A LIMITED SLIP COUPLING WITH AN ACTUATOR

Technical Field

The present invention relates to a limited slip coupling for distributing drive torque between axles or wheels in a road vehicle, the coupling engagement being controlled by hydraulic pressure acting on at least one coupling-engaging piston in a cylinder. The invention also relates to an actuator for use in such a limited slip coupling.

Background of the Invention

A known four-wheel drive system for a road vehicle, preferably a car, includes a multi-plate limited slip coupling for distributing drive torque to the front and rear axles of the vehicle. The coupling is controlled by a hydraulic system, which in a simple embodiment can be limited, apart from connecting hydraulic conduits, to an oil reservoir, a pump, an oil accumulator and a control valve for admitting oil under pressure to pistons for actuating the coupling. Even if this hydraulic system is very simple, there is a desire to simplify it even more.

The main object of the invention is accordingly to obtain a simple and cheap but yet safe and effective way of controlling the engagement of a limited slip coupling for a road vehicle, especially a car. The Invention

This is according to the invention attained by an actuator having an armature magnetically movable by a solenoid and being connected to means for generating the hydraulic pressure in response to the movement of the armature.

In several practical embodiments, a plunger connected to the armature is movable in a bore in communication with the cylinder of the coupling. Hydraulic oil in the bore and the cylinder of the coupling is thus pressurized by the plunger.

The armature may be axially movable within the solenoid, which normally is tubular.

In one embodiment, the armature may be ring-shaped and arranged around one of the shafts connectable by the coupling.

In a special embodiment, providing for an especially compact design, the axially movable armature is provided with a longitudinal blind hole for hydraulic oil, into which hole a fixed plunger with a longitudinal bore sealingly extends.

The invention also relates to an actuator for use in a limited slip coupling according to the invention.

Brief Description of the Drawings

The invention will be described in further detail below under reference to the accompanying drawings, in which

Fig 1 is a schematical view of a hydraulic circuit for a four-wheel drive system according to the prior art,

Figs 2 and 3 are schematical side views - mainly in section - of a first embodiment of an actuator according to the invention in a coupling arrangement for a four-wheel drive system and show two different operating conditions,

Figs 4 and 5 are a side-view - mainly in section - and a view along the line V- V in Fig 4, respectively, of a second embodiment of an actuator according to the invention,

Fig 6 is a section through a third embodiment of an actuator according to the invention,

Fig 7 is a section through a fourth embodiment of an actuator according to the invention, and

Fig 8 is a section through a fifth embodiment of an actuator according to the invention.

Detailed Description of Embodiments A prior art all- wheel drive system for a road vehicle is shown in Fig 1. In such a system a limited slip coupling A is incorporated in a drive train between different driven axles in the vehicle, preferably a front axle and a rear axle, both with two wheels. The coupling A is controlled or actuated by pistons in hydraulic cylinders B.

The coupling A is very schematically depicted and is known per se. It is preferably a multi-plate coupling.

The hydraulic system to which the cylinders B belong may have the following general layout. Oil is transferred from an oil reservoir C to a spring-biased accumulator D by means of a pump E driven by an electric motor F controlled from an ECU G. The pressurized oil from the accumulator D can reach the coupling cylinders B via a control valve H, which sets the pressure of the oil under the control from the ECU G. After operation, oil can be transferred from the cylinders B back to the oil reservoir C.

The function of the limited slip coupling A is not described in detail here; for a further description reference may for example be made to WO-A-97/04245.

Although the all-wheel drive system shown in Fig 1 may seem simple, it still contains a number of mechanical and hydraulic components, such as the oil reservoir C,

the motor-driven pump E, the accumulator D, and the control valve H as well as connecting hydraulic conduits.

A simpler solution to the problem of controlling or actuating a limited slip coupling in an all- wheel drive vehicle is shown in Figs 2 and 3, of which only Fig 2 is provided with reference numerals for the sake of clarity.

A multi-plate limited slip coupling 1 in a housing 2 is connected to an input shaft 3 and an output shaft 4. The coupling is normally unengaged but may be brought into engagement by pistons 5 in hydraulic cylinders 6.

In order to control the supply of oil under pressure to the cylinders 6, a solenoid-operated actuator 7 is arranged in or attached to the housing 2. The actuator 7 comprises a cylindrical solenoid 8 and an armature 9 axially movable therein. The armature is provided with an actuator plunger 10 axially movable in a sealed fashion in a bore 11, which is provided with a return spring 12 for the actuator plunger 10.

