Tensioner

[0002] The present application claims priority to U.S. Provisional Application of Frankowski et al., Ser. No. 60/111,378, filed Dec. 8, 1998, the entirety of which is hereby incorporated into the present application by reference.

[0003] The present invention relates to tensioners for tensioning engine driven elements such as timing belts or chains. In particular, the present invention is primarily concerned with timing belt tensioners, although the principles of the present invention may also be applied to accessory belt and chain tensioners and timing chain tensioners.

[0004] In prior art tensioners, the tensioner geometry and characteristics of the tensioner's spring are selected to ensure that the belt tension required to move the tensioner through its range of operating positions remains relatively constant throughout the range. That is, these tensioners are designed so that as belt tension increases due to engine conditions, such as thermal expansion or increased operational belt loads, the tensioner moves under the increased belt tension to compensate for such increases and maintain the belt tension relatively constant.

[0005] Prior art tensioners are normally provided with a pair of stops, one at the maximum travel position of the tensioner arm and one at the free arm position of the tensioner arm. These stops restrict the pivotal movement of the tensioner arm and provide the same with a limited range of movement. Because the belt tension required through the range is relatively constant, increases in belt tension can cause the tensioner arm to travel through the range of operating positions until tensioner arm contacts the stop at the maximum travel position thereof. When the increase in belt tension is rapid, the contact between the tensioner arm can create undesirable noises or, in the worst case scenario, damage-the tensioner. This type of increase occurs most commonly as a result of engine kickback at shutdown. If the tensioner is damaged, the engine itself may suffer extensive damage as a result of the timing belt or chain failing to operate the component(s) connected thereto in proper timing with respect to the engine cycles.

[0006] Consequently, there exists a need in the art for a tensioner that can be used in combination with vehicle engine that eliminates the problems discussed above with respect to prior art tensioners.

[0007] It is an object of the present invention to meet the above-described need. To achieve this object, one aspect of the present invention provides a combination comprising a vehicle engine, an endless flexible driving element driven by the engine, and a tensioner. The tensioner comprises a fixed structure mounted on the engine, a pivot structure pivotally mounted on the fixed structure for pivotal movement about a pivot axis, spring structure constructed and arranged to apply a tensioning torque to the pivot structure that tends to pivot the pivot structure in a tension applying direction, and a rotatable member rotatably mounted on the pivot structure for rotation about a rotational axis spaced radially from the pivot axis by a radius. The rotatable member engages the driving element in a tension applying relationship such that the driving element is tensioned and in reaction applies a hub load force to the rotatable member at an angle with respect to the radius.

[0008] The tensioner is mounted on the engine such that when the engine is in an initial condition the pivot structure is angularly positioned at an initial angular position spaced from a perpendicular angular position at which the hub load force would be applied to the rotatable member perpendicularly to the radius. The initial angular position is spaced from the perpendicular angular position in an opposite direction opposite the tension applying direction. In the initial angular position, the spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a first mean dynamic tension.

[0009] As the engine thermally expands to its hot engine condition, the mean dynamic tension in the driving element increases so that the hub load force applied by the driving element pivots the pivot structure in the opposite direction away from the initial angular position thereof to a hot engine angular position. In the hot engine angular position, the spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a second mean dynamic tension greater than the first mean dynamic tension. The tensioner is constructed and arranged such that, as the pivot structure is pivoted from the initial angular position thereof to the hot engine angular position thereof, the angle between the hub load force and the radius continually increases and the spring structure is continually increasingly stressed so that the mean dynamic tension in the driving element continually increases from the first mean dynamic tension to the second mean dynamic tension during the thermal expansion of the engine. The tensioner is also constructed and arranged such that the mean dynamic tension of the driving element required to continue pivoting the pivot structure in the opposite direction from the hot engine position thereof continually increases as a result of the angle between the hub load force and the radius continually increasing and the spring structure being continually increasingly stressed the further the pivot structure is pivoted in the opposite direction from the hot engine position.

