tensioned. By contrast, the second belt
portion 56
becomes the slack side and it is relieved of tension. The tensioner 1 pivots
about the
first pivot axis 4 toward the first belt portion 55 and it thus compensates
the resulting
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longitudinal shift of the belt portions, namely the shortening of the first
belt portion 55
and the lengthening of the second belt portion 56. After the pivoting
movement, the
tensioner 1 is in a position as seen in Fig. 5 that differs from the initial
position shown
in Fig. 4. The first angle 13 between a first hub load force 14 on the first
pulley 6 and
the first line 11 has dropped to a value of less than 300, even a value less
than 25 in the
present exemplary embodiment. Likewise, the angle a between force 14 and line
AA
has dropped. Thus a significant component of the first hub load force 14 is
absorbed in
the form of a tensile force by the first tensioning arm 2 and by the bearing
that is part of
the first pivot axis 4. Only a small component of the first resulting force
acts at right
angles to the first line 11 or line AA and it must be absorbed by the coil
spring 38
tensioning the two tensioning arms, the second tensioning arm 3 being
supported
appropriately by means of its second tensioning pulley 8 against the second
belt portion
56.
IS [00039] A second hub load force 16 acting on the second
tensioning pulley 8 is
small and the necessary tension in the second belt portion 56 is maintained
easily by the
coil spring 38.
[00040] The opening angle 10 is substantially constant as compared to
the
illustration shown in Fig. 4. It has opened only slightly by less than 100,
and by less
than 50 in the present exemplary embodiment. Thus the wrap angle of the belt
54
around the belt pulley 52 remains substantially constant so that the force-
transmission
capacity between the belt 54 and the belt pulley 52 is substantially constant.
[00041] A similar operating state as the one shown in Fig. 5 occurs when
the
engine is started by the starter-generator unit.
[00042] The starter-generator unit must be driven by the internal
combustion
engine by means of the belt 54 when it switches from the starter or engine
mode to the
generator mode. The second belt portion 56 becomes the tight side and it is
tensioned.
By contrast, the first belt portion 55 becomes the slack side and it is
relieved of tension.
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The tensioner 1 pivots about the first pivot axis 4 toward the second belt
portion 56 and
it thus compensates the longitudinal shift of the belt portions, namely the
shortening of
the second belt portion 56 and the lengthening of the first belt portion 55.
On
completion of the pivoting movement, the tensioner 1 achieves a second
position as
seen in Fig. 6 that differs from the initial position shown in Fig. 4. The
second hub load
force 16 is greater than the first hub load force 14. The second angle 15 has
reduced to a
value that is clearly less than 300, and even to a value less than 20 . In the
illustrated
embodiment, it is less than 150. Likewise the angle 13 between force 16 and
line BB has
dropped in comparison to Fig. 4. As a result, the orthogonal component about
the pivot
axis 5 is reduced, reducing the tendency of the arms 2,3 to open for a given
unit force.
In addition, a significant component of the second hub load force 16 is
introduced in the
form of a tensile force into the second tensioning arm 3 and into the first
tensioning arm
2 by means of the bearing that is part of the second pivot axis 5. This force
is absorbed,
on the one hand, by the bearing that is part of the fixed first pivot axis 4
and, on the
other hand, by virtue of the fact that the first tensioning arm 2 is supported
against the
first belt portion 55 by means of the first tensioning pulley 6. In spite of
that, the first
hub load force 14 is less than the second hub load force 16. The coil spring
38 can
easily compensate the orthogonal components of the hub load forces 14, 16 that
push
the tensioning arms apart.
[00043] The eccentric arrangement of the second pivot axis 5 helps in
this
arrangement because a significant component of the second hub load force 16 is
directed along line BB passing through the first pivot axis 4, which is fixed
in position.
[00044] The system shown in Figs. 4-6 can be mathematically understood by
the
following simplified equations.
[00045] The torque about pivot axis 4, which sets the angular position
of the
system as a whole, is
L4,7 X Fi4 + L4,9 X F16 = 0, or
L4,7 ' F14 sina = L4,9 ' F16 = sin/3
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where L4,7 is a vector between axes 4 and 7; L4,9 is a vector between axes 4
and 9.
[00046] The torque about pivot axis 5, which determines the opening
angle 10, is
4,9 X Fi6 k = Op + 010), or
L5,9 = F16 = Sti7/015 = k = OP + eio)
where L5,9 is a vector between axes 5 and 9; 010 is the opening angle 10 and
Op is a
preload angle (in the case where the spring delivers a preload torque).
[00047] From the foregoing it will be seen that when the tensioner
switches to the
second position, the opening angle 10 does not alter substantially. As
compared to the
operating state shown in Fig. 5, the opening angle 10 has reduced here by less
than 10
and even by less than 5 in this case. This contributes toward maintaining a
good wrap
angle.
