Decoupler assembly having limited overrunning capability

In one aspect, a decoupler assembly is provided for use between a shaft and an endless drive member that is used to drive the shaft. The decoupler assembly includes a pulley, a hub and an isolator spring that is preferably a coiled torsion spring. The two ends of the spring are engageable, at least indirectly, with the pulley and the hub for the transfer of torque therebetween. At least one of the ends of the spring engages an engagement structure (on either the pulley or the hub) that includes a helical axial shoulder and a driver wall. The spring transfers torque in one direction through the driver wall (e.g. when the pulley overruns the hub), but the spring end is not fixedly connected to the driver wall. When the hub overruns the pulley, there is relative rotation between the spring and whichever of the hub and pulley it is not fixedly connected to. Accordingly, there is relative rotation between the spring end and the helical axial shoulder and the driver wall. This causes the spring end to separate from the driver wall and ride up the helical axial shoulder. This causes the spring to compress axially. The spring coils have a selected amount of spacing so that the spring can be compressed by a selected amount axially. This sets the amount of relative rotation (and the amount of overrun) that is available between the pulley and the hub in the situation when the hub overruns the pulley.

FIELD OF THE INVENTION

The present invention relates to decoupler assemblies and more particularly to decoupler assemblies for alternators.

BACKGROUND OF THE INVENTION

It is known to provide a decoupling mechanism on an accessory, such as an alternator, that is driven by a belt from an engine in a vehicle. Such a decoupling mechanism, which may be referred to as a decoupler, permits the associated accessory to operate temporarily at a speed that is different than the speed of the belt. For example, when there is a sudden stoppage of the belt when the belt was running and driving rotation of the alternator shaft, the decoupler permits the alternator shaft to continue rotating temporarily as a result of inertia until it decelerates to a stop as a result of drag, thereby reducing the stress on the alternator shaft. As another example, the decoupler permits the alternator shaft to rotate at a relatively constant speed even though the crankshaft from the engine undergoes a cycle of decelerations and accelerations associated with the movement of the pistons.

Such a decoupler is a valuable additions to the powertrain of the vehicle. However, it can be costly to manufacture for various reasons. One example that drives up its cost is the pulley that is included with it. In certain decouplers the pulley is typically made from steel because it is engaged with the wrap spring that is in the decoupler. The pulley may have to coated for appearance reasons. The interior surface of the pulley, however, is machined to have selected dimensions with very tight tolerances to provide predictability in its engagement with the wrap spring. Thus, coatings, which typically have a relatively high variability in their thickness, cannot typically be applied to its interior surface that engages the wrap spring. Thus the coating process is made more difficult and expensive than it would otherwise be. Additionally, the coating itself can be prone to scratches, which could cause the entire decoupler to be rejected upon inspection.

Other problems arise when a decoupler with a wrap spring is used in conjunction with a BAS (Belt-Alternator-Start) system on a vehicle. In such a system, the alternator is driven as a motor and is used to drive the belt, so that the belt drives the engine's crankshaft, in order to start the engine. The wrap spring, however, prevents the alternator shaft from driving the pulley however, and so a separate electric clutch has been proposed to overcome this issue. Such clutches are expensive and complex however.

There is a continuing need to reduce their cost, to improve their operating life, to reduce their complexity and to simplify their manufacture. It would thus be beneficial to provide a decoupler that addresses one or more of these continuing needs.