[0010] The key feature to note of this aspect to the invention is that in the initial angular position of the pivot structure is spaced from the perpendicular angular position in the opposite direction. As a result, as the pivot structure is pivoted from the initial angular position thereof to the hot engine angular position thereof, the angle between the hub load force and the radius continually increases beyond 90 degrees and the spring structure is continually increasingly stressed so that the mean dynamic tension in the driving element continually increases from the first mean dynamic tension to the second mean dynamic tension during the thermal expansion of the engine. Likewise, the mean dynamic tension of the driving element required to continue pivoting the pivot structure in the opposite direction from the hot engine position thereof continually increases as a result of the angle between the hub load force and the radius continually increasing and the spring structure being continually increasingly stressed the further the pivot structure is pivoted in the opposite direction from the hot engine position. In prior art tensioners, the initial angular position of the pivot structure is spaced from the perpendicular angular position in the tension applying direction. Because the torque acting against the spring structure is related to the sine of the angle between the hub load force applied to the rotatable member and the radius, the sine of this angle increases as the pivot structure approaches the perpendicular angular position, thus maximizing the contribution of the tensioner's geometry to that torque. By spacing the initial angular position of the pivot structure in the opposite direction from the perpendicular angular position, the contribution offered to that torque by the tensioner's geometry is reduced, and this reduction increases as a function of the angle between the hub load force and the radius continuing to increase (and hence the sine of that angle decreasing).

[0011] In another way to achieve the object of the present invention, another aspect of the present invention provides a vehicle engine, an endless belt driven by the engine, and a tensioner. The engine is capable of applying a maximum tension to the driving element during operation thereof. This maximum tension is the known maximum tension which the engine is capable of creating, and is normally determined from either manufacturer specifications or testing.

[0012] The tensioner comprises a fixed structure mounted on the engine, a pivot structure pivotally mounted on the fixed structure for pivotal movement about a pivot axis within a range of angular positions, spring structure constructed and arranged to apply a tensioning torque to the pivot structure that tends to pivot the pivot structure in a tension applying direction within the range of angular positions, and a rotatable member rotatably mounted on the pivot structure for rotation about a rotational axis spaced radially from the pivot axis by a radius. The rotatable member engages the driving element in a tension applying relationship such that the driving element is tensioned and in reaction applies a hub load force to the rotatable member at an angle with respect to the radius. The range of angular positions of the pivot structure includes a potential tooth skip position. This potential tooth skip position is the point at which, if the pivot structure were moved into the potential tooth skip position under driving element tension and then the tension in the driving element were decreased, tooth skip would be allowed to occur between the driving element and the engine if the spring structure failed to move the pivot structure in the tension applying direction to maintain the rotatable member in the tension applying relationship with the driving element. It is important to understand that the tensioner in accordance with this aspect of the invention is designed to prevent the pivot structure from moving into this potential tooth skip position, and that the pivot structure does not necessarily have to move into this position during operation. Instead, this potential tooth skip position is a position at which such tooth skip would occur if the pivot structure were moved to that position and the spring structure failed to move the pivot structure in the tension applying direction. This failure can possibly occur from dirt and other particulate material jamming the pivot structure's movement, or from water on the tensioner freezing during winter conditions and hence jamming the pivot structure's movement.

[0013] The tensioner is constructed and arranged such that the mean dynamic tension of the driving element required to pivot the pivot structure in the opposite direction from the hot engine position thereof towards and into the potential tooth skip position is greater than the aforesaid maximum tension the engine is capable of applying to the driving element. Because the mean dynamic tension required to pivot the pivot structure into the potential tooth skip position thereof is greater than the maximum tension that the engine is capable of creating, the tensioner can be considered self-limiting and the need for a maximum travel stop can be obviated. It should be noted, however, that a maximum travel stop may be provided within this aspect of the invention as a safety feature in order accommodate for incorrect installations and the like.

