TENSIONING SYSTEM FOR SAME - WITH DIRECTIONAL DAMPING

FIELD OF THE DISCLOSURE [0001] 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

[0002] 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.

[0003] 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 as the 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. SUMMARY

[0004] 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 about a first tensioner arm axis and which has a first tensioner pulley rotatably mounted thereto for rotation about a first tensioner pulley axis that is spaced from the first tensioner arm axis. 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 about a second tensioner arm axis and which has a second tensioner pulley rotatably mounted thereto for rotation about a second tensioner pulley axis that is spaced from the second tensioner arm axis. 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 further includes a second tensioner arm stop surface that is positioned to limit the movement of the second tensioner arm in a direction opposite the second free arm direction. The second tensioner arm stop surface is positioned such that, in use, the second tensioner pulley is engaged with the endless drive member while the second tensioner arm is engaged with the second tensioner arm stop surface throughout a first selected range of operating conditions. Under static equilibrium, the second tensioner arm has a preload torque from at least the endless drive member and the tensioner biasing member, wherein the preload torque urges the second tensioner arm into engagement with the second tensioner arm stop surface of between about 1 Nm and about 15 Nm.

[0005] In another aspect, an endless drive arrangement for an engine is provided and includes a crankshaft pulley connected to a crankshaft, a secondary drive device pulley connected to a shaft of a secondary drive device, an endless drive member that is engaged with the crankshaft pulley and with the secondary drive device pulley and a tensioner. The endless drive arrangement is operable in a first mode in which the crankshaft pulley drives the endless drive member and the secondary drive device does not drive the endless drive member such that tension in a first span of the endless drive member is lower than tension in a second span of the endless drive member, and in a second mode in which the secondary drive device drives the endless drive member. The tensioner includes a first tensioner arm, a second tensioner arm and a tensioner biasing member. The first tensioner arm has a first tensioner pulley rotatably mounted thereto. The first tensioner pulley is engaged with the first span of the endless drive member. The first tensioner arm is movable about a first tensioner arm axis. The second tensioner arm has a second tensioner pulley rotatably mounted thereto. The second tensioner pulley is engaged with the second span of the endless drive member. The second tensioner arm is movable about a second tensioner arm axis. The tensioner biasing member is positioned to bias the first and second tensioner arms in respective first and second free arm directions. The tensioner further includes a first tensioner arm stop surface that is positioned to limit the movement of the first tensioner arm in a direction opposite the first free arm direction. The second tensioner arm stop surface that is positioned to limit the movement of the second tensioner arm in a direction opposite the second free arm direction. The second tensioner arm has a preload torque from a combination of torques applied by at least the endless drive member and the tensioner biasing member, wherein the preload torque urges the second tensioner arm into engagement with the second tensioner arm stop surface such that, when the endless drive arrangement operates in the first mode, the second tensioner arm remains engaged with the second tensioner arm stop surface and the first tensioner arm remains spaced from the first tensioner arm stop surface throughout operation of the engine where transient torque on the second tensioner arm acts against the preload torque and is below the preload torque, and wherein, when the endless drive arrangement operates in the first mode, the first tensioner arm remains engaged with the first tensioner arm stop surface and the second tensioner arm remains spaced from the second tensioner arm stop surface throughout operation of the engine where transient torque on the second tensioner arm acts against the preload torque and is sufficiently above the preload torque. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing and other aspects of the invention will be better appreciated with reference to the attached drawings, wherein: [0007] Figure 1 is a plan view of an endless drive arrangement including a tensioner, in accordance with an embodiment of the present disclosure;

[0008] Figure 2 is a plan view of a variation of the endless drive arrangement shown in Figure 1;

[0009] Figure 3 is a perspective view of an element of the endless drive arrangement shown in Figure 1;

[0010] Figure 4 is a plan view of the endless drive arrangement shown in Figure 1, operating in a first mode;

[0011] 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;

[0012] 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

[0013] 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.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0014] 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.

[0015] 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.

[0016] 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).

[0017] 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).

