Tensioners for driving belts

A tensioner for belts including a stationary structure (3), an arm (4) movable with respect to a shaft (7) of the stationary structure, a torsion spring (6), a pair of dampening cones (8), (9) associated with the stationary structure and the arm, a compression spring (10) acting on the dampening cones. The mean radius of the cones is smaller than the maximum radial dimension of the torsion spring. The value "1" of the arm is correlated to the mean radius of the cones determining a frictional dampening between the cones smaller than that existing between the shaft and the rotating arm.

BACKGROUND OF THE INVENTION 
The present invention relates to improvements to tensioners used in power 
transmissions which comprise a belt and at least two pulleys, that is a 
driving pulley and a driven pulley, respectively. 
In such power transmissions, the tensioner acts with its pulley on the back 
of the belt to tension it to a value that assures normal working of the 
power transmissions. 
It is known to use a tensioner for driving belts which comprise a 
stationary structure, a lever arm supported by the stationary structure 
and movable with respect to the latter, a pulley associated with the arm, 
a first mechanical or torsion spring means between the stationary 
structure and the arm to move the arm with respect to the stationary 
structure and to urge the pulley against the belt, frictional dampening 
means associated with the stationary structure and the arm to dampen the 
movements of the arm rotating with respect to the stationary structure in 
both rotation directions. The stationary structure comprises a shaft 
having a longitudinal axis mounted in a stationary manner with respect to 
the movements of the rotating arm and the arm has a portion carried by the 
shaft so as to rotate relative thereto. 
A tensioner as set forth above is described in greater detail in U.S. Pat. 
No. 4,473,362, the disclosure of which is hereby incorporated by 
reference. 
The tensioner of U.S. Pat. No. 4,473,362 is provided with a torsion spring 
having at the ends thereof a rotating arm and a stationary structure and a 
plastic spring holder ring with cylindrical walls and supported by the 
lower portion of the stationary structure coaxially to the shaft. The 
lever arm lengthens considerably starting from the tensioner shaft over 
its radial path which is determined by the torsion spring and carries at 
its free end an idle pulley. 
During operation one end of the torsion spring for the rotation of the 
lever arm is clamped on the lateral wall of the spring holder element 
which is urged against rotating parts of the tensioner giving rise to a 
frictional dampening. 
In this tensioner the dampening is proportional to the force of the torsion 
spring. 
Tensioners having the above mentioned characteristics are known, in which 
however the dampening has constant values independently of the values of 
the forces of the mechanical spring means that urge the tensioner pulley 
towards the belt. 
Such a tensioner is described in the U.S. Pat. No. 4,596,538 the disclosure 
of which is hereby incorporated by reference. 
In this tensioner, a flat torsion spring wound coaxially to the shaft is 
arranged in a first plane and a dampening device, formed by two plates 
opposite to each other and second mechanical spring means, is arranged in 
a second plane lying below the first plane. 
One of the plates is mounted coaxially to the shaft, rotates with the arm 
and is axially movable relative to the second plate integral with the 
stationary structure. 
The second spring means comprises a wavy spring that urges one of the 
plates against the other to determine the desired frictional value during 
the rotation of the arm and to compensate for wear appearing in the form 
of dust due to the friction between the plates. 
The tensioner arm extends considerably over the radial encumbrance of the 
flat torsion spring and the underlying damper and carries at its end an 
idle pulley for tensioning the belt. 
A further tensioner having a constant frictional dampening is described in 
U.S. Pat. No. 4,971,589 the disclosure of which is hereby incorporated by 
reference. This tensioner provides a flat torsion spring wound coaxially 
to the shaft and on the shaft itself. 
The stationary structure provides a cup-shaped envelope having a base to 
which the shaft is secured and lateral walls ending in an outwardly 
flaring shaped with a conical surface portion having an inclination 
converging towards the shaft. The arm has a conical surface complementary 
to that of the envelope. A layer of suitable anti-friction material is 
arranged between the two conical surfaces. The second mechanical means is 
represented by a wavy spring that, from the tensioner upper part, urges 
the conical surface of the arm against that of the envelope. 