[0014] In yet another way to achieve the object of the present invention, yet another aspect of the present invention provides a combination comprising a vehicle engine adapted to thermally expand from an initial condition at an ambient temperature to a hot engine condition due to an increase in engine temperature during engine operation; an endless flexible driving element driven by the engine; and a tensioner. The tensioner comprises a fixed structure mounted on the engine; a pivot structure pivotally mounted on the fixed structure for pivotal movement about a pivot axis; spring structure constructed and arranged to apply a tensioning torque to the pivot structure that tends to pivot the pivot structure in a tension applying direction; a maximum travel stop constructed and arranged to engage the pivot structure pivoting in an opposite direction opposite the tension applying thereof to thereby prevent further pivotal movement of the pivot structure in the opposite direction and provide the pivot structure with a maximum travel angular position; and a rotatable member rotatably mounted on the pivot structure for rotation about a rotational axis spaced radially from the pivot axis by a radius. The rotatable member engages the driving element in a tension applying relationship such that the driving element is tensioned and in reaction applies a hub load force to the rotatable member at an angle with respect to the radius.

[0015] The tensioner is mounted on the engine such that when the engine is in the initial condition the pivot structure is angularly positioned at an initial angular position. In the initial angular position, the spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a first mean dynamic tension when the pivot structure is in the initial angular position thereof. As the engine thermally expands to the hot engine condition, the mean dynamic tension in the driving element increases so that the hub load force applied by the driving element pivots the pivot structure in the opposite direction away from the initial angular position thereof to a hot engine angular position. In the hot engine angular position, the spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a second mean dynamic tension when the pivot structure is in the hot engine angular position thereof.

[0016] The spring structure and the initial angular position of the pivot structure are selected such that the mean dynamic tension of the driving element required to pivot the pivot structure in the opposite direction from the hot engine position to the maximum travel angular position continually increases in such a manner that the mean dynamic tension required to move the pivot structure from the hot engine position to the maximum travel position is at least 75% greater than the second mean dynamic tension. This feature effectively reduces the contact between the pivot structure and the maximum travel stop, unless the specified driving element tension is created. Further, in the event that the tension is high enough to create contact between the pivot structure and the stop, this increased resistance effectively “cushions” the upstroke of the pivot structure and reduces the force with which such contact is made. In prior art tensioners, there is normally some slight increase in belt tension as the pivot structure moves beyond the hot engine angular position thereof. However, the goal in these prior art tensioners is to maintain a constant belt tension and the increase is not enough to have as significant of an effect as this aspect of the invention, wherein a 75% or greater increase over the second dynamic tension is required. The minimum threshold of 75% is where significant improvement in this type of behavior is typically, seen, and thus is to be regarded as a commercially valuable lower end for this range.

[0017] In still another way to achieve the object of the present invention, still another aspect of the present invention provides a combination comprising a vehicle engine adapted to thermally expand from an initial condition at an ambient temperature to a hot engine condition due to an increase in engine temperature during engine operation; an endless flexible driving element driven by the engine; and a tensioner. The tensioner comprises a fixed structure mounted on the engine; a pivot structure pivotally mounted on the fixed structure for pivotal movement about a pivot axis within a predetermined range of angular positions; spring structure constructed and arranged to apply a tensioning torque to the pivot structure that tends to pivot the pivot structure in a tension applying direction within the predetermined range of angular positions; and a rotatable member rotatably mounted on the pivot structure for rotation about a rotational axis spaced radially from the pivot axis by a radius. The rotatable member engages the driving element in a tension applying relationship such that the driving element is tensioned and in reaction applies a hub load force to the rotatable member at an angle with respect to the radius.

