CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/582, §§941 filed November 7, 2017, the contents of which are incorporated herein in their entirety.

FIELD OF THE DISCLOSURE

[0002] This disclosure relates generally to the art of endless drive arrangements and more particularly to systems for vehicular front engine accessory drive arrangements that employ a motor/generator unit or other secondary motive unit in addition to an engine and a two-armed tensioner.

BACKGROUND [0003] Vehicular engines typically employ a front engine accessory drive to transfer power to one or more accessories, such as an alternator, an air conditioner compressor, a water pump and various other accessories. Some vehicles are hybrids and employ both an internal combustion engine, along with an electric drive. There are many possible configurations of such vehicles. For example, in some configurations, the electric motor is used to assist the engine in driving the vehicle (i.e. the electric motor is used to temporarily boost the amount of power being sent to the driven wheels of the vehicle). In some configurations, the electric motor is used to drive the driven wheels of the vehicle by itself and only after the battery is exhausted to a sufficient level does the engine turn on to take over the function of driving the vehicle.

[0004] While hybrid vehicles are advantageous in terms of improved fuel economy, their operation can result in higher stresses and different stresses on certain components such asihe belt from the front engine accessory drive, which can lead to a reduction in the operating life of these components. It would be advantageous to provide improved operating life for components of the front engine accessory drive in a hybrid vehicle.

[0005] The outlined issues complicate the development and implementation of belt tensioner on modern engines.

SUMMARY

[0006] In an aspect, a tensioner is provided for tensioning an endless drive member on an engine. The tensioner includes a first tensioner arm that is movable relative to the engine and which has a first tensioner pulley rotatably mounted thereto. The first tensioner pulley is configured for engagement with a first span of the endless drive member. The tensioner further includes a second tensioner arm that is movable relative to the engine and which has a second tensioner pulley rotatably mounted. The second tensioner pulley is configured for engagement with a second span of the endless drive member. The tensioner further includes a tensioner biasing member that is positioned to bias the first and second tensioner arms in a first free arm direction and in a second free arm direction, respectively. In use, the tensioner biasing member has a biasing member force/biasing member length relationship which is a relationship between a spring force exerted by the tensioner biasing member against the first and second tensioner arms, and a length of the endless drive member. The tensioner further includes a tensioner biasing member adjustment structure that is controllable to change the biasing member force/biasing member length relationship. [0007] In another aspect, a tensioner is provided for tensioning an endless drive member on an engine. The tensioner includes a first tensioner arm that is movable relative to the engine and which has a first tensioner pulley rotatably mounted thereto. The first tensioner pulley is configured for engagement with a first span of the endless drive member. The tensioner further includes a second tensioner arm that is movable relative to the engine and which has a second tensioner pulley rotatably mounted. The second tensioner pulley is configured for engagement with a second span of the endless drive member. The tensioner further includes a tensioner biasing member that is positioned to bias the first and second tensioner arms in a first free arm direction and in a second free arm direction, respectively. The tensioner biasing member has a first end and has an end engagement member at the first end, wherein the end engagement member has a retainer aperture therein that is oriented generally perpendicularly to a tensioner biasing member axis. The retainer aperture receives a projection on one of the first and second tensioner arms so as to hold the first end of the tensioner biasing member to the one of the first and second tensioner arms.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The foregoing and other aspects of the invention will be better appreciated with reference to the attached drawings, wherein:

[0009] Figure 1 is a plan view of an endless drive arrangement including a tensioner, in accordance with an embodiment of the present disclosure;

[0010] Figure 2 is a plan view of a variation of the endless drive arrangement shown in Figure 1 ; [0011] Figure 3 is a perspective view of an element of the endless drive arrangement shown in Figure 1 ;

[0012] Figure 4 is a plan view of the endless drive arrangement shown in Figure 1 , operating in a first mode;

[0013] Figure 5A is a schematic representation of the endless drive arrangement shown in Figure 1 , operating in the first mode, illustrating forces exerted on tensioner arms that are part of the tensioner; [0014] Figure 5B is a schematic representation of the endless drive arrangement shown in Figure 1 , operating in the first mode, further illustrating forces and moment arms in relation to the tensioner arms; and

[0015] Figure 5C is a schematic representation of the endless drive arrangement shown in Figure 1 , operating in a second mode, illustrating forces exerted on the tensioner arms;

[0016] Figure 6A is a perspective view of a tensioner with an optional tensioner biasing member adjustment structure in accordance with another embodiment of the present disclosure, in a first position; [0017] Figure 6B is a perspective view of the tensioner shown in Figure 6A, with the tensioner biasing member adjustment structure in a second position;

[0018] Figure 7 is a graph of tension in an endless drive member in relation to a length of the endless drive member;

[0019] Figure 8 a magnified perspective view of a portion of the tensioner shown in Figures 6A and 6B;

[0020] Figure 9 is a magnified perspective view of a portion of the tensioner shown in Figures 6A and 6B with a different locking member than that shown in Figure 8;

[0021] Figure 10 is a magnified perspective view of a portion of the tensioner shown in Figures 6A and 6B with another different locking member than that shown in Figure 8;

[0022] Figure 11 is a magnified perspective view of a portion of a tensioner with a tensioner biasing member adjustment structure wherein a tensioner biasing member is a compression spring; [0023] Figure 12 is a perspective view of a tensioner with another example of an optional tensioner biasing member adjustment structure in accordance with another embodiment of the present disclosure; [0024] Figure 13 a magnified perspective view of a portion of the tensioner shown in Figure 12;

[0025] Figure 14 is a perspective view of a tensioner with yet another example of an optional tensioner biasing member adjustment structure in accordance with another embodiment of the present disclosure;

[0026] Figure 15 is a perspective view of a tensioner with yet another example of an optional tensioner biasing member adjustment structure in accordance with another embodiment of the present disclosure;

[0027] Figures 16 and 17 are perspective views of elements that facilitate removal and installation of a spring on a tensioner, in accordance with another embodiment of the present disclosure; and

[0028] Figure 18 is a view of the elements shown in Figures 16 and 17 assembled together; and

[0029] Figures 19A-19C show elevation views of a biasing member output control member that is usable during installation of a tensioner biasing member on a tensioner.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0030] Figure 1 shows an endless drive arrangement 10 for an engine, schematically represented by a dashed-line rectangle and shown at 12. In embodiments wherein the engine 12 is mounted in a vehicle, the endless drive arrangement 10 may be a front engine accessory drive. The engine 12 includes a crankshaft 14 that has a crankshaft pulley 16 mounted thereon. The crankshaft pulley 16 is drivable by the crankshaft 14 of the engine 12 and itself drives one or more vehicle accessories 18 via an endless drive member 20, such as a belt. For convenience the endless drive member 20 will be referred to as a belt 20, however it will be understood that it could be any other type of endless drive member. The accessories 18 may include a motor-generator unit (MGU) 18a, an air conditioning compressor 18b, a water pump (not shown), a power steering pump (not shown) and/or any other suitable accessory.