The conical surfaces are adopted to oppose a "cocking" phenomena, i.e. the 
inclination of the tubular portion of the arm on the shaft. Such "cocking" 
is due to the fact that the horizontal force transmitted by the belt to 
the pulley is applied at a certain distance from the lower portion to 
which the shaft is secured thus originating a bending moment with a 
resulting inclination and a consequent wear of the anti-friction material 
layer between the shaft and the portion of the arm rotating around the 
shaft. In this tensioner the dampening is determined by the friction due 
to the movements relating to the rotation between said conical surfaces at 
the periphery of the envelope and assumes high values due to the sensible 
mean radius of the conical surfaces measured with regard to the axis of 
the shaft. 
A further tensioner is known from U.S. Pat. No. 4,826,471, the disclosure 
of which is hereby incorporated by reference, which comprises first 
mechanical spring means formed by a torsion helical spring mounted 
coaxially to the shaft between the arm and the stationary structure. 
This tensioner makes use of a dampening device comprising two cams having 
inclined surfaces cooperating with each other and second mechanical spring 
means formed by a compression spring. One of the cams is mounted on the 
lower base of the arm and the second cam having an inclined surface 
directed towards the first inclined surface is mounted coaxially to the 
shaft on which it can slide only axially through a guide which is 
longitudinally notched to the shaft. 
The second cam is urged against the first one by the compression spring 
disposed with its lower end on a stationary base, coaxially to the shaft 
and inside the torsion spring. The arm lengthens over an envelope of space 
defined by the two springs. 
During operation, in relation to a predetermined direction of rotation of 
the arm, one of the cams goes up with its inclined surface towards the 
other one, compressing the spring which when unloading its force obliges 
the cam to go down towards the other one with a movement of the arm 
against the back of the belt. 
In practice, in this tensioner the torsion spring and the compression 
spring act as they were in parallel to tension the belt. The working of 
the dampening device is unidirectional. The tensioners of the state of the 
art set forth above also have a dampening effect independent of the 
dampening device. This independent dampening is due to the friction 
between the shaft and the portion of the arm in rotation around the shaft 
since the force of the belt acting on the pulley at the end of the arm is 
balanced by an equal and opposite reaction directed by the portion of the 
arm in rotation around the shaft towards the shaft. 
Consequently, the tensioners of the state of the art provide a dampening, 
even if minimal, due to the essential structural parts and additional 
dampening devices that can be grouped in two types different from each 
other that is, a first type in which the dampening can depend on the force 
of the spring or also unidirectionally and a second type in which the 
friction is constant for both the directions of rotation of the arm. 
The present invention includes dampening devices of the second type. 
The tensioners provided with devices of the second type have in several of 
their embodiments frictional surfaces exposed to the outside with the 
possible risk of embedding particles of the surrounding environment and 
consequent risk of jamming. 
Moreover said tensioner are based on dampening devices which substantially 
make use of very large frictional surfaces and heavy compression loads 
with a consequent and not indifferent wear-in time with formation of dust 
and risks of jamming between the frictional surfaces in the relative 
rotary motion. 
SUMMARY OF THE INVENTION 
The Applicant has found that it is possible to improve the tensioners of 
the state of the art with reference to the regularity of working, its 
lifetime, as well as resistance to "cocking" by making use of a dampening 
unit comprising opposed conical surfaces, subjected to pressure between 
them by mechanical spring means and by linking the frictional dampening 
percentage obtainable from said unit to that obtainable from the friction 
produced between the surfaces of the tensioner stationary shaft and the 
tensioner arm rotating around said shaft. 
More particularly, it was found that it is possible to achieve the desired 
improvements when the value of the arm measured between the centers of the 
shaft and the pulley acting on the belt and the value of the mean radius 
of the conical surfaces are correlated to each other in a predetermined 
manner to control when the arm rotates relative to the stationary 
structure with a frictional dampening between the conical surfaces being 
sensibly smaller than the frictional dampening between the shaft and the 
arm portion carried by the shaft, for example a dampening of between 15% 
and 40%, also for example a dampening of 20% of the total dampening. 
