Electric power steering device and method of assembling the same

An electric power steering device includes a steering torque detector for detecting a steering torque in a steering system by detecting relative rotation between an input shaft and an output shaft, an electric motor for generating an auxiliary torque corresponding to a detection signal from the steering torque detector, and a torque transmitting mechanism for transmitting the auxiliary torque from the electric motor to the output shaft. The steering torque detector is composed of a detector body directly attached to the input shaft, and an actuator engaged with the output shaft for activating the detector body. With the steering torque detector thus arranged, an adjustment of the neutral position of the steering torque detector can be performed after the input and output shafts, the steering torque detector, and the torque transmitting mechanism are assembled together to form a single semi-finished steering assembly, and before the semi-finished steering assembly is received in a housing. Since this adjustment is effected on the steering torque detector while the semi-finished steering assembly is standing alone before it is assembled in the housing, the neutral position of the steering torque detector can be obtained with excellent accuracy but through a simple adjustment work.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to improvements in an electric power steering 
device and a method of assembling the electric power steering device. 
2. Description of the Related Art 
In recent years, electric power steering devices have been used extensively 
as they can reduce the driver's muscular effort or force required to turn 
the steering handle and thus provide a comfortable steering touch to the 
driver. The electric power steering devices of the type concerned are 
constructed such that an auxiliary torque generated by an electric motor 
in proportion to the steering torque is transmitted via a mechanical 
clutch to the steering system. One example of such electric power steering 
devices is disclosed in Japanese Patent Laid-open Publication No. HEI 
1-172058, entitled "Electric Power Steering Device". 
The disclosed electric power steering device includes a steering torque 
detecting means for detecting a steering torque in a steering system, and 
an electric motor for generating an auxiliary torque according to a 
detection signal from the steering torque detecting means. The electric 
motor is connected to a sleeve, and the sleeve is connected by a clutch 
mechanism to a hexagonal portion of an output shaft. The output shaft is 
connected to steered wheels. Thus, the auxiliary torque from the electric 
motor is transmitted to the output shaft. The sleeve forms part of the 
clutch mechanism and is rotatably supported on an upper housing. The 
output shaft is rotatably supported on the upper housing and a lower 
housing. The output shaft also has a portion supported on the upper 
housing via the sleeve. The conventional electric power steering device 
having such support or bearing structure has a limited assembling 
accuracy. 
The steering torque detecting means, in one form, includes a ring fitted 
around an input shaft, and a strain gauge attached to a cantilevered 
resilient metal strip. The resilient metal strip has a free end held in 
engagement with the ring. The ring is movable or displaceable in the axial 
direction of the input shaft in response to a relative torsional 
displacement or rotation between the input shaft and the output shaft. 
With this displacement of the ring, the resilient metal strip is 
resiliently deformed or flexed whereupon the strain gauge produces an 
electric signal corresponding to the relative torsional displacement 
between the input shaft and the output shaft. 
In an alternative form, the steering torque detecting means includes two 
confronting annular flanges formed on the input shaft and the output 
shaft, respectively, and a brush slidably disposed between the annular 
flanges so as to form, jointly with the annular flanges, a slip ring. The 
thus formed slip ring generates an electric signal directly corresponding 
to a relative torsional displacement between the input shaft and the 
output shaft. 
In the first-mentioned form of the steering torque detecting means, the 
resilient metal strip carrying thereon the strain gauge is cantilevered 
within a case. Since the case is attached to the upper housing, mounting 
of the steering torque detecting means should be achieved in the course of 
an assembling process which is performed to assemble a body of the 
electric power steering device in a housing composed of the upper housing 
and the lower housing. Such a mounting procedure is quite laborious and 
makes it difficult to replace the steering torque detecting means. In 
addition, since an adjustment of the neutral position of the steering 
torque detecting means is performed under the condition that the steering 
torque detecting means is held in the housing, it is very difficult to 
carry out the neutral position adjustment work with sufficient efficiency 
and accuracy. 
