EP2705269A1

EP2705269A1 - Pulley unit for bi-directional torque transfer

Info

Publication number: EP2705269A1
Application number: EP12719379.5A
Authority: EP
Inventors: Alessandro GARRONE
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2011-05-06
Filing date: 2012-05-07
Publication date: 2014-03-12
Also published as: WO2012152757A1

Abstract

The invention relates to a pulley unit for bi-directional torque-transfer, comprising a first cutch (30) and a second clutch (40) arranged between an inner member (10) and an outer member (20) of the pulley. The unit is configured such that when the outer member (20) is being driven, the first clutch (30) enables torque to be transferred from the outer member to the inner member. When the inner member (10) is being driven, torque is transferred from the inner member to the outer member via the second clutch (40). The second clutch is a sprag clutch comprising an inner cage (42) and an outer cage (43), each cage having a number of pockets for retaining a corresponding number of sprags (41). When the outer member is being driven, the sprags have a first orientation, in which the sprags are not in engagement with the inner and outer members. In a second orientation of the sprags, the sprags engage with the inner and outer members and enable torque transfer. According to the invention, one of the inner and outer cages is rotatable relative to the other of the inner and outer cages, whereby the relative rotation causes the sprags to move from the first orientation to the second orientation. The unit further comprises actuation means (60, 80) for engaging with the rotatable cage (42) and changing the orientation of the sprags.

Description

Pulley unit for bi-directional torque transfer

FIELD OF THE INVENTION

The present invention relates to a pulley unit that is configured for bi-directional torque transfer between an outer member and an inner member of the pulley unit.

BACKGROUND TO THE INVENTION

A unit of this kind is known from WO 2004/06581 1 , which describes a pulley device and a starter/generator comprising the device. The pulley device comprises a shaft, a pulley that is rotatably mounted on the shaft, and a one-way clutch for providing a freely rotatable condition when the shaft and pulley are relatively rotated in a first direction of rotation, and for providing a first coupling between the shaft and the pulley when they are relatively rotated in an opposite direction. The device further includes a locking clutch, which provides a second coupling between the shaft and pulley. The locking clutch prevents relative rotation between the shaft and pulley in both relative rotational directions. In one embodiment, the locking clutch is an electromagnetic clutch comprising a first armature that can slide axially, but is rotationally fixed to one of the shaft and pulley. A second armature is fixedly connected to the other of the shaft and pulley. The armatures are further provided with friction means, which engage with each other when the locking clutch is energized.

There is still room for improvement however. SUMMARY OF THE INVENTION

The present invention resides in a pulley unit as defined in claim 1 , whereby further developments of the invention are specified in the dependent claims.

Specifically, the inventive pulley unit comprises an inner member that is rotatably mounted to an outer member, and further comprises a first clutch and a second clutch arranged between the inner and outer members. The unit is configured such that when the outer member is being driven, the first clutch enables torque to be transferred from the outer member to the inner member, while allowing the inner member to rotate relative to the outer member. When the inner member is being driven, the second clutch is actively engaged to enable torque transfer from the inner member to the outer member. The second clutch is a sprag clutch comprising an inner cage and an outer cage, each cage having a number of pockets for retaining a corresponding number of sprags. When the outer member is being driven, the sprags have a first orientation, in which the sprags are not in engagement with the inner and outer members. In a second orientation of the sprags, the sprags engage with the inner and outer members and enable torque transfer from the inner to the outer member. According to the invention, one of the inner and outer cages is partly rotatable relative to the other of the inner and outer cages, whereby the relative rotation causes the sprags to move from the first orientation to the second orientation. The unit further comprises actuation means for moving the rotatable cage, changing the orientation of the sprags and engaging the second clutch. The first clutch may be a one-way clutch, such as a sprag clutch, a roller clutch or a wrap spring. The first clutch may also comprise a torsion spring, whereby one end of the spring is coupled to the inner member and an opposite end of the spring is coupled to the outer member. The function of the first clutch is to transfer torque in the first direction and to enable the inner member of the pulley to overrun the outer member when the outer member is being driven by e.g. an engine crankshaft belt.

