Stretcher for a transmission belt

Stretcher for a transmission belt comprising a fixed part suitable for being rigidly connected to a support structure and provided with a pin defining a first axis, a movable part comprising a hub rotating on the pin about the first axis, a pulley carried by the movable part and rotating with respect thereto about a second axis distinct from the first axis and parallel thereto, and a spiral spring interposed between the fixed part and the movable part and suitable for loading in use the movable part in the direction tending to keep the pulley in engagement with the relative belt. The spring has an internal end secured to the pin, and an external end secured to the hub, and is interposed between two bushes which support the hub on the fixed part and are provided with respective flanges cooperating axially with the spring.

BACKGROUND OF THE POTION 
The invention relates to a stretcher for a transmission belt and 
particularly but not exclusively for a belt for transmitting motion from 
the drive shaft of an internal combustion engine to auxiliary elements 
driven by the said engine. 
Stretchers are known which comprise a fixed part suitable for being rigidly 
connected to a support structure integral with the engine block, a movable 
part hinged to the fixed part about a first axis, a pulley carried by the 
movable part and rotating with respect thereto about a second axis 
distinct from the first axis and parallel thereto, and a spring interposed 
between the fixed part and the movable part, which loads the movable part 
in the direction tending to keep the pulley in engagement with the 
relative belt. 
Variations in belt tension due to the selective actuation of the various 
accessories and to wear are therefore compensated by consequent variations 
in the deformation of the spring and corresponding movements of the axis 
of rotation of the pulley. 
According to a known embodiment, the fixed part comprises a pin defining 
the first axis, and the movable part comprises a hub rotating on the pin 
with the interposition of a bush, also having the function of damping by 
friction the oscillations of the movable part under the pulsating load 
transmitted by the belt to the pulley, and thus to the said movable part. 
The spring, a spiral one, is wound round the hub and has an internal end 
secured to the said hub and an external end secured to a flange of the 
fixed part. 
The known stretchers of the type briefly described have a number of 
disadvantages. 
Because the pulley is supported in projecting manner by the movable part, 
the pull of the belt causes a tilting moment on the said movable part 
(i.e. tending to rotate the latter about an axis transverse to its own 
axis of rotation). The tilting moment is balanced by the restraining 
reaction of the fixed part through the bush; this tends, however, to 
stress the bush in anomalous manner, creating non-uniform wear in it. 
Obviously, the effects of the tilting moment could be reduced by increasing 
the dimensions of the pin and the bush whilst retaining the same 
configuration; this would, however, involve an undesirable increase in the 
size of the hub and hence also of the fixed part. 
SUMMARY OF THE INVENTION 
The object of the invention is to produce a stretcher which does not have 
the disadvantages associated with the known stretchers specified above, 
and which is of modest size. 
This object is achieved by the invention in that it relates to a stretcher 
for a transmission belt of the type comprising a fixed part suitable for 
being rigidly connected to a support structure and provided with a pin 
defining a first axis, a movable part provided with a hub mounted on the 
pin rotating about the said first axis with the interposition of a first 
bush, a pulley carried by the said movable part and rotating with respect 
thereto about a second axis distinct from the first axis and parallel 
thereto and flexible means interposed between the said fixed part and the 
said movable part and suitable for loading in use the movable part in the 
direction tending to keep the pulley in engagement with the relative belt, 
wherein the said flexible means comprise a spiral spring wound round the 
pin and having an internal end secured to the said pin and an external end 
secured to an annular portion of the hub, the stretcher also comprising a 
second bush interposed between the annular portion of the hub and the 
fixed part, the bushes being disposed on parts axially opposite with 
respect to the spring.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the illustration, in its entirety 1 denotes a stretcher 
for a belt 2 with multiple grooves (multi-V) for driving the auxiliary 
elements (not shown) of an internal combustion engine (not shown). 
In known manner the stretcher 1 substantially comprises a fixed part 3 
suitable for being rigidly connected to a support structure (not shown), 
such as a support plate for the auxiliary elements which is fixed to the 
engine block, a movable part 4 hinged to the fixed part 3 about a first 
axis A, a pulley 5 carried in projecting manner by the movable part 4 and 
rotating with respect thereto about a second axis B distinct from the 
first axis A and parallel thereto, and a spring 6 interposed between the 
fixed part 3 and the movable part 4, which loads the movable part in the 
direction tending to keep the pulley 5 in engagement with the belt 2. 
