Tensioner for a power transmission belt

A tensioner is provided for a power transmission belt comprises a tensioning roller with which the belt cooperates, a return spring for controlling the displacement of said roller when the tension of the belt decreases and means for damping the vibrations likely to occur in the belt and comprising two variable volume chambers filled with a liquid and communicating together at one of their ends through a valve device through which the passage of the liquid entering and/or leaving said chambers introduces the damping effect, wherein the sidewall at least of one of the chambers is a membrane made from an elastomer material, such as rubber, having a general bell-shape and without spring, and the return spring associated with the other chamber is formed by a coil spring working under compression, prestressed in its rest state.

BACKGROUND OF THE INVENTION 
The invention relates to a tensioner for a power transmission belt. 
Tensioners for belt power transmission systems have already been proposed, 
in particular by the Applicant in FR-A-2 597 565 or FR-A-2 612 591. In the 
embodiments according to these documents, a linear tensioner for a power 
transmission belt comprises a return spring associated with the shaft of a 
tensioner roller with which the belt cooperates for controlling the 
displacement of said roller and the action of said spring, which is coated 
with elastomer forming a sheath defining at least a variable volume 
chamber filled with a liquid, said chamber being closed at one of its ends 
by a valve device, through which the passage of the liquid entering or 
leaving said chamber introduces an effect of damping the vibrations likely 
to occur in the belt. Although devices of this type are satisfactory for 
obtaining the desired results, it has proved that their construction, in 
particular in so far as the manufacture of the elastomer sheaths and 
fixing them at their ends are concerned, lead to manufacturing costs which 
are too high for devices for used in the automobile industry where the 
price factor is of great importance. 
In other documents, for example in FR-A-2 617 556, also in the name of the 
Applicant, a tensioner of rotary type is described having a helical spring 
exerting a resilient return torque and comprising a flexible member of a 
general toroidal shape divided into two fluid chambers separated at one of 
their ends by a valve with dissymmetric operation so as to introduce the 
desired damping effect. The manufacture of such a device is also 
relatively expensive and leads to a relatively cumbersome assembly. 
Since, however, the natural extension of the belt during use, as well as 
the vibrations generated in the belt by cyclic irregularities of the 
engine speed, are accompanied by slipping of the belt over the pulleys of 
the driven members--with consequently transmission defects--it is 
important to remedy this drawback of belt power transmissions using a 
tensioner, either rotary or linear depending on the organization of the 
belt power transmission, but which in all cases must be of as low a size 
and cost as possible, while offering great safety and reliability. 
SUMMARY OF THE INVENTION 
Consequently, an object of the invention is to provide a tensioner for a 
power transmission belt which, while offering the same results as known 
devices, is nevertheless much simpler in structure and therefore of a 
lower manufacturing cost. 
Another object of the invention is to provide such a tensioner of smaller 
size, particularly shorter than known devices, and which thus has the 
advantage of requiring less space for use thereof, which makes application 
thereof possible in cases where the available volume is limited, for 
example in some systems associated with motor vehicle engines. 
A tensioner for a power transmission belt according to the invention 
comprises a tensioning roller with which the belt cooperates, a return 
spring for controlling the displacement of the roller during a decrease of 
tension of the belt and means for damping the vibrations likely to occur 
in the belt and comprising two variable volume chambers filled with a 
liquid and communicating together at one of their ends through a valve 
device through which the passage of the liquid entering and/or leaving 
said chambers introduces the damping effect, is characterized in that the 
sidewall at least of one of the chambers is a membrane made from an 
elastomer material, such as rubber, having a general bell-shape and 
without spring, and in that the return spring associated with the other 
chamber is formed by a coil spring working under compression, prestressed 
in its rest state. 
In the preferred embodiment, the return spring is fast by coating with an 
elastomer material sheath forming the sidewall of the chamber with which 
it is associated. 
The ends of the sheath fast with the spring and the ends of the membrane 
having a general bell-shape are advantageously fixed to their respective 
mounting members by adherence during molding. 
In a variant, the ends of the sheath fast with the spring, as well as the 
ends of the membrane having a general bell-shape are fixed to their 
respective mounting members by crimping means. 
Thus, the cost of manufacturing the devices in accordance with the 
invention is considerably reduced with respect to known tensioner 
constructions, which comprise machined parts for mounting them which are 
expensive to manufacture. 
