Eccentric type belt tensioner with cam operated damping means

A spring bias tensioner of the eccentric type with a fixed cam ring and rotatable cam ring activated by a torsion spring wherein the cam rings generate an axially force to engage radial friction sliding surfaces to affect damping as a pivot arm of the tensioner is moved.

BACKGROUND OF THE INVENTION 
The invention relates to a spring biased tensioner of the eccentric type 
with an effective pivot arm as part of a stub shaft and located within the 
inside diameter of a pulley bearing, but more particularly, the invention 
relates to an eccentric type tensioner with a damping mechanism utilizing 
friction surface sliding for inhibiting pivotal oscillations of the pivot 
arm. 
Various mechanisms are used to inhibit angular movements of a tensioner's 
pivot arm about its pivot axis. An example of a non-eccentric type 
tensioner (i.e., where the pivot arm has a length that is greater than the 
radius of a pulley carried by the pivot arm) utilizing a cam activated 
damping mechanism is disclosed in U.S. Pat. No. 4,557,707. Besides its 
overall large size comparative to an eccentric type tensioner, the cam 
operated mechanism of the '707 tensioner operates at an effective radius 
that is less than the length of the tensioner's pivot arm which affects an 
insensitivity of the damping torque relative to the output torque of the 
tensioner as reacted through the pivot arm by way of a pulley. 
Another problem associated with the size of the non-eccentric belt 
tensioner of the type disclosed in U.S. Pat. No. 4,557,707, is that the 
cam operated damping mechanism is located outside a radial boundary of the 
pulley. 
Still another problem associated with the '707 tensioning device is that 
the cam mechanism includes a singular, polymeric cam ring in which cam 
followers in the form of bent tabs are located. With a single, plastic cam 
follower ring, it is difficult to change operating characteristics of the 
friction sliding surfaces, the slope of the surfaces, as the cam is in the 
form of bent tabs of metal. 
An example of an eccentric type tensioner that includes a cam activated 
mechanism, is disclosed in U.S. Pat. No. 4,834,694. The cam activated 
mechanism of the tensioner does not operate to provide a variable axial 
force associated with damping pivotal movements of the pivot arm, but 
rather, the cam mechanism operates as a one-way clutch where a radial 
force is generated to move a follower to lockup with an inner surface of a 
pulley supporting housing whereby movements of the pivot arm in a 
non-tensioning direction, are prevented or substantially inhibited. 
Another example of an eccentric type tensioner is disclosed in U.S. Pat. 
No. 4,808,148. While a cam operated mechanism is not used, a locking 
mechanism in the form of a ratchet and pawl, is used to prevent the pivot 
arm from rotating in a non-tensioning direction which is similar to the 
operation of the aforementioned '694 tensioner. However, the '148 device 
presents a size problem in that the rachet and pawl mechanism is located 
within the inside diameter of a pulley bearing making the bearing overly 
large and expensive in comparison to other eccentric type tensioning 
devices that are without additional mechanisms located within a bearing 
bore. 
Still another problem associated with the aforementioned tensioners is that 
the location of friction sliding surfaces are at an effective radius that 
is equal to less than the length of the pivot arm, or the inside radius of 
its pulley bearing. 
The present invention primarily concerns belt tensioners of the eccentric 
category and makes possible the use of small pulley bearings. 
SUMMARY OF THE INVENTION 
In accordance with the invention, an eccentric type tensioner is provided 
with the centers for a pulley bearing and a pivot positioned within the 
inside diameter of the pulley bearing. A pivot arm is formed as a portion 
of a stub shaft to which the pulley bearing is attached. A pivot-pin 
extends through the stub shaft such that the pivot-pin is radially 
eccentric to the pulley bearing axis. The eccentric radius defines a pivot 
arm length for the tensioner. A spring means biases rotation of the pivot 
arm about the pivot-pin. 
A feature of the invention is a damping mechanism which includes a fixed 
cam ring coaxially positioned in relation to the pivot pin and having a 
plurality of axially facing, circumjacent tapered cam surfaces. A 
rotatable cam ring has a plurality of oppositely facing tapered cam 
surfaces that substantially compliment and engage the cam surfaces of the 
fixed cam ring. First and second disc-like members are operatively 
connected at oppositely facing ends of the stub shaft and sides of a 
pulley bearing which each define a surface for friction surface sliding 
against a friction material positioned at an effective radius from the 
pivot pin. Angular movement of the pivot arm operates to activate the cam 
surfaces which separates the cam rings so as to vary an axial force 
applied to the friction sliding surfaces. In one embodiment of the 
invention, the friction sliding surfaces operate at an effective radius 
from the pivot pin axis where such radius is greater than the pivot arm 
length.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the Figures, a belt tensioner 10 of the invention is provided 
with: a bearing 12 having a bore 14 at an inside diameter; a stub shaft 
16; a pivot arm 18 formed as a portion of the stub shaft 16; a pivot 
bushing 20; a pivot pin 22; and a spring 24. For some applications, the 
outer race 26 of the bearing 12 may be used directly as a pulley member. 