[0018] The tensioner is mounted on the engine such that when the engine is in the initial condition the pivot structure is angularly positioned at an initial angular position. In this initial angular position, the spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a first mean dynamic tension when the pivot structure is in the initial angular position thereof. As the engine thermally expands! to the hot engine condition, the mean dynamic tension in the driving element increases so that the hub load force applied by the driving element pivots the pivot structure in an opposite direction opposite the tension applying direction away from the initial angular position thereof to a hot engine angular position. The hot engine angular position is spaced from an end of the predetermined range of angular positions in the tension applying direction. The spring structure applies the tensioning torque to the pivot structure such that the driving element is tensioned to a second mean dynamic tension when the pivot structure is in the hot engine angular position thereof. The aforementioned end of the predetermined range may be a point determined by engine manufacturer specifications/requirements, or a maximum travel stop as discussed above. Further, this end of the predetermined range may be the potential tooth skip position in an arrangement wherein no maximum travel stop is used, and the increase in mean dynamic tension necessary to reach this position, which is specified below as being 75% or more greater than the tension at the hot engine angular position, would be set high enough to prevent the pivot structure from moving into this position. This aspect of the invention should not be considered as being limited to a tensioner with a tooth skip position, a stop, or other structures discussed in connection with other aspects of the invention as being determinative of the end of a predetermined range. The spring structure and the initial angular position of the pivot structure are selected such that the mean dynamic tension of the driving element required to pivot the pivot structure in the opposite direction from the hot engine position to the end of the predetermined range continually increases in such a manner that the mean dynamic tension required to pivot structure from the hot engine position to the end of the predetermined range is at least 75% greater than the second mean dynamic tension.

[0019] Other objects, features, and advantages of the present application will become appreciated from the following detailed description, the accompanying drawings, and the appended claims.

[0020] FIG. 1 is a schematic front elevational view of a vehicle's internal combustion engine with a timing belt driven by the engine and a timing belt tensioner constructed in accordance with the principles of the present invention mounted to the engine and engaging the belt in a tension applying relationship;

[0021] FIG. 2 is a cross-sectional view of the tensioner shown in FIG. 1 taken along a line passing through both the pivotal axis of the tensioner's pivot structure and the rotational axis of the tensioner's rotatable member;

[0022] FIG. 3 is a schematic diagram illustrating the forces and torques that are applied to the belt and components of the tensioner during operation;

[0023] FIG. 4 is a graph depicting a curve representing a theoretical torque requirement for the spring structure that would provide for constant belt tension and a straight line illustrating the actual torque output of a spring structure selected based on the theoretical requirement for use in a prior art tensioner;

[0024] FIG. 5 is a graph illustrating actual belt tension achieved using the spring structure in FIG. 4;

[0025] FIG. 6 is a graph similar to FIG. 4 for a tensioner constructed in accordance with the principles of the present invention;

[0026] FIG. 7 is a graph similar to FIG. 5 for a tensioner constructed in accordance with the principles of the present invention.

[0027] FIG. 1 shows a schematic front elevational view of a vehicle's internal combustion engine 100 with an endless driving element in the form of an internally toothed timing belt 114 driven by the engine 100 and a timing belt tensioner, generally indicated at 10, constructed in accordance with the principles of the present invention mounted to the engine 100. The tensioner 10 engages the belt 114 in a tension applying relationship. A toothed pulley 112 is fixed to the end of the engine's crankshaft 113 and the toothed side of the belt 114 is trained over the pulley 112 in an intermeshed relationship. The toothed side of the belt 114 is also trained in an intermeshed relationship over a toothed pulley 116 that is fixed to a cam shaft 118 of the engine 100. Rotation of the crankshaft 113 drives the belt 114 via the intermeshed relationship between the belt 114 and the pulley 112, which in turn drives the camshaft 118 via the intermeshed relationship between the belt 114 and the pulley 116. This ensures that the camshaft 118 is driven in time with the engine crankshaft 113, as is conventional in internal combustion engines.

[0028] The belts may be trained over the pulley or sprocket of any engine driven component/accessory, and the invention is not necessarily limited to an arrangement wherein the camshaft is being driven. Thus, the belt 114 can be broadly considered to be driving one or more engine driven components.