[0031] In Figure 1 , two accessories 18 are shown, however there could be more or fewer accessories. Each of the driven accessories has a drive shaft 22 and a pulley 24. The MGU 18a has an MGU drive shaft 22a and an MGU pulley 24a.

[0032] As can be seen in Figure 1 , the belt 20 is engaged with the crankshaft pulley 16 and the MGU pulley shown at 24a (and the other accessory pulleys 24). Under normal operating conditions the endless drive arrangement 10 is operable in a first mode in which the endless drive arrangement 10 may be driven by the engine 12, and in turn drives the pulleys 24 of the accessories 18. In the first mode, the tension in the first belt span 20a is lower than the tension in the second belt span 20b. The MGU 18a may be operable to as an alternator in the first mode, in order to charge the vehicle’s battery (not shown).

[0033] The MGU 18a is also operable as a motor, wherein it drives the MGU pulley 24a, which in turn drives the belt 20. During such events where the MGU 18a is operated as a motor, the endless drive arrangement 10 may be considered to be operable in a second mode, in which the tension in the second belt span 20b is lower than the tension in the first belt span 20a. This may be during a‘boost’ event when the engine is driving the wheels of the vehicle, but additional power is desired to supply further power to the wheels indirectly by transferring power to the engine’s crankshaft 14 via the belt 20. Another situation in which the MGU 18a is operated as a motor include a BAS (Belt-Alternator Start) event, in which the MGU 18a drives the belt 20 in order to cause rotation of the crankshaft 14, and thereby start the engine 12. Yet another situation in which the MGU 18a is operated as a motor is an ISAF (Idle/Stop Accessory Function) event, when the MGU 18a is used to drive the belt 20 in order to drive one or more accessories when the engine is off (e.g. in some hybrid vehicles where the engine is turned off automatically when the vehicle is at a stoplight or is otherwise stopped briefly). [0034] In the present disclosure, the span 20a of the belt 20 may be referred to at the belt span 20a, and the span 20b of the belt 20 may be referred to as the belt span 20b.

[0035] It will be noted that the MGU 18a is but one example of a secondary drive device that can be used as a motor to drive the belt 20 for any of the purposes ascribed above to the MGU 18a. In an alternative example, the accessory 18a may be a typical alternator and a separate electric motor may be provided adjacent to the alternator (either upstream or downstream on the belt 20 from the alternator) to driving the belt 20 when it is desired to boost acceleration of the vehicle, in BAS operation, and/or in ISAF operation.

[0036] A tensioner 25 for the endless drive arrangement 10 is shown in Figure 1. The first tensioner pulley 26 is rotatably mounted on a first tensioner arm 30 for rotational movement of the pulley about a first arm pulley axis APA1 (Figure 4). The second tensioner pulley 28 is rotatably mounted on a second tensioner arm 32 for rotational movement of the pulley about a first arm pulley axis APA2. The rotational mounting to each tensioner arm 30 and 32, may be provided by a shoulder bolt 52 that passes through an aperture in each tensioner arm 30 and 32 and into a threaded aperture in the base 48.

[0037] The first and second tensioner arms 30 and 32 are pivotally mounted to a base 48 for pivotal movement about first and second tensioner arm pivot axes AP1 and AP2, respectively. The pivotal mounting to the base 48 may be provided by a shoulder bolt 57 that passes through an aperture in each of the tensioner arms 30 and 32 and into a threaded aperture in the base 48.

[0038] The base 48 mounts fixedly to the housing of the MGU 18a or any other suitable stationary member.

[0039] The first and second tensioner pulleys 26 and 28 are biased in first and second free arm directions (shown in Figure 1 at DFA1 and DFA2 respectively). More specifically, a tensioner biasing member 41 may be positioned to apply a tensioner biasing force F on the first and second tensioner arms 30 and 32 in the respective first and second free arm directions DFA1 and DFA2.

[0040] The tensioner biasing member 41 may have any suitable structure, such as, for example, a linear helical compression spring that extends between the first and second tensioner arms 30 and 32. In an alternative embodiment, shown in Figure 2, the tensioner biasing member 41 may, for example, be a torsion spring that abuts first and second drive surfaces 43 and 45 on the first and second arms 30 and 32 and urges the arms 30 and 32 in directions to drive the first and second tensioner pulleys 26 (shown partially in Figure 2) and 28 (not shown in Figure 2) into the belt 20.

[0041] In the embodiments shown in Figures 1 and 2, the first tensioner pulley 26 is on a first side of the first tensioner arm pivot axis AP1 , in the sense that the tensioner pulley 26 is positioned to, in use, apply a moment in a first rotational direction on the first tensioner arm 30 about the pivot axis AP1. The tensioner biasing member 41 is positioned to apply the tensioner biasing force F on a second side of the first tensioner arm pivot axis AP1 , in the sense that the tensioner biasing member 41 is positioned to, in use, apply a moment in a second rotational direction (that is opposite the first rotational direction) on the first tensioner arm 30 about the pivot axis AP1. [0042] Analogously, the second tensioner pulley 28 is on a first side of the second tensioner arm pivot axis AP2, in the sense that the tensioner pulley 28 is positioned to, in use, apply a moment in a first rotational direction on the second tensioner arm 32 about the pivot axis AP2, and the tensioner biasing member 41 is positioned to apply the tensioner biasing force F on a second side of the second tensioner arm pivot axis AP2, in the sense that the tensioner biasing member 41 is positioned to, in use, apply a moment in a second rotational direction (that is opposite this immediately aforementioned first rotational direction) on the second tensioner arm 32 about the pivot axis AP2. [0043] Several features of the tensioner 25 may be advantageous and are described further below.

