Electro-magnetic actuator for torque coupling with variable pressure-relief valve

1. A torque-coupling assembly comprising:

a rotatable torque-coupling case;

at least one output shaft drivingly operatively connected to said torque-coupling case;

a friction clutch assembly for selectively engaging and disengaging said torque-coupling case and said at least one output shaft;

a hydraulic pump for generating a hydraulic fluid pressure to frictionally load said clutch assembly;

a variable pressure-relief valve assembly to selectively control said friction clutch assembly, said variable pressure-relief valve assembly including a valve closure member, a valve seat complementary to said valve closure member, and an electro-magnetic actuator for engaging said valve closure member and generating a variable axial electro-magnetic force urging said valve closure member against said valve seat so as to selectively vary a release pressure of said pressure-relief valve assembly based on a magnitude of an electric current supplied to said etectro-magnetic actuator;

said electro-magnetic actuator including an eleetro-magnetic coil assembly and an armature disposed radially inwardly of said electro-magnetic coil assembly and axially movable relative thereto;

a plenum passage provided adjacent to a radially inner peripheral surface of said armature for providing fluid communication through said electro-magnetic actuator; and

an annular bushing disposed within an annular bore of said armature and rotatably supporting said electro-magnetic actuator to said torque-coupling case so that said plenum passage is formed between said armature and said bushing.

2. The torque-coupling assembly as defined in claim 1 , further comprising a non-rotatable hydraulic fluid plenum plate mounted to said torque-coupling case so as to form an annular hydraulic plenum chamber defined between said plenum plate and the torque-coupling case, said plenum chamber housing said electro-magnetic actuator.

3. The torque-coupling assembly as defined in claim 2 , wherein said electro-magnetic actuator is non-rotatably secured to said plenum plate.

4. The torque-coupling assembly as defined in claim 3 , further comprising an annular bushing rotatably supporting said plenum plate to said torque-coupling case, said bushing is disposed within an annular bore of said armature and said plenum passage is defined by a set of circumferentially distributed axial grooves formed in said bushing.

5. The torque-coupling assembly of claim 1 , wherein said bushing is made of a plastic material.

6. The torque-coupling assembly as defined in claim 1 , further including a piston assembly disposed within said torque-coupling case between said pump and said clutch assembly and defining a pressure chamber, wherein said variable pressure relief valve assembly selectively controls a maximum hydraulic pressure attainable within said pressure chamber.

7. The torque-coupling assembly as defined in claim 6 , wherein said variable pressure relief valve assembly selectively controls said maximum pressure attainable within said pressure chamber between a maximum release pressure and a minimum release pressure.

8. The torque-coupling assembly as defined in claim 7 , wherein said minimum release pressure is at a level that prevents actuation of said friction clutch assembly.

9. The torque-coupling assembly as defined in claim 7 , wherein said maximum release pressure is at a level that enables complete actuation of said friction clutch assembly.