Electric power steering assembly

This invention relates to an improved structure for a rack drive assembly adapted for use in a vehicle electric power steering assembly. The electric power steering assembly includes a steering member (32), a ball nut assembly (70), a bearing assembly (80) and an electric motor (60). The steering member (32) is disposed in a rack housing (30) and includes a rack portion (34) operatively connected to a vehicle steering wheel (12) and a screw portion (40) having an external thread convolution (42). The ball nut assembly (70) is operatively connected to the screw portion (40) of the rack portion (34) of the steering member (32) for effecting axial movement of the steering member (32) upon rotation of the vehicle steering wheel (12). The ball nut assembly (70) includes a rotor shaft (76) and a ball nut (72) fixed to the shaft (76) for rotation therewith. The ball nut (72) includes an outer surface (96) and the rotor shaft (76) includes an inner surface (114). The bearing assembly (80) is carried by the rotor shaft (76) for rotatably supporting the ball nut assembly (70) relative to the steering member (32). The electric motor (60) is operatively coupled to the rotor shaft (76) via a pulley assembly (82). The power steering assembly further includes an insulator (74) disposed between at least a portion of the outer surface (96) of the ball nut (72) and the inner surface (114) of the rotor shaft (76). The insulator (74) isolates the ball nut (72) from at least one of the pulley assembly (82) and the bearing assembly (80) to thereby reduce the noise in the ball nut assembly (70).

BACKGROUND OF THE INVENTION

This invention relates in general to vehicle electric power steering assemblies and in particular to an improved rack drive assembly adapted for use in such a vehicle electric power steering assembly.

One known electric power steering assembly for turning steerable wheels of a vehicle includes a ball nut for transmitting force between an axially movable rack member and an electric motor. Upon actuation of the electric motor of the power steering assembly, the ball nut is driven to rotate relative to the rack member. The rotational force of the ball nut is transmitted to the rack member by balls to drive the rack member axially. The axial movement of the rack member by the balls effects turning movement of the steerable wheels.

The electric motor in the above-described electric power steering assembly can be coupled to drive the ball nut in various known methods. One such method includes a belt drive assembly wherein an output shaft of the electric motor is connected via a belt to a pulley assembly. The pulley assembly typically includes a pulley which is operatively fixed to the ball nut so that the ball nut rotates with the pulley when the electric motor is actuated. This type of electric power steering assembly has advantages over other known electric power steering assemblies. However, when this type of electric power steering assembly is actuated, undesirable noise can be produced from one or more of the associated components of the rack drive assembly. Thus, it would be desirable to provide an improved structure for a rack drive assembly of an electric power steering assembly which reduces noise in the assembly and yet is simple and inexpensive.

SUMMARY OF THE INVENTION

This invention relates to an improved structure for a rack drive assembly adapted for use in a vehicle electric power steering assembly. The electric power steering assembly includes a steering member, a ball nut assembly, a bearing assembly and an electric motor. The steering member is disposed in a rack housing and includes a rack portion operatively connected to a vehicle steering wheel and a screw portion having an external thread convolution. The ball nut assembly is operatively connected to the screw portion of the rack portion of the steering member for effecting axial movement of the steering member upon rotation of the vehicle steering wheel. The ball nut assembly includes a rotor shaft and a ball nut fixed to the shaft for rotation therewith. The ball nut includes an outer surface and the rotor shaft includes an inner surface. The bearing assembly is carried by the rotor shaft for rotatably supporting the ball nut assembly relative to the steering member. The electric motor is operatively coupled to the rotor shaft via a pulley assembly. The power steering assembly further includes an insulator disposed between at least a portion of the outer surface of the ball nut and the inner surface of the rotor shaft. The insulator isolates the ball nut from at least one of the pulley assembly and the bearing assembly to thereby reduce the noise in the ball nut assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now toFIGS. 1-3, there is illustrated a first embodiment of a vehicle electric power steering assembly, indicated generally at10, constructed in accordance with the present invention. The illustrated vehicle electric power steering assembly10is a vehicle electric belt driven rack drive steering assembly and is associated with the front driven wheels (not shown) of the vehicle. The general structure and operation of the electric power steering assembly10is conventional in the art. Thus, only those portions of the electric power steering assembly10which are necessary for a full understanding of this invention will be explained and illustrated in detail. Also, although this invention will be described and illustrated in connection with the particular electric power steering assembly10disclosed herein, it will be appreciated that this invention may be used in connection with other electric power steering assemblies.

The illustrated electric power steering assembly10includes a vehicle steering wheel12and a rotatable input shaft14which is operatively coupled in a manner not shown, to the steering wheel12for rotation therewith about a steering axis X1. A torque sensor16is located inside a pinion housing18and encircles the input shaft12. The torque sensor16includes coils (not shown) which respond to the rotation of the input shaft12and which generate over electrical lines20an electrical signal indicative of the direction and magnitude of the applied steering torque.