SUMMARY OF THE INVENTION

In a first aspect, the invention is directed to a decoupler assembly for use between a rotating member such as an alternator shaft and a belt or other endless drive member that is used to drive the rotating member. The decoupler assembly includes a pulley, a hub and an isolator spring that is preferably a coiled, torsion spring. The two ends of the spring are engageable with, at least indirectly, the pulley and the hub for the transfer of torque therebetween. At least one of the ends of the spring engages an engagement structure (on either the pulley or the hub) that includes a helical axial shoulder and a driver wall. The spring transfers torque in one direction through the driver wall (e.g. when the pulley overruns the hub), but the spring end is not fixedly connected to the driver wall. As a result, when the hub overruns the pulley, there is relative rotation between the spring and whichever of the hub and pulley it is not fixedly connected to. Accordingly, there is relatively rotation between the spring end and the engagement structure (i.e. the helical axial shoulder and the driver wall). This causes the spring end to separate from the driver wall and to ride up the helical axial shoulder. This causes the spring to compress axially. The coils of the spring have a selected amount of spacing so that the spring can be compressed by a selected amount axially. This sets the amount of relative rotation (and therefore the amount of overrun) that is available between the pulley and the hub in that situation (e.g. in the situation when the hub overruns the pulley).

In a particular embodiment of the first aspect, the invention is directed to a decoupler assembly for transferring torque between a shaft and an endless drive member. The decoupler assembly includes a hub that is adapted to be coupled to the shaft such that the shaft co-rotates with the hub about a rotational axis, a pulley rotatably coupled to the hub, and having an outer periphery that is adapted to engage the endless drive member, a helical torsion spring concentric with the rotational axis and having a first axial face and a second axial face, and having a plurality of coils which are spaced apart by a plurality of gaps, a first engagement structure positioned between the torsion spring and one of the hub and the pulley, and a second engagement structure positioned between the torsion spring and the other of the hub and the pulley. The first engagement structure includes a helical first axial shoulder for engaging the first axial face of the torsion spring. The second engagement structure includes a second axial shoulder engageable with the second axial face of the torsion spring. Rotation of the pulley in a first rotational direction relative to the hub drives rotation of the hub through the torsion spring. Rotation of the hub in the first direction relative to the pulley generates relative rotation between the torsion spring and the helical first axial shoulder which causes axial compression of the torsion spring between the first and second axial shoulders, wherein the plurality of gaps are sized to provide a selected amount of axial compression of the torsion spring.

The decoupler assembly may be used as part of a BAS (Belt-Alternator-Start) system for a vehicle. In an embodiment, the vehicle includes an engine that has crankshaft, a crankshaft pulley, and a belt that is engaged with the crankshaft pulley and with an alternator. The BAS system includes a decoupler assembly mountable to the shaft of the alternator. The decoupler assembly includes a hub that is adapted to be coupled to the shaft such that the shaft co-rotates with the hub about a rotational axis, a pulley rotatably coupled to the hub, and having an outer periphery that is adapted to engage the endless drive member, a helical torsion spring concentric with the rotational axis and having a first axial face and a second axial face, and having a plurality of coils which are spaced apart by a plurality of gaps, a first engagement structure positioned between the torsion spring and one of the hub and the pulley, and a second engagement structure positioned between the torsion spring and the other of the hub and the pulley. The first engagement structure includes a helical first axial shoulder for engaging the first axial face of the torsion spring. The second engagement structure includes a second axial shoulder engageable with the second axial face of the torsion spring. Rotation of the pulley in a first rotational direction relative to the hub drives rotation of the hub through the torsion spring. Rotation of the hub in the first direction relative to the pulley generates relative rotation between the torsion spring and the helical first axial shoulder which causes axial compression of the torsion spring between the first and second axial shoulders. The plurality of gaps are sized to provide a selected amount of axial compression of the torsion spring. The selected amount of compression of the torsion spring is reached in less than 360 degrees of rotation of the hub relative to the pulley.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made toFIG. 1, which shows an engine10for a vehicle. The engine10includes a crankshaft12which drives an endless drive element, which may be, for example, a belt14. Via the belt14, the engine10drives a plurality of accessories16(shown in dashed outlines), such as an alternator and a compressor. Each accessory16includes an input drive shaft15with a pulley13thereon, which is driven by the belt14. A decoupler assembly20is provided instead of a pulley, between the belt14and the input shaft15of any one or more of the belt driven accessories16. The decoupler assembly20transfers torque between the belt14and the shaft15but automatically decouples the shaft15from the belt14when the belt14decelerates relative to the shaft15. Additionally, the decoupler assembly20allows the speed of the belt14to oscillate relative to the shaft15. Thus, oscillations in the belt speed that are the result of oscillations in the speed of the crankshaft (an inherent property of internal combustion piston engines), are dampened by the decoupler assembly20, and as a result, the stresses that would otherwise be incurred by the shaft15and the component16are reduced.