[0044] In an embodiment, the base 48 for the tensioner 25 may be generally C-shaped as shown in Figure 3. In the embodiment shown in Figure 3, the base 48 has a base body 47, and first and second mounting apertures 49 and 51 proximate the circumferential ends of the base body 47, wherein the first and second apertures 49 and 51 are configured for mounting the base 28 to the housing of the MGU 18a or another suitable member. The mounting apertures 49 and 51 may also be used to receive pins (shown at 53 in Figures 1 and 2) for supporting the pivoting movement of the first and second tensioner arms 30 and 32 and may thus define the first and second pivot axes AP1 and AP2. Furthermore, the opening that is defined by the C-shape of the base 48, is free of any obstructions in an axial direction. As a result, the tensioner 25 is configured to facilitate dissipation of heat from the MGU 18a. [0045] In the embodiment shown in Figure 4, the tensioner 25 includes a first tensioner arm stop 60 that is positioned to limit the movement of the first tensioner arm 30 in a direction opposite the first free arm direction. The direction opposite the first free arm direction may be referred to as a first load stop direction. The tensioner 25 includes a second tensioner arm stop 62 that is positioned to limit the movement of the second tensioner arm 32 in a direction opposite the second free arm direction (i.e. a second load stop direction). The tensioner arm stops 60 and 62 have first and second base-mounted stop surfaces 64 and 66 respectively that are engageable with first and second arm- mounted stop surfaces 68 and 70 on the first and second tensioner arms 30 and 32 respectively.

[0046] The tensioner 25 is configured such that, in use, the second tensioner arm 32 is engaged with the second tensioner arm stop 62 throughout a first selected range of operating conditions. [0047] Optionally, the tensioner 25 is configured such that, in use, the first tensioner arm 30 is engaged with the first tensioner arm stop 60 throughout a second selected range of operating conditions that is different from the first range of operating conditions.

[0048] As a further option, the tensioner 25 is configured such that, in use, the first and second tensioner arms 30 and 32 are disengaged from the first and second tensioner arm stops 60 and 62 throughout a third selected range of operating conditions that is different from the first and second ranges of operating conditions.

[0049] Reference is made to Figures 5A-5C, in which there is a schematic representation of the tensioner 25 to show the forces and moments acting thereon. In Figures 5A-5C, the tensioner arms 30 and 32, the belt 20 and the biasing member 41 are represented as single lines and the pulleys 24a, 26 and 28 are shown in outline only, to avoid visual clutter in these figures.

[0050] The forces that act on the tensioner 25 will create moments that urge the tensioner arms 30 and 32 to swing one way or the other include the forces applied to the tensioner arms 30 and 32 by the belt 20 and these forces applied to the tensioner arms 30 and 32 by the biasing member 41. These forces are shown in Figure 5A. The belt tension in a belt span 20-2 is shown as T2; the belt tension in a belt span 20-3 is shown as T3; the belt tension in a belt span 20-4 is shown as T4; and the belt tension in a belt span 20-5 is shown as T5. The force of the biasing member 41 is shown as FL. As can be seen the biasing member 41 applies the force FL at one end on the first tensioner arm 30 and at the other end on the second tensioner arm 32. Under static equilibrium, the belt tension is considered to be substantially equal everywhere (i.e., throughout all of the spans 20-2, 20-3, 20-4 and 20-5). Thus, for the purposes of the present mathematical derivation, T2=T3=T4=T5. These tensions result in hub loads shown at HL23 and HL45 on the first and second tensioner arms 32 and 30 respectively. The hub loads HL23 and HL45 act on the tensioner arms 32 and 30 at the centers of rotation of the pulleys 28 and 26 respectively (i.e. axes APA2 and APA1 ). The directions of the hub loads is dependent on the respective wrap angles of the belt 20 on the pulleys 26 and 28, as will be understood by those skilled in the art.

[0051] The hub load HL23 can be divided into a vector component HLVC2 which is parallel to the belt span 20-2 and a vector component HLVC3 which is parallel to the belt span 20-3. The magnitudes of HLVC2 and HLVC3 are the same as the tension forces T2 and T3, but act on the arm 32, whereas the tension forces T2 and T3 act on the pulley 28. Similarly, the hub load HL45 can be divided into a vector component HLVC4 which is parallel to the belt span 20-4 and a vector component HLVC5 which is parallel to the belt span 20-5. The magnitudes of HLVC4 and HLVC5 are the same as the tension forces T4 and T5, but act on the arm 30, whereas the tension forces T4 and T5 act on the pulley 26.

[0052] Put another way, the belt tension forces T2 and T3, which act on the surface of the pulley 28, which is itself rotatable about the axis APA2, are transferred to the tensioner arm 32 at the center of rotation of the pulley 28 (i.e., along axis APA2), resulting in forces HLVC2 and HLVC3.

[0053] Figure 5B illustrates the moment arms that are associated with each of the hub load component forces, HLVC2, HLVC3, HLVC4 and HLVC5 (i.e., the perpendicular distances of the lines of action of each of the forces and the pivot axes AP1 and AP2). The moment arms associated with the forces T2 and T3 relative to the pivot axis AP2 are shown at TR2 and TR3, respectively. Similarly, the forces T4 and T5 are shown in Figure 5B acting through the axis APA1 of the pulley 26 and the moment arms associated with the forces T4 and T5 relative to the pivot axis AP1 are shown at TR4 and TR5, respectively. Additionally, the moment arms of the forces H L acting on each tensioner arm 30 and 32 are shown as HF1 and HF2, respectively.

[0054] In general, when the tensioner 25 is in static equilibrium, the stop 62 applies a force and therefore a moment to compensate for the moments applied by the belt 20 and the biasing member 41 so that the net moment on the tensioner arm 32 is zero. The moment applied by the stop 62 is represented as Mstop2. Similarly, the stop 60 applies a force and therefore a moment to compensate for the moments applied by the belt 20 and the biasing member 41 so that the net moment on the tensioner arm 30 is zero. The moment applied by the stop 60 is represented as Mstopl . It will be understood that, in any equilibrium position in which the first arm 30 is not in contact with the first stop 60, then the moment Mstopl is zero, and similarly, in any equilibrium position in which the second arm 32 is not in contact with the second stop 62, then the moment Mstop2 is zero. The mathematical expressions relating to a static equilibrium condition are: HLVC4 TR4 - HLVC 5 TR5 + FL HF1 + Mstopl = 0

HLVC2- TR2 - HLVC3 TR3 - FL HF2 - Mstop2 = 0

Because HLVC2=T2, HLVC3=T3, HLVC4=T4, and HLVC5=T5, we can express these two aforementioned equations as: T4 TR4 - T5 TR5 + FL HF1 + Mstopl = 0