A torsion bar20is provided to connect the input shaft12to a pinion22located inside the pinion housing18. The torsion bar20twists in response to the steering torque applied to the steering wheel12. When the torsion bar20twists, relative rotation occurs between the input shaft12and the pinion22.

The pinion housing18is attached to a rack housing, indicted generally at30. A linearly movable steering member32extends axially through the rack housing30. The steering member32is linearly (or axially) movable along a rack axis X2. A rack portion34of the steering member32is provided with a series of rack teeth (not shown) which meshingly engage gear teeth (not shown) provided on the pinion22. The steering member32further includes a screw portion40having an external thread convolution42. The steering member32is connected with steerable wheels (not shown) of the vehicle through tie rods (not shown) located at the distal ends of the steering member32. Linear movement of the steering member32along the rack axis X2results in steering movement of the steerable wheels as is known manner.

The rack housing30has a generally cylindrical configuration and includes a first section50, a second section52and a third section54. The first section50is connected to the second section52by suitable means, such as for example by a plurality of bolts and nuts (not shown). Similarly, the second section54is connected to the third section54by suitable means, such as for example by a plurality of bolts and nuts (only the bolts shown inFIG. 1by reference numbers170). The first section50is provided with a radially enlarged end50A, and the third section54is provided with a radially enlarged end54A. The enlarged ends50A and54A of the respective sections50and54cooperate with the second section52to define an annular chamber56. Alternatively, the structure of the rack housing30can be other than illustrated if so desired. For example, the rack housing30can include less than three sections or more than three sections if so desired.

The steering assembly10further includes an electric motor60which, as will be discussed below in detail, is drivably connected to a ball nut assembly, indicated generally at70for effecting axial movement of the steering member32upon rotation of the steering wheel12. In the event of the inability of the electric motor60to effect axial movement of the steering member32, the mechanical connection between the gear teeth on the pinion24and the rack teeth on the rack portion34of the steering member32permits manual steering of the vehicle. The ball nut assembly70is located in the chamber56of the rack housing30and encircles the screw portion40of the steering member32. As best shown inFIGS. 2 and 3, the ball nut assembly70includes a ball nut72, an insulator74, and a rotor shaft76. As will be discussed below, the ball nut72is fixed to the rotor shaft76for rotation therewith.

The ball nut72has oppositely disposed first and second end portions90and92, respectively, and generally cylindrical inner and outer surfaces94and96, respectively, extending between the end portions. The first end portion90of the ball nut72projects axially toward the end50A of the first portion50of the rack housing30. The second end portion92of the ball nut72projects toward the end54A of the third portion54of the rack housing30. The second end portion92is provided with a screw thread convolution98formed on the cylindrical inner surface96thereof. The ball nut72further includes a raised shoulder or flange102provided on the cylindrical outer surface96thereof for a purpose to be discussed below.

The rotor shaft76has oppositely disposed first and second end portions110and112, respectively, and generally cylindrical inner and outer surfaces114and116, respectively, extending between the end portions. The first end portion110of the rotor shaft76projects axially toward the end50A of the first portion50of the rack housing30and is supported by a bearing assembly80. In the illustrated embodiment, the bearing assembly80is a ball bearing or a journal bearing. The first end portion110further includes a raised shoulder120provided on the inner cylindrical surface114thereof.

The second end portion112of the rotor shaft76projects toward the end54A of the third portion54of the rack housing30. The second end portion112is provided with a first an annular groove or recess122formed in the cylindrical inner surface114thereof, and with a second annular groove124formed in the cylindrical outer surface116thereof. The rotor shaft76further includes a raised shoulder or flange118provided on the cylindrical outer surface116thereof for a purpose to be discussed below.

The bearing assembly80is disposed on the cylindrical outer surface116of first end portion110of the rotor shaft76. To accomplish this in the illustrated embodiment, a spanner nut78screws onto threads130provided in the cylindrical outer surface116in the first end portion110of the rotor shaft76. As the spanner nut78is tightened against the adjacent surface of the bearing assembly80, the opposite surface of the bearing assembly80engages the associated surface of the shoulder126of the rotor shaft76and loads the bearing assembly80. As will be discussed below, the bearing assembly80is operative to rotatably support the ball nut72relative to the steering member32. Alternatively, other methods can be used to retain the bearing assembly80on the rotor shaft76and/or to rotatably support the ball nut72relative to the steering member32. In the illustrated embodiment, the bearing assembly80is a ball bearing. Alternatively, other types of bearing assemblies can be used if so desired.

