CA2392560C - Timing belt tensioner having a floating backstop - Google Patents

Abstract

A tensioner (10) comprises a base (20) and a pivot shaft (16) extending from the base.
A spring (18) is connected with the pivoting arm (14) to bias the arm (14) into tensioning engagement with an endless drive component of an engine.
A floating backstop assembly comprises a lever member (36) and an arm engagement member (30).
The lever member (36) is rotatably mounted on the base and the arm engagement member (30) is engaged with the lever member. The lever member (36) and the arm engagement member (30) are oriented such that the arm engagement member (30) can be biased by the spring (18) into a direction towards the arcuate surface structure of the pivoting arm (14) so as to apply a frictional force sufficient to stop movement of the arm (14).

Description

TIMING BELT TENSIONER HAVING A FLOATING BACKSTOP
Field of the Invention This invention relates to a timing belt tensioner having a floating backstop assembly which allows a controlled movement of the tensioner arm.

Background of the Invention Belt tensioners are well known in the prior art and have been used in many belt systems heretofore. A belt tensioner is a movable support structure that rotatably supports a portion of a belt in an engine or other mechanical system. A belt tensioner is movable to compensate for increases or decreases in belt path length due to wear and other factors to provide a constant belt tensioning force on a timing or drive belt.

A common type of conventional belt tensioner includes a fixed structure and a pivoted structure pivotally mounted on the fixed structure. The pivoted structure carries a belt-engaging pulley. A coil spring has ends thereof connected between the fixed and pivoted structures to bias the latter with respect to the former toward a position of maximum belt take-up. The spring biasing force decreases as the pivoted structure moves from a position of minimum belt take-up to a position of maximum belt take-up.
Although the spring force varies within the range of movement provided, a substantially constant tension is maintained on the belt. The principles stated above can be appreciated from U.S.
Patent No. 4,473,362.

Current timing belt and chain tensioners are normally equipped with a stroke limiter. A stroke limiter comprises fixed stops which will physically stop the rotation of the pivot arm a predetermined distance from the nominal pivot arm position. A
first stop will limit the arm rotation towards the belt and is commonly called a free arm stop. A
second stop limits the arm movement away from the belt and is commonly known as a back stop. The back stop is normally positioned in such a way that even if the pivot arm is rotated against the back stop, there will not be enough slack in the belt for it to rise above teeth in any of the sprockets in the drive and jump over the teeth. In other words, the back stop is designed to prevent tooth skip which would cause error in timing between the sprockets and consequent equipment errors and damage.

The common problem with the positioning of the back stop is that it often has to be placed quite close to the nominal pivot arm position. Therefore it is possible, in some operating conditions, that the pivot arm will contact the back stop. This is particularly common during periods of engine kick back experienced during engiiie shu'L
down. The possibility of the back stop contact may be further increased by minor inaccuracies or errors in the tensioner installation process whenever the pivot arm is installed away from its nominal position towards the back stop.

Back stop contact during the normal operating conditions can result in two different problems. In the most typical case involving motor car engines, back stop contact will generate excessive noise from the timing belt drive. This is a source of numerous consumer complaints. In addition, the continuous hitting of the back stop by the arm can damage the stop mechanism or arm.

The positioning of the back stop has become even more complicated with the requirement that belt tensioners be capable of adjusting their initial running position after installation without any interference or alignment by the operator. In addition, it is desirable for tensioners to be able to compensate for extremely long belt stretch due to the increased life of the belt. These requirements mean that the position of the back stop cannot be predetermined by design or adjusted by the operator; the operation of the tensioner must be able to automatically seek a proper position for the back stop at any engine running condition and limit movement of the tensioner arm during engine kick-back.

Several known designs have attempted to accommodate the need for the self-adjustment of the tensioner back stop. United States Patent no. 4,145,934 describes a wedge which is pushed against the arm eccentric (lever) so that the arm cannot rotate outwards once the tensioner arm is biased towards the belt by a tensioning spring. United States Patent no. 4,351,636 describes a tensioner similar in principle to the '934 patent except that the one-way wedge is replaced by a ratchet/pawl assembly. Another ratchet and pawl mechanism is described in United States Patent no. 4,634,407.