[0029] FIG. 2 is a cross-sectional view of the tensioner 10 shown in FIG. 1 taken along a line passing through both the pivotal axis of the tensioner's pivot structure and the rotational axis of the tensioner's rotatable member. The pivot structure is provided by a tensioner arm 14 and the rotatable member is provided by an annular pulley 12. A sprocket (not shown) may be substituted for the pulley 12 in a timing chain system.

[0030] The pivot arm 14 is pivotally mounted to a fixed structure in the form of pivot shaft 16 for pivotal movements about a pivot axis 26. Damping structure in the form of annular sleeve 24 is press-fit over the pivot shaft 16 and located between the arm 14 and the pivot shaft 16. As the arm 14 pivots relative to the pivot shaft 16, the sleeve 24 creates friction that resists the pivoting of the arm 14. Friction may also be created between the spring 18 and the spring mounting bracket 20. Suitable damping structures are described in further detail in U.S. Reissue Pat. No. 34,543, the entirety of which is hereby incorporated into the present application by reference, and European Patent Application. No. 0294919, the entirety of which is hereby incorporated into the present application by reference. The pivot shaft 16 is constructed and arranged to mount to the engine 100 using a bolt. An installation structure for facilitating installation and adjustment of the tensioner 10 is indicated at 22.

[0031] The pulley 12 is rotatably mounted to the pivot structure by a ball bearing assembly 28. The inner race 30 of assembly 28 is press-fit to the exterior cylindrical surface of the arm 14 and the pulley 12 is press-fit to the exterior surface of the outer race 32 of the assembly 28. The pulley 12 rotates about a rotation axis 34 that is spaced from the pivot axis 26 by a radius r, shown in FIG. 3.

[0032] The tensioner 10 also comprises an annular spring mounting bracket 20 fixedly secured to the pivot shaft 16. Spring structure in the form of a wound linear torsion spring 18 is carried within the spring mounting bracket 20. One end of the spring 18 is connected to the arm 14 and the other end of the spring is connected to the bracket 20. The spring 18 applies a tension applying torque to the arm 14 that tends to pivot the arm 14 about the pivot axis 26 thereof in a tension applying direction.

[0033] FIG. 3 shows a schematic diagram illustrating the forces and torques that are applied to the belt and components of the tensioner 10 during operation. In FIG. 3, the belt tension is indicated at T_band the hub load force applied to pulley 12 in the radial direction thereof by the belt 114 is indicated at F_b. The angle at which the hub load force F_bis applied with respect to the radius r extending between the pivot axis 26 and the rotation axis 34 is indicated at β and the wrap angle of the belt 114 with respect to the pulley 12 is indicated at α.

[0034] The torque M_bapplied to the tensioner arm 14 is related to the hub load force F_bas follows: M_b=(F_b)(r)(SINβ). The hub load force F_bis related to the belt tension T_bby the following relationship: F_b=2(T_b)(SIN(α/2)). By combining these two equations using substitution, the torque M_bapplied to the tensioner arm 14 can be expressed in relation to the belt tension T_bas follows: M_b=2(T_b)(r)(SIN(α/2))(SINβ).

[0035] Using these equations, it is possible to determine a theoretical amount of tensioning torque (which is indicated as M_sin FIG. 3) that the spring 18 must apply to the tensioner arm 14 over a range of angular positions in order to maintain the belt tension at a constant level. That is, it is possible to determine the amount of resistance to movement that theoretically must be offered by the spring 18 over the pivot arm's range of angular positions in order to allow tensioner arm 14 to move in a suitable manner to compensate for increases/decreases in belt tension whereby the arm moves a sufficient amount to reestablish the belt tension at its proper amount. Keep in mind that this is a “theoretical” amount and, as will become better appreciated hereinbelow, there is no commercially available spring that is capable of behaving commensurate with these theoretical requirements. Further, the belt driven system is a dynamic system and thus the damping structure will play a role in the overall resistance to pivot structure movement that is offered by the tensioner 10 as a whole.