Referring toFIGS. 2 and 3, the decoupler assembly20includes a hub22, a pulley24, a first bearing member26, a second bearing member27, and an isolation spring28.

The hub22may be adapted to mount to the accessory shaft15(FIG. 1) in any suitable way. For example, the hub22may have a shaft-mounting aperture36therethrough that is used for the mounting of the hub22to the end of the shaft15, for co-rotation of the hub22and the shaft15about an axis A.

The pulley24is rotatably coupled to the hub22. The pulley24has an outer surface40which is configured to engage the belt14. The outer surface40is shown as having grooves42. The belt14may thus be a multiple-V belt. It will be understood however, that the outer surface40of the pulley24may have any other suitable configuration and the belt14need not be a multiple-V belt. For example, the pulley24could have a single groove and the belt14could be a single V belt, or the pulley24may have a generally flat portion for engaging a flat belt14. The pulley24further includes an inner surface43. Unlike some decoupler assemblies of the prior art, the inner surface43of the pulley24does not engage a one-way clutch spring and as a result, the pulley24need not be made of a material that resists galling or wear from such a clutch spring. The pulley24may thus be made from any suitable material, such as a polymeric material, such as a type of phenolic, or an up to 50% glass-reinforced nylon-6. As a result, the pulley can be injection molded, and can easily have any suitable finish provided on it. Furthermore, the material can be of a selected colour, so that the pulley is a selected colour for appearance purposes, without the need for paint. Paint or some similar coating is needed for metallic pulleys, however it is susceptible to scratches which can reveal the base material underneath, leading to a rejection of the assembly during the inspection process. A polymeric pulley, however, even if scratched, remains the same colour since the colour extends throughout it, therefore making it less susceptible to rejection from being scratched. This reduced potential for rejection reduces the overall average cost of manufacture of the pulley. Furthermore, a polymeric pulley24can be significantly less expensive to manufacture than a coated steel pulley due to lower cost of materials, elimination of the coating step. Additionally, because the pulley24is not engaged with a wrap spring, the inner surface43of the pulley24need not be formed with very tight tolerances. By contrast, pulleys of the prior art that directly engage a clutch spring can in some cases require strict dimensional control on the pulley inner surface that engages the clutch spring so that the clutch spring operates as intended.

The pulley24may nonetheless be made from a metallic material, such as a steel, or aluminum. Even when made from steel, however, the pulley24may be less expensive then some pulleys of the prior art used in decoupler assemblies. For example, the pulley24may be made from a spinning and forming process as necessary to achieve a pulley shape. Such as pulley is described in U.S. Pat. No. 4,273,547.

Whether made from a polymeric material, or a metallic material the pulley24may be lighter than some pulleys of the prior art because it is not required to withstand the stresses associated with engagement with a wrap spring. Furthermore, it is not required to have the tight tolerances associated with some pulleys of the prior art, and so wall thicknesses and the like may be selected with the goal of lightness and with less emphasis on ensuring the capability of providing tight tolerances on its inner surface. This reduced weight translates into reduced rotational inertia, which can result in reduced energy consumption associated with its rotation. This translates into reduced emissions and/or increased fuel economy for the vehicle in which it is employed.