Each of the above-mentioned tensioners describe a one-way mechanism, which does not allow the tensioner ~um to rotate away from the belt once the arm is allowed to move inwards towards the belt.

United States Patent no. 4,583,962 offers an improvement to these designs by describing a mechanism which allows a limited return stroke of the arm towards the backstop when required by the thermal expansion of the engine. 'L he detail design of this patent describes a spring clutch type one-way device and an arc shape slot wherein the arm is free to rotate backwards.

United States Patent no. 4,808,148 describes a tensioner wherein the slot controlled reverse stroke is replaced by a resilient biasing element such as elastomeric spring located between the ratchet and pawl assembly and the stationary mounting member.

United States Patent nos. 4,822,322 and 4,834,694 describe tensioners wherein the one-way mechanisms are conventional one-way (roller) clutches and the arm return strokes are controlled by arc shaped slots.

All of the above-mentioned tensioner designs have a draw-back that they will not allow the back stop to move towards an "away-from-the-belt" direction once the back stop has moved towards the free arm position and beyond an optimum hot engine running condition. The back stop will move beyond the optimum position usually during cold starts. Consequently, the tensioner arm will frequently contact the back stop resulting in the previously mentioned operational problems.

United States Patent no. 4,923,435 describes a tensioner wherein the arm can have a return stroke controlled by a viscous clutch between the arm and the one-way mechanism.
This particular design does not guarantee that the tensioned belt will not skip a tooth, because the clutch can only act as a damper and not as a stop. This is due to the fact that viscous clutches will always move, even under a small load, providing the load will remain long enough. Consequently, a viscous clutch cannot be considered as a properly fimctional stop.

Summary of the Invention The disadvantages of the prior art may be overcome by providing a timing belt tensioner having a floating backstop assembly, which can resist an arm movement with a force exceeding the belt load to the tensioner.

In one aspect of the invention, there is provided a belt tensioner with a backstop assembly which can restrict the arm movement wilth forces which are of different magnitudes into back stop and free arm directions and which forces can vary according to the particular running conditions of the engine.

In accordance with another aspect of the invention, the restricting forces are controlled in such a way that the pivoting arm can more easily move towards the free arm position.

In accordance with another aspect of the invention, restricting forces are created with a lever-action frictional floating backstop assembly which creates radial forces on one or several diametrical or arcuate surfaces of the pivoting arm and/or on stationary arcuate sliding surfaces, thereby creating tangential frictional forces, which act on the arm either slowing down or completely stopping the arm movement.

In accordance with another aspect of the invention, the lever-action floating backstop assembly is designed in such a way that it results in a frictional wedging action taking place between the backstop assembly and the pivoting arm, which wedging action has an increased wedge angle when the frictional wedge is driven further towards the arm surface. In other words, the force acting perpendicular to the arm surface has a force component, resisting the movement of the lever member towards the arm surface, which component will increase the more the lever member rotates towards the arm surface.

In accordance with another aspect of the invention, the above-mentioned frictional wedging is created by constructing the lever member and the sliding surface for the wedging frictional element in such a way than the radius of the arcuate sliding surface is smaller than the pivoting arm length of the lever member.

According to another aspect of the invention, there is provided a tensioner including a base and a pivot shaft extending from the base having a pivot axis. A pivoting arm is rotatably mounted on the pivot shaft and has an arcuate surface structure. A pulley is rotatably mounted on the pivoting arm and has an axis of rotation extending parallel to and spaced from the pivot axis. A spring is connected with the pivoting arm to bias the arm into tensioning engagement with an endless drive component of an engine. A floating backstop assembly comprises a lever member and an arm engagement member. The lever member is rotatably mounted on the base and the arm engagement member is engaged with the lever member. The lever member and arm engagement member are oriented such that the arm engagement member can be biased by the spring into a direction towards the arcuate surface structure of the pivoting arm so as to apply a frictional force sufficient to stop movement of the arm.