[00048] The wrap-around angle of the belt pulley 52 of the starter-
generator unit
has increased additionally as a result of the geometry of the tensioner and
the
positioning of the pivot axis 4. The first pivot axis 4 is provided at a
position in which
the first tensioning pulley 6 reduces its distance from the belt pulley 52 of
the starter-
generator unit when the tensioner 1 pivots from the first position (Fig. 5)
into the
second position (Fig. 6). The first axis of rotation 7 pivots toward a line
(not referenced
in the drawings) between the first pivot axis 4 and an axis of rotation 57 of
the belt
pulley 52.
[00049] In addition, the geometry of the tensioner may be selected such
that the
distance of the second axis of rotation 9 from the pivot axis 4 is somewhat
smaller than
the distance of the first axis of rotation 7 from the first pivot axis 4. Thus
the first
tensioning pulley 6 draws close to the belt pulley 52 of the starter-generator
unit more
strongly than the second tensioning pulley 8 moves away from the belt pulley
52 of the
starter-generator unit when the tensioner pivots into the second position.
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[00050] In the preferred embodiment, the first tensioning arm 2 is
assigned to the
strand in which maximum belt tension occurs during the operation of the belt
drive
arrangement, namely the first belt portion 55. Thus a significant component of
the
maximum resulting force is introduced in the form of tensile force into the
first
tensioning arm 2 and is directly absorbed by the bearing of the tensioner 1
that is part of
the first pivot axis 4. This likewise contributes toward a reduction in the
spring force
required for tensioning the two tensioning arms.
[00051] From the foregoing, it will be appreciated that a tensioner
according to
the invention can maintain a good wrap angle around an endless driving wheel
by
means of the tensioning arms that are spring-loaded toward each other even
during a
changeover between the tight side and the slack side, and the tensioner can
effectively
compensate longitudinal shifts in portions of the endless drive accompanying
this
changeover by means of the eccentricity between the first and the second pivot
axes.
Furthermore, it is possible for this tensioner at the same time to realize a
moderate level
of basic tension of the endless drive.
[00052] The tensioner can be moved between a first position, in which,
when the
first strand is the tight side, a resulting force on the first tensioning
pulley and a line
connecting the second pivot axis to the first axis of rotation forms a first
angle that is
smaller than 30 , and a second position, in which, when the second strand is
the tight
side, a resulting force on the second tensioning pulley and a line connecting
the second
pivot axis to the second axis of rotation forms a second angle that is smaller
than 30 .
By virtue of the fact that the first angle is smaller than 30 when the first
strand is the
tight side and the second angle is smaller than 30 when the second strand is
the tight
side, a considerable component of the resulting force in question is absorbed
by the
respective tensioning arm in the form of a tensile force. Thus less spring
force is
required for tensioning the two tensioning arms. The basic tension level of
the endless
drive is reduced as a result of the reduced spring tension force.
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CA 02773016 2012-03-28
[00053] Advantageously, the first angle in the first position and/or
the second
angle in the second position can be smaller than 25 , preferably smaller than
20 , and
even more preferably smaller than 150. The smaller the angle, the higher is
the
component of the resulting force that can be absorbed by the tensioning arm in
question
in the form of a tensile force. Accordingly, it is possible to use a smaller
amount of
spring tension force for tensioning the two tensioning arms, as a result of
which the
level of basic tension of the endless drive can be reduced still further. In
spite of that,
the tensioner is able to effectively attenuate tension peaks in the endless
drive.
I 0 [00054] Preferably, the first tensioning arm can be assigned to
the strand, in
which the maximum tension occurs during the operation of the endless drive
arrangement. Thus a large component of the resulting force on the first
tensioning
pulley can be absorbed by the bearing of the tensioner on the first pivot
axis. Thus a
smaller amount of spring force is sufficient for tensioning the tensioning
arms, as a
result of which the level of basic tension of the endless drive can be
reduced.
[00055] Advantageously, an angle formed between a line connecting the
second
pivot axis to the first axis of rotation and a line connecting the second
pivot axis to the
second axis of rotation during a movement of the tensioner from a first
position, in
which the first strand is the tight side, into a second position, in which the
second strand
is the tight side, and/or vice versa remains substantially constant. Thus only
a small
amount or no amount of spring work is required during a changeover between the
tight
side and the slack side, and the wrap angle around the endless driving wheel
remains
substantially constant.
Very advantageously, the angle can alter by less than 10 , and preferably by
less than
5 . The smaller the amount by which the angle alters, the lesser is the spring
work
required and the better is the wrap angle retained.
[00056] Very advantageously, an angle formed between a line connecting the
second pivot axis to the first axis of rotation and a line connecting the
second pivot axis
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CA 02773016 2012-03-28
to the second axis of rotation can range from approximately 600 to 90 . Thus
force can
be absorbed effectively on the respective tensioning arm tensioning the tight
side, a
considerable component of the resulting force on the tensioning pulley in
question being
absorbed in the form of a tensile force by the tensioning arm in question, as
a result of
which it is possible to apply lesser spring force to the tensioning arms.
[00057] Advantageously, the endless driving wheel can be part of that
equipment
assembly of the endless drive arrangement which has the greatest moment of
inertia
and/or the greatest rotational non-uniformities. Thus longitudinal shifts in
the endless
drive can be compensated very effectively.