The second clutch is engaged when the pulley inner member is being driven by e.g. an alternator shaft. The inner and outer cages of the second clutch are mounted to the same pulley member, which may be the inner member or the outer member. One of the inner and outer cages is mounted to the pulley member in a fixed manner, and will be referred to as the fixed cage. The other of the inner and outer cages is mounted to the pulley member in a manner that permits a small amount of angular displacement of the cage, and this cage will be referred to as the rotatable cage. To engage the second clutch, the actuation means are activated and angularly displace the rotatable cage. The fixed cage then serves as a fulcrum around which the sprags are tiled to adopt their engaged position. Preferably, the rotatable cage is resiliently mounted to the pulley member via one or more springs, so that when the actuation means are de-activated, the spring force returns the rotatable cage and the sprags to their disengaged position. In a preferred embodiment, the actuation means comprises a cam and the rotatable cage is provided with one or more axially extending slots for receiving a cam. Specifically, each slot has an angled camming surface and the actuation means comprises an actuation disc with one or more protrusions that can be received within the one or more slots. Each protrusion has a camming surface that is in contact with the angled camming surface of each slot.

In a first embodiment, the angled camming surface of each slot is undercut relative to the camming surface of each protrusion. Furthermore, when the second clutch is in the disengaged position, the one or more protrusions are arranged axially within the one or more slots, such that an axial retraction of the actuation disc causes an angular displacement of the rotatable cage. In other words, a pulling force is exerted on the actuation disc in order to engage the second clutch.

In a second embodiment, the angled camming surface of each slot is angled in an opposite direction, such that at an axially outermost edge of each cage protrusion, the width of the slot appears to become narrower. Furthermore, when the second clutch is in the disengaged position, the one or more protrusions are predominantly arranged axially outside of the one or more slots, such that an axial displacement of the actuation disc towards the rotatable cage causes an angular displacement of the rotatable cage. In other words, a pushing force is exerted on the actuation disc in order to engage the second clutch. Suitably, the actuation disc is mounted to the same pulley member as the inner and outer cages, in a manner which allows an axial displacement of the actuation disc, but which does not permit angular displacement of the actuation disc relative to the pulley member. The sprags in the sprag clutch are spring-loaded, whereby the springs keep the sprags in their disengaged position. In order to overcome the resistance of the spring-loaded sprags and the resistance of the resiliently mounted rotatable cage, a certain tangential force is required. The tangential force is produced as a force component of the axial force exerted on the angled camming surface when the actuation disc is axially displaced. The magnitude of the axial force and the tangential force is dependent on an angle a of the angled camming surface relative to the axial force direction. In a further development of the invention, the angle a of the angled camming surface is non-constant and varies with respect to the axial force direction. The advantage of a variable angle of the angled camming surface is that the resulting axial and tangential forces can be tuned to the actuation forces that are required to change the orientation of the sprags.

The actuation disc is axially displaced by an actuating element. Preferably, the actuating element is mounted to a stationary component, to simply the construction of the inventive pulley unit. In a preferred embodiment, the actuation disc is made of or comprises a ferromagnetic material, and the axial force is provided by magnetic attraction. Suitably, the actuating element is an electromagnet, meaning that the second clutch can easily be activated and de-activated by energizing and de- energizing the electromagnet. Other actuation means are possible, however. For example, the actuation disc may be moved by one or more linear actuators that are mounted to the same pulley member as the actuation disc. Alternatively, the actuation disc may be rotatably mounted to a fixed part via a rolling element bearing, whereby e.g. the bearing outer ring is mounted to the fixed part and the actuation disc is mounted to the bearing inner ring. An axial force exerted on the bearing outer ring will then be transmitted to the inner ring and to the actuation disc via the rolling elements. Thus, a pulley unit according to the invention may be executed in a variety of manners and may be adapted to the requirements of the specific application.

These and other advantages of the present invention will become apparent from the following detailed description and accompanying drawings.

DESCRIPTION OF THE FIGURES

In the following, the invention is described with reference to the accompanying drawings, in which: Fig. 1 a shows a cut perspective view of an example of a pulley unit according to the invention;

Fig. 1 b, 1 c show details of a second clutch of the pulley unit;

Fig. 2 shows a perspective view of part of the second clutch and actuation means;

Fig. 3 is a perspective view of the pulley unit, from the actuation side, with the actuation means removed;

Fig. 4 is a detail of part of a cage of the second clutch;

Fig. 5 is a detail of part of a cage of a further embodiment of a second clutch.