The pivoting point of the lever member is to be arranged in such a way that the distance between the pivoting point and the contact point between the arm engagement member and its sliding surface remains longer that the curvature radius of the sliding surface. The forces to bias the lever member and the arm engagement member towards the sliding surface of the pivoting arm can be created by weights, the main spring member, one or more separate springs and/or by the size, shape and flexibility of the lever member itself acting as a compression spring. The construction of the lever member can take several forms depending the space available under and inside the circumferential face of the bottom cavity of the pivoting arm. The pivoting point of the lever member can be of any known pivot construction, such as a rigid pin, cup joint with a semicircular swivel, etc.

The main body of the lever member can take any shape, which can connect the pivoting point and the arm engagement member and the body can be designed to be flexible thereby limiting the radial force the arm engagement member can compel on the pivoting arm.

According to another aspect of the invention, the self-limiting wedging action can also be achieved by using an arcuate sliding surface, which is rigidly connected to the base as long as the construction radius of this surface is smaller than the pivoting arm length of the lever member. In this case the rotational movement of the pivoting arm of the tensioner must be transmitted to the lever member by a joint mechanism which can of any known construction, e.g. a pin and slot, or another arm engagement member between the pivoting arm and the lever member.

These and other objects, features and advantages of the present invention will be more fully appreciated from the following detailed description when considered in connection with the accompanying drawings, in which the same or like reference numerals designate the same or corresponding parts throughout.

Brief Description of the Drawings FIG. 1 is fragmentary front elevational view illustrating a portion of an automobile internal combustion engine having a timing belt assembly including a tensioner of the present invention;

FIG. 2 is a side sectional view of the tensioner of FIG. 1;

FIG. 3 is a partial perspective view of the tensioner of FIG. 2, with the pulley assembly removed; and FIG. 4 is a partial top view of the tensioner of FIG. 2, with the pulley assembly and the coil spring removed.

Detailed Description of the Invention Referring to Figure 1, there is illustrated a timing belt system for an internal combustion engine. A toothed pulley 112 is fixed to an output shaft 113 of the engine and an internally toothed belt 114 is driven by the pulley 112. The toothed belt 114 is trained about a second externally toothed pulley 116 which is fixed to a cam shaft 118 of the engine. A tensioner 10 is mounted in operative tensioning relation with the belt 114.
Referring to Figure 2 there is illustrated the tensioner 10 embodying the principles of the present invention. The tensioner 10 generally comprises a pulley 12 journal mounted on a pivoting arm 14 which is pivotally mounted on a pivot shaft 16.
The pulley 12 has an axis of rotation. The pivot shaft 16 has a longitudinal axis which is generally parallel to and spaced from the axis of rotation of the pulley 12. A coil spring 18 is mounted in the manner described below about the pivot shaft 16 and operably extends between the pivoting arm 14 and a tensioner base 20. A bolt 22 extends through the pivot shaft 16 to engage the tensioner 10 to an engine mounting surface.

The bottom section of the pivoting arm 14 is in a shape of a cylindrical skirt 46.
The skirt 46 is preferably mad.- of steel and it can be either an integral part of the arm 14 or connected to the separate arm part 14 with any known techniques, such as press fit, cast in, or riveted. A sector of the internal circumferential surface of the skirt 46 is especially prepared to engage an arm engagement member 30, in the form of a floating shoe, as will be described later.

Referring to Figures 3 and 4, the base 20 is generally cup shaped with a central opening and boss formation which engages the pivot shaft 16 in a friction fit.
The base 20 has a lip 24 extending about the perimeter of the base 20. A lever pivot cradling structure 26 extends upwardly from the lip 24. The cradling structure 26 is generally arcuate in shape.
A lever-action floating backstop assembly of the present invention, generally indicated at 11, generally comprises the arm engagement member 30, (also termed as a "floating shoe" or "shoe member"), a pivoting shoe 34 and a lever member 36.