Therefore an object of the present invention is a tensioner for driving 
belts comprising a stationary structure, a lever arm carried by said 
stationary structure and movable relative to said stationary structure, a 
pulley associated with the arm, first mechanical spring means associated 
with the stationary structure and the arm to move the arm with respect to 
the stationary structure and to urge the pulley on the belt. The present 
invention also includes frictional dampening means associated with the 
stationary structure and the arm to dampen the movements of the arm 
rotating with respect to the stationary structure in both rotation 
directions, said stationary structure comprising a shaft having a 
longitudinal axis and being mounted in a stationary manner relative to the 
movements of the rotating arm, said arm having a portion carried by the 
shaft so as to rotate with respect to the shaft, said tensioner being 
characterized by the fact that: 
a) said dampening means comprises a pair of annular elements provided with 
first and second conical surfaces, and second mechanical compression 
spring means, said first surface comprising frictional means and being 
mounted coaxially to the shaft and axially movable with respect thereto as 
regards the second surface, said second spring means biasing the conical 
surfaces toward each other and into contact under pressure, said conical 
surfaces having a mean radius smaller than the maximum radial dimension of 
the first mechanical torsion spring means; 
b) the length values (1) of the arm between the centers of the shaft and 
the pulley and the mean radius values (Rm) of the conical surfaces are 
correlated between them in a predetermined manner to produce when the arm 
rotates relative to the stationary structure, a frictional dampening 
between the conical surfaces smaller than the frictional dampening 
occurring between the shaft and the portion of the arm carried by the 
shaft. 
Hereinafter the value of the mean radius Rm of the conical surfaces is the 
value that for a determinate height h of the conical surface is measured 
at half height. 
Preferably the tensioner is characterized by the fact that the ratio 
between the length of the arm and the mean radius Rm of the conical 
surfaces is comprised between 0.1 and 0.5. 
Also preferably, the tensioner is characterized by the fact that the arm 
comprises a cylinder mounted in an eccentric manner with respect to the 
longitudinal axis of the shaft, said pulley being mounted in freely 
rotatable manner on said cylinder by means of a suitable bearing. 
In some particularly advantageous embodiments, the tensioner is 
characterized by the fact that in an axial section of the tensioner the 
angle .beta. (FIG. 6) between the line of the conical surface in the axial 
plane and the longitudinal axis of the shaft is comprised between 20 and 
70 degrees. 
In a preferred embodiment the tensioner comprises means for connection to 
the rotation and contemporaneously means to guide the axial movement 
between the element comprising the first conical surface and the arm. 
The two conical annular elements can be made of plastic material or 
metallic material, for instance a bronze and brass alloy or of polyamide. 
In particular and preferably, the invention uses an annular element of 
bronze alloy axially movable along the shaft and confronting a stationary 
annular element of plastic material having fibers embedded therein, for 
example aromatic polyamide fibers; it is possible to use two annular 
elements, both of plastic material. 
Alternatively to the previous solution, the annular element having a 
conical surface, for example of bronze alloy, could be mounted on the 
stationary structure with ways for axial sliding along which said annular 
element is urged by a compression spring against a further annular element 
having a complementary conical surface fixed to the lever arm. 
Further characteristics and advantages will better appear from the detailed 
description of a preferred, but not exclusive, embodiment of a tensioner 
for driving belts according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows a flexible belt "a" in a driving device provided with two 
pulleys, a driving pulley "m" and a driven pulley "c", respectively. 
The belt of FIG. 1 is represented for the sake of simplicity only by its 
longitudinal development; the belt can be of any known type, for example a 
motor vehicle toothed timing belt of elastomeric material as used in an 
internal-combustion engine for the control of the camshaft. 
Further, FIG. 1 represents diagrammatically by a circle the pulley of the 
tensioner 1 having a rotation center Y, which is mounted on a support 
having a center X. The features of the tensioner forming the object of the 
present application are described later, illustrated in general in FIG. 2 
and more in detail in FIGS. 3-7. 