[0018] 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. [0019] 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.

[0020] A tensioner 25 for the endless drive arrangement 10 is shown in Figure 1. A 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). A second tensioner pulley 28 is rotatably mounted on a second tensioner arm

32 for rotational movement of the pulley about a second arm pulley axis APA2. The first and second tensioner pulleys 26, 28 are rotatably mounted to the first and second tensioner arms 30, 32, respectively, via shoulder bolts 57 (Fig. 4).

[0021] 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 52 that passes through an aperture in each of the tensioner arms 30 and 32 and into a threaded aperture in the base 48. [0022] The base 48 mounts fixedly to the housing of the MGU 18a or any other suitable stationary member.

[0023] 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.

[0024] 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.

[0025] 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. [0026] Analogously, the second tensioner pulley 28 is positioned on a first side of the second tensioner arm pivot axis AP2, in the sense that, in use, as a result of its engagement with the belt span 20b, the tensioner pulley 28 applies 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, in use, the tensioner biasing member 41 applies 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. [0027] Several features of the tensioner 25 may be advantageous and are described further below.

[0028] 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.

[0029] 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. [0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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. [0037] 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 HL acting on each tensioner arm 30 and 32 are shown as HF1 and HF2, respectively. [0038] 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 [0039] 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. [0040] 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

[0041] 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

[0042] 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 - HPT (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

[0043] 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.

[0044] As a result, the expression shown above is applicable, namely that: TR > HF2

TL HF1

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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 preload. 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.

[0049] 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.

[0050] 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.

[0051] 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. [0052] 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.

[0053] 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.

[0054] 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).

[0055] 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 with an MGU torque of about 10 Nm.

[0056] 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.

[0057] 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 or second 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 or second 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] Reference is made to Figure 6, which shows the tensioner 25, with an optional first arm damping structure 102, and an optional second arm damping structure 104 in accordance with another embodiment of the present disclosure.

[0065] The first and second arm damping structures 102 and 104 are configured to control the speed of the first and second tensioner arms 30 and 32 during impact of the tensioner arms 30 and 32 with the first and second tensioner arm stop surfaces 64 and 66, respectively.

[0066] The first and second arm damping structures 102 and 104 may both be constructed similarly. The second arm damping structure 104 is shown in Figures 7 an 8 and will be described herein. The second arm damping structure 104 includes a first friction surface 106 for the second tensioner arm 32, and a second friction surface 108 for the second tensioner arm 32. The first and second friction surfaces 106 and 108 engage one another frictionally to generate all or part of a second arm damping force for the second tensioner arm 32. The second arm damping structure 104 further includes a second arm ramp surface 110 and a second arm ramp rider surface 112. The second arm ramp surface 110 is mounted to one of the second tensioner arm 32 and the base 48. The second arm ramp rider surface 112 is mounted to the other of the second tensioner arm 32 and the base 48. In the embodiment shown, the second arm ramp surface 110 is mounted to one of the base 48 and the second arm ramp rider surface 112 is mounted to the second tensioner arm 32. In the embodiment shown, the second arm ramp surface 110 is on a second arm ramp member 114, which is mounted fixedly to the base 48 in any suitable way. In the example shown, the base 48 includes a base plate 116, and a second arm pivot support 118 that is fixedly mounted in an aperture 120 in the base plate 48a (e.g. by a press-fit). At an upper end of the second arm pivot support 118 there is an orientation feature 122, which may be, for example an axially extending flat surface. The second arm ramp member 114 has a mounting aperture that has a corresponding orientation feature, which may also be an axially extending flat surface 124. Mounting the second arm ramp member 114 to the second arm pivot support 118, fixes the second arm ramp member 114 rotationally to the base 48. A mounting bolt 126 may be used to mount in a threaded aperture 128 of the second arm pivot support 118 to hold the second arm ramp member 114 (and the rest of the second arm damping structure 104) in place.

