WO2013124009A1

WO2013124009A1 - Damping pulley for bi-directional torque transfer

Info

Publication number: WO2013124009A1
Application number: PCT/EP2012/069519
Authority: WO
Inventors: Alessandro GARRONE; Marien VAN DITTEN; Gilbert PETERS
Original assignee: Skf B.V.
Priority date: 2012-02-20
Filing date: 2012-10-03
Publication date: 2013-08-29

Abstract

The invention relates to a pulley unit that comprises an outer member (120) and an inner member (110) which are elastically coupled by means of a spring arrangement (150) and which are rotatable relative to each other via a bearing arrangement (135, 137). The spring arrangement comprises a first spring end (151a) that is rotationally fixed relative to the outer member and further comprises a second spring end (152b) that is rotationally fixed relative to the inner member. Torque is therefore transferable through the spring arrangement in a first direction, from the outer member to the inner member, and is transferable in a second direction, from the inner member to the outer member. According to the invention, the spring arrangement comprises a series connection of a first torsional spring (151) and a second torsional spring (152), whereby the first torsional spring has a first spring stiffness that is smaller than a second spring stiffness of the second torsional spring.

Description

Damping pulley 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. The pulley unit is further configured to provide torsional vibration damping, and is particularly suitable for use in vehicle engines.

BACKGROUND TO THE INVENTION

A unit of this kind is known from US 8021253, which describes a one way isolator for high torque devices, such as alternator-starters, driven by a flexible drive means. The isolator includes a hub (inner member) and a sheave (outer member), each of which includes at least one stop member. The hub and sheave are linked by an isolating spring and, via a bearing and bushing, can rotate with respect to each other to provide isolation through the spring from torque variations when torque is transferred from the flexible drive means to the device. When substantial amounts of torque are transferred from the device to the flexible drive means, the sheave rotates with respect to the hub to bring the stop members into contact, such that the isolator then acts like a solid pulley to facilitate the transfer of the torque from the device.

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 outer member and an inner member which are elastically coupled by means of a spring arrangement and which are rotatable relative to each other via a bearing arrangement. The spring arrangement comprises a first spring end that is rotationally fixed relative to the outer member and further comprises a second spring end that is rotationally fixed relative to the inner member. Torque is therefore transferable through the spring arrangement in a first direction, from the outer member to the inner member, and in a second direction, from the inner member to the outer member. According to the invention, the spring arrangement comprises a first torsional spring and a second torsional spring, whereby the first torsional spring has a first spring stiffness that is greater than a second spring stiffness of the second torsional spring.

In other words, the spring arrangement comprises at least one relatively stiff spring and at least one relatively unstiff spring. A typical application of the inventive pulley unit is to connect a device to an engine belt drive. The spring arrangement of the pulley unit is designed for the dual functionality of transferring torque in both the first and second directions, and damping torsional fluctuations. The damping functionality is particularly important in the first direction of torque transfer, when the pulley unit is driven by e.g. a crankshaft belt. In the second direction of torque transfer, the applied torque is generally much higher than in the first direction, making the torque transfer capacity of the spring arrangement more important. Thus, the advantage of a spring arrangement according to the invention is that the at least one relatively unstiff spring (first spring) can be optimally adapted for damping torsional fluctuations, while the at least one relatively stiff spring (second spring) can be optimally adapted for providing the magnitude of torque transfer that the application requires.

The first spring end of the spring arrangement (which is rotationally fixed relative to the outer member of the pulley unit) may form part of the first spring or the second spring. Preferably, the first spring end is part of the first spring, since the first spring can be dedicated to damping and the torsional fluctuations are most prevalent when the outer member is being belt-driven.