[0036] FIG. 4 shows a graph depicting the theoretical requirement for M_sover a 180 degree range of pivot arm movement in a prior art tensioner. The vertical axis of this graph is torque in Newton-meters and the horizontal axis of this graph is the angle in degrees between the hub load force F_band the radius r of the pivot arm 14. The line indicated at 200 in FIG. 4 is M_s. Note that this curve has a sinusoidal profile, which is a result of being related to the equation 2(T_b)(r)(SIN(α2))(SINβ). Also, note that the torque requirement in theory is at a maximum where the angle between the hub load force and the radius is 90 degrees, which is defined for the purposes of this application as the perpendicular angular position of the pivot arm 14. Likewise, the torque requirement of the spring M_sis approaches zero towards the 0 and 180 degree positions of the pivot arm 14.

[0037] The predetermined range of operative positions for this prior art tensioner is defined between positions β0 and β3, which correspond to the free arm and maximum travel positions of the pivot arm 14, respectively. In this prior art tensioner, a stop is positioned at each of these positions to ensure that the pivot arm does not travel beyond then. Position β1 represents an initial angular position, which is commonly referred to in the art as the nominal position. This position β3 is the position at which the arm is angularly positioned when the tensioner is initially installed on the engine 100 while the engine is in its initial condition and engaged with a new belt 114, assuming installation is performed correctly as per the tensioner design and engine specifications. The initial condition of the engine is the condition of the engine when it is at ambient room temperature. It should be noted that this initial angular position is spaced in the tension applying direction (to the left) away from the aforementioned perpendicular angular position in this prior art arrangement.

[0038] Position β2 represents a hot engine angular position of the tensioner arm 14. This position β2 is the position at which the arm 14 is angularly positioned when the engine is in its hot engine condition. The hot engine condition is the condition of the engine when its temperature increases to its operating temperature. As a result of this temperature increase, the engine thermally expands and the components with which the belt 114 is engaged are moved relatively apart from one another. This movement causes the tension of the belt to increase.

[0039] Continuing to refer to FIG. 4, the line at 202 therein represents the actual behavior of the torsion spring 18 used in this prior art tensioner. The slope of line 202 is the spring rate of the spring 18. As is customary with prior art tensioner design, the actual spring torque line 202 and the range between β0 and β3 have been selected so that the actual spring torque line matches up somewhat closely to the theoretical torque line 200 over a narrow portion thereof within the predetermined range of angular positions. The spring behavior can be altered by selecting the spring rate and pretensioning with which the spring is installed. The range of positions provided by β0 and β3 can be altered by varying the mounting location of the tensioner, the pulley diameter, the pivot arm radius, the locations of the stops. This is the manner in which conventional tensioner design methods have attempted to achieve a relatively constant belt tension over the operative range. Line 204 in FIG. 4 shows the cumulative belt take-up for the arrangement of the tensioner designed in accordance with FIG. 4.

[0040] FIG. 5 shows a graph depicting the performance of the tensioner designed in accordance with FIG. 4 in terms of the belt tension achieved. The vertical axis shows the amount of belt tension that is created in the belt when using the belt tensioner of FIG. 4 and the horizontal axis is the angular position of the tensioner arm 14 radius with respect to the hub load force. Line 206 represents the tension that the tensioner should apply to the belt 114 on the upstroke of the tensioner (movement of the tensioner arm in the direction opposite the tension applying direction) and line 208 represents the tension that the tensioner should apply to the belt 114 on the downstroke of the tensioner (movement of the tensioner arm in the tension applying direction). The reason for the disparity between belt tension in the upstroke and downstroke is attributable to the damping structure. Specifically, on the upstroke, the damping structure frictionally resists movement of the tensioner arm 14 in the aforementioned direction opposite the tension applying direction and hence the spring 18 and damping structure work together to resist pivot arm movement. On the downstroke, the damping structure frictionally resists movement of the tensioner arm 14 in the tension applying direction and hence the damping structure is working against the action of the spring 18. Line 210 represents the mean dynamic tension of the belt, which is the average of lines 206 and 208. This mean dynamic tension is more representative of what actually happens during engine operation because the belt tension is dynamic and rapidly changing. As a result, the tensioner acts in a dynamic manner and the arm 14 thereof oscillates rapidly back and forth between up and downstrokes. Further, when evaluating tensioner performance during engine operation, upstroke and downstroke belt tension cannot readily be measured with any accuracy and normally such testing is done by measuring the mean dynamic belt tension. As such, the claims of this application present each of the aspects of the invention in terms of dynamic belt tension rather than tension during either of the up and downstrokes, although the scope of the invention could be expressed in terms of the other tensions mentioned herein.