The first bearing member26rotatably supports the pulley24on the hub22at a first (proximal) axial end44of the pulley24. The first bearing member26may be any suitable type of bearing member, such as a bushing. In cases where it is a bushing it may be made from nylon-4-6 or for some applications it could be PX9A which is made by DSM in Birmingham, Mich., USA, or some other suitable polymeric material, and may be molded directly on the pulley24in a two step molding process in embodiments wherein a molded pulley is provided. In such a case, the bearing could be inserted into a mold cavity and the pulley24could be molded over the bearing26. Instead of a polymeric bushing, a metallic (e.g. bronze) bushing may be provided, which can be inserted into a mold cavity for the pulley molding process in similar fashion to the aforementioned bearing. The first bearing member26could alternatively be a bearing (e.g. a ball bearing, or a roller bearing).

The second bearing member27is positioned at a second (distal) axial end46of the pulley24so as to rotatably support the pulley24on a pulley support surface48of the hub22. The second bearing member27may be any suitable type of bearing member such as a ball bearing, a roller bearing, or a bushing.

The isolation spring28is provided to accommodate oscillations in the speed of the belt14relative to the shaft15. The isolation spring28may be a helical torsion spring that has a first helical end50that abuts a radially extending driver wall52(FIG. 4a) and a first helical axial face63that is engaged with a first helical axial shoulder51on the hub22(FIGS. 4aand4b). The isolation spring28has a second helical end53(FIG. 3) that engages a radially extending driver wall54on the pulley24, and a second helical axial face65that is engaged with a second helical axial shoulder67.

In the embodiment shown, the isolation spring28has a plurality of coils58between the first and second ends50and53. The coils58are preferably spaced apart by a plurality of gaps69(FIG. 4a) and the isolation spring28is preferably under a selected amount of axial compression to ensure that the first and second helical ends50and53of the spring28are abutted with the helical axial shoulder51with the driver walls52and54respectively.

The first helical axial shoulder51and the first driver wall52may together be referred to as a first engagement structure. The second helical axial shoulder67and the second driver wall54may together be referred to as a second engagement structure.

Rotation of the pulley24in a first rotational direction relative to the hub22drives rotation of the hub22through the torsion spring28. Rotation of the hub22in the first direction relative to the pulley24generates relative rotation between the torsion spring28and the helical first axial shoulder51which causes axial compression of the torsion spring28between the first and second axial shoulders51and67. The plurality of gaps69are sized to provide a selected amount of axial compression of the torsion spring28when the decoupler assembly20is in a rest state.

The isolation spring28may be made from any suitable material, such as a suitable spring steel. The isolation spring28may have any suitable cross-sectional shape. In the figures, the isolation spring28is shown as having a rectangular cross-sectional shape, which provides it with a relatively torsional resistance (i.e. spring rate) for a given occupied volume. A suitable spring rate may be obtained with other cross-sectional shapes, such as a circular cross-sectional shape or a square cross-sectional shape. This may be advantageous in that it may reduce the cost of the isolation spring as compared to one made from a wire having a rectangular cross-section.

During use, when the pulley24is being driven by the belt14, the pulley24drives the rotation of the alternator shaft (or shaft from another other accessory through the engagement of the torsion spring28with the first and second driver walls52and54. During a transient event such as when the engine stops, the pulley24will be stopped by the belt14, but the alternator shaft15will continue to turn for a small period of time. As shown inFIGS. 4aand4b, the hub22will rotate with the shaft15which will bring the first driver wall52away from the end50of the spring28. The helical axial face51rotates with the hub22as well however, and as it rotates, it pushes the axial face63of the spring28axially proximally (FIG. 4b), thereby compressing the spring28axially. This continues until either: the shaft15stops rotating due to frictional forces, or the hub22rotates far enough to drive the axial compression of the spring28until the coils58all contact each other, at which point the spring28locks (i.e. no further axial compression is possible) and no longer permits the hub22to further overrun the pulley24. In the embodiment shown inFIGS. 4aand4b, there is a selected relative angle between the hub22and pulley24at which the spring28locks up. This means that the decoupler20provides less than 360 degrees of relative movement by the hub22relative to the pulley24. The particular amount of relative movement available prior to spring lock up can be selected however, based on the size of the gaps69. In particular, the amount of relative movement available can be selected to exceed the amount needed for most situations. It has been determined that under many circumstances, there is less than 70 degrees of relative movement between the hub and the pulley in a decoupler. Thus, if the amount of relative movement available is selected to be greater than about 70 degrees then many circumstances could be handled by the decoupler20. It will be noted that the amount of relative movement prior to spring lock up could be selected to be any amount up to 360 degrees, or even more in some embodiments. In one particular embodiment the amount of relative movement available is less than approximately 360 degrees and is more preferably less than about 350 degrees.