In another embodiment, the sidewall of the chamber with which the spring is 
associated is an indeformable rigid tube inside which said spring is 
housed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first of all to FIG. 1 which illustrates very schematically a 
power transmission system in which a belt 10 cooperates with the flanges 
of pulleys 12 and 13 fast for rotation with driven shafts and with the 
flanges of a pulley 14 fast with a driving shaft for example the 
crankshaft of a motor, truck, agricultural machine or similar engine. To 
provide tensioning of belt 10, a tensioner 15 is associated with the 
system and is fastened at point 18 on a lever 19, which is mounted for 
pivoting at one of its ends about a shaft 20 and having at its other end a 
roller 16 rotating about a shaft 21. Tensioner 15, whose end opposite that 
of the fastening point to the lever is fixed to the engine block, 
referenced schematically at 22, is intended to take up the extension of 
belt 10 and prevent it from slipping on the flanges of the pulleys and 
also to damp the vibrations generated in the belt 10 by the cyclic 
irregularities of the engine speed, particularly when idling, or during 
shocks generated during start-up and/or stopping of the apparatus driven 
by the shafts fixed to pulleys 12 and 13. 
In accordance with the invention, tensioner 15 (FIG. 2) comprises a coil 
spring 25 fast by coating with a sheath 26 made from an elastomer 
material, such as nitrile rubber, and which forms the sidewall of a first 
liquid filled chamber 27. Said chamber, of a general cylindrical shape, is 
closed at one of its ends by a first closure means 28, sealingly fixed by 
bonding to the assembly formed by sheath 26 and spring 25 during molding 
of this assembly, whereas the other end of chamber 27 is closed by a 
second closure means 30, of revolution, with a T shaped cross section and 
which thus has a base 31 with circular contour on which a barrel 32 is 
erected. Both base 31 and barrel 32 are pierced with a bore 33 for 
guiding, preferably with interpositioning of rings such as 34, a rod 35 
fast at one of its ends, by a pin 36, with closure means 28. 
In chamber 27 a valve device 40 is also housed, in the vicinity of closure 
means 30, illustrated here schematically in the form of a valve head 41 on 
which the action of a return spring 42 surrounding rod 35 is exerted, said 
device being organized so as to permit the flow, advantageously 
dissymmetrically, of the liquid contained in chamber 27 between said 
chamber and a second chamber 43. The latter is defined laterally by a wall 
formed by a membrane 44 made from an elastomer material, such as rubber, 
having a general bell-shape, which also serves as bottom wall and whose 
free edge 45 is bonded to base 31 and to a clamping disk 47 covered by a 
cap 46. The bottom zone of membrane 44, namely that which is the furthest 
away from base 31, is pierced with an orifice for passing therethrough the 
second end of rod 35 which is fixed, by a sealed mounting, shown 
schematically at 50, to a short spout 51 fast with a ring 52 for 
attachment to lever 19. 
A similar ring 53 is provided at the other end of the tensioner, ring 53 
being fast with a bell 54 whose length is substantially that of spring 25 
which it encloses thus totally or partially while being fixed, at its 
portion opposite ring 53, to cap 46. 
The operation of the tensioner--thus without spring operating under 
traction as in known tensioners and in which the spring 25 is a spring 
acting under compression, prestressed in its rest state--is clear from the 
foregoing: as soon as the tension of belt 10 decreases, spring 25 expands, 
causing displacement of closure means 28 and simultaneously of rod 35 in 
the direction of arrow F with the corresponding passage of liquid from 
chamber 43 to chamber 27 when the valve head 41 is lifted. Thus, weak 
damping is obtained which permits lever 19 and roller 21 to pivot rapidly 
in the direction of arrow a so as to adjust the tension in the belt in a 
very short time. 
If, on the other hand, the tension of the latter increases, lever 19 pivots 
in the opposite direction to arrow a and rod 35 is displaced in the 
direction of arrow F; the liquid enclosed in chamber 27 reaches chamber 
43, and while passing through valve 40 causes a maximum damping effect. 
The structure of the tensioner illustrated in FIG. 3 is very close to that 
of the tensioner shown in FIG. 2 and only differs therefrom by the fact 
that the closure means 28' and 30' defining the chamber 27' are here fixed 
to spring 25' and the elastomer sheath 26' which is fast therewith, not by 
bonding but by crimping, as shown schematically at 60 and 61, the ends of 
the elastomer sheath 26' being in the form of sleeves 62 and 63, 
respectively, of a greater length and greater thickness than the 
corresponding end portions of sheath 26 of the preceding embodiments so as 
to permit satisfactory crimping. 