T2- TR2 - T3 TR3 - FL HF2 - Mstop2 = 0

[0055] In these two aforementioned mathematical expressions, moments in the counterclockwise direction were considered to be positive and moments in the clockwise direction were considered to be negative. Because the tension values are all equal to one another, T2, T3, T4 and T5 may all be represented by a single term, TO. When the tensioner 25 is in a static equilibrium condition as shown in Figures 5A and 5B, the moment applied by the first stop 60 is zero, since, as noted above, it is not in contact with the first tensioner arm 30. Thus, Mstopl =0 in such situations. [0056] Thus, in such situations, the equations above may be rewritten as follows:

T0 TR4 - T0 TR5 + FL HF1 = 0 T0 TR2 - T0 TR3 - FL HF2 - Mstop2 = 0

[0057] By solving the first equation above for FL and inserting this expression into the second equation, the result is:

T0 TR2 - T0 TR3 - (T0 TR5 - T0 TR4VHF2 - Mstop2 = 0

HF1

Thus:

T0(TR2 - TR3 - HFT (TR5 - TR4)) = Mstop2

HF2

[0058] It will be understood that TO is always positive since a negative value for TO would indicate that the belt 20 has less than zero tension. Furthermore, it will be understood that Mstop2 is positive in the context of the above equation at least for the equilibrium position shown in Figures 5A and 5B, since it is desired for the stop 62 to apply a moment in the selected direction on the tensioner arm 32. Since Mstop2 and TO must both be positive, it can readily be seen that the expression:

TR2 - TR3 - HFT (TR5 - TR4) must be positive.

HF2

If a value TR is used to represent TR2-TR3, and TL is used to represent TR5- TR4, then the expression above can be rewritten as:

TR - HFT TL > 0

HF2

If the value of TL is greater than zero, then the expression can be rewritten as:

TR > HF2

TL HF1

To determine if the value of TL is greater than zero, one can review the equation shown above:

T0 TR4 - T0 TR5 + FL HF1 = 0

This can be rewritten as: T0(TR5 - TR4) = FL-HF1 , and therefore:

T0-TL = FL HF1

[0059] Since the moment applied by the biasing member 41 is positive, and, as noted above, the tension TO is positive, then the value of TL must be positive, for the situation shown in Figures 5A and 5B.

[0060] As a result, the expression shown above is applicable, namely that:

TR > HF2

TL HF1

[0061] By meeting the above noted relationship, the tensioner 25 remains stable against the second base-mounted stop surface 66 when the endless drive arrangement is in static equilibrium. It will be noted that static equilibrium is reached when the engine is off. In other words, it is desirable for the second tensioner arm 32 to abut the second arm stop 62 when the engine is off, in at least some embodiments.

[0062] Meeting the aforementioned relationship entails some preload torque urging the second tensioner arm 32 against the second base-mounted stop surface 66. This preload torque can be selected to cause the second tensioner arm 32 to remain against the stop surface 66 during certain operating conditions (referred to above as the first set of operating conditions) which are described further below. It is valuable to set the preload torque in the tensioner 25 to be sufficiently high that certain transient events that occur during operation do not cause movement of the tensioner arm 32 away from the stop 62. As noted above, such movement has a number of deleterious consequences including, but not limited to, contributing to NVH (noise, vibration and harshness), energy wastage (such as the energy associated with causing the movement of the tensioner arms and the rapid changes in direction of travel of the tensioner arms that are associated with torsional vibrations), and reduction in the operating life of the tensioner due to component wear and dynamic stresses associated with the accelerations and decelerations on the components of the tensioner during such movement.

[0063] Another advantage to maintaining the second arm 32 against the stop surface 66 is that any damping structures that are provided on the tensioner 25 in association with the second arm 32 incur a reduced amount of wear. Additionally, the stop surface 66 on the base (and the corresponding surface 70 on the second arm 32) incur a reduced amount of wear as compared to a situation where there is repeated impact with a stop surface.

[0064] However, it is desirable for the preload torque to not be so high that the tensioner arm 32 always remains engaged with the stop 62 under all operating conditions. If the preload torque was so high as to always maintain engagement between the tensioner arm 32 and the stop 62, then the resulting tension in the belt 20 would be so high that a significant amount of parasitic loss would be incurred, resulting in a reduction in available power for the engine and a reduction in fuel efficiency. Therefore, it is desirable for the preload torque to be high enough that under certain operating conditions the preload torque is sufficient to maintain engagement of the tensioner arm 32 with the stop 62, and to permit the second tensioner arm 32 to leave the stop 62 under certain other operation conditions. For example, with a preload torque of between about 1 Nm and about 15 Nm of torque on the second tensioner arm 32, movement of the tensioner arm 32 away from the stop 62 is prevented during the majority of events that would cause movement of a tensioner that does not incorporate such stops. Such events are a by-product of the operation of an engine and the commonly available accessories available in a modern hybrid vehicle, such as the air conditioning compressor, or the water pump, while adding relatively little to the tension in the belt 20 and therefore adding relatively little to the parasitic losses that are associated with high belt tension. These are elements that are part of the vehicle, but do not contribute directly to the generation of motive power for the vehicle, in contrast to elements such as the MGU 18a. It has been determined that the aforementioned range of preloads (about 1 Nm to about 15 Nm) on the second tensioner arm 32 is particularly desirable in terms of improving operating life of the components, reducing parasitic losses that are associated with high belt tension, while also reducing energy losses that result from the energy expended in moving tensioner arms during operation of the engine.

[0065] It may be desirable, in embodiments in which the first arm stop 60 is provided, for the tensioner 25 to move to the position shown in Figure 5C, wherein the first tensioner arm 30 abuts the first arm stop 60, under certain conditions, such as certain conditions when the engine operates in the second mode.

[0066] Some of the events that cause a set of conditions that can generate a torque that urges the tensioner arm 32 in a direction away from stop 62 will now be described. As will be seen, some of the events cause a torque that is low enough that the second tensioner arm 32 does not come off of its stop 62. Some of the events may cause a torque high enough to bring the second tensioner arm 32 off of its stop 62.

[0067] One event is the activation of the air conditioning clutch to initiate operation of the compressor 18b (Figure 1). Inertia in the rotor of the air conditioning compressor 18b and a resistance to movement due to the compression of any refrigerant gas that is in the compressor 18b at that time can cause a transient event which results in a momentary reduction in the belt tension in the belt spans 20-2 and 20-3 momentarily. In some embodiments, the tensioner arm 32 will be preloaded sufficiently to ensure that it remains against the stop 62 during this transient event. Flowever, in some embodiments, it is possible to set the preload on the arm 32 such that the arm 32 would momentarily come off the stop 62 during an air conditioning engagement event.

