Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
1999-10-08
2001-01-16
Hurley, Kevin (Department: 3619)
Motor vehicles
Steering gear
With electric power assist
Reexamination Certificate
active
06173802
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric power steering device, and in particular to an electric power steering device having a hollow motor shaft, and a drive shaft coaxially received therein and adapted to be axially actuated via a ball nut arrangement interposed between them.
BACKGROUND OF THE INVENTION
According to a known electric power steering device for motor vehicles, the motor shaft consists of a hollow shaft, and a drive shaft connected to the right and left steerable wheels via tie rods is coaxially received in the motor shaft. A ball nut arrangement is interposed between the hollow motor shaft and the drive shaft for converting the rotational movement of the hollow motor shaft into the axial movement of the drive shaft.
In such an electric power steering device, the electric motor must have a required level of performance for the electric power steering device to operate in a satisfactory manner. However, once the power steering device is fully assembled, it is not practical to replace or repair the electric motor even if the electric motor is found to be faulty or otherwise incapable of producing a required level of performance. Therefore, it has been a common procedure in the manufacturing of electric power steering devices to test the operation of the electric motor as a part of the assembling process.
FIG. 6
illustrates a conventional electric power steering device which comprises a yoke
1
consisting of an axially elongated tube member serving also as a part of the housing for the power steering device, and a gear casing which accommodates a rack and pinion gear device and coaxially attached to the yoke
1
via a radial flange to form the rest of the housing for the power steering device. The power steering device normally extends laterally across the vehicle body which is not shown in the drawing, and is attached to the vehicle body via a bracket
2
a
formed in the gear casing
2
and a mounting bracket
13
partly surrounding a small diameter portion of the yoke
1
. These brackets
2
a
and
13
are normally secured to the vehicle body by fasteners such as threaded bolts.
The yoke
1
and the gear casing
2
thus jointly form the housing for the power steering device, and coaxially receive therein a drive shaft
3
which is axially slidable so as to steer the right and left front wheels via the tie rods attached to either axial end thereof. The drive shaft
3
is connected to the rack of a rack and pinion gear device, and the pinion thereof is attached to a pinion shaft
4
which is connected to a steering shaft.
An intermediate part of the drive shaft
3
is coaxially received in a hollow motor shaft
5
. A motor armature core
6
a
and a commutator
6
b
are attached to the outer circumferential surface of the hollow motor shaft
5
so that the armature core
6
a
and the hollow motor shaft
5
rotate integrally together. A ball nut mechanism is interposed between the inner circumferential surface of the hollow motor shaft
5
and the outer circumferential surface of the drive shaft
3
to convert the rotational movement of the hollow motor shaft
5
to the axial movement of the drive shaft
3
. The rotational torque produced from the motor armature
6
is converted into the axial assist force for the drive shaft
3
so that the effort required for turning the steering wheel attached to the pinion shaft
4
may be reduced.
According to this electric power steering device, the outer diameter of the ball nut is greater than the outer diameter of the drive shaft
3
. Further, because of the need to restrict the axial movement of the drive shaft
3
within a prescribed range at an open end of the yoke
1
, and the need to seal the annular gap between the open end of the yoke
1
and the outer circumferential surface of the drive shaft
3
, the inner diameter of the open end of the yoke
1
was conventionally smaller than the outer diameter of the ball nut. Therefore, conventionally, there was some difficulty in directly measuring the output torque of the electric motor. To overcome this difficulty, conventionally, the thrust force of the drive shaft
3
was measured instead of directly measuring the output torque of the electric motor.
However, the thrust force of the drive shaft
3
does not necessarily represent the output torque of the electric motor because of the friction loss in the ball nut mechanism, manufacturing errors in the ball nut mechanism, and the machining errors in the drive shaft. Therefore, conventionally, to be able to accurately determine the torque output of an electric motor for an electric power steering device of this type, it was necessary either to measure the mechanical loss that may be present in the ball nut mechanism or to test the motor armature in a special jig incorporated with permanent magnets and a yoke. In either case, extra preparatory steps and extra testing steps must be added to the manufacturing process, and this leads to the reduction in the manufacturing efficiency. Also, the test precision may still not be very high because of a number of factors which arise from the fact that the motor output torque has to be measured indirectly.
BRIEF SUMMARY OF THE INVENTION
In view of such problems of the prior art, a primary object of the present invention is to provide an improved electric power steering device which allows a motor output torque or a rotational speed thereof to be measured directly.
A second object of the present invention is to provide an electric power steering device which is adapted for an efficient manufacturing process.
A third object of the present invention is to provide an electric power steering device which allows an electric motor thereof to be tested in a highly accurate manner.
According to the present invention, these and other objects can be accomplished by providing an electric power steering device, comprising: a yoke made of a tubular member, and carrying magnetic field generating means on an inner circumferential surface thereof; a hollow motor shaft coaxially and rotatably received in the yoke, and carrying a motor armature which cooperates with the magnetic field generating means to form an electric motor for the power steering device; a drive shaft coaxially and axially slidably received in the hollow motor shaft, and provided with a threaded section; and a threaded nut mechanism including a nut member which is coaxially and fixedly secured in the hollow motor shaft and threadably engages with the threaded section of the drive shaft so as to convert a rotational movement of the motor shaft into an axial movement of the drive shaft; wherein a section of the yoke extending from a part thereof surrounding an axial end of the motor shaft adjacent to the threaded nut mechanism to an adjacent axial end of the yoke defines an inner diameter which is greater than an outer diameter of the nut member.
Thus, the output torque of the electric motor can be measured during the course of assembling the power steering device by allowing a testing tool to be inserted from the open end of the yoke without any interference and to be applied to a suitable part of the nut member or the motor shaft. It means that the performance of the electric motor can be directly evaluated before the power steering device is fully assembled so that any faulty electric motor can be either rejected or repaired before any further assembling work is conducted. In either case, a substantial gain in the manufacturing cost can be achieved by minimizing the wasteful use of labor and material.
The magnetic field generating means comprises either electromagnetic coils or permanent magnets. Typically, to minimize the frictional resistance in the threaded nut mechanism, a plurality of steel balls are received in a spiral passage defined by a threaded section formed in an inner circumferential surface of the nut member, and the threaded section of the drive shaft.
According to a preferred embodiment of the present invention, the hollow motor shaft comprises a nut retaining portion having a slightly en
Aoki Susumu
Kodaira Tadao
Onodera Tsugio
Hurley Kevin
Marshall & Melhorn
Mitsuba Corporation
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