The sizing of the gaps69may be selected so that there is enough clearance to prevent the spring28from locking up even if there was a full 360 degrees of relatively rotation by the hub22with respect to the pulley24.

Reference is made toFIGS. 5aand5b, which show a decoupler assembly129which is similar to the decoupler assembly20but which includes means for damping oscillations transferring through the spring28(e.g. from the pulley24to the hub22), and also includes means for limiting the amount of torque that the spring28has to handle on its own. In the embodiment shown inFIGS. 5aand5b, a separate carrier130is provided between the spring28and the pulley24. The carrier130may be made from any suitable material such as a polymeric material. The carrier130may be fixedly connected rotationally to the pulley24by means of a key, a press-fit, a spline or any other suitable structure. A key131that is integral with the pulley24is shown engaged with a keyway133in the carrier130inFIG. 5d. The carrier130may have the second engagement structure thereon. As the spring28expands during use, it may expand sufficiently to rub against a damping surface132on the carrier130. When this occurs, some damping occurs when there are differences in speed between the spring28and the pulley24.

Also shown inFIGS. 5a,5band5c, is a sleeve134. The sleeve134may be in contact with the inner surface of the pulley24but may be unconnected to it (i.e. the sleeve134may be capable of movement relative to the pulley24). The sleeve134may have any suitable structure. For example, in the embodiment shown, the sleeve134is a nearly complete cylindrical shape, as shown inFIG. 5c. In another embodiment the sleeve134may be shaped like a coil spring. In yet another embodiment, the sleeve134may be in the form of a complete cylinder. The sleeve134surrounds the spring28and limits the amount of radial expansion that is available to the spring28. If a torque that is sufficiently large is applied through the spring28, the spring28will expand sufficiently to engage the sleeve134. As shown inFIGS. 5aand5b, the sleeve134is engaged with the inner surface of the pulley24and so once the spring28engages the sleeve134, the spring28can expand no further radially. Any greater torque applied through the spring28is then supported by the sleeve134. In this way, the sleeve134limits the amount of torque that the spring28is required to handle on its own. Furthermore, the engagement of the spring28with the sleeve134and the sleeve134with the inner surface of the pulley28acts to dampen oscillations that are transmitted through the spring28. The sleeve134may be made from any suitable material, such as a plastic material (e.g. nylon), or a metal (e.g. steel). In embodiments wherein the sleeve134is provided it may simply ‘float’ axially between the carrier130and an analogous portion136on the hub22.

As shown inFIGS. 5aand5b, the decoupler assembly129further includes a retainer138which captures the bearing member27. Also, as shown inFIGS. 5aand5b, the bearing member26is shown as a bushing140which is positioned radially between the pulley24and the hub22, and which is also positioned axially between the pulley24and the carrier130.

Reference is made toFIGS. 8aand8b, which show a decoupler assembly150in accordance with yet another embodiment of the present invention, which may be similar to the decoupler assembly129, but which includes a further means for damping oscillations. In the decoupler assembly150the bearing member27is a bushing152, not a ball bearing. The bushing152is positioned radially between the hub22and the pulley24, and is also positioned axially between the distal end of the hub22and the retainer shown at154. The bushing152provides additional damping to the decoupler assembly150as compared to the damping provided in the decoupler assembly129.