The second-mentioned form of the steering torque detecting means also has a 
drawback that since two confronting annular flanges forming the slip ring 
are formed on the input shaft and the output shaft, respectively, 
replacement of the steering torque detecting means is practically 
impossible. The input shaft further has a large-diameter tubular portion 
forming one part of the clutch mechanism, and the output shaft further has 
a hexagonal portion forming another part of the clutch mechanism. With 
this arrangement, an adjustment of the neutral position of the steering 
torque detecting means and an adjustment of the neutral position of the 
clutch mechanism cannot be achieved separately. The neutral position 
adjustments are, therefore, rendered tedious and time-consuming and 
insufficient in accuracy. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide an electric 
power steering device which is capable of being assembled easily, 
efficiently and accurately. 
Another object of the present invention is to provide an electric power 
steering device including a steering torque detecting means or unit which 
can be easily assembled with a body of the electric power steering device 
and is able to secure an easy and accurate neutral position adjustment. 
A further object of the present invention is to provide an electric power 
steering device including a clutch mechanism which is easy to assemble and 
able to secure an easy and accurate neutral position adjustment. 
A still further object of the present invention is to provide a method of 
assembling an electric power steering device, which makes it possible to 
perform an adjustment of the neutral position of a clutch mechanism and an 
adjustment of the neutral position of a steering torque detecting means or 
unit with utmost ease and improved accuracy. 
Yet another object of the present invention is to provide a method which is 
capable of assembling an electric power steering device with improved 
efficiency. 
The present invention, in one aspect, seeks to provide an electric power 
steering device comprising: an input shaft connected to a steering wheel; 
an output shaft connected to steered wheels; a resilient member connecting 
together the input shaft and the output shaft while allowing the input and 
output shafts to rotate relatively to each other; a steering torque 
detecting means for detecting a steering torque in a steering system by 
detecting relative rotation between the input shaft and the output shaft; 
an electric motor for generating an auxiliary torque corresponding to a 
detection signal from the steering torque detecting means; a torque 
transmitting means for transmitting the auxiliary torque from the electric 
motor to the output shaft; and a housing containing therein at least part 
of the input shaft, at least part of the output shaft, at least part of 
the resilient member, and the torque transmitting means. The steering 
torque detecting means is composed of a detector body directly attached to 
one of the input shaft and the output shaft, and an actuator engaged with 
the other of the input shaft and the output shaft for activating the 
detector body. The detector body and the actuator are received in the 
housing together with the input shaft and the output shaft. 
Since the steering torque detecting means composed of the detector body and 
the actuator is attached to the input and output shafts, an adjustment of 
the neutral position of the steering torque detecting means can be 
performed after the input shaft, the output shaft, the steering torque 
detecting means, and the torque transmitting means are assembled together 
to form a single semi-finished steering assembly, and before the 
semi-finished steering assembly is received in the housing. More 
particularly, since the neutral position adjustment is effected in such a 
condition that the steering torque detecting means is already assembled 
with the semi-finished steering assembly but the semi-finished steering 
assembly is still standing alone before it is assembled into the housing, 
the neutral position of the steering torque detecting means can be 
obtained with excellent accuracy but through a simple adjustment work or 
operation. 
In addition, since the detector body and the actuator of the steering 
torque detecting means are received at one time in the housing 
automatically when the semi-finished steering assembly is set or installed 
in the housing, the efficiency of the assembling operation is extremely 
high. The steering torque detecting means of the foregoing construction is 
easy to replace. 
The present invention, in another aspect, seeks to provide an electric 
power steering device of the type wherein an auxiliary torque generated by 
an electric motor is transmitted via a torque transmitting means and a 
clutch mechanism to a pinion shaft of a rack-and-pinion mechanism in a 
steering system, characterized in that: the clutch mechanism is disposed 
between the torque transmitting means and the pinion shaft and received in 
a housing together with the torque transmitting means and the pinion 
shaft; the pinion shaft is rotatably mounted only on the housing; and the 
torque transmitting means includes a wheel for connecting together the 
torque transmitting means and the clutch mechanism, the wheel being 
rotatably mounted only on the pinion shaft. 