More particularly, the fixed part 3 integrally comprises a flange 10 
suitable for cooperating axially with the support structure and a tubular 
pin 11 of axis A extending axially in projecting manner from the flange 
10. The flange 10 has a plurality of frontal teeth 12 suitable for 
engaging respective seats (not shown) of the support structure to anchor 
the fixed part 3 in the angular direction. 
The movable part 4 integrally comprises a hub 13 coaxial to the pin 11 and 
mounted in a rotating manner thereon, an arm 15 extending radially in a 
projecting manner from the hub 13, and a tubular portion 16, of axis B, 
integrally connected to an end of the arm 15 and having the function of 
supporting the pulley 5. 
The hub 13 of the movable part 4 is substantially bell-shaped and comprises 
a first portion 18 and a second portion 19 which are axially adjacent and 
have a larger and smaller diameter respectively. 
For a section of its own axial length, the first portion 18 is disposed 
round the flange 10 and is supported radially thereon by means of a bush 
20 which, with the flange 10, defines a friction torque of pre-determined 
coefficient. For the remaining section of its own length the first portion 
18 delimits with the pin 11 an annular chamber 21 housing the spring 6. 
The latter comprises a strip of steel spiral-wound round the pin 11 and 
having an internal end 24 secured in known manner to the pin 11 and an 
external end 25 secured in known manner to the portion 18. 
The second portion 19 is supported radially by the pin 11 with the 
interposition of a bush 26 which defines with the pin 11 a friction torque 
of pre-determined coefficient. 
The bush 20 and the bush 26 have respective end, internal and external 
respectively, flat annular flanges 28, 29 which cooperate axially with 
opposite flanks of the spring 6 and are interposed between the latter and, 
respectively, the flange 10 and the second portion 19 of the hub 13. 
Finally the stretcher 1 comprises a damping element 30 interposed between 
the pin 11 and the movable part 4. 
The damping element 30 substantially comprises a metallic cupped core 34 
having a substantially flat circular base wall 35 rigidly fixed onto a 
free end 36 of reduced diameter of the pin 11 and a substantially 
cylindrical lateral wall 37 extending in projecting manner from an 
external periphery of the base wall 35. 
The damping element 30 also has a coating 38 of plastic material extending 
over the face of the base wall 35 facing towards the hub 13 and over the 
entire surface of the lateral wall 37, so as to form therewith an annular 
friction crown 39 engaging an annular seat 40, of complementary form, 
produced on a front face 41 of the portion 19 of the hub 13. The material 
of the coating 38 is selected so as to define with the hub 13 a friction 
torque of pre-determined coefficient to ensure, in use, adequate 
oscillation damping properties. The contact between the annular crown 39 
and the relative seat 40 conveniently takes place either along the 
internal and external cylindrical surfaces, or along the annular front 
surface of the crown 39. 
The core 34 is provided with an axial hole 44, engaged in forced manner by 
the end 36 of the pin 11; the end 36 is thus pressed onto the internal 
edge of the base wall 35 so as to ensure a "pack" mounting on the pin 11, 
i.e. substantially with no axial play, of the group comprising the bush 
20, the spring 6, the bush 26 and the hub 13. 
The coating 38 is applied to the core 34 by means of over-moulding; for 
this purpose the core 34 is provided with a plurality of holes 45 
surrounding the axial hole 44 and suitable for being engaged by the 
material of the coating 38, during the molding phase, to ensure the 
anchorage of the coating 38 to the core 34. 
The operation ok the stretcher 1 is per se known and will not therefore be 
described in detail. 
The arrangement of the spring 6, with the internal end 24 secured to the 
pin 11 and the external end 25 to the portion 18 of the hub 13 enables the 
diameter of the pin 11 to be increased without penalizing the overall size 
of the hub. The tilting moment caused by the pull of the belt is 
efficiently balanced by the bushes 20, 26, which have a large diameter and 
are spaced between them axially so as to generate restraining reactions 
the resultants of which have a relatively long arm. 
Furthermore, the radial flanges 28, 29 of the bushes 20, 26 and the damping 
element 30 help both to damp the oscillations and to support the movable 
part 4 with respect to the fixed part 3, further reducing the effects of 
the tilting moment. 
Finally, it will be clear that modifications and variants which do not 
depart from the protective scope of the invention may be introduced to the 
stretcher 1 described.