[0067] The second arm ramp rider surface 112 is on a second arm ramp rider 130 that is operatively engaged with the second tensioner arm 32. The second arm ramp surface 110 and the second arm ramp rider surface 112 are oriented such that, during movement of the second tensioner arm 32 towards the second tensioner arm stop surface 66 relative movement between the second arm ramp surface 110 and the second arm ramp rider surface 112 drives the first friction surface 106 for the second tensioner arm 32 axially into engagement with the second friction surface 108 for the second tensioner arm 32 with a progressively increasing normal force so as to generate a progressively increasing damping force (i.e. frictionally) for the second tensioner arm 32. During movement of the second tensioner arm 32 away from the second tensioner arm stop surface 66, relative movement between the second arm ramp surface 110 and the second arm ramp rider surface 112 drives the first friction surface 106 for the second tensioner arm 32 into engagement with the second friction surface 108 for the second tensioner arm 32 with a progressively decreasing normal force so as to generate a progressively decreasing frictional damping force for the second tensioner arm 32. More particularly, pivoting of the second tensioner arm 32 in the direction opposite the free arm direction (i.e. in the direction shown by the arrow 134 in Figure 6) drives pivoting of the ramp rider 130 against the ramp member 114 in such a way to cause the ramp rider 130 to be driven axially downwards. The first friction surface 106 is provided in the present embodiment on the ramp rider 130. The second friction surface 108 is provided on a friction member 136 (e.g. a washer) that is engaged with the first friction surface 106.

[0068] A reason that the normal force between the first and second friction surfaces 106 and 108 changes as described above, is because there is provided a second damping biasing structure 140 that is positioned to apply a second arm axial force to urge the second arm ramp surface 110 and the second arm ramp rider surface 112 into engagement with one another and to urge the first and second friction surfaces 106 and 108 for the second tensioner arm 32 against one another.

[0069] The second damping biasing structure 140 may include at least one Belleville washer 142 (in the present embodiment four Belleville washers 142 are shown, oriented with two lower ones arranged as regular frustums, and two upper ones oriented as inverted frustums). Alternatively any other biasing structure could be used, such as a helical compression spring.

[0070] It will be noted that, reference to an axial force herein in relation to the embodiment shown in Figures 6-10 is a force that is orientated in the direction of the pivot axis of the respective tensioner arm 30 or 32 being described. The normal force between the first and second friction members 106 and 108 is also an axial force in this embodiment.

[0071] The operative connection between the second tensioner arm 32 and the second arm ramp rider 130 will now be described. The second tensioner arm 32 has a second arm aperture 144 therein, and includes a plurality of second arm drive surfaces 146 which are positioned on a wall 148 of the second arm aperture 144 and project radially inwardly. The second arm ramp rider 130 includes a plurality of second arm driven surfaces 150 that project radially outwardly. [0072] In the embodiment shown, there is a dimensional clearance shown at 152 between the second arm drive surfaces 146 and the second arm driven surfaces 150 in the sense that, if there were no force urging relative movement between the ramp rider 130 and the tensioner arm 32, there would be a space between each of the second arm drive surfaces 146 and the corresponding second arm driven surfaces 150. However, the plurality of second arm drive surfaces 146 and the plurality of second arm driven surfaces 150 are driven into contact with one another by rotation of the second arm ramp rider 130 against the second arm ramp member 114 due to sliding between the second arm ramp rider surface 112 and the second arm ramp surface 110 as a result of the second axial force applied by the second arm damping biasing structure 140.

[0073] As can be seen, the second arm ramp surface 110 and the second arm ramp rider surface 112 are both generally helical surfaces and have a selected pitch (which is a measure of the steepness of the angles of the helical surfaces). [0074] However it is alternatively possible for one of them to be a helical surface and for the other to be some other shape that is slidable the helical surface, such as a suitably angled planar surface.

[0075] The first arm damping structure 102 may be the same as the second arm damping structure 104, and is thus considered to be represented by the images shown in Figures 7-9, with the primary differences being that certain properties such as helical angles (i.e. helical pitch), coefficient of friction, and number of individual segments of the ramp surface 110 and the ramp rider surface 112 are provided (in the embodiment shown the ramp member 114 and the ramp rider 130 both have four segments to their respective ramp and ramp rider surfaces 110 and 112.