The first spring end may be executed in the form of a radial extension that fits directly into a recess in the outer member. In a preferred embodiment, the pulley unit comprises a fixed spring carrier, which is e.g. mounted to the bore of the outer member via a press fit. This removes the need for machining the outer member and facilitates the mounting of the spring arrangement. In a further development, a friction liner is provided on a radially inner surface of the fixed spring carrier, which liner is in frictional contact with the inner member. Thus, the unit can be additionally provided with friction damping functionality. In a preferred embodiment, the fixed spring carrier comprises an axial recess and the first spring end is executed as an axial extension. Preferably, the ends of each torsional spring in the spring arrangement are executed with an axial extension. It is thought that such a design provides for a more robust torque connection in both directions of torque transfer.

The spring arrangement may comprise two or more torsional springs connected in series. Moreover, the spring arrangement may comprise at least one spring that is nested within the first spring or the second spring. For cost reasons and simplicity of construction, a preferred embodiment of the invention comprises a spring arrangement with only a first torsional spring and a second torsional spring.

Further, the torsional springs used in the spring arrangement may have a circular, a square or rectangular cross-section and may have a linear spring stiffness or a non-linear spring stiffness. The spring arrangement may comprise at least one spring with coils having a constant diameter and/or at least one spring with coils having a varying diameter. Thus, the spring arrangement may be adapted depending on the torque response that is required in each direction of torque transfer. In a further development of the invention, the first and second springs are connected in series via a rotationally-free spring carrier. Suitably, one side of the spring carrier comprises a recess (preferably an axial recess) for receiving an end of the first spring, while the opposite side of the spring carrier comprises a recess (preferably an axial recess) for receiving an end of the second torsional spring. The aforementioned spring ends are connected to the spring carrier in a rotationally fixed manner. The rotationally-free spring carrier not only simplifies the connection of the two springs, but also facilitates the mounting of the spring arrangement between the inner and outer members of the pulley unit. Furthermore, a radially outer surface of the rotationally-free spring carrier may be provided with a friction liner in frictional contact with a radially inner surface of the outer member, and/or a radially inner surface of the rotationally-free spring carrier may be provided with a friction liner in frictional contact with a radially outer surface of the inner member. Again the purpose of the friction liner(s) is to provide frictional damping

In a preferred example, the rotationally-free spring carrier comprises a cylindrical portion around which the coils of the first torsional spring are arranged. The first spring is configured such that its coils are not contact with the cylindrical portion of the spring carrier, or with the outer member, when the applied torque in the first direction is at an expected normal level. This ensures that the first spring's damping performance remains optimal. Preferably, the first spring is further configured such that torque in the second direction causes compression of the spring (coil diameter increases), while torque in the first direction causes the spring to elongate (coil diameter decreases). The cylindrical portion of the spring carrier prevents the coils from contacting the inner member when the torque in the first direction exceeds the "normal" level. If contact were to happen, there is a risk that the first spring would grip the inner member and form a solid connection between the outer and inner members. The full torque would then be transferred through the first spring only, and not through the second, stiffer spring. Such a situation is prevented by the cylindrical portion of the rotationally free spring carrier. If the applied torque in the first direction causes the coils of the first spring to grip the rotationally-free spring carrier, deformation of the second spring is still possible, which protects the first spring from damage

Furthermore, the first spring is preferably configured to make contact with the outer member when the applied torque in the second direction has caused the first spring to be fully compressed. This helps ensure that further torque is then transferred through deformation of the second spring.

In a still further development, the second spring is configured to come into contact with the outer member when its coil(s) have been compressed. The second spring then acts as a solid member for transferring further torque. In applications where the torque in the second direction is too high to be transferred via the spring arrangement on its own, the pulley unit preferably further comprises mechanical stops. Specifically, the inner member has a least one mechanical stop, such as a radially outward protrusion, and the outer member has at least one mechanical stop, such as a radially inward protrusion. These stops engage with each other when the inner member has rotated relative to the outer member through a maximum angular displacement. To distribute the load, each of the inner and outer member preferably comprises two or three mechanical stops. The mechanical stops may be spaced at regular intervals or irregular intervals, depending on the required maximum angular displacement.