The tensioner 1 shown in FIG. 2 comprises a stationary structure 3, a lever 
arm 4, preferably of aluminum, carried by the stationary structure, a 
pulley 5 associated with the arm, a torsion spring (first spring means) 6 
between the stationary structure and the arm to move the arm relative to 
the stationary structure, frictional dampening means associated with the 
stationary structure and the arm to dampen the movements of the arm 
rotating in both rotation directions relative to the stationary structure. 
A compression spring (second spring means) 10 biases the frictional 
dampening means and is positioned concentrically inside torsion spring 6. 
The stationary structure comprises a shaft 7 having a longitudinal axis 
mounted in a stationary manner with respect to the movements of the 
rotating arm. The arm comprises a portion 4' supported by the shaft 7 so 
as to rotate relative thereto. 
The stationary structure 3 is also anchored to parts of the engine, for 
instance to the face of the engine through holes 2 inside which 
appropriate fastening means (not shown) are introduced. 
The dampening means comprise a pair of annular elements provided with 
conical surfaces 8, 9 and compression spring 10 inside and coaxial to the 
torsion spring 6. The angle between the cones and the shaft axis can range 
between 20 and 70 degrees with a commensurate change in the dampening 
effect (angle .beta., FIG. 6). In the example of FIG. 2, the angle of the 
cones with respect to the longitudinal axis of shaft 7 is about 45 
degrees. 
The whole dampening device is positioned coaxially inside the torsion 
spring. 
In the application to a toothed belt driving device the tensioner makes use 
of an arm comprising an upper cylindrical portion 4' having its center on 
axis Y--Y eccentric relative to the axis X--X of the shaft 7. 
Pulley 5 is mounted in a freely rotational manner around the cylindrical 
portion of the arm by interposition of a bearing 5'. 
The length values (l) of the arm measured between the centers of the shaft 
and the pulley and the values of the mean radius (Rm) of the conical 
surfaces are characteristics of the invention. 
It was found that said values must be correlated to each other in a 
predetermined way to originate, when the arm 4 rotates relative to the 
stationary structure 3, a frictional dampening between the conical 
surfaces 8 and 9 is sensibly smaller than the frictional dampening between 
the shaft and the arm portion carried by the shaft. 
To achieve a reduced wear of the material of the cones and a longer 
lifetime of the tensioner and also maintaining an adequate resistance to 
cocking, the values of the cited parameters "l" and "Rm" (FIG. 6) are 
correlated to determine a dampening due to the friction between the cones 
comprised between 15% and 40% of the total potential dampening due to the 
friction between shaft and arm portion and dampening device. 
In the particular example described, the ratio between the parameter "l" 
and the mean radius Rm of the conical surfaces is preferably within the 
range of 0.1 to 0.5 and more preferably equal to about 0.4. 
Preferably in absolute the values of "l" in the case of the application of 
FIG. 1 are between 3 and 7 mm and the values of Rm between 10 and 17 mm. 
Moreover it was found particularly adequate in relation to the 
predetermined parameters "l" and "Rm" to utilize the structural part and 
the dampening device as described in detail hereinafter. 
As clearly visible in FIG. 3, the tensioner 1 comprises a shaft 7 defined 
by various parts consisting of a pivot 12 and a steel sleeve 13 placed 
around the pivot. The pivot has a head 14 housed in a seat 15 of the 
stationary structure and a cylindrical body provided with an end thread 16 
projecting towards the inner space of the tensioner. The steel sleeve 13 
has its base supported by the stationary structure with which it is 
integral through a nut 17 screwed on the end thread 16 of the pivot 16 so 
as to press through a flange 18 on the upper end of the sleeve 13. 
Between the arm and the sleeve and between the arm upper end 20 and the 
flange 18 there is a tubular layer 19 of anti-friction material, for 
instance said layer is constituted by a bronze mesh impregnated in 
particular with plastic material and for example with 
polytetrafluoroethylene. The arm 4, advantageously formed in a single 
piece, comprises various parts to perform different tasks. 