[0068] Another event is the disengagement of the air conditioning compressor 18b, the belt tension on the spans engaging the tensioner arm 32 will increase, thereby increasing the amount of torque urging the arm 32 into the stop 62. Thus, the tensioner arm 32 will not lift off the stop 62 during such an event.

[0069] During a key start event (i.e., when the engine 12 is started via the electric starter motor that is provided on any modern non-hybrid engine) the inertias of the components that are to be driven by the belt 20, in combination with the abrupt generation of torques as combustion is initiated in the cylinders of the engine 12, may result in the tensioner arm 32 lifting off its stop 62 and the tensioner arm 30 engaging its stop surface 64 during the key start event.

[0070] During an MGU start event (i.e., when the engine 12 is started via the belt 20 by operation of the MGU 18a as a motor), the MGU 18a generates a torque that varies depending on the type of engine and the specific details of the application. In other words, the quickness of the ramp up for the MGU 18a varies based on the application and may vary during an MGU start event. In at least some embodiments, the torque applied by the MGU 18a during an MGU start event changes the belt tension sufficiently to overcome the preload on the tensioner arm 32, thereby lifting the arm 32 off of the stop 62 (i.e., lifting the arm 32 away from the stop surface 66) and for the tensioner arm 30 to engage its stop 60 (i.e., to engage the stop surface 64).

[0071] During a boost event (i.e., when the engine 12 is operating but is assisted via the belt 20 by operation of the MGU 18a as a motor), the MGU 18a can generate a varying boost torque based on how much boost is called for by the driver’s depression of the accelerator pedal, among other things. It has been determined that, for at least some embodiments, if the MGU torque is less than about 10 Nm, then there does not need to be a change in belt tension (i.e., the belt tension throughout the belt 20 is sufficient even without engagement of the arm 30 on the stop 60), whereas if the MGU torque is greater than about 10 Nm, then it is preferable to shift the tensioner arms 30 and 32 such that the tensioner arm 30 is against the stop 60. By setting the preload torque on the tensioner arm 32 to be less than about 10 Nm, (e.g., about 3 to about 5 Nm), the arms 30 and 32 are ensured of switching over to the position shown in Figure 5C below an MGU torque of about 10 Nm.

[0072] In terms of torsional vibrations, it has been found that it is advantageous to ensure that any torsional vibrations in the endless drive arrangements 10 do not result in accelerations of a pulley (and therefore the belt 20) of more than about 1500 radians/s ^A 2. In some embodiments, where torsional vibrations can exceed this value (or some other selected value), it may be desirable to provide an isolator on the MGU pulley 24a in order to reduce the severity of any torsional vibrations. Additionally, damping that is provided to resist movement of the tensioner arms 30 and 32 can be provided in order to reduce torsional vibrations.

[0073] Providing a non-zero preload torque, as has been described above, may be considered as a kind of filtering structure, in the sense that the tensioner 25 remains in a position with the second tensioner arm 32 against the stop 62 until a transient torque event is sufficiently higher than the preload torque, at which point the tensioner 25 moves so that the first arm 30 is against the stop 60. For example, in some embodiments if the torque on the second tensioner arm 32 is more than 1 Nm greater than the preload torque then the tensioner 25 may move to bring the first arm 30 against the stop 60 (and to cause the second arm 32 to be spaced from the stop 62). In some other embodiments, the torque on the second arm 32 may need to be more than the preload torque by 2 Nm, or by some other value in order to bring the first tensioner arm 30 against the stop 60. For completeness, it will be noted that, if during the transient event the torque is greater than the preload torque, but is not sufficiently higher to bring the first tensioner arm 30 against the stop 60, this corresponds to a narrow window (identified above as the third range of operating conditions), in which the tensioner arms 30 and 32 are not against either of the stops 60 and 62. This filtering aspect of the tensioner 25 may be described as follows: The second tensioner arm 32 has a non-zero preload torque from a combination of torques applied by at least the endless drive member 20 and the tensioner biasing member 41 , wherein the preload torque urges the second tensioner arm 32 into engagement with the second tensioner arm stop surface 66 such that, when the endless drive arrangement 10 operates in the first mode, the second tensioner arm 32 remains engaged with the second tensioner arm stop surface 66 and the first tensioner arm 30 remains spaced from the first tensioner arm stop surface 60 throughout operation of the engine 12 where transient torque on the second tensioner arm 32 acts against the preload torque but is below the preload torque, and wherein, when the endless drive arrangement 10 operates in the first mode, the first tensioner arm 30 remains engaged with the first tensioner arm stop surface 60 and the second tensioner arm 32 remains spaced from the second tensioner arm stop surface 66 throughout operation of the engine 12 where transient torque on the second tensioner arm 32 acts against the preload torque and is sufficiently above the preload torque. The first mode may in some embodiments include when the engine 12 is at constant RPM at idle.

[0074] The stop 62 and the optionally provided stop 60 may be made from a material that is suitably resistant to wear and fracture. A suitable material may be, for example, Hytrel™, by E.l. Dupont de Nemours. The material may have a hardness in a range of about 25 Shore D to about 75 Shore D. It will be noted that, during operation, the stop 62 (and the optional stop 60) have some amount of compliance, in the sense that, during use, while the second tensioner arm 32 is engaged with the stop 62, there may be some small amount of displacement as the hub load (i.e., the combination of T2 and T3 acting on the pulley 28) varies due to changes in the operating conditions of the engine. As a result, because of the compliance that is present in the stop 62, the tensioner arm 32 may undergo a range of displacement of between about 0.15mm to about 2.25mm for applied loads of up to about 3000N against the stop 62 or 60 as the case may be. Example spring rates that have been determined to be effective are between about 4000N/mm to about 10000N/mm of travel. The displacement of the second tensioner arm 32 while against the stop 62 (and, similarly, the displacement of the arm 30 while against the stop 60) is so small that it is considered negligible in terms of energy lost to movement of the arm. Additionally, such displacement is so small that any sound generated from this movement is inaudible to occupants inside the vehicle. Accordingly, it can be said that the tensioner arm 32 is substantially stationary. By contrast, the movement that occurs in some tensioner arms of some prior art tensioners that do not incorporate stops can be 20mm to 30mm (which corresponds to about 20 degrees to about 30 degrees of angular travel) or even more in some instances, thereby expending significant amounts of energy in the movement of these tensioner arms, and generating sounds that are audible to vehicle occupants in some instances. Furthermore, the high accelerations that occur for the tensioner arms of certain prior art tensioners causes tremors that could be felt by vehicle occupants, whereas the accelerations seen when the overall movement is kept below about 2.25mm are sufficiently low that such tremors are undetectable by vehicle occupants. It is important for sounds and tremors associated with movement of the tensioner to not be detectable as this can affect the perception of quality of the vehicle by the vehicle occupants. Furthermore, by keeping the total movement down to less than about 2.25mm while the tensioner arm 32 (or 30) against its stop 62 (or 60) the accelerations and therefore the corresponding stresses in the arm 32 (or 30) and associated components have been reduced such that they have no negative impact on the longevity of the tensioner. By contrast, the accelerations that have been seen in some prior art tensioners have reached levels where the elements of the tensioner are subjected to fatigue and therefore premature failure.