The arm engagement member 30 is generally sector shaped with an arcuate outer surface 38. The inner face of the arm engagement member 30 has a groove for receiving and pivotally interlocking with the lever member 36. Diametrically opposed to the cradling structure 26, the lip 24 has a gap 28, defined by stops 29 and 31, in which the arm engagement member 30 is fitted.

The structure 26 receives the pivoting shoe 34. The shoe 34 has a pivot lobe which engages the cradling structure 26. The pivot lobe 35 minimizes circumferential sliding movement and allows the shoe 34 to pivot about a pivot point. The shoe 34 is configured to receive and interlock with the preferably ring-shaped lever member 36. An upwardly extending pin 40 is press-fitted into the lever member 36 and acts as a support for the spring 18. It is contemplated that the shoe 34 is a part of a larger plastic piece which is molded over and attached to the base 20. In this contemplated embodiment, the shoe 34 would not pivot on the cradling structure 26.

Preferably, the arm engagement member 30 and the shoe 34 are made from known frictional materials.

One end of the spring 18 is provided with a tang 42 which is inserted in a correspondingly sized slot in the lip 24 of the base 20, as shown in Fig. 3.
The opposite end of the spring 18 has a tang 44 which engages the skirt 46, as shown in Fig. 2.
Spring 18 provides a bias for the pivoting arm 14 to urge the pulley 12 into tensioning engagement with the timing belt 114 towards the free arm position.

The skirt 46 has generally a dov.mwardly extending cup shape. The skirt 46 has a central opening for frictionally engaging the pivoting arm 14. The skirt 46 is sized to fit within the inner diameter of the pulley 12 and has an axial extent to nestingly receive the base 20. The arm engagement member 30 frictionally engages the inner circumferential face of the skirt 46. The inner circumferential face of the skirt 46 has a curvature that corresponds with the arcuate outer surface 38. The inner circumferential face of the skirt 46 also has a suitable surface finish for engagement with the arm engagement member 30 and may be coated with a friction controlling material.

The lever member 36 can be of any shape which can connect to the lever pivot at the shoe 34 and the arm engagement member 30, allowing the pivotal movement of the lever member 36 even when the wear of the arm engagement member 30 allows further rotational movement of the lever member 36. The lever member 36 is preferably made of steel and can be designed to act as a compression member allowing a radial movement of the arm engagement member 30 in relation to the pivot point of the shoe 34.
Preferably, the lever member 36 has a generally circular outline extending about the pivot shaft 16 but not necessarily coaxial thereto. The distance between the pivoting point at the shoe 34 and the arm engagement member 30 may vary according to the spring rate of the lever member 36 and the force acting on the arm engagement member 30.

In the embodiment of the invention shown in FIG. 3, the biasing force to drive the arm engagement member 30 into a frictional contact with the arm skirt 46 is created by the coil spring 18. The base end tang 42 of the spring 18 is inserted in a correspondingly sized slot in the lip 24 of the base 20 located in such an angular position in relation to the pivoting shoe 34 that at least a bottom coil 50 of the spring 18 rests against the pin 40 attached to the lever 36, so that the torsional action on the spring 18 pushes the bottom coil 50 and the pin 40 towards the arm engagement member 30 forcing the latter against the skirt 46. It is also possible to control the force by which the arm engagement member 30 is brought into contact with the skirt 46 by moving by design the angular position of the spring tang 42 and by changing the position where the coil spring 18 acts on the lever member 36 in relation to the lever pivot point within the pivot shoe 34. For this purpose, another pin or tab type feature would be formed or attached to the lever member 36 at this intended contact point.

In one contemplated embodiment, the arm engagement member 30 may change orientation in relation to the lever member 36 during operation. Specifically, the arm engagement member 30 may pivot on the lever member 36. As shown in Fig. 4, it is contemplated that a rear surface of the groove of the arm engagement member 30 has a protuberance 33. The protuberance 33 is received within a notch 37 formed in an outer peripheral surface of the lever member 36 when the arm engagement member 30 is interlocked with the lever member 36. Thus, the arm engagement member 30 may pivot on the lever member 36 about the protuberance 33. This pivotal movement allows the entire outer surface 38 of the arm engagement member 30 to maintain engagement with the skirt 46 during operation. It is further contemplated that the arm engagement member 30 may have some sliding free stroke in relation to the lever member 36.