Since the clutch mechanism is disposed between the torque transmitting 
means and the pinion shaft, it is possible to perform an adjustment of the 
neutral position of the clutch mechanism after the pinion shaft, the 
torque transmitting means, and the clutch mechanism are assembled together 
to form a semi-finished steering assembly and before the semi-finished 
steering assembly is set or installed in the housing. More particularly, 
since the neutral position adjustment is effected in such a condition that 
the clutch mechanism is already assembled with the semi-finished steering 
assembly but the semi-finished steering assembly is still standing alone 
before it is assembled in the housing, the neutral position of the clutch 
mechanism can be obtained with high accuracy but through a simple 
adjustment work. 
In addition, since the clutch mechanism is automatically received when the 
semi-finished steering assembly is set in the housing, assembling 
efficiency of the electric power steering device increases greatly. 
Furthermore, since the pinion shaft is rotatably mounted only on the 
housing, and since the wheel of the torque transmitting means is rotatably 
mounted only on the pinion shaft, it is possible to fit the pinion shaft 
directly with the housing. By virtue of this supporting structure, the 
assembling accuracy of the electric power steering device is greatly 
improved. 
The present invention, in still another aspect, seeks to provide an 
electric power steering device comprising: an input shaft connected to a 
steering wheel; an output shaft connected to steered wheels; a resilient 
member connecting together the input shaft and the output shaft while 
allowing the input and output shafts to rotate relatively to each other; a 
steering torque detecting means for detecting a steering torque in a 
steering system by detecting relative rotation between the input shaft and 
the output shaft; an electric motor for generating an auxiliary torque 
corresponding to a detection signal from the steering torque detecting 
means; a torque transmitting means for transmitting the auxiliary torque 
from the electric motor to the output shaft via a clutch mechanism; and a 
housing containing therein at least part of the input shaft, at least part 
of the output shaft, at least part of the resilient member, and the torque 
transmitting means. The steering torque detecting means is composed of a 
detector body directly attached to one of the input shaft and the output 
shaft, and an actuator engaged with the other of the input shaft and the 
output shaft for activating the detector body. The clutch mechanism is 
disposed between the torque transmitting means and the output shaft. The 
torque transmitting means includes a wheel for connecting together the 
torque transmitting means and the clutch mechanism, the wheel being 
rotatably supported solely on the output shaft. And, the steering torque 
detecting means and the clutch mechanism are received in the housing 
together with the input shaft and the output shaft. 
Since the steering torque detecting means which is composed of the detector 
body and the actuator is attached to the input and output shafts, and 
since the clutch mechanism is disposed between the torque transmitting 
means and the output shaft, an adjustment of the neutral position of the 
clutch mechanism and an adjustment of the neutral position of the steering 
torque detecting means can be performed after the input shaft, the output 
shaft, the steering torque detecting means, the torque transmitting means, 
and the clutch mechanism are assembled together to form a single 
semi-finished steering assembly, and before the semi-finished steering 
assembly is received in the housing. More particularly, since the neutral 
position adjustments are effected in such a condition that the clutch 
mechanism and the steering torque detecting means are already assembled 
with the semi-finished steering assembly but the semi-finished steering 
assembly is still standing alone before it is assembled into the housing, 
the respective neutral positions of the clutch mechanism and the steering 
torque detecting means can be obtained with excellent accuracy but through 
a simple adjustment work. 
In addition, since the clutch mechanism and the steering torque detecting 
means are received at one time in the housing automatically when the 
semi-finished steering assembly is set or installed in the housing, the 
electric power steering device can be assembled with improved efficiency. 