Reference is made toFIG. 9, which shows a cartridge160that may be used during assembly of the decoupler assembly. The cartridge160may be made up of a pulley-associated carrier162, a sleeve164and a hub-associated carrier166. The three components162,164and166may be assembled together and held together with the spring28(not shown in this figure) captured therein, by a robot or by an assembly line worker and may be mounted all together onto the hub shown at168. The hub-associated carrier166may sit on a support surface170on the hub168. A key that extends in a keyway (similar to that shown inFIG. 5d) in the support surface170may be provided. A similar arrangement may be provided between the pulley24and the carrier162. Bearing members for supporting the pulley24on the hub are not shown, but would be provided.

Reference is made toFIG. 10which shows a cartridge180that may be similar to the cartridge160, except that the cartridge180includes only two components: a pulley-associated carrier182that may be keyed, for example, to the pulley24, and a hub-associated carrier184that includes a sleeve portion, shown at186that may be keyed to a support surface189on the hub shown at188. Bearing members for supporting the pulley24on the hub are not shown, but would be provided.

Reference is made toFIG. 11which shows a cartridge190that may be similar to the cartridge180except that the hub-associated and pulley-associated carriers, shown at192and194respectively, are connected together by a clip connection or the like, shown at196. The clip connection196holds the cartridge190together for easy transport and handling by an assembly line worker or by a robot during manufacturing of the decoupler assembly. Once the cartridge190is mounted onto the hub, shown at198, the pulley-associated and hub-associated carriers192and194may be disconnected from each other by any suitable means. For example as shown inFIG. 11arotation of the two carriers192and194relative to each other may slide the two clip elements shown at200and202apart so that they no longer overlap, permitting the spring28to push the two carriers192and194apart (the spring28may be in compression when the two carriers192and194are clipped together). During use, the two carriers192and194would remain sufficiently separated that they would not be at any significant risk of rejoining together. Bearing members for supporting the pulley24on the hub are not shown, but would be provided.

Reference is made toFIG. 12, which shows a decoupler assembly210in accordance with another embodiment of the present invention. In the decoupler assembly210, the first engagement structure includes a helical axial shoulder212on the hub shown at214, which engages a first axial end216of a hub-associated carrier218. The carrier218is engaged with the spring28for co-rotation therewith. A driver wall shown at220inFIG. 12aon the hub214engages a corresponding wall222on the carrier218. When the hub214overruns the pulley24, the relative rotation of the hub214(and therefore the helical axial shoulder212) with respect to the carrier218compresses the spring28(not shown in this figure) axially so as to permit the overrun in similar fashion to that described elsewhere herein. A sleeve is shown at224and a pulley-associated carrier is shown at226. Bearing members for supporting the pulley24on the hub are not shown, but would be provided.

During use of a decoupler assembly according to at least some of the embodiments described above, it can be seen that the damping force (i.e. the frictional force) is at least in part dependent on the axial force exerted by the spring28. In such embodiments, as the spring28is axially compressed by rotation of the first engagement surface, the axial force exerted by the spring28increases and so the damping force provided by the decoupler assembly increases.

Damping has been described as being provided by a carrier in conjunction with a friction surface associated with the hub. It will be noted that some or substantially all of the damping may be provided in conjunction with a friction surface provided on or associated with the pulley.

As shown and described, in some embodiments, both the first and second engagement structures include driver walls and helical axial shoulders so that the spring28is not fixedly connected at either end to the hub or pulley. It is alternatively possible, however to fixedly connect one end of the spring28to the pulley or to the hub and to leave the other end of the spring not fixedly connected to the other of the hub or pulley. The unconnected end of the spring28may be on the hub or it may be on the pulley.