Suitably, the maximum angular displacement corresponds to the maximum relative rotation permitted by the spring arrangement before the constituent springs are fully compressed. Due to the presence of the second (stiff) spring, the spring arrangement is able to transfer a relatively large portion of the torque, before full compression is reached and further torque is transferred via the mechanical stops. Thus, the impact on the mechanical stops is much lower than if the spring arrangement comprised only one spring (with characteristics suitable for damping). The mechanical stops are designed to withstand the maximum impact. As a result of lessening the impact, the mechanical stops may be made smaller, thereby reducing the weight of the unit. Preferably, the portion of the torque taken up by the spring arrangement, at full compression, is at least 50% of the maximum applied torque.

Thus a pulley unit according to the invention is able to transfer relatively large torques in both directions of torque transfer, while retaining optimal damping of torsional fluctuations. 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 shows an exploded perspective view of an example of pulley unit according to the invention;

Fig. 2 shows a cut perspective view of the pulley unit of Fig. 1 in an initial position;

Fig. 3 shows a cut perspective view of the pulley unit of Fig. 1 in an operating position;

Fig. 4 shows a graph of relative angular displacement between inner and outer members of the pulley in response to an applied torque.

DETAILED DESCRIPTION

Components of an example of a pulley unit according to the invention are shown in the exploded view of Figure 1 . The assembled pulley unit is depicted in Figure 2. 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 designated as a first direction of torque transfer. In start/boost mode, torque is transferred from the alternator shaft to the belt, which will be designated 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 100 comprises an inner member 1 10 and outer member 120. The inner member has a cylindrical part 1 12 and a flange part 1 15. Further, the inner member has a bore for receiving the alternator shaft. The alternator shaft is coupled to the inner member by means of e.g. a screw connection. The outer member 120 has a grooved section 122 for receiving the crankshaft belt.

When the engine's cylinders fire, torque is imparted to the crankshaft. The crankshaft deflects under this torque, which sets up vibrations when the torque is released. The crankshaft rotation therefore fluctuates, causing momentary and recurring increases and decreases in rotational speed (torsional fluctuations). In turn, these fluctuations can cause slippage between the belt and pulley, leading to unwanted noise. Furthermore, torsional fluctuations imparted to the alternator shaft could adversely affect power generation capability.

One way of absorbing the fluctuations is to provide an elastic coupling between the inner and outer members of the pulley, to enable a limited amount of relative rotation therebetween. Accordingly, the inner member 1 10 is supported relative to the outer member 120 via a bearing arrangement, which in this example comprises a rolling element bearing 135 and a bushing 137. Furthermore, a spring arrangement 150 is disposed coaxially between the inner and outer members. In generating mode, torque is transferred from the outer member to the inner member via the spring arrangement, which also damps the torsional fluctuations from the belt.

In this first direction of torque transfer, the applied torque is much lower than in the second direction, when torque is transferred from the inner member 1 10 to the outer member 120. Typically, the maximum torque in the second direction is up to five times higher than in the first direction. In order to provide effective damping, a relatively low spring stiffness is required. For transferring large amounts of torque, however, a relatively high spring stiffness would be beneficial.

In a pulley unit according to the invention, optimal damping behaviour is combined with high torque-transfer capability. This is achieved in that the spring arrangement comprises a first torsional spring 151 with a relatively low spring stiffness and further comprises a second torsional spring 152 with a relatively high spring stiffness. Typically, the second spring stiffness (measured in Nm/rad) is at least 3 times greater than the first spring stiffness. Thus, the first spring may be optimised in terms of damping behaviour, while the second spring is optimised in terms of torque-transfer capacity.

The first and second springs are connected in series, meaning that the torsional response of the spring arrangement is initially governed by the effective stiffness of the spring arrangement. The effective stiffness is less than the stiffness of the first spring 151 on its own. As a result, damping behaviour is enhanced. When an applied torque is sufficiently large to cause the first spring to act as a solid body, the stiffness of the spring arrangement is then governed by the stiff second spring 152, allowing high torques to be transferred.