As mentioned before, the initial bias urging the arm engagement member 30 into contact with the skirt 46 can also arranged by using other means such as weights, separate springs or the spring action of the lever body. In these cases the coil spring 18 is not brought into contact with the lever member 36 and the location of the tang 42 can be made totally independent on the lever member 36. In addition, by controlling the force by which the arm engagement member 30 is initially pressed against the sliding surface of the skirt 46, one can also provide some frictional damping of the tensioner 10.

In the preferred embodiment of the invention, during the free standing condition before the initial installation of the tensioner onto the engine, the arm engagement member will abut against the stop 31 in the base 20. The angular position of the stop 31 is preferably selected in such a way that it does not allow the lever member 36 beyond its neutral position, i.e. the position where there is a least radial force between the arm 30 engagement member 30 and the skirt 46, during its stroke towards free arm.
After the tensioner has been installed onto engine using a bolt or stud applied through the longitudinal hole in the pivot shaft 16, the pivoting arm 14 must be rotated towards the back stop direction towards stop 29 until there is enough clearance around the pulley 12 to place the belt member around the pulley 12. While the pivoting arm 14 is rotated into back stop direction, the arm engagement member 30, which has a frictional connection with the arm skirt 46 will initially, together with the iever member 36, follow the rotational movement of the arm 14. During the rotation of the lever member 36, the arm engagement member 30 will be wedged towards the arm skirt 46 under the radially inwardly directed force applied by spring 18 on pin 40 increasing the frictional forces between the arm engagement member 30 and the skirt 46. Because of the geometric relationship between the skirt 46, the arm engagement member 30 and the lever pivot point within pivot shoe 34 and the optional compression spring action of the lever member 36, the torque to resist the rotation of the lever member 36 will become greater than the rotational torque created by the friction force between the arm engagement member 30 and the skirt 46, and the arm engagement member 30 will start sliding on the skirt 46 allowing the pivoting arm 14 to continue its rotation towards back stop direction while the arm engagement member 30 stops moving.

Once the belt member has been installed over the tensioner pulley 12, the operator releases the pivoting ann 14, which will now be rotated towards the belt by the spring 18.
During this return stroke of the arm 14, the arm engagement member 30 and the lever member 36 will follow the arm 14. Provided that the return stroke is long enough, the arm engagement member 30 may be able to return as far as the stop 31. It should also be noted that it is possible to bring the pivoting arm 14 all the way to the belt installation clearance position at the factory, and lock the arm 14 into this position by a locking element, such as a pin, clip or alike. In this case the operator only needs to bolt the tensioner down onto the engine, install the belt and remove the locking element.

During the normal running life of the engine, the engine will go through numerous relatively slow heat cycles, thereby decreasing and increasing the length of the belt path over the pulleys and sprockets in the belt drive. Simultaneously, the slowly occurring belt stretch and wear makes the belt behave longer. During these slowly occurring belt path length variations, the tensioner arm 14 can continuously follow the movement of the belt as the tensioner will vibrationally release, as described by U.S. Patent No.
4,824,421 to Komorowski. However, if the engine creates sudden variations in the belt path length due dynamic imbalance or kick back, which would suddenly require a large angular rotation of the arm 14 towards back stop direction, the increased frictional forces between the arm engagement member 30 and the skirt 46 will stop the arm 14. Consequently, the geometry of the lever mechanism and/or frictional characteristics between the arm engagement member 30 and the skirt 46, as well as within the lever mechanism itself, should be selected in such a way that they will not prohibit the easy return stroke of the arm 14, but will effectively stop the excessive arm movement towards the back stop direction during engine kick backs.