The electric power steering assembly10further includes a pulley assembly82disposed on the cylindrical outer surface116of the second end portion112of the rotor shaft76. To accomplish this, one end of the pulley assembly82abuts the shoulder126of the rotor shaft76and a retaining ring140disposed in the groove124abuts the opposite second end of the pulley assembly82. The rotor shaft76is secured to the pulley assembly82by a suitable means for rotation therewith. In the illustrated embodiment, the rotor shaft76includes a female keyway142which receives a male key144of the pulley assembly82in order to key to the rotor shaft76to the pulley assembly82for rotation therewith. Alternatively, other methods can be used to secure the rotor shaft76to the pulley assembly82.

In the illustrated embodiment, the insulator74is disposed between selected adjacent surfaces of the ball nut72and the rotor shaft76. In particular, in the illustrated embodiment, the insulator74is disposed between the cylindrical outer surface96of the first end portion90of the ball nut72and the cylindrical inner surface114of the first end portion110of the rotor shaft76, and between the cylindrical outer surface96of the second end portion92of the ball nut72and the cylindrical inner surface114of the second end portion112of the rotor shaft76. Preferably, the insulator74is formed from a suitable damping material and is molded in place between the ball nut72and the rotor shaft76during a molding process so as to fixedly secure the ball nut72to the rotor shaft76for rotation therewith. A suitable material for the insulator74is a rubber material, such as for example, a rubber from the nitrile rubber family. Preferably, the rubber is dead compound rubber and has a low durometer hardness Shore A. Alternatively, the insulator74can be formed from other suitable type of materials if so desired.

In the illustrated embodiment, the insulator74operatively fixes or couples the ball nut72to the rotor shaft76for rotation therewith. Also, as shown inFIG. 3, a retaining ring150is disposed in the groove122of the rotor shaft76. The ring150is operative to capture the flange52of the ball nut72between itself and the shoulder120of the rotor shaft76to assist in connecting the ball nut72and the rotor shaft76together. Alternatively, the location and/or the composition of one or more of the ball nut72, the insulator74and/or the rotor shaft76can be other than illustrated if so desired. For example, as shown in the embodiment illustrated inFIG. 4, the insulator74could be provided only between the second end portions92and112of the ball nut72and the rotor shaft76, respectively, if so desired. If this is the case, then the rotor shaft76could also only include the second end portion92if so desired (not shown). In addition, the rotor shaft76and the ball nut72can be connected together by other suitable means if so desired.

The ball nut assembly70further includes a plurality of force-transmitting members160. The force transmitting members160comprise balls (shown in FIG.1), which are disposed between the internal screw thread convolution98of the ball nut72and the external thread convolution42on the screw portion40of the steering member32. The balls160are loaded into the ball nut assembly70in a known manner. The ball nut assembly70further includes a recirculation passage (not shown) for recirculating the balls160upon axial movement of the steering member32relative to the ball nut assembly70.

The electric motor60is supported relative to the rack housing30by suitable means. The electric motor60includes an output shaft162which is connected to a member164for rotation therewith. In the illustrated embodiment, the member164is a timing belt. The timing belt164is operatively connected to the pulley assembly82. Thus, when the electric motor60is actuated, the timing belt164is rotated via the output shaft162so as to rotate the pulley assembly82. The rotation of the pulley assembly82causes the ball nut assembly72to be rotated thereby effecting axial movement of the steering member32via the balls160.

The steering assembly10further includes an electronic control unit or controller180. The controller180is preferably secured to the rack housing30in a manner not shown. The controller180is electrically connected to the electric motor60by electrical lines182and is electrically connected by the electrical lines20to the torque sensor16. The controller180is operable to receive electrical signals from the torque sensor16and to control the electric motor60in accordance with the received electrical signals. When steering torque is applied to the vehicle steering wheel12, the input shaft14rotates about the axis X1. The direction and magnitude of the applied steering torque are sensed by the torque sensor16. The torque sensor16outputs an electrical signal to the controller180. The electric motor60is energized by a control signal transmitted to the electric motor60by the controller180, and the motor output shaft162of the electric motor60is caused to rotate about a motor axis X3.

The rotating motor shaft162rotates the pulley assembly82via the belt164which in turn transmit the drive force of the electric motor60to the ball nut72of the ball nut assembly70. The rotation of the ball nut72results in linear movement of the steering member32. The balls160transmit the rotation force of the ball nut72to the rack portion34of the steering member32. Because the ball nut72is fixed in position axially, the steering member32is driven to move axially in response, effecting steering movement of the steerable wheels of the vehicle. The electric motor60thus provides steering assist in response to the applied steering torque.

One advantage of the electric power steering assembly10of the present invention is that the insulator74is effective to dampen or reduce the noise in the ball nut assembly70by isolating the ball nut72from at least one of the pulley assembly82and the bearing assembly80. Preferably, as illustrated in the preferred first embodiment of the present invention, the insulator74is effective to isolate the ball nut72from both the pulley assembly82and the bearing assembly80.