The first and second springs 151 , 152 in this example are coil springs with a circular cross-section. Other cross-sections are possible. Further, the springs may be arranged such that torque in the first direction causes the coils to become more tightly wound, such that spring length decreases and outside diameter increases. In the depicted example, the first spring and the second spring are configured to unwind in the first direction of torque transfer and to become more tightly wound in the second direction. Preferably, the first spring 151 and the second spring 152 are connected in series via an intermediate spring carrier 155, which is rotationally free relative to the inner member 1 10 and the outer member 120. In some embodiments, an outer cylindrical surface of the spring carrier 155 is provided with a friction liner which is contact with an opposing cylindrical surface of the outer member. Alternatively, an inner cylindrical surface of the intermediate spring carrier 155 may be provided with a friction liner that is in contact with an opposing cylindrical surface of the inner member. The provision of a friction liner is advantageous if frictional damping of the torsional fluctuations is additionally required. The first spring 151 has a first end 151 a that is rotationally fixed relative to the outer member 120. The first spring end 151 a in this example is formed by an axial extension that is received in an axially extending recess 158 in a second spring carrier 157. The second spring carrier 157 is rotationally fixed relative to the pulley outer member 120 by means of e.g. a press fit. In this example, a friction liner 160 is provided at an inner cylindrical surface of the second spring carrier 157, which is in contact with an opposing surface of the inner member. As mentioned, the friction liner provides frictional damping. The first end 151 a of the first spring 151 may alternatively comprise a radial extension that fits into a radially extending recess in the pulley outer member 120. The second spring carrier is thus an optional component, but has the advantage of facilitating the mounting of the spring arrangement as a whole and also enables the first end 151 a of the first spring to have an axial extension. The axial extension and correspondingly shaped recess are thought to provide a robust torque-transfer connection that will resist slipping in both directions of torque transfer.

Thus, a second end 151 b of the first spring is also executed with an axial extension, which is received in a corresponding recess in the intermediate spring carrier 155. The second end 151 b of the first spring is rotationally fixed relative to the intermediate spring carrier.

The intermediate spring carrier preferably further comprises an axial extension 156 around which the coils of the first spring 151 are arranged. When the outer member is being driven by the crankshaft belt, the stiffness and diameter of the first spring are selected such that the coils of the first spring are not in contact with the bore of the outer member 120 or with the axial extension 156, when the torque in the first direction is at normal operating levels. If exceeded, the diameter of the first spring 151 and the diameter of the axial extension 156 are tuned such that the coils come into contact with a radially outer surface of the axial extension 156. The first spring therefore grips the spring carrier 155, meaning that any further applied torque is transferred via deformation of the second spring 152. If the coils of the first spring were unwound to such an extent that they came into direct contact with the inner member, there is a risk that further torque would be transferred to the inner member through the first spring alone, leading to damage.

When torque is transferred in the second direction, the diameter and stiffness of the first spring 151 are suitably tuned such that when the spring coils are fully wound up (compressed), the coils make contact with the outer member of the pulley.

The second spring 152 is also a coil spring in this example, and has a first end 152a and a second end 152b. As with the first spring, the spring ends 152a and 152b are executed with axial extensions. The first end 152a is rotationally fixed relative to the intermediate spring carrier 155, which comprises a corresponding axial recess for receiving the first end 152a. The second end 152a is rotationally fixed relative to the inner member 1 10, whereby the flange part 1 15 comprises an axial recess for receiving the second end 152b.