However, and as illustrated in FIGS. 1 and 2, a pulley 28 is attached to 
the outer race. 
The bearing is positioned with its inside bore 14 substantially at the 
circumference 34 of the stub shaft 16. The bearing may be attached by any 
desired means to the stub shaft such as by an interference fit between the 
bore of the bearing and the outside circumference 34 of the stub shaft. 
The pivot bushing 20 and pivot-pin 22 extend through and are eccentric to 
the stub shaft at a radius defining a pivot arm length A. Pivot-pin 22 has 
a mounting means 36 for an attachment to a nonrotatable member such as 
some part of an engine, not shown. The mounting means 36 may also extend 
through and attached to a nonrotating base member 38. 
The spring is preferably of the torsional type, and preferably, has bent 
ends 42, 44 as means for operatively interconnecting the spring so as to 
bias rotation of the pivot arm about the pivot-pin causing eccentric 
movements of the bearing and pulley. The spring is interconnected to a cam 
operated damping means 46 to inhibit rotation of the pivot arm about the 
pivot pin. 
Referring more particularly to FIG. 2, a first disc-like member 48 with a 
radially oriented friction sliding surface 50 and a tang 52 is affixed to 
the stub shaft 34 such as by being an integral component part thereof. A 
second disc-like member 53 is affixed at the opposite end of the stub 
shaft 50 such as by means of a collar 54, a pin 56, and slot 58. The 
second disc-like member 53 has a radially oriented friction sliding 
surface 60, and both the first and second disc-like members rotate with 
the stub shaft. A third disc-like member 62 with a radially oriented 
friction sliding surface 64 may optionally be included and fixed against 
rotation with the pivot-pin by means of a flat 66 formed as part of the 
pivot-pin, and flat 68 formed as part of a wrenching surface of the third 
disc-like member. A fastener 70 holds the assembly together. 
A fixed cam ring 72 is positioned substantially coaxially with the pivot 
pin and has a plurality of circumjacent and tapered cam surfaces 74 
oriented in the same circular direction. The affixed cam ring is prevented 
from rotating by means of positioning pins 76 formed as part of the cam 
ring, meshed with apertures 78 formed in the base member 38. 
A rotatable cam ring 80 is positioned substantially coaxially with the 
fixed cam ring and has a plurality of oppositely facing, circumjacent and 
tapered cam surfaces 82. 
The fixed cam ring and rotatable cam ring are preferably made of plastic 
materials, and optionally may be made of different plastic materials so as 
to characterize a coefficient of friction of the engaged cam surfaces 
where the friction is in a range that is greater than zero and less than 
about 0.15. Any suitable plastic material may be used. Examples of such 
materials are nylon such as manufactured by LNP Engineering Plastics Inc. 
and sold under the brand Lubricimp 189; and acetol such as manufactured by 
DuPont and sold under the brand 500 TL. The cam rings may be made of the 
same or different plastic materials. 
Rotatable cam ring 80 defines a radially oriented friction sliding surface 
84 that engages the friction sliding surface 50 of the first disc member. 
A disc like washer 86 of a suitable plastic material such as those 
exemplified for the fixed and rotatable cam rings, has oppositely axially 
facing friction sliding surfaces 88, 90 which engage flat friction sliding 
surfaces 64, 60. 
The spring 24 is operatively connected with end 44 hooked around tang 52, 
and the opposite end 42, hooked around and positioned in a slot 92 of the 
rotatable cam ring. 
Rotation of the pivot arm winds the spring whereby end 42 of the spring 
pulls against the rotatable cam ring causing the rotation thereof. The cam 
rings, with the engaged tapered surfaces move to axially separate the 
rings from each other as the spring is wound. The friction sliding 
surfaces including the engaged tapered surfaces are pulled into operative 
engagement by the axial force which varies in accordance with a force as 
induced by the torsional spring. As best shown in FIG. 1, the friction 
sliding surfaces of the cam rings and radial surfaces juxtaposed thereto, 
are at an effective radius R that is greater than the pivot-arm length. 
The greater arm length assures a greater percentage of achievable damping 
and is present in prior art devices. 
The foregoing description is made for purpose of illustration only and the 
scope of the invention is to be determined from the appended claims.