Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-06-23
2002-08-06
Mai, Lanna (Department: 3619)
Motor vehicles
Steering gear
With electric power assist
Reexamination Certificate
active
06427799
ABSTRACT:
TECHNICAL FIELD
This invention relates to an electric power steering unit to be used for a vehicle and, more particularly, it relates to a field of technology effectively applicable to electric power steering gears to be used for rack and pinion type steering systems.
BACKGROUND ART
For assisting steering power of vehicles, a so-called power steering unit is equipped to many vehicles in recent years, and various power steering units such as hydraulic type, electric type, etc. are proposed. Of these power steering units, for the electric type power steering unit applied to rack and pinion type steering units (so-called electric power steering), the one called rack-assist type that obtains steering assisting power by a motor equipped close to the rack shaft is known.
FIG. 5
is an explanatory view showing one example of this kind of rack-assist type power steering unit. As shown in
FIG. 5
, the electric power steering unit has a motor
52
installed coaxially with a rack shaft
51
, and the steering assisting power generated by the motor
52
is transmitted to the rack shaft
51
via a ball screw mechanism
53
. And with this steering assisting power and manual steering power, guiding wheels are turned to alleviate steering loads of a driver.
The rack-shaft
51
is linked to the guiding wheels typically by way of tie rods or knuckle arms arranged at the respective opposite ends thereof and also linked to the steering column
54
that is coupled to the steering wheel by way of a rack and pinion gear so that it may be reciprocatively moved in the horizontal directions of
FIG. 5
as the steering operation by the driver. The motor
52
is arranged in a housing
71
containing coupling section
68
between the steering column
54
and the rack shaft
51
, and a cylindrical armature shaft
56
and a field device
57
coaxially inserted in a cylindrical yoke
55
. In the power steering unit of
FIG. 5
, the yoke
55
is formed integral in the housing
71
. In addition, the field device
57
comprises a magnet
59
installed to the inner circumferential portion of the yoke
55
and an armature core
60
installed to the outer circumferential portion of the armature shaft
56
. And electric power is supplied to this motor
52
via a power supply section
58
equipped with a brush
61
and a commutator
76
in slidably contact therewith.
On the other hand, the ball screw mechanism
53
is of a generally known construction in which a large number of balls
64
are intermediately inserted between a nut section
62
and a screw section
63
. In this case, the nut section
62
is held rotatably in the housing
72
by an angular bearing
65
in the condition housed in a holder
75
. The rotating force of the motor
52
is transmitted from the armature shaft
56
to the rack shaft
51
via the nut section
62
, balls
64
, and screw section
63
in the form of axial reciprocating motion, and assists the steering power.
Now, in the power steering unit of
FIG. 5
comprising in this way, both housings
71
and
72
are assembled by directly coupling the housings with screws. In a conventional power steering unit, right and left housings have a flange protrudably formed at each end portion, respectively, and by connecting these with bolts, the housings are coupled. However, since the power steering unit is generally installed near the road surface of the car body and must be housed in a limited space below the engine room, it is not desirable to have protrusions on the outer circumference of the device and the improvements have been waited.
Therefore, in the power steering unit of
FIG. 5
, a system for coupling the housings with screws as shown in
FIG. 6
is adopted, and here, the device configuration free of protrusions with the flange removed has been achieved.
FIG. 6
is an enlarged sectional view of a main portion showing the configuration of the coupling portion. In the figure, a female threaded portion
73
is formed on the left inner circumference of the housing
71
, while a male threaded portion
74
is formed on the right end outer circumference of the housing
72
. And by driving a male threaded portion
74
into this female threaded portion
73
, both housings
71
and
72
are coupled together.
However, in the power steering unit of
FIG. 5
, since both housings
71
and
72
are formed with same material by aluminum die casting, etc., there is a problem in that the contact surface wears and is easy to adhere by surface pressure and friction caused by tightening. Consequently, lubrication and surface treatment are required between both housings, also resulting in a problem of disadvantages from the viewpoint of man-hour or parts cost.
In addition, because both housings
71
and
72
are formed with nonferrous metals such as aluminum, etc. for weight reduction, there was a problem of decreased coupling strength or rigidity at the threaded portion. Consequently, as shown in
FIG. 6
, the wall thickness at the threaded portion must be increased to secure the strength of the coupling portion, also producing a problem of going against needs of reduced weight, reduced thickness, and reduced size.
It is an object of the present invention to prevent wear and adhesion and to reduce the wall thickness with the coupling strength of housings secured in an electric power steering unit of a configuration in which housings are coupled with screws.
The above-described and other objects and novel feature of the present invention will become apparent more fully from the description of the following specification in conjunction with the accompanying drawings.
DISCLOSURE OF INVENTION
The electric power steering unit according to the present invention comprise a rack- shaft to be coupled to the guiding wheels of a vehicle, a first housing for movably containing the rack-shaft and holding therein a coupling section for causing the teeth of the rack-shaft to engage those of a pinion arranged on the steering column linked to the steering handle of the vehicle, an electric motor equipped with a yoke integrally formed with the first housing and coaxially arranged around the rack-shaft to supply assisting power to the rack-shaft, a ball screw mechanism for connecting a nut section linked to the armature shaft of the electric motor with a screw section formed on the rack-shaft by disposing ball members therebetween and for transmitting the rotary power of the electric motor to the rack-shaft as assisting power for steering, and a second housing for containing the ball screw mechanism, characterized in that a first threaded portion is formed at the end portion of the yoke on the second housing side, a second threaded portion is formed to fit to the first threaded portion on the end portion of the second housing on the first housing side; and the first housing and the second housing are coupled together by fitting the first threaded portion to the second threaded portion.
With this configuration, the first threaded portion and the second threaded portion fitted to it are able to be formed with different materials, and it becomes possible to prevent wear or adhesion on the contact surface caused by fitting screws of the same material.
On the other hand, the first threaded portion may be formed into female screw and the second threaded portion into male screw, and in such case, it is possible to form the female screw on the steel-made yoke, increasing the coupling strength and rigidity at the threaded portion, and this no longer requires to form the coupling portion to have large wall thickness, and the housing is able to reduce wall thickness.
In addition, the first threaded portion may be made into a male screw and the second threaded portion into a female screw. Preferably, the first threaded portion may be made of steel, and the second threaded portion may be made of aluminum.
Furthermore, caulking grooves may be formed on the outer circumference of the second housing, and the end portion of the first housing on the second housing is caulked to these caulking groove, thereby forming caulking portions to fi
Mai Lanna
McCormick Paulding & Huber LLP
Mitsuba Corporation
Royal, Jr. Paul
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