In other words, as the arm 14 is rotated towards the back stop direction, the spring 18 engages the pin 40 to bring the lever member 36 and hence the arm engagement member 30 into frictional engagement with the arm 14. As the arm 14, lever member 36, and arm engagement member 30 rotate together, the pivoting arm length decreases which wedges the lever member 36 into the arm 14 and thus increases the frictional force between the arm engagement member 30 and the arm 14. The increased frictional force is sufficient to stop movement of the arm 14 towards the back stop direction or a position of minimum belt take-up. Thus, the tensioner multiplies the initial spring force by the wedging action of the lever member 36 and the arm engagement member 30. During operation, there will be some movement of the arm 14 towards the backstop direction before stoppage 14 of the arm occurs.

When the arm 14 moves towards the free arm direction, the bias of the spring and wedging action of the lever member 36 is not sufficient to increase the friction between the arm 14 and the arm engagement member 30 such that the arm 14 is permitted to move towards the free arm direction or a maximum belt take-up position. The arm engagement member 30 is adapted to slide on the arm 14 if readjustment is necessary.
Relative movement between the arm 14 and the arm engagement member 30 occurs only on readjustment of the belt and also on installation.

Thus, movement of the arm 14 is inhibited only when the arm 14 moves towards the back stop direction, not towards the free arm direction.

During operation, the lever member 36 pivots as much as the tensioner geometry, engine driving forces, coefficients of friction in both shoe 34 and arm engagement member 30, and overall spring rate of the design allows.

Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the scope of the invention as defined by the attached claims.

Claims (11)

What is claimed is

1. A tensioner comprising:
a base;
a pivot shaft extending from said base, said pivot shaft having a pivot axis;
a pivoting arm rotatably mounted on said pivot shaft, said pivoting arm having an arcuate surface structure;
a pulley rotatably mounted on said pivoting arm, said pulley having an axis of rotation extending parallel to and spaced from said pivot axis;
a spring connected between said pivoting arm and said base, biasing said arm into tensioning engagement with an endless drive component of an engine; and a floating backstop assembly comprising a lever member and an arm engagement member, said lever member being pivotally mounted on said base at a lever pivot and having said arm engagement member mounted thereon in frictional engagement with said arcuate surface structure whereby said arm engagement member engages said arcuate surface structure at a position opposite from said lever pivot, said spring engaging said arm engagement member biasing said arm engagement member towards said arcuate surface structure, applying a frictional force sufficient to stop movement of said arm as said arm moves in a direction away from tensioning engagement with said endless drive component during engine kick backs.

2. A tensioner according to claim 1, wherein said arcuate surface structure of the pivoting arm has a smaller radius than a pivoting arm length of said lever member.

3. A tensioner according to claim 1, wherein said spring biases said lever member such that said arm engagement member is biased into frictional engagement with the pivoting arm when the pivoting arm is rotated towards a back stop direction, said lever member and said arm engagement member being adapted to apply frictional force to stop movement of the pivoting arm as the pivoting arm moves towards the backstop direction.

4. A tensioner according to claim 1, wherein the bias of the spring is not sufficient to increase friction force between the pivoting arm and the arm engagement member when the pivoting arm moves toward a free arm direction such that the pivoting arm is permitted to move towards the free arm direction, said arm engagement member being adapted to slide on the pivoting arm in order to readjust.

5. A tensioner according to claim 1, wherein said lever member acts as a compression member allowing radial movement of said arm engagement member.

6. A tensioner according to claims 1 to 5, wherein said base has a lip extending about a perimeter thereof and said lip has a pair of stops limiting travel of said arm engagement member.

7. A tensioner according to claim 6, wherein said lever member has a circular outline.

8. A tensioner according to claim 7, wherein said lever member has a shoe frictionally supporting said lever member on said base at said lever pivot.

9. A tensioner according to claim 8, wherein said base has a cradling structure frictionally receiving said shoe.

10. A tensioner according to claim 9, wherein said pivoting arm has a skirt extending towards said base and said arcuate surface structure is on an inner face of said skirt.

11. A tensioner according to claim 10, wherein said shoe and said arm engagement member are made from a friction material.