The bore 11 is in communication with the cylinders 6, and these compartments are filled with hydraulic oil. Oil can if needed be supplied from a small reservoir 13 therefore in the housing 2 through a conduit 14 opening into the bore 11 just in front of the actuator plunger 10 in its rest position shown in Fig 2.

If the solenoid 8 is energized (under the control of an ECU not shown in Figs 2 and 3), the plunger 10 connected to the armature 9 will be moved to the right in the bore 11 , so that oil will be pressed into the cylinders 6 applying the pistons 5 against the coupling 1 for engagement thereof. This situation is depicted in Fig 3.

In order to control the pressure, a pressure sensor or a position sensor is probably to be used in a closed loop. An open control may possibly be used, if the position precision in the solenoid operated actuator 7 is high or if the force characteristic of the solenoid has been especially adapted herefore.

It may be shown that it is necessary that the derivative of the solenoid force is strictly less than the derivative of the added force from the coupling plates and the return spring 12 in the vicinity of the equilibrium position.

A precondition for the use of a simple actuation according to the invention may be that the coupling design is very stiff or in other words that the elasticity is very low.

The use of the design according to the invention provides for an automatic slack adjusting for the wear in the coupling.

A second embodiment of an actuator according to the invention is shown in Figs 4 and 5.

A multi-plate coupling 20 is connected to an input shaft 21 and an output shaft 22 (here in fact represented by a sleeve attached thereto). An actuator for the coupling 20 is arranged in a housing 23, in which the input shaft 21 is journaled by means of a bearing 24. A ring-shaped piston 25 with a coupling pad 26 is arranged in a cylinder 27 in a cylinder body 28 in an end of the housing 23 facing the coupling 20.

A ring-shaped solenoid 29 is arranged in a solenoid body 30 attached to the cylinder body 28. Actuator plungers 31 (in the shown case two plungers) are axially movably arranged in corresponding bores in the solenoid body 30. Each plunger 31 is connected to an armature 32, which either may be ring-shaped for all plungers and is arranged around the front shaft 21 or have the form of a discrete armature for each plunger). When the solenoid 29 is energized, the armature 32 will be transferred to the right in Fig 4, bringing the plungers 31 with it, as is shown in Fig 4.

The bores in the solenoid body 30 for the plungers 31 are connected to the cylinder 27 by conduits 33. The bores, the conduit 33 and the cylinder 27 are filled with hydraulic oil. In order to provide for aeration of the system, it may be provided with one-way valves 34 and an aeration hole 35 in conjunction with the bore for the upper plunger 31.

Replacement oil to the system may be sucked in from a supply thereof in the housing 23 at the bottom of the cylinder 27 through an oil hole 36 provided with a one- way valve.

Fig 6 illustrates a portion of a third embodiment of an actuator according to the invention. A preferably cylindrical solenoid 40 houses an armature 41 with a plunger 42. These parts are arranged in a solenoid frame 43 of an "open frame"-type, i e it is open in the shown view but closed in a view rotated 90° from this view. An open solenoid frame 44 is also used in a fourth embodiment shown in Fig 7.

In this case two solenoids 45 are used, each arranged around a wall portion of the open frame. The advantage with this arrangement is that more thread may be used in the solenoids, for example if aluminum instead of copper is to be utilized. Further, a more efficient cooling may be obtained. Each of the two embodiments according to Figs 6 and 7 may be combined into a general design as shown in Figs 2 and 3.

A fifth and slightly different embodiment is shown in Fig 8. The arrangement with the solenoid(s) and the solenoid frame can be either as shown in Fig 6 or Fig 7. The reference numeral 43 for the solenoid frame is employed. No solenoid is shown. Also a combination with the arrangement shown in Figs 2 and 3 is possible.

In this case the axially movable armature 46 is provided with a longitudinal blind hole 47, into which a plunger 48 sealingly extends, said plunger being attached to the solenoid frame 43. The plunger 48 is provided with a longitudinal bore 49 to be connected to the coupling cylinder 6 (Fig 2). The blind hole 47 is oil-filled. At the movement of the armature 46 to the right in Fig 8 under the influence of the solenoid, oil will thus be pressed into the cylinder 6 (Fig 2) for engaging the coupling 1.

The main advantage with this design is that it can be more compact and space saving than the other designs.

In all embodiments the solenoid-operated actuator is shown to be arranged in close vicinity to the coupling itself, but it may alternatively be arranged at distance therefrom and be connected to the coupling cylinder by a conduit.

It should be observed that the actuator does not function as a pump, providing a continuous or intermittent flow, but rather as a pressure generator.

The actuator according to the invention may be used for limited slip couplings used in a variety of positions and for a variety of purposes in a road vehicle, and the invention is not limited in its use to any of these applications.