In particular the arm 4 comprises: 
a tubular member 22 to be mounted around the shaft; 
an upper full cylinder 4' arranged in an eccentric manner relative to the 
axis of the tubular member and extending between two bases, an upper base 
20 and a lower base 23, respectively, for a short length with regards the 
tubular member; the pulley 5 with its bearing 5' is applied on said 
cylindrical portion 4--4'; 
a sectional reduction from the periphery of the lower base of the arm 
extends towards the center of the tubular member to form a shank 24 
extending along the remaining part of the tubular member; said shank is 
useful both to increase the area of contact with the anti-friction layer 
around the sleeve and to form, as it will be explained later, some guide 
grooves for the annular element 8 preserving the shaft 7 from incidental 
notches for the same purpose; 
a lateral cylindrical wall 25 extending in a projecting manner from the 
periphery of the lower base 23 as far as a minimum distance with respect 
to corresponding lateral walls of the stationary structure; said wall is 
useful to enclose the damping device preserving it from any dust coming 
from outside; 
a hole 26 (FIG. 1) is in the lateral wall 25 to receive one end of the 
torsion spring 6. 
Very advantageously the shank 24 extends towards the stationary structure 
without contact with adjacent parts. This permits fittings of the length 
of the tubular member in presence of thermal expansions avoiding therefore 
the risk of a contact between the lower end of the shank and the 
stationary structure with a consequent risk of blocking the rotation of 
the arm caused by the torsion spring 6 acting to tension the belt. 
In one embodiment of the present invention now described, the steel sleeve 
13 has an outer diameter of about 16 mm and the length of the tubular 
member 22 is about 32 mm. In accordance with one embodiment of the 
invention the friction coefficient between sleeve 13 and layer 19 is about 
0.12 and that between layer 19 and arm is about 0.20 so that the torsion 
spring 6 can pull the layer 19 into rotation. 
The stationary structure comprises a base 27 in which a cavity 28 is 
formed. Said cavity 28 is delimited by a cylindrical lateral wall 29 
projecting towards a corresponding lateral wall of the lever arm; 
therefore the lateral walls of the arm and of the stationary structure 
form the previously mentioned substantially closed cylindrical space where 
the springs and the dampening device are arranged. 
The cavity 28 forms a supporting and fastening seat of the element 
comprising the second conical surface 9 of the dampening means. 
In the embodiment of FIG. 3 the cavity is shaped in such a way that the 
material of the base 27 is the same material as the annular element having 
the conical surface 9. 
The lateral wall of the cavity 28 on the base 27 has a hole 30 wherein one 
end of the torsion spring 6 is fixed. 
Further, a plastic annular crown 31 is preferably on the base 27, said 
plastic annular crown being arranged coaxially to the annular element 
having the conical surface 9 to form a support for the end of the torsion 
spring 6. 
Alternatively, it is possible to utilize a single plastic unit shaped in a 
way corresponding both to the annular element having the conical surface 
and the crown for supporting the helical spring; said unit is force fitted 
into a corresponding cavity in the base material. 
A further particular characteristic of the invention is given by the 
embodiment of the dampening device as pointed out in detail in FIGS. 4 to 
7. 
As seen in said figures, the element comprising the first conical surface 
which is applied around the cylindrical shank 24 of the arm and between 
the arm and the shank there are means for connection to the rotation of 
the element together with the shank and means to guide the axial movement 
of the element itself in respect to the second conical surface. 
The connecting and guiding means comprise teeth 33 extending radially from 
the inner annular surface of the annular element towards the shaft and 
grooves 34 in a number corresponding to the teeth on the outer surface of 
the shank; the grooves 34 are formed and extend in the axial direction of 
the shank. 
The annular element 8 inserted together with the teeth 33 in the grooves 34 
is urged with a pressure against the element having the conical surface 9 
by the compression spring 10 arranged coaxially to the shaft 7, inside the 
torsion spring 6 and with one end in contact with a seat 23' of the arm 
lower base 23. 
Very advantageously, the annular element having a conical surface 8 is made 
of a brass and bronze alloy and the annular element having the conical 
surface 9 is of plastic material. 