Furthermore, replacement of the steering torque detecting means is easy to 
perform. Since the output shaft is rotatably mounted only on the housing, 
and since the wheel of the torque transmitting means is rotatably mounted 
only on the output shaft, the output shaft can be directly fitted with the 
housing. By virtue of this supporting structure, the assembling accuracy 
of the electric power steering device is greatly improved. 
Preferably, the actuator comprises a lever-like configuration. The 
lever-like actuator is pivotally connected at one end to the detector body 
attached to the one shaft and, at the opposite end, slidably engaged in a 
diagonal groove formed in an outer peripheral surface of the other shaft. 
It is further preferable that the wheel is a hollow cylindrical member 
rotatably fitted around the output shaft, the hollow cylindrical member 
being formed from a self-lubricating material. 
The present invention, in yet another aspect, seeks to provide a method of 
assembling an electric power steering device, comprising the steps of: 
assembling together an input shaft adapted to be connected to a steering 
wheel, an output shaft adapted to be connected to steered wheels, a 
resilient member for connecting together the input shaft and the output 
shaft while allowing them to rotate relatively to each other, a steering 
torque detecting means for detecting relative rotation between the input 
shaft and the output shaft, a torque transmitting means for transmitting 
an auxiliary torque generated by an electric motor to the output shaft, 
and a clutch mechanism thereby to form a semi-finished steering assembly; 
then, inserting the semi-finished steering assembly into a housing through 
an opening at one end of the housing such that part of the input shaft, 
the output shaft, the steering torque detecting means, the torque 
transmitting means, and the clutch mechanism are all received at one time 
in the housing; and thereafter, attaching a lid to the one end of the 
housing to close the opening. 
The method may further include, between the assembling step and the 
inserting step, a step of adjusting the neutral position of the clutch 
mechanism, and a step of adjusting the neutral position of the steering 
torque detecting means. The step of adjusting the neutral position of the 
steering torque detecting means and the step of adjusting the neutral 
position of the clutch mechanism are performed separately from each other. 
Since both adjustment processes are effected in such a condition that the 
clutch mechanism and the steering torque detecting means are already 
assembled with the semi-finished steering assembly but the semi-finished 
steering assembly is still standing alone before it is assembled in the 
housing, the neutral position can be obtained with excellent accuracy but 
through a simple adjustment work. 
In addition, since the clutch mechanism and the steering torque detecting 
means are received in the housing automatically when the semi-finished 
steering assembly is set or installed in the housing, the electric power 
steering device can be assembled with improved efficiency. 
The above and other object, features and advantages of the present 
invention will become manifest to those versed in the art upon making 
reference to the detailed description and accompanying sheets of drawings 
in which preferred structural embodiments incorporating the principles of 
the present invention are shown by way of illustrative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, one preferred structural embodiment of the present invention will be 
described below in greater detail with reference to the accompanying 
sheets of drawings, wherein like reference characters designate like or 
corresponding parts throughout the several views. 
FIG. 1 diagrammatically shows the general construction of an electric power 
steering device 1 embodying the present invention. The electric power 
steering device 1 includes a steering torque detecting means or detector 3 
for detecting a steering torque created in a steering system when a 
steering wheel 2 is actuated, a control unit or controller 4 for 
generating a control signal based on a detection signal from the steering 
torque detector 3, an electric motor 5 for producing, based on the control 
signal from the controller 4, an auxiliary torque proportional to the 
steering torque, and a torque transmitting means or mechanism 6 and a 
mechanical clutch 40 that cooperate with each other to transmit the 
auxiliary torque from the electric motor 4 to a steering system. The 
electric power steering device 1 thus constructed is operative to steer a 
pair of wheels (steered wheels) 9, 9 via a rack-and-pinion mechanism 
including a pinion 7 and a rack 8a. 