In a typical (non-overrunning) isolator of the prior art, both the first and second ends of the torsion spring are fixedly connected to the hub and pulley respectively (by being bent to form tangs that engage slots in the hub and pulley).FIG. 6aillustrates the response curve70of such a prior art isolator. As can be seen, a first part72of the curve70shows the linear relationship between the relative angle between the hub and pulley and the torque transferred through the torsion spring. When the pulley drives the hub, for example, the torque applied by the pulley through the spring to the hub may be considered to be positive and the angular change associated with it may be considered to be positive. As the torque increases, the relative angle increases relatively linearly.

In the isolator modeled inFIG. 6a, a sleeve was provided which provides a constraint as to the maximum amount of radial expansion that the torsion spring can undergo during use. The second part of the curve shown at74illustrates what happens when the spring expands and is constrained by the sleeve. As can be seen the torque increases nearly vertically with essentially no change in the relative angle of the hub and the pulley. As can be seen in curve part76, as the torque transferred is reduced, the relative angle reduces, essentially mirroring the second curve part74. Once the spring pulls away inwardly from the sleeve, the reduction in relative angle between the hub and pulley is relatively linear and parallel to the first curve part72. As can be seen at78, when the hub drives the pulley (e.g. when it pulls the pulley to rotate during shutdown of the engine), the spring ends move past the rest position and transition from being pushed towards each other, to being pulled by the hub and pulley away from each other (which is considered to be a negative angular change). During this transition however, a spike can be observed in the curve. This spike occurs as the spring ends adjust in the slots from being pushed to being pulled. Repeated passing through this region of the graph during use of the isolator eventually can lead to noise, and/or yield and failure of the spring, the hub and/or the pulley. It can be seen that a similar transition region80may exist on the part of the curve illustrating the transition from when the hub pulls the pulley to when the pulley pushes the hub, which again contributes to wear, noise and failure of the spring, the hub and/or the pulley during use.

In general, an isolator which has both ends of the spring fixedly connected to the hub and pulley benefits greatly from a sleeve because it helps increase the operating life of the spring. More specifically, when the spring expands radially (i.e. when transferring torque) the ends of the spring, which are fixed in position, are stressed. Repeated stressing of the ends eventually can cause failure of the spring at these points due to fatigue. A sleeve improves this situation by restricting how much the spring can expand radially, however this restricts how much isolation the spring is capable of providing. By contrast, both ends of the spring28in at least some embodiments shown and described herein are not fixedly connected to the hub and pulley. As a result, the spring28is not subjected to these aforementioned stresses. As a result, the spring28can operate without a sleeve (so as to have a greater range of torques that it can handle while providing isolation), without risk of fatigue and failure at its ends in this way. If a sleeve is provided (as is shown inFIGS. 5a-5cfor example), the sleeve may have a greater spacing from the spring28than would be practical for the spring in the prior art isolator described above due to the risk of fatigue and failure.

FIG. 6bshows a curve81that illustrates the response of the decoupler20during use (without a sleeve). The first parts of the curve shown at82and88may be very similar to the parts72and78on the curve70inFIG. 6a. As can be seen, as the curve transitions (at region90) from a situation where the pulley drives the hub to a situation where the hub overruns the pulley, the curve then extends horizontally, illustrating that there is angular displacement with no torque transfer (at portion92). This illustrates when the first spring end50has separated from the driving wall52. Eventually if the overrunning extended sufficiently long, the hub and pulley would reach a relative angle at which the spring28locks up (i.e. there are no remaining gaps between the coils58), and the torque would increase (in the negative direction) with essentially no change in relative angle as seen at94. As can be seen in the curve81, there are no spikes that occur as the spring transitions between the hub overrunning the pulley and between the pulley overrunning the hub. This is because at least one end of the spring is not fixedly connected to the hub or pulley with which it is engageable.