The second spring 152, having a relatively high stiffness, allows high torque transfer in both the first direction and the second direction. In one embodiment, the pulley unit is configured to transfer the maximum torque in both directions via the spring arrangement alone. Suitably, the second spring is tuned such that when the applied torque has caused compression of the second spring, the coils come into contact with an overhanging surface on the inner member 1 10. Further applied torque is then transferred via the second spring, acting as a solid body. In applications where the full torque in the second direction is very high, the unit is preferably configured such that a first portion of the full torque is taken up by the spring arrangement, while the remaining torque is transferred via mechanical stops. A pulley unit of this kind is depicted in Figures 1 , 2 and 3. Specifically, the outer member 120 and the inner member 1 10 have at least one mechanical stop which engage with each other after a maximum angular displacement of the inner member relative to the outer member. Preferably, each member has more than one mechanical stop, to distribute the load. In the depicted example, the inner member 1 10 has three radial protrusions 1 19 (outward protrusions) which are arranged with an even spacing on a radially outer surface of the flange part 1 15 of the inner member 1 10. A radially inner surface of the outer member 120 is likewise provided with three evenly spaced radial protrusions 129 (outward protrusions). When the unit is in a first position, as depicted in Figure 2, the inner member is arranged relative to the outer member such that the outward protrusions 1 19 are essentially halfway between the inward protrusions 129. Consequently, a maximum relative angular displacement of around 50 degrees is possible between the inner and outer members (depending on the circumferential dimensions of the stops 1 19, 129). When an applied torque leads to this maximum relative angular displacement, the inward and outward protrusions come into contact with each other and take over the torque transfer.

Let us assume that in the second direction, the maximum applied torque is 70 nM and that in the first direction, the maximum applied torque is 20 nM. If the spring arrangement was capable of transferring only 20 nM, before full deformation, then the remaining 50 Nm would cause a heavy impact on the mechanical stops (protrusions) when these engage. The impact could be lessened by using a stiffer spring that is able to carry more torque, but this would reduce the spring's ability to absorb torsional fluctuations. In a pulley unit according to the invention, this problem is solved in that the first spring 151 is optimally adapted for damping, while the second, stiffer spring 152 has a high torque-transfer capability. Consequently, the spring arrangement is able to take up a large proportion of the maximum applied torque before the mechanical stops engage, thereby lessening the impact. Preferably, the spring arrangement is able to transfer at least half of the maximum torque.

As mentioned, the mechanical stops mean that there is a maximum angular displacement of the inner member relative to the outer member before the pulley unit acts as a solid pulley. The maximum relative angular displacement is indicated by reference numeral 9_max in Figure 3. The maximum value is applicable to both directions of torque transfer, but in practice, the torques in the first direction are not high enough to cause the maximum amount of relative rotation. The effect of the inventive spring arrangement, in conjunction with the mechanical stops, will therefore be explained in detail with respect to the second direction of torque flow. Reference will also be made to Figure 4, which shows a graph of angular displacement between the inner and outer members (relative to the first position as depicted in Figure 2) in response to the applied torque. When, for example, the alternator is used to start the engine, the applied torque causes the coils of the springs in the spring arrangement to compress (spring length shortens) or to decompress (spring length increases). As mentioned, this depends on how the springs are wound relative to the direction of torque transfer. In the depicted example, the springs of the spring arrangement are configured to compress under the influence of torque in the second direction. Initially, a first amount of relative angular displacement θι takes place. In Figure 3, the unit is shown in an operating position whereby this first amount of relative angular displacement θι has occurred. The magnitude of θι is governed by the effective stiffness of the first spring and the second spring. Further the magnitude of θι corresponds to a first amount of torque τ·ι . Suitably, the first spring 151 is configured to be fully compressed under the first amount of torque , Furthermore, the diameter of the first spring is preferably tuned such that when fully compressed, the coils come into contact with the outer member 120 of the pulley.