DETAILED DESCRIPTION

The invention will be described with reference to an application in a vehicle engine whereby the pulley unit is connected between an alternator shaft and a crankshaft belt. The pulley unit enables bidirectional torque-transfer between the alternator shaft and the belt, meaning that the alternator can function not only as a generator of electrical power, but can also serve as a starter motor and can boost power to the engine. In generating mode, torque is transferred from the belt to the alternator shaft, which will be referred to as a first direction of torque transfer. In start/boost mode, torque is transferred from the alternator shaft to the belt, which will be referred to as a second direction of torque transfer.

The unit is not restricted to this application, however, and may be used to couple any device with a flexible drive means, whereby bidirectional torque transfer is desirable.

The pulley unit comprises an inner member 10 and an outer member 20. The inner member has a bore for receiving a shaft (not shown) of the alternator device 70. The alternator shaft is coupled to the inner member 10 by means of e.g. a screw connection. The outer member 20 has a grooved section 22 for receiving the crankshaft belt.

For each combustion stroke of the engine, the crankshaft accelerates then decelerates until the next combustion stroke. These engine velocity fluctuations are transferred to the crankshaft belt, and the inertia of the driven components, particularly the alternator, generates dynamic tensions as the belt continuously tries to accelerate and decelerate these components. Belt slippage and unwanted noise are examples of the problems associated with fluctuating belt tension. One way of absorbing the fluctuations is to allow the alternator to overrun the belt, by means of an over-running clutch arranged between the inner and outer members. In the depicted example, the over-running clutch 30 is a one-way clutch, such as a sprag clutch, which prevents a relative rotation between the inner member 10 and the outer member 20 when relatively rotating in a first direction of rotation. In an opposite direction, the one-way clutch 30 allows a free rotation between the inner member 10 and the outer member 20. When the outer member is being driven, torque is transferred to the inner member 10 via the one-way clutch 30. When, however, the belt imparts a negative torque to the outer member 20, the one-way clutch 30 disengages and the inner member 10 is able to rotate relative to the outer member 20. The relative rotation is further possible in that the outer member is mounted to the inner member via a bearing arrangement. In the depicted example, the bearing arrangement comprises first 51 and second 52 axially spaced ball bearings. For transferring torque in the second direction, such that the alternator 70 may function as a starter motor, the pulley unit further comprises a second clutch 40. The second clutch is a sprag clutch, comprising a plurality of sprags 41 retained in pockets of an inner cage 42 and an outer cage 43. In a first orientation of the sprags, as depicted in Fig. 1 b, the sprags are not in engagement with a radially inner surface 21 of the outer member 20 and a radially outer surface 1 1 of the inner member 10. The sprags are in the first (disengaged) orientation when the outer member 20 is being driven in the direction shown by the arrow 25 (and torque is transferred in the first direction via the first clutch 30). In a second orientation of the sprags, as depicted in Fig. 1 c, the sprags engage with the radially inner surface 21 of the outer member and the radially outer surface 1 1 of the inner member, thereby enabling torque transfer in the second direction when the inner member 10 is being driven in the direction indicated by the arrow 15. The sprags 41 are moved from the first orientation to the second orientation by rotating one of the inner and outer cages 42, 43 relative to the other. In the depicted example, the inner cage 42 is the rotatable cage and is moved by an actuation disc 60. Figure 2 shows a perspective view of the inner cage and actuation disc, when these components are in a default position, before the inner cage has been moved. Furthermore, the sprags 41 are in the first orientation in the default position.

The inner cage 42 comprises axial extensions 44 with slots 45 in between. The axial extensions 44 are provided with an angled surface 47. The actuation disc 60 has protrusions 62, which are less wide than the slots 45 and which are received in the slots. The protrusions 62 of the actuation disc also have an angled surface 67, which is in contact with the angled surface 47 of the cage extensions 44. The angled surface 47 of the cage extensions is undercut relative to the contacting surface 67 of the actuation disc 60. Thus, an axial retraction of the actuation disc 60 (away from the cage) will cause an angular displacement of the inner cage 42. The axial retraction may be performed by mechanical, electromechanical or magnetic means. In a preferred embodiment, the actuation disc 60 is made of a steel material which can be magnetically attracted. Suitably, an electromagnet 80 is provided, which may be mounted on a housing of the alternator 70. In start mode, the electromagnet is energized and the actuation disc 60 is axially displaced towards the electromagnet 80, which rotates the inner cage 42 and changes the orientation of the sprags 41 to the engaged position.