FIGS. 7a-7gillustrate a comparison of the decoupler20with a decoupler of the prior art that includes a one-way wrap spring clutch. The graph inFIG. 7ashows a steady state test that was carried out on both the decoupler20and the decoupler with the wrap spring. In this test, a sinusoidal torsional vibration was applied to the decouplers where the torque applied was 2000 Nm+/−300 Nm, at a frequency of 21.7 Hz as shown by curve93.FIG. 7bshows the performance of the prior art decoupler with the wrap spring. The curve shown at95is the torque applied by the pulley. The curve shown at96is the torque applied to the hub. As can be seen, the torque at the hub is phase shifted in time and is lower than the torque applied at the pulley.FIG. 7cshows the performance of the decoupler20. The curve shown at98is the torque applied by the pulley24. The curve shown at100is the torque applied to the hub22. As can be seen, here too the torque at the hub22is phase shifted in time and is lower than the torque applied at the pulley24.

FIG. 7dis a graph that illustrates the performance of the prior art decoupler under a first type of transient condition, which is during start up of the engine. The speed of the pulley is represented by curve102and the speed of the hub is represented by curve104. As can be seen in exemplary region106, there are situations wherein the speed of the hub is greater than the speed of the pulley, (i.e. the hub is overrunning the pulley).FIG. 7eis an analogous graph for the decoupler20. The pulley speed curve is shown at108and the hub speed curve is shown at110. As can be seen in exemplary regions112, here too the hub overruns the pulley at certain points during engine start up.

FIG. 7fillustrates the response of the prior art decoupler during another transient condition, which is engine shutdown. The pulley speed and hub speed are represented by curves114and116respectively. As can be seen, the wrap spring permits a relatively long period of overrun (about 0.4 second) by the hub relative to the pulley as shown at region118.FIG. 7gillustrates the response of the decoupler20during engine shutdown. The pulley speed and hub speed are represented by curves120and122respectively. As can be seen, the hub overruns the pulley repeatedly for shorter periods of time (see at regions124) during shutdown, somewhat mirroring the performance during a startup situation. While this may in some situations permit a chirp to be emitted due to some degree of belt slip, in many situations belt chirp is prevented and in any case whether belt chirp is present, the overall stresses on the pulley, the shaft and the belt are reduced as compared to an arrangement without any isolation or decoupling.

A particularly advantageous application for the decoupler assemblies described herein is as part of a BAS (Belt-Alternator-Start) system for the engine10. A BAS system starts the engine by turning the crankshaft via the belt instead of via a starter motor. The belt is driven by the alternator, which is powered to operate as a motor temporarily. In such situations a prior art decoupler that is equipped with a one-way wrap spring clutch would be operable, since the clutch would prevent the hub from driving the pulley. To overcome this, some systems have been proposed whereby an electrically actuated clutch is provided which is actuated during engine startup where the hub has to drive the pulley. Such an arrangement may work, but it can be relatively expensive, relatively complex, and may occupy a relatively large space in the already cramped engine bays of many vehicles. By contrast, the decoupler assemblies described herein that lock the spring28within 360 degrees of relative rotation between the hub and pulley automatically permit the hub to drive the pulley and therefore do not require a complex and expensive electrically actuated clutch.

It can be seen the decoupler assemblies described herein provide some overrunning capability while doing away with the cost and complexity associated with a wrap spring and the precisely machined pulley associated therewith. In addition to the reduced cost of manufacture of the pulley there are other advantages provided by the decoupler assembly described herein. For example, in decouplers that include wrap springs that engage the inner surface of the pulley, it is difficult to efficiently change the design to accommodate a larger pulley. If the inner diameter of the pulley is changed, then the wrap spring needs to be changed and the design will potentially have to be revalidated. If the inner diameter of the pulley is not changed even though the outer diameter is increased, then the pulley becomes unnecessarily heavy. By contrast, the decoupler assemblies described herein do not need to employ a wrap spring and accordingly can easily accommodate an increase in both the outer diameter and the inner diameter of the pulley.

While the above description 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.