[0076] It has been found that, it is desirable to control the speed of the first and second tensioner arms 30 and 32 when impacting the respective stop surfaces 64 and 66. It has also been found that the damping that is provided in the first tensioner arm 30 affects the speed of impact of the second tensioner arm 32 on its stop surface 66, and similarly the damping that is provided in the second tensioner arm 32 affects the speed of impact of the first tensioner arm 30 on its stop surface 64. It has been found that, by controlling the speed of each of the first and second tensioner arms 30 to be less than about 20 radians per second, the impact noise is below a selected level which has been found to be acceptable. In some embodiments it has been found that, by keeping the speed of the first and second tensioner arms 30 and 32 to be less than about 19.5 radians per second, the impact noise heard in the vehicle passenger cabin is less than about 85 dB, which is considered to be a suitable upper threshold for impact noise. Alternatively other upper thresholds may be used for the impact noise and correspondingly for the speed of the tensioner arms 30 and 32 upon impact with their respective stop surface 64 and 66.

[0077] There is, as mentioned above, a first coefficient of friction between the first arm ramp surface 110 and the first arm ramp rider surface 112, and a first ramp surface pitch (i.e. a helical pitch, which is determinative of the steepness of the helical surface). The first coefficient of friction and the first ramp surface pitch are selected such that a damping force (which is directly related to the normal force F1A (not shown in Figure 7) generated by the first damping biasing structure 140) generated by the first arm damping structure 140 increases during travel of the first tensioner arm 30 in a first arm direction that is opposite the first free arm direction (i.e. in the direction shown by arrow 160 shown in Figure 6), and during travel of the first tensioner arm 30 in the first free arm direction, and such that a second arm speed during impact of the second tensioner arm 32 with the second tensioner arm stop surface 66 as a result of movement of the first tensioner arm 30 in the first free arm direction is less than 20 radians per second.

[0078] There is a second coefficient of friction between the second arm ramp surface 110 and the second arm ramp rider surface 112, and wherein there is a second ramp surface pitch, wherein the second coefficient of friction and the second ramp surface pitch are selected such that a damping force (which is directly related to the normal force F2A generated by the second arm damping structure 104 increases during travel of the second tensioner arm 32 in a second arm direction that is opposite the second free arm direction, and during travel of the second tensioner arm 32 in the second free arm direction, and such that a first arm speed during impact of the first tensioner arm 30 with the first tensioner arm stop surface 64 as a result of movement of the second arm in the second free arm direction is less than 20 radians per second.

[0079] Figure 10A shows a torque displacement curve for the first tensioner arm 30. It has been found that it is beneficial for the ramp surfaces 110 and 112 (which affect the damping force provided on the tensioner arm 30 during movement thereof) to be angled sufficiently steeply that the torque on the first tensioner arm 30 decreases during movement of the first tensioner arm in the free arm direction, as this has been found to reduce the speed of the second tensioner arm 32 upon impact with its stop surface 66. Analogously, the torque curves shown in Figure 10B have a selected shape achieved by angling the ramp and ramp rider surfaces 110 and 112 for the second tensioner arm 32 sufficiently steeply that the torque on the second tensioner arm 32 decreases during movement of the first tensioner arm in the free arm direction, as this has been found to reduce the speed of the first tensioner arm 30 upon impact with its stop surface 64.

[0080] The curves shown in Figure 10A (for the first tensioner 30) are for nominal construction (curve 300), a first stack up tolerance (curve 302) which determines the spring force applied by the first damping biasing structure 140 and a second stack up tolerance (curve 304) which determines the spring force applied by the first damping biasing structure 140.

[0081] The curves shown in Figure 10B (for the second tensioner arm 32) are for nominal construction (curve 400), a first stack up tolerance (curve 402) which determines the spring force applied by the first damping biasing structure 140 and a second stack up tolerance (curve 404) which determines the spring force applied by the first damping biasing structure 140. [0082] 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.