According to some embodiments the annular element having a conical surface 
has a height h (FIG. 6) of between 2 and 20 mm, preferably between 3 and 
10 mm, and a radius Rm between 10 and 17.5 mm. 
According to one embodiment of the invention the friction coefficient 
between the cones 8 and 9 is equal to 0.20 and the compression spring 10 
exerts on the cones an axial force of 30N (Newton). 
According to one application of the tensioner the force that the belt 
exerts on the tensioner arm is about 200N. 
The just described tensioner operates as follows. 
In presence of pulsating forces, for instance the oscillating forces 
produced by the springs of the valves of the camshafts, a variation of the 
force transmitted by the belt to the pulley of the tensioner takes place 
with a consequent balancing reaction of equal value on the shaft. The arm 
tends to rotate finding an obstacle in the friction generation that arises 
between the surfaces of contact in the relative rotary motion of the shaft 
and the portion of the arm around the shaft: consequently a first 
dampening action takes place. 
At the same time the conical surface 8 carried by the arm 4 and integral 
therewith through the connection between the teeth 33 and the grooves 34, 
is obliged to rotate, remaining subjected to the action of the compression 
spring 10 that urges it with a pressure against the opposite conical 
surface 9; in such a way a friction is generated between the conical parts 
coaxial to the shaft together with a dampening action. 
The two cited dampenings together impede the tensioner pulley 5 to move 
away and gives rise to the risk of an incidental skipping phenomenon from 
the grooves of the driving and driven pulleys on the toothed belt. 
The solution of the present invention achieves the previously established 
objects of the invention through the values cited in the description of 
the distance "l" between the centers of the pulley and the shaft 7 and 
those of the mean radius "Rm" of the cones, as well as through the values 
of their ratio. 
In fact, it was found that with equal force transmitted by the belt towards 
the pulley, equal compression load of the spring 10 on the cones, equal 
angle of the cones, same materials, the force that must be applied on the 
arm to overcome the friction on the shaft increases with decreasing values 
of the arm "l" and the force that must be applied by the belt on the arm 
to overcome the friction of the cones decreases on decreasing the mean 
radii of the cones, so that establishing the value "l" then it is possible 
to control the ratio "l" on "Rm" so as to obtain the desired dampening 
ascribing however the maximum part to the friction between the shaft and 
the arm and reducing that of the cones. In such a way a reduction of the 
wear of the materials of the cones is obtained and their capability of 
withstanding "cocking" is maintained unchanged in time. 
As regards the predetermined value "l", it was found that it is function of 
various geometric parameters as the value of the angle alpha (FIG. 1) 
assumed by the belt on the tensioner pulley, the belt length, the belt 
materials, the supporting materials of the driving and driven pulleys as 
for instance the base and the engine head. 
The value of "l" is also a function of physical parameters, such as the 
elongation of the belt in time as a function of the wear, and the modulus 
of elasticity of the belt. 
In particular the expansions to which the whole system is subject 
determines a variation of the belt development which is balanced by the 
movement of the pulley of the present tensioner. 
It was found that the value "l" of the arm can range between 3 and 7 mm. 
For example, it was found that the minimum value "l" of the arm to maintain 
constant tension values in the belt branches is about 5 mm in the case of 
a belt having a circumferential length of 1200 mm, an angle alpha of 70 
degrees, an engine of aluminum, a permanent elongation of the belt of 
0.1%, a variation of temperature comprised between -30 degrees and +130 
degrees celsius. 
In particular it is pointed out that the dampening due to the friction 
between the shaft and the portion of the arm can determine a certain wear 
of the anti-friction material interposed therebetween, but thanks to the 
characteristic of the shaft surrounded by a tubular member extending in 
practice for the whole length of the shaft, a specific pressure takes 
place which has such a value to limit considerably the wear phenomena. 
Also it is understood that the invention is not strictly limited to what is 
previously described, but it includes also all those solutions and 
alternative expedients, even if not explicitly described here, but easily 
deducible by anyone skilled in the art on the basis of the present 
inventive idea.