[0041] Line 212 in FIG. 5 represents the static belt tension. This static belt tension is unaffected by the damping structure because the pivot arm does not move while the system is static. This line is useful for understanding how the spring and tensioner geometry relate to one another without taking into account the complexities involved with understanding the dynamic system behavior and the damping structure's effect thereon.

[0042] It should be noted that the mean dynamic belt tension as shown in FIG. 5 increases from about 245N to about 260N from β2 (hot engine) to β3 (maximum travel), an increase of approximately 6.1%.

[0043] In accordance with the present invention, the parameters concerning the tensioner geometry and spring and damping characteristics are selected so that the mean dynamic belt tension increases rapidly as the pivot arm 14 moves opposite the tension applying direction past the hot engine angular position β2 and towards position β3. In accordance with the broad aspects of the invention, position β3 is not necessarily a maximum travel position which is determined by the presence of travel stop. Instead, position β3 may be the end position of a predetermined range of angular positions which has been specified by engine manufacturer requirements. Likewise, position β3 may be a potential tooth skip position at which, if the pivot structure were moved into the potential tooth skip position under driving element tension and then the tension in the driving element were decreased, tooth skip would be allowed to occur between the driving element and the engine if the spring structure failed to move the pivot structure in the tension applying direction to maintain the rotatable member in the tension applying relationship with the driving element. This failure can possibly occur from dirt and other particulate material jamming the pivot arm's movement, or from water on the tensioner freezing during winter conditions and hence jamming the pivot structure's movement. Of course, position β3 may be determined by the presence of a stop which it is desired to avoid contacting, or at least cushion the pivot arm movement prior to contacting the stop.

[0044] In the arrangement wherein position β3 corresponds to the potential tooth skip position and the tensioner is constructed and arranged such that the tension required to move the arm 14 into the tooth skip position β3 is greater than the amount of belt tension that the engine is capable of creating, the use of a stop at position β3 may be omitted. However, a stop may be placed at that position as a redundant safety feature and as a safeguard against incorrect installations.

[0045] FIGS. 6 and 7 show graphs similar to FIGS. 4 and 5, respectively, for a tensioner 10 constructed in accordance with the principles of the present invention. Line 300 in FIG. 6 represents the aforementioned theoretical value for spring torque M_s, line 302 represents the actual spring torque M_s, and line 304 represents the cumulative belt take-up. Line 306 in FIG. 7 represents the upstroke belt tension, line 308 represents the downstroke belt tension, line 310 represents the mean dynamic tension, and line 312 represents the static belt tension.