When the amount of applied torque exceeds τ-ι, the first spring 151 acts as a solid body and a further amount of angular displacement θ₂ is enabled via the second spring 152, until the maximum value 9_max is reached. Suitably, the maximum angular displacement is reached at a second torque τ₂, at which the second spring 152 is fully compressed. Assuming that a maximum torque T_max is applied, the remaining amount of torque (x_max - ₂) is then transferred via the mechanical stops. As a result of the inventive spring arrangement, the impact on the mechanical stops is lessened, thereby increasing the service life of the pulley unit. 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 outer member (120) and an inner member (1 10) which are elastically coupled by means of a spring arrangement (150) and which are rotatable relative to each other via a bearing arrangement (135,

137), the spring arrangement comprising a first spring end (151 a) that is rotationally fixed relative to the outer member and further comprising a second spring end (152b) that is rotationally fixed relative to the inner member, such that torque is transferable through the spring arrangement in a first direction from the outer member to the inner member and a second direction from the inner member to the outer member,

characterized in that,

the spring arrangement (150) comprises a first torsional spring (151 ) having a first spring stiffness and comprises a second torsional spring (152) having a second spring stiffness greater than the first spring stiffness.

2. The pulley unit according to claim 1 , further comprising a fixed spring carrier (157), mounted to the outer member (120) in a rotationally fixed manner, the fixed spring carrier comprising a recess for receiving the first spring end (151 a) in a rotationally fixed manner.

3. The pulley unit according to claim 2, wherein a radially inner surface of the fixed spring carrier is provided with a friction liner (160) in frictional contact with a radially outer surface of the inner member (1 10)

4. The pulley unit according to any preceding claim, further comprising a rotationally free spring carrier (155) for connecting the first and second springs, the rotationally free spring carrier comprising:

- a recess for receiving a spring end (151 b) of the first torsional spring (151 ) of the arrangement in a rotationally fixed manner, and

- a further recess for receiving a spring end (152a) of the second torsional spring (152) in a rotationally fixed manner.

5. The pulley unit according to claim 4, wherein a radially outer surface of the rotationally-free spring carrier is provided with a friction liner in frictional contact with radially inner surface of the outer member (120), and/or a radially inner surface of the rotationally-free spring carrier is provided with a friction liner in frictional contact with a radially outer surface of the inner member

(1 10).

6. The pulley unit according to any preceding claim, wherein the spring arrangement (150) comprises at least one spring end (151 a, 151 b, 152a, 152b) that extends in a direction essentially parallel to an axial centreline of the spring arrangement.

7. The pulley unit according to any preceding claim, wherein one of the first and second torsional springs has a linear spring stiffness or a non-linear spring stiffness.

8. The pulley unit according to any preceding claim, wherein one of the first and second torsional springs has a constant coil diameter or a varying coil diameter.

9. The pulley unit according to any preceding claim, wherein the first spring end (151 a) forms part of the first spring (151 ) and the second spring end forms part of the second spring. 10. The pulley unit according to claim 9, dependent on claim 4, wherein the first spring (151 ) has a second end (151 b) that is coupled to the rotationally-free spring carrier (155), and wherein the first spring is configured to grip a radially outer surface of the spring carrier when the torque in the first direction exceeds a first toque threshold, and is further configured to grip a radially inner surface of the outer member when the torque in the second direction exceeds a second torque threshold.

1 1 . The pulley unit according to any preceding claim, wherein the second spring is configured to grip a radially inner surface of the outer member when the torque in the second direction exceeds a further torque threshold.

The pulley unit according to any preceding claim, wherein the outer member comprises at least one mechanical stop (129) and the inner member comprises at least one mechanical stop (1 19), which stops engage with each other when the inner member has rotated relative to the outer member through a maximum angular displacement.

Pulley unit according to claim 12, wherein each of the inner member and the outer member comprises two or three evenly spaced mechanical stops.

The pulley unit according to claim 12 or 13, wherein the maximum angular displacement is reached at a spring torque limit (τ-ι) at which the springs of the spring arrangement have fully deformed, further torque being transferred via the mechanical stops.

15. The pulley unit according to claim 14, wherein the spring torque limit (τ-ι) is at least 50 % of a maximum applied torque.