The outer cage 43 in this example is fixedly mounted to the pulley outer member 20; i.e. the outer cage does not move relative to the outer member. The outer cage may, for example, comprise one or more radial extensions 48 which fit exactly into first recesses in the outer member. The fixed outer cage serves as a fulcrum around which the sprags are pivoted. The inner cage 43 is mounted to the outer member 20 in a manner that does permit a limited about of relative rotation. In the depicted example, the inner cage 42 comprises three evenly spaced radial extensions 49, which are received in second recesses 27 in the outer member 20 (Refer Fig. 3). The second recesses 27 may be the same recesses that receive the radial extensions 48 of the outer cage. In relation to the radial extensions 49 of the inner cage, the second recesses 27 are slightly wider, thereby creating a gap which permits angular displacement of the inner cage 42 relative to the outer member 20 and the outer cage 43. Preferably, a spring element 50 is provided in each gap, such that rotation of the inner cage causes a deformation of the spring. Thus, when the electromagnet 80 is de-energized, the spring force causes the inner cage 42 to return to the default position. Due to the geometry of the slots 45 and disc protrusions 62, the actuation disc is also returned to the default postion as shown in Fig. 2. The actuation disc 60 is therefore mounted to the pulley outer member in a manner which allows axial displacement of the actuation disc 60 relative to the outer member 20, but does not permit relative angular displacement. Suitably, the actuation disc 60 comprises axial extensions 65 that are slidingly received in third recesses 29 in the outer member 20.

In effect, the actuation disc 60 is a cam and the inner cage 42 of the second clutch 40 is a cam follower, whereby the angle of the angled surface 47 on the cage extensions 44 defines the camming angle, also known as the pressure angle. The sprags in the sprag clutch 40 are spring-loaded, whereby the springs keep the sprags in their disengaged position. In order to overcome the resistance of the spring-loaded sprags and the resistance of the resiliently mounted rotatable cage, a certain tangential force is required. The tangential force is produced as a force component of the axial force exerted on the angled camming surface 47 when the actuation disc is axially displaced. The magnitude of the axial force and the tangential force is dependent on an angle a of the angled camming surface relative to the axial force direction. Therefore, by varying the angle of the angled camming surface, the peak forces needed to change the orientation of the sprags can be modulated, depending on the force that is actually needed during a particular stage of the actuation cycle.

A detail of the angled camming surface 47 is shown in Fig. 4, in which said surface is provided a non-constant pressure angle. For the sake of clarity, the angles have been exaggerated somewhat. The actuation disc (not shown) is retracted in an axial direction, which exerts a force F_a on the camming surface. The camming surface has a first portion 47A which presents a first angle αι relative to the axial force direction. The camming surface further has a second portion 47B, which presents a second angle a₂ relative to the axial force direction. The second angle a₂ is steeper than the first angle αι.

During an early stage of the actuation cycle, when the actuation disc starts to be retracted, a high tangential force is beneficial in order to overcome the resistance of the spring-loaded sprags 41 and the resiliently mounted cage. Thus, to produce a relatively high tangential force F_t, the angle αι of the first portion 147A of the camming surface is relatively shallow. Once the initial resistance has been overcome, a lower tangential force is needed to keep tilting the sprags. Furthermore, as the sprags start to come into contact with the pulley inner member, they will tend to self-engage, meaning than an even lower tangential force is required to act on the rotatable cage. Thus, in a later stage of the actuation cycle, when the camming surface of the actuation disc is in contact with the second portion 47B of the cage camming surface, the steeper second angle a₂ produces a relatively smaller tangential force F_t. Additionally, the axial force F_a that is needed to retract the actuation disc is smaller, meaning that the electromagnet or other actuating element has a lower current requirement. Suitably, the angle of the camming surface is varied in accordance with the required actuation forces. In the embodiment described with reference to Figs 1 - 4, the rotatable cage of the second clutch is moved by means of a pulling force, away from the cage. Also, each of the inner cage, outer cage and actuation disc are mounted to the poulley outer member. In an alternative embodiment, the inner cage, the outer cage and the actuation disc are mounted to the inner member and the outer cage is the rotatable cage which is angularly displaced by means of an axial force on the actuation disc in a direction towards the cage. A detail of the outer cage 543 is shown in Fig. 5. Again the cage has an extension 544 with an angled camming surface 547, which presents first and second angles to an actuation disc (not shown). A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.