[0075] While it is beneficial to keep the amount of travel small, some amount of compliance in the stops 62 and 60 is desirable. Such compliance is valuable as it reduces the severity of the impact between the second tensioner arm 32 and the stop 62 (or between the first tensioner arm 30 and the stop 60) during a transition from one set of conditions to another. If the stops 62 and 60 were too rigid, such impacts could result in an audible click that could be heard (and possibly felt) by vehicle occupants, which would detract from their perception of vehicle quality. Furthermore, such impacts could result in hub load spikes and could negatively affect the life of the components. It has been found that, for impact speeds of up to 50 rad/s in combination with compliance levels as described above (i.e., the hardnesses and/or spring rates described above) the impact stresses and impact noise and vibration have been sufficiently low to not harm longevity of the components and to not be detectable by vehicle occupants. A preferred maximum speed for the tensioner arm 32 at impact with the stop 62 is about 25 rad/s, for a stop having the compliance levels described above.

[0076] In order to design the tensioner, a tensioner manufacturer may be provided with certain data from a vehicle manufacturer, such as the positions of the pulleys that make up the front engine accessory drive system, the torque required to start the engine via the belt 20, and various other design parameters. The tensioner manufacturer can determine a suitable belt tension that would be sufficient to drive the crankshaft to start the engine from the MGU. This belt tension can be used to determine the spring force that can be applied to drive the tensioner pulleys 26 and 28 into the belt 20 with sufficient force to achieve the selected belt tension. Using these values, the resulting torques on the tensioner arms 30 and 32 can be determined based on different positions of the arms 30 and 32. When the resulting torque is proximate a selected value, such as a selected value that is within a range of about 1 Nm and about 15 Nm, the stop 62 can be selectively positioned to abut the tensioner arm 32 at that point.

[0077] The wrap angle of the belt 20 on the first and second tensioner arm pulleys 26 and 28 directly impacts the belt tension. By selecting a relatively shallow wrap angle, the resulting amount of biasing torque that urges the second tensioner arm 32 into the stop 62 is kept relatively small, while maintaining a selected tension in the belt 20 that is sufficient to transmit drive torque from the engine to the accessories while preventing belt squeal, during events such as a key-start. The wrap angle is preferably also not so small that it risks the occurrence of bearing hoot during operation of the engine. Hoot is a type of undesirable noise that occurs when the wrap angle of a belt on a pulley is so low that there is not sufficient frictional engagement between the rolling elements (e.g. the balls) of a bearing and the corresponding bearing races to cause the rolling elements to roll. Instead there is sliding movement of the rolling elements on the races. By ensuring that the wrap angle is sufficiently high, such as, above about 10 degrees, hoot can be substantially eliminated. By keeping the wrap angle below a selected value, such as about 90 degrees, the preload of the tensioner arm 32 against the stop 62 is kept sufficiently low that standard bearings and the like may be used on the pulley 28. In some embodiments, the wrap angle of the belt 20 on the pulley 28 (and on the pulley 26) is between about 25 degrees and about 60 degrees.

[0078] As can be seen from the description above, under static equilibrium, the second tensioner arm 32 has been described as having a preload torque resulting from a combination of torques applied by at least the endless drive member 20 and the tensioner biasing member 41 , wherein the preload torque urges the second tensioner arm 32 into engagement with the second tensioner arm stop surface 66 and is between about 1 Nm and about 15 Nm. Furthermore, the second tensioner pulley 28 is engaged with the endless drive member 20 while the second tensioner arm 32 is engaged with the second tensioner arm stop surface 66 throughout a first selected range of operating conditions, which include, for example, conditions in which: the crankshaft pulley 16 drives the endless drive member 20, the secondary drive device 18a (e.g. the MGU) does not drive the endless drive member 20. In some embodiments, the preload torque is between about 3 Nm and about 5 Nm. As can be seen from the description above, the tensioner biasing member 41 may in some embodiments be positioned to apply a torque to the second tensioner arm 32 (by way of the force FL) that is opposed to a torque exerted on the second tensioner arm 32 by the endless drive member 20 (by way of the tensions T2 and T3), during use. In some embodiments, the first tensioner arm stop surface 64 is provided and is positioned to limit the movement of the first tensioner arm 30 in a direction opposite the first free arm direction. The first tensioner arm stop surface 64 is positioned such that, in use, the first tensioner pulley 26 is engaged with the endless drive member 20 while the first tensioner arm 30 is engaged with the first tensioner arm stop surface 64 throughout a second selected range of operating conditions that is different from the first range of operating conditions. For example, in the second selected set of operating conditions: the secondary drive device pulley 24a drives the endless drive member 20, the crankshaft pulley 16 may optionally drive the endless drive member 20, and substantially any transient torque on the first tensioner arm 32 that oppose the preload torque is greater than the preload torque. Furthermore, in use, it is optionally possible for the first and second tensioner pulleys to be engaged with the endless drive member while the first and second tensioner arms are disengaged from the first and second tensioner arm stop surfaces throughout a third selected range of operating conditions that is different from the first and second ranges of operating conditions.