The operation of this embodiment is identical to that illustrated in FIG. 
2, in particular in so far as the valve device separating chamber 27' from 
chamber 43' is concerned, and which is not shown integrally in FIG. 3. 
In the embodiment illustrated in FIG. 4, the coil spring 65--similar to 
springs 25 and 25' of the preceding embodiments--is not coated with an 
elastomer material but is housed in a rigid and indeformable tube 66 
forming the sidewall of a liquid filled chamber 67 and which is similar to 
chambers 27 and 27' of the preceding embodiments. Chamber 67 is closed at 
one of its ends by a closure means 68 mounted for sliding in tube 66 with 
interpositioning between said tube and the closure means of a seal 69 and 
the other end of the chamber is closed by a closure means 70, similar to 
closure means 30 and 30' of the preceding embodiments, and with which is 
associated a valve 71 similar to valve 40 of the embodiment shown in FIG. 
2. As in the tensioner illustrated in this FIGURE, valve 71 permits the 
liquid contained in chamber 67 to pass to chamber 72 and vice versa, said 
chamber 72 being defined by the closure means 70 and an elastomer membrane 
73 having a general bell-shape and which is quite similar to membrane 44 
of the preceding embodiments. 
The operation of this tensioner, in which the coil spring 65 prestressed in 
its rest state is provided for working under compression only, is quite 
similar to that of the embodiments described with reference to FIGS. 2 and 
3. 
Reference will now be made to FIGS. 5 and 6 which show two embodiments of a 
very compact tensioner particularly well suited for regulating the tension 
of the belt with which it cooperates without special mounting and 
measuring tools. 
In the embodiment of FIG. 5, roller 80 intended to cooperate with belt 10 
is mounted for rotation at the end of a cranked apertured arm 81 rotatable 
about a shaft 82, the portion 83 of arm 81 distant from that carrying 
roller 81 facing an arm 84. Between the latter and portion 83 is mounted a 
device 85 similar to the compression spring devices, fluid chambers and 
valve which have just been described with reference to FIGS. 2 to 4. After 
device 85, whose spring has been prestressed, has been positioned between 
the ends of arms 81 and 84, said arms are immobilized against rotation 
with respect to each other by a pin 86 and the tensioner is fixed to the 
engine block which it is to equip, so as to place roller 80 in contact 
with the belt 10, by rotating the whole of the tensioner about shaft 82; 
with this contact made between the roller and the belt, the tensioner is 
fixed in position on the engine block by a means, not shown, which 
immobilizes arm 84 with respect to said engine block. Pin 86 is then 
removed and with device 85 operative, the cranked arm 81 pivots about 
shaft 82 in the direction of arrow f so as to give belt 10 the desired 
tension, defined by the prestress rate imposed on the spring of device 85. 
The tensioner may thus be put into position in an assembly chain without 
special tooling and the initial tension of the belt may be provided 
without a special measuring tool. 
When, in time, the tension of the belt decreases, the expansion of the 
spring included in device 85 causes a pivoting movement of lever 81 in the 
direction of arrow f, which takes up the tension of the belt with the 
desired damping effect introduced by the valve device separating the two 
fluid chambers which said device 85 comprises. 
In the embodiment of FIG. 6, similar to that which has just been described 
immediately above, roller 80' which cooperates with belt 10 is mounted for 
rotation at the end of a cranked arm 81' similar to arm 81 of the 
preceding embodiment but whose portion 83' is articulated by a pin 87 to a 
ring 88 of a device 89 similar to the compression spring device, fluid 
chambers and valve described in connection with FIGS. 2 to 4. Arm 84', 
which is the one fixed to the engine block after the device has been 
positioned, is connected by a shaft 90 to the casing of device 89, the 
latter then being mounted under "traction" for applications requiring 
reversal of the tensioning direction of the roller with respect to the 
embodiment of FIG. 5. As in the latter embodiment, however, the tensioner 
is first of all positioned on the engine block with a pin 86'--similar to 
pin 86--which immobilizes arms 81' and 84' with respect to each other 
then, after rotation about shaft 82' bringing roller 80' in contact with 
belt 10, said pin is withdrawn. 
In one and other embodiment described immediately above, the weakest 
damping corresponds to the direction of movement of the tensioning roller 
80 or 80' which is that for which belt 10 is expanded in the loop of said 
roller, whereas the highest damping is that corresponding to an increase 
of the tension of the belt.