Claims

1 . A pulley unit comprising an inner member (10) rotatably mounted to an outer member (20), further comprising a first clutch (30) and a second clutch (40), the pulley unit being configured such that:

when the outer member (20) is being driven, the first clutch (30) enables torque to be transferred from the outer member to the inner member; and

when the inner member is being driven, the second clutch (40) enables torque to be transferred from the inner member (10) to the outer member, (20)

characterized in that,

the second clutch (40) is a sprag clutch comprising an inner cage (42) and an outer cage (43), each cage having a number of pockets for retaining a corresponding number of sprags (41 ), and in that

one of the inner and outer cages (42, 43) is partly rotatable relative to the other of the inner and outer cages, the relative rotation causing the sprags (41 ) to tilt from a disengaged orientation to an engaged orientation in which torque is transferable,

the pulley unit further comprising actuation means (60, 80) for angularly displacing the rotatable cage and changing the orientation of the sprags (41 ).

2. The pulley unit according to claim 1 , wherein an axial side of the rotatable cage (42) comprises at least one slot (45) that is provided with an angled camming surface (47), the unit further comprising an actuation disc (60) with at least one protrustion (62) having a camming surface (67) that is in contact with the angled camming surface (47) of the slot (45), whereby an axial displacement of the actuation disc (60) causes angular displacement of the rotatable cage.

3. The pulley unit according to claim 2, wherein the angled camming surface of the at least one slot (45) comprises an undercut and the at least one protrusion (62) is arranged within the at least one slot such that an axial displacement of the actuation disc away from the rotatable cage produces an angular displacement that causes the sprags (41 ) to tilt from the disengaged orientation to the engaged orientation.

4. The pulley unit according to claim 2, wherein the angled camming surface (47) of the at least one slot (45) and the camming surface (67) of the at least one protrusion are arranged such that an axial displacement of the actuation disc towards the rotatable cage (42) cage produces an angular displacement that causes the sprags (41 ) to tilt from the disengaged orientation to the engaged orientation.

5. The pulley unit according to claim 2 or 3, wherein the angled camming surface (47) of the at least one slot (45) has a varying angle (a), such that an axial force and a corresponding tangential force on the rotatable cage (42) varies as the actuation disc (60) is axially displaced.

6. The pulley unit according to any of claims 2 to 5, wherein the inner cage (42), the outer cage (43) and the actuation disc (60) are mounted to the same pulley member, being one of the inner member (10) and the outer member (20).

7. The pulley unit according to claim 6, wherein the partly rotatable one of the inner and outer cages is resiliently mounted to the pulley member (10, 20) via one or more springs (50).

8. The pulley unit according to claim 6 or 7, wherein the actuation disc (60) is mounted to the pulley member (10, 20) in a rotationally fixed manner, which allows axial displacement of the actuation disc (60) relative to the pulley member.

9. The pulley unit according to any preceding claim, wherein the actuation means comprises an actuating element (80) that is mounted to a non-rotating component.

10. The pulley unit according to claim 9, wherein the actuation disc is further mounted to the non rotating component via a rolling element bearing, and the actuating element exerts a linear force on a non-rotatable ring of the rolling element bearing

1 1 . The pulley unit according to any of claims 2 to 9, wherein the actuation disc (60) comprises a ferromagnetic material and the actuating element an electromagnet (80).

12. The pulley unit according to any preceding claim, wherein the first clutch (30) is a one-way clutch.

13. Assembly comprising an alternator (70) and a pulley unit according to any preceding claim, wherein the pulley inner member (10) is mounted to a shaft of the alternator.