[0079] As described above, in some embodiments, the endless drive arrangement is operable in a first mode (Figures 5A and 5B) in which the crankshaft pulley 16 drives the endless drive member 20 and the secondary drive device 18a (such as an MGU) does not drive the endless drive member 20 such that tension in a first span 20-3 of the endless drive member 20 is lower than tension in a second span 20-4 of the endless drive member 20, and in a second mode (Figure 5C) in which the secondary drive device 18a drives the endless drive member 20. In some instances during the second mode, (e.g. during a BAS event), the crankshaft pulley 16 does not drive the endless drive member 20. In some instances of the second mode (e.g. during a boost event), the crankshaft pulley 16 drives the endless drive member 20 along with the secondary drive device 18a. The first and second tensioner arm stop surfaces 64 and 66 are positioned such that, in use, at least some of the time that the endless drive arrangement operates in the first mode the second tensioner arm 32 is engaged with the second tensioner arm stop surface 66 and the first tensioner arm 30 is spaced from the first tensioner arm stop surface 64, and at least some of the time that the endless drive arrangement operates in the second mode, the second tensioner arm 32 is spaced from the second tensioner arm stop surface 66 and the first tensioner arm 30 is engaged with the first tensioner arm stop surface 64. In some embodiments, the first and second tensioner arm stop surfaces 64 and 66 are positioned such that, in use, substantially all of the time that the endless drive arrangement operates in the first mode, the second tensioner arm 32 is engaged with the second tensioner arm stop surface 66 and the first tensioner arm 30 is spaced from the first tensioner arm stop surface 64. In some embodiments, the first and second tensioner arm stop surfaces are positioned such that, in use, some of the time that the endless drive arrangement operates in the second mode (e.g. during boost events wherein the amount of torque being delivered by the secondary drive device 18a is small enough to generate a torque on the second tensioner arm 32 that is below the amount of preload torque in the second tensioner arm 32), the second tensioner arm 32 is engaged with the second tensioner arm stop surface 66 and the first tensioner arm 30 is spaced from the first tensioner arm stop surface 64.

[0080] Reference is made to Figure 6, which shows the tensioner 25 with an optional tensioner biasing member adjustment structure 400. The tensioner biasing member 41 has a biasing member force/endless drive member length relationship which is a relationship between a spring force exerted by the tensioner biasing member 41 against the first and second tensioner arms 30 and 32, and a length of the endless drive member 20. The graph in Figure 7 shows the tension in the endless drive member 20 as a function of the length of the endless drive member 20 on an engine. As the endless drive member 20 ages over time, it stretches permanently, which, as can be seen in the dropoff of the curves shown at 402, 404 and 406 towards the right, results in a dropoff in the tension. Additionally, in other conditions the length of the endless drive member 20 can effectively increase or decrease, such as when the engine and the endless drive member 20 are very hot or very cold, which correspondingly affects the tension at that moment.

[0081] Because the tension in the endless drive member 20 is directly related to the biasing force applied by the tensioner biasing member 41 on the first and second tensioner arms 30 and 32, the graph in Figure 7 may be said to represent the biasing member force/endless drive member length relationship. [0082] There are a number of sources of variation in the aforementioned relationship. For example, there are tolerances in the manufacture of a tensioner, and a tensioner biasing member, the engine, and the endless drive member 20 all of which contribute to a variation in the relationship. If the overall result is to increase the amount of slack in the endless drive member 20 when installed, then the curve that represents the relationship may be curve 402. If the overall result is that the endless drive member 20 is at the nominal state, the curve may be the curve 404. If the overall result is that there is less slack in the endless drive member 20 than nominal, then the curve 406 may be the representative curve of the relationship. By providing the tensioner biasing member adjustment structure 400 (Figure 6A), the installer of the endless drive member 20 can adjust the amount of preload that is present in the tensioner biasing member 41 , thereby adjusting the relationship, so that the resultant curve can be made to match the curve 404. Accordingly, it can be said that the tensioner biasing member adjustment structure 400 is controllable to change the biasing member force/endless drive member length relationship.

[0083] The tensioner biasing member adjustment structure 400 may be positionable in a first position (Figure 6A) which provides a first biasing member force at a first length of the endless drive member (corresponding to point 408 on curve 406), and which may be positionable in a second position (Figure 6B) which provides a second biasing member force at a second length of the endless drive member (corresponding to point 410 on curve 402). In the second position of the tensioner biasing member adjustment structure 400, the tensioner biasing member 41 is more relaxed (i.e. closer to its neutral position) than in the first position.

[0084] An example of the tensioner biasing member adjustment structure 400 is shown in Figure 8. The tensioner biasing member 41 shown in the examples in Figures 8-15 is a tension spring 411 , however it will be understood that other types of spring may be used, such as a leaf spring a tension spring, a torsion spring or any other type of spring. In the example shown in Figure 8, the tensioner biasing member adjustment structure 400 includes an end engagement member 412 that is movable relative to one of the first and second tensioner arms 30 and 32 (in this example it is tensioner arm 32) and engages an end of the tensioner biasing member, such that movement of the end engagement member 412 relative to the base 48 adjusts an amount of tension (preload) in the tension spring 411.

[0085] The end engagement member 412 may be a member that holds an end (e.g. a first end 414) of the tension spring 411. The second end shown at 416 (Figures 6A and 6B) of the tension spring 411 is mounted to the other of the first and second tensioner arms 30 and 32. In the example shown in Figure 8, the end engagement member 412 has a helical groove 418 therein, which receives at least one helical coil 420 at the first end 414 of the tension spring 411. This captures the at least one helical coil 420 to substantially prevent relative movement of the first end 414 of the tension spring 411 relative to the end engagement member 412.

[0086] In the embodiment shown in Figure 8, the tensioner biasing member adjustment structure 400 includes threaded end engagement member driver member 422 that is threadedly connected to said one of the first and second tensioner arms 30 and 32 (in this example, the tensioner arm 32). Furthermore a locking member 424 is provided to lock the threaded end engagement member driver member 422 in a selected position relative to said one of the first and second tensioner arms 30 and 32. In the present example, the threaded end engagement member driver member 422 is rotatable so as to advance it or retract it relative to said one of the first and second tensioner arms 30 and 32, which drives the end engagement member 412 and therefore the first end 414 of the tension spring 411 one way or the other, so as to adjust the amount of preload in the tension spring 411. The locking member 424 in the present example is a nut that can be tightened down against the tensioner arm 32 so as to put the threaded end engagement member driver member 422 in tension and lock the threaded end engagement member driver member 422 in position. Figures 6A and 6B show two positions for the threaded end engagement member driver member 422 and therefore for the tensioner biasing member adjustment structure 400. In the first position (Figure 6A) the end engagement member 412 compresses the tension spring 411 by a first amount. In the second position (Figure 6B) the end engagement member 412 compresses the tension spring 411 by a second amount that is greater than the first amount.

[0087] The arrangement shown in Figure 9 is similar to that shown in Figure 8, but includes a different locking member 424. In the example shown in Figure 9, the locking member 424 is a push member that engages the thread on the threaded end engagement member driver member 422 and causes it to be in tension, achieving a similar effect to the nut shown in Figure 8.