[00058] Preferably, the endless driving wheel can be part of the
starter-generator
unit. In a starter-generator unit, the strand switches between the tight side
and the slack
side during a changeover of the starter-generator unit from the starter mode
to the
generator mode and vice versa. Thus the accompanying longitudinal shifts in
the
endless drive are compensated at the locus of their origin.
[00059] Preferably, the distance of the first pivot axis from the
second pivot axis
can be at least a quarter of the distance of the second pivot axis from the
first axis of
rotation and/or the second axis of rotation. Thus the tensioner of the
invention achieves
a performance characteristic that differs clearly from that of a conventional
two-armed
tensioner comprising tensioning arms disposed in a V-shaped arrangement, that
is to
say, comprising only one pivot axis. As a result of the reduction in the
distance between
the axes of rotation from the pivot axis responsible for the relative rotation
of the
tensioning arms, the angle between a line connecting the first axis of
rotation to the
second pivot axis and a line connecting the second axis of rotation to the
second pivot
axis is large enough to absorb a considerable component of the resulting force
of the
tensioning pulley that tensions the tight side by means of the articulated
connection of
the second pivot axis. As a result, lesser spring tension force is required
for tensioning
the endless drive, that is to say, a clearly reduced level of basic tension is
possible in the
endless drive. At the same time, the aforementioned geometry contributes
toward
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CA 02773016 2012-03-28
maintaining a good wrap angle. In spite of that, the tensioner is able to
effectively
attenuate tension peaks occurring in the endless drive. By means of the
distance
between the first and the second pivot axes, the tensioner can compensate
longitudinal
shifts in the endless drive when there is a changeover between the tight side
and the
slack side. Consequently, such a tensioner of the invention enables distances
to be
realized between the axes of rotation and the second pivot axis, which would
have made
it difficult for a conventional tensioner comprising tensioning arms disposed
in a V-
shaped arrangement and only one pivot axis or a tensioner behaving almost like
such a
V-shaped tensioner to effectively compensate longitudinal shifts in the
endless drive
during a changeover between slack side and tight side without excessively
reducing the
wrap angle or without excessively increasing the level of basic tension of the
endless
drive.
[000601 Advantageously, the distance of the first pivot axis from the
second pivot
axis can be at least a third, preferably at least half of the distance of the
second pivot
axis from the first axis of rotation and/or the second axis of rotation. The
wrap angle can
then be retained even better and the spring tension force required for
tensioning the two
tensioning arms can be reduced still further. At the same time, the tensioner
compensates longitudinal shifts in the endless drive effectively.
[00061] Very preferably, the distance of the first pivot axis from the
second pivot
axis can be approximately as large as the distance of the second pivot axis
from the first
axis of rotation and/or the second axis of rotation. In this arrangement, the
wrap angle
can be maintained particularly effectively, it being possible for the force
required for
tensioning the two tensioning arms to be reduced once again. At the same time,
the
tensioner can effectively compensate tension peaks and longitudinal shifts in
the endless
drive during a changeover between the tight side and the slack side.
[00062] Advantageously, a line connecting the first and second pivot
axes and a
line connecting the second pivot axis to the first axis of rotation form an
obtuse angle,
preferably an angle ranging from approximately 140 to 175 . As a result, the
length of
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CA 02773016 2012-03-28
the second tensioning arm can be shorter compared to a stretched form of the
first
tensioning arm, and a greater opening of the angle between the two tensioning
arms is
possible consequently. This proves advantageous for maintaining a good wrap
angle
and enables a further reduction in the force required for tensioning the two
tensioning
arms.
[00063] Very advantageously, provision can be made for a damping bush
along a
periphery of a coil spring that spring-loads the first and the second
tensioning arms
relative to each other, and the coil spring presses against this damping bush
radially
when its diameter alters during a movement of the tensioning arms relative to
each
other. Thus a damping effect is achieved that alters increasingly with the
increasing
change in the diameter of the coil spring.
[00064] Very preferably, the distance of the first pivot axis from the
second pivot
axis can be at least a third, and even more preferably at least half of the
distance of the
second pivot axis from the first axis of rotation and/or the second axis of
rotation.
[00065] Very advantageously, the distance of the first pivot axis from
the second
pivot axis can be approximately as large as the distance of the second pivot
axis from
the first axis of rotation and/or the second axis of rotation.
[00066] Advantageously, a line connecting the first and the second
pivot axes and
a line connecting the second pivot axis to the first axis of rotation form an
obtuse angle,
preferably an angle ranging from approximately 140 to 1750
.
[00067] Very advantageously, an angle formed between a line connecting
the
second pivot axis to the first axis of rotation and a line connecting the
second pivot axis
to the second axis of rotation can range from approximately 60 to 90 .
[00068] Very preferably, provision can be made for a damping bush along a
periphery of a coil spring that spring-loads the first and the second
tensioning arms
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CA 02773016 2012-03-28
relative to each other, and the coil spring presses against this damping bush
radially
when its diameter alters during a movement of the tensioning arms relative to
each
other.
[00069] Those skilled in
the art will appreciate that a variety of modifications
may be made to the embodiments described herein without departing from the
fair
meaning of the accompanying claims.
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