[0046] As can be appreciated from viewing FIGS. 6 and 7, the spring rate and tensioner geometry have been selected such that the mean dynamic belt tension required to move the pivot arm 14 in the direction opposite the tension applying direction continually increases between the initial angular position β1 and the hot engine position β2. In accordance with one aspect of the invention, this is accomplished by selecting an initial angular position that is spaced in the direction opposite the tension applying direction (i.e. to the right in FIG. 6) from the initial angular position β1. As a result, as the pivot arm 14 is pivoted from the initial angular position β1 thereof to the hot engine angular position thereof, the angle between the hub load force and the radius continually increases beyond 90 degrees and the spring structure is continually increasingly stressed so that the mean dynamic tension in the driving element continually increases from the first mean dynamic tension to the second mean dynamic tension during the thermal expansion of the engine. In prior art tensioners, because the torque acting against the spring structure is related to the sine of the angle between the hub load force applied to the rotatable member and the radius, the sine of this angle increases as the pivot structure approaches the perpendicular angular position, thus maximizing the contribution of the tensioner's geometry to that torque. By spacing the initial angular position of the pivot structure in the opposite direction from the perpendicular angular position, the contribution offered to that torque by the tensioner's geometry is reduced, and this reduction increases as a function of the angle between the hub load force and the radius continuing to increase (and hence the sine of that angle decreasing). Further, positioning the initial angular position to the right of the perpendicular angular position ensures that the hot engine will be moved further in the opposite direction, whereat the angle between the radius and the hub load force becomes increasingly smaller and hence the sine of that angle decreases incrementally at a faster rate.

[0047] Preferably, the initial angular position is spaced at least 5 or 10 degrees from the perpendicular angular position in the direction opposite the tension applying direction.

[0048] As can be seen from reviewing FIG. 7, the mean dynamic belt tension required to move the pivot arm 14 to the hot engine position β2 is about 300N and the mean dynamic belt tension required to move the pivot arm 14 to position β3 is about 710N. This is approximately a 130% increase. Preferably, in accordance with one aspect of the invention, the minimum amount of increase between these two is at least 75%, although in other aspects of the invention this amount may be lower. Specifically, in accordance with some aspects of the invention the threshold may be as low as 30 or 50%, although it is preferred that they be 100% or greater. In other aspects of the invention, the specific amount of increase in terms of percentage is unimportant. For example, in the aspect of the invention wherein β3 represents the potential tooth skip position, the tensioner 10 is constructed and arranged by way of selecting appropriate tensioner geometry and spring characteristic to ensure that the belt tension required to move the tensioner arm 14 into the potential tooth skip position is greater than the maximum belt tension of which the engine is capable of creating irrespective of a predefined minimum belt tension increase.

[0049] In addition, in vehicles the engine thereof is constructed and arranged such that the crankshaft can be turned in a reverse rotating direction opposite its normal forward rotating direction. This typically can occur when a vehicle is left in gear and parked on a hill. Backwards rolling of the vehicle backdrives the engine in this reverse rotating manner. During this backdriving, the engine is capable of applying a reverse operation maximum tension to said driving element. In the normal arrangement of most vehicle engines, the tensioner 10 is located downstream of the operative components in the normal running direction of the belt (i.e. the belt runs clockwise in FIG. 1). As a result, a significant portion of the belt load is transferred to the one or more operative components located between the crankshaft and the tensioner 10. However, when the engine is backdriven as mentioned above, the tensioner is the first element downstream of the belts reverse running direction and thus there are no intervening components to absorb this belt load. As a result, tensioner must be designed to take this type of loading into account in addition to the types of belts load seen during normal (i.e. forward crankshaft rotation) engine operation. Thus, in accordance with a further aspect of the invention, the tensioner 10 is constructed and arranged, specifically by carefully choosing spring characteristics and tensioner geometry, such that the tension required to move the arm 14 into the potential tooth skip position thereof is greater than the maximum reverse operation tension that the engine is capable of creating. This prevents tooth skip from occuring during these backdrive conditions. The exact amount of this maximum reverse operation tension can ascertain either through testing or from engine manufacturer specifications.

[0050] It will thus be seen that the objective of the present invention have been fully and effectively accomplished. The foregoing specific embodiments have been provided to illustrate the structural and functional principles of the present invention and are not intended to be limiting. To the contrary, the present invention is intended to encompass all modifications, alterations, substitutions, and changes within the spirit and scope of the following appended claims.