[0088] Figure 10 is a sectional view of an embodiment in which the threaded end engagement member driver member 422 is threadingly engaged with a pull nut as the locking member 424. This permits the end engagement member 412 to extend slidingly farther into the associated aperture (shown at 426 in Figure 10) in the tensioner arm 32 than is possible in the embodiments shown in Figures 8 and 9, which employ a push nut and a push member respectively.

[0089] It will be noted that the tensioner biasing member 41 shown in Figures 6A-10 is positioned on the same side of the pivot axes of the tensioner arms 30 and 32 and is a tension spring. Figure 11 shows an example in which the tensioner biasing member 41 is a compression spring in similar manner to the tensioner biasing member 41 shown in Figures 2-5C. In the example shown in Figure 11 , the tensioner biasing member adjustment structure 400 may be similar to that shown in Figure 8.

[0090] Another example of a tensioner biasing member adjustment structure 400 is shown in Figure 12. In this example, the tensioner biasing member 41 is a tension spring, has a longitudinal axis Aspr, and is not rotatable about the axis Aspr (i.e. it is stationary rotationally about the axis Aspr), due to its connection at its second end 416 with the other tensioner arm (arm 30). The end engagement member 412 is rotatable relative to said one of the first and second tensioner arms 30 and 32 and is rotatable relative to the one of the first and second tensioner arms 30 and 32 while remaining stationary along the longitudinal axis Aspr of the tensioner biasing member 41 , to adjust how much of the at least one helical coil 420 is received in the helical groove 418. Rotation of the end engagement member 412 drives relative helical movement of the at least one helical coil 420 along the helical groove 418, thereby adjusting how much of the at least one helical coil 420 is received in the helical groove 418. This draws the helical coil up or down in the helical groove 418, thereby changing the length of the tension spring 411 , which changes the amount of tension (preload) it has.

[0091] Figure 14 shows another example of the tensioner biasing member adjustment structure 400. In this example, the tensioner biasing member adjustment structure 400 includes a locking member 428, which has a locking member tooth 430 thereon and which is mounted to one of the end engagement member 412 and the one of the first and second tensioner arms 30 and 32. A plurality of receiving teeth 432 are provided on the other of the end engagement member and the one of the first and second tensioner arms 30 and 32. The locking member tooth 430 is engageable with the plurality of receiving teeth 432 to lock the position of the end engagement member 412 relative to the one of the first and second tensioner arms 30 and 32 and is resiliently releasable from the plurality of receiving teeth 432 to unlock the position of the end engagement member relative to the one of the first and second tensioner arms 30 and 32. The resiliently releasability of the locking member 428 is by way of a leaf spring 434 (more broadly referred to as a locking member tooth biasing member 434) that urges the locking member tooth 430 to engage the plurality of receiving teeth 432.

[0092] Reference is made to Figure 15, which shows another example of the tensioner biasing member adjustment structure 400. In this example, the threaded end engagement member driver member 422 is threadedly engaged with the end engagement member 412, and is rotatable relative to the one of the first and second tensioner arms 30 and 32, but is stationary along the axis Aspr. Rotation of the threaded end engagement member driver member 422 causes the threaded end engagement member driver member 422 to act similarly to a leadscrew and to drive the end engagement member therealong axially in one direction or another so as to control the length of the tension spring 411. [0093] Reference is made to Figures 16-18, which show a structure permitting easy removal and installation of the tensioner biasing member 41 from and on the tensioner 25. As can be seen, the tensioner biasing member 41 includes an end engagement member 450 that has retainer aperture 452 thereon. The retainer aperture 452 extends generally perpendicularly to the tensioner biasing member axis Aspr. The tensioner arm on which the end of the tensioner biasing member 41 mounts (in this example tensioner arm 32) includes a projection 454 that projects into the retainer aperture 452 to hold the end engagement member 450 and therefore the end of the tensioner biasing member 41. In the example shown the tensioner biasing member 41 is a tension spring, however, in other embodiments it could be a compression spring or any other suitable kind of spring. Optionally a locking member such as an insert (not shown) could be connected to a free end 456 of the projection 454 so as to lock the end engagement member 450 thereon.

[0094] Reference is made to Figures 19A and 19B which show a tensioner biasing member output control member 500 that can be used to control the output (i.e. the force) of the tensioner biasing member 41 on the first and second tensioner arms 30 and 32. In the present example the tensioner biasing member 41 (See Figure 19C) is a compression spring. The tensioner biasing member output control member 500 has a first end portion 502 and a second end portion 504 and a failure portion 506 connecting the first and second end portions 502 and 504. While the failure portion 506 is intact (Figure 19A), the spring 41 is prevented from applying too large a force on the first and second tensioner arms 30 and 32. Rotation of at least one of the first and second end portions relative to each other, causes failure of the failure portion 506 (Figure 19B) which permits the spring 41 to expand to apply a greater force on the first and second tensioner arms 30 and 32, after the spring 41 has been installed on the arms 30 and 32.

[0095] While the description contained herein constitutes a plurality of embodiments of the present invention, it will be appreciated that the present invention is susceptible to further modification and change without departing from the fair meaning of the accompanying claims.

CLAIMS

1. A tensioner for tensioning an endless drive member on an engine, comprising:

a first tensioner arm that is movable relative to the engine and which has a first tensioner pulley rotatably mounted thereto for rotation about a first tensioner pulley axis, wherein the first tensioner pulley is positioned for engagement with a first span of the endless drive member;

a second tensioner arm that is movable relative to the engine and which has a second tensioner pulley rotatably mounted thereto for rotation about a second tensioner pulley axis, wherein the second tensioner pulley is positioned for engagement with a second span of the endless drive member;

a tensioner biasing member that is positioned to bias the first and second tensioner arms in a first free arm direction and in a second free arm direction, respectively, wherein, in use, the tensioner biasing member applies a biasing member force according to a biasing member force/endless drive member length relationship which is a relationship between the biasing member force exerted by the tensioner biasing member against the first and second tensioner arms, and a length of the endless drive member; and

a tensioner biasing member adjustment structure that is controllable to change the biasing member force/endless drive member length relationship.

2. A tensioner as claimed in claim 1 ,

wherein the first tensioner arm is pivotable about a first arm pivot axis, wherein the first tensioner pulley axis is spaced from the first arm pivot axis;

wherein the second tensioner arm is pivotable about a second arm pivot axis, wherein the second tensioner pulley axis is spaced from the second arm pivot axis;