FIG. 2 shows, in enlarged cross section, a main portion of the electric 
power steering device 1. As shown in the same figure, the steering system 
is composed of a tubular input shaft 11 connected to the steering wheel 2 
(FIG. 1), a torsion bar (resilient member) 13 inserted in the tubular 
input shaft 11 and connected at its upper end portion to the input shaft 
11 by means of a pin 12, and an output shaft (pinion shaft) 15 connected 
by a pin 14 to a lower end portion of the torsion bar 13, the output shaft 
15 having a lower portion on which the pinion 7 of the rack-and-pinion 
mechanism is formed. 
The torsion bar 13 is a resilient member which is capable of producing a 
torsional angle or twist exactly proportional to the steering torque and 
which allows the input shaft 11 and the output shaft 15 to rotate 
relatively to each other to thereby create a relative torsional 
displacement therebetween. The rack 8a is toothed on a rack shaft 8 
extending in a direction perpendicular to the sheet of FIG. 2. The rack 8 
is held in mesh with the pinion 7. The input shaft 11, the torsion bar 13 
and the output shaft 15 are concentric with each other. 
The steering torque detector 3 is constructed to detect a steering torque 
in the steering system by detecting a relative torsional angle (relative 
torsional displacement or twist) between the input shaft 11 and the output 
shaft 15. In the illustrated embodiment, the detector 3 comprises a 
potentiometer. 
The torque transmitting mechanism 6 includes a toothed wheel 32. The wheel 
32 is formed by a relatively thick hollow cylindrical member mounted 
directly and rotatably on an upper portion of the output shaft 15. The 
cylindrical member (wheel) 32 has on its outside surface a toothed gear 
portion 32a, and an annular or ring-like input member 32b disposed above 
the toothed gear portion 32. The cylindrical member (wheel) 32 is formed 
from a self-lubricating material, such as oil-impregnated sintered metal. 
The mechanical clutch 40 is disposed inside the input member 32b and has a 
structure detailed below with reference to FIG. 6. 
The electric power steering device 1 of the present invention further 
includes a housing 26 which houses or contains therein the torque 
transmitting mechanism 6, part of the input shaft, part of the torsion bar 
13, the output shaft 15, and mechanical clutch 40. The housing has an 
opening 26a at its upper end, the opening 26a being closed by a lid 71. 
The lid 71 is attached by screws 72 (only one being shown) to an upper end 
flange 26b of the housing 26, with an O-ring 73 disposed between the lid 
71 and the end flange 26b to provide a hermetic seal therebetween. 
The lid 71 has an upwardly swelled, annular central portion 71a forming a 
bearing retainer in which a bearing 74 is retained for rotatably 
supporting an intermediate portion of the input shaft 11 within the 
housing 26. Though not shown, the bearing 74 has a water-tight structure 
including a seal attached to an upper side (or both sides) of the bearing 
74. A typical example of such water-tight bearing is the so-called "slide 
contact rubber seal bearing" equipped with a rubber seal held in sliding 
contact with an inner race of the bearing. 
A dust cover 74 made of rubber and having a generally inverted cup-like 
configuration is firmly fitted on an intermediate portion of the input 
shaft 11 so as to cover the bearing retainer portion 71a of the lid 71. 
The dust cover 74 has a pair of spaced annular seal lips 75a projecting 
from an inner peripheral surface of the cup-like dust cover 75 and held in 
sealing contact with an outer peripheral surface of the bearing retainer 
portion 71a. The seal lips 75a provide a hermetic seal between the dust 
cover 75 and the bearing retainer portion 71a of the lid 71. 
Thus, the bearing portion provided at an upper end of the housing 26 for 
the input shaft 11 has a water-tight structure which is composed of a 
small number of component and hence is simple in construction, easy to 
assemble, and reliable in operation. 
In FIG. 2, designated by 66 is a plain washer press-fitted with the output 
shaft 15. Numerals 76, 77 denote a pair of bearings mounted on the housing 
26 for rotatably supporting the output shaft 15. Numeral 78 denotes a nut 
threaded over an externally threaded lower end portion (not designated) of 
the output shaft 15 to set the output shaft 15 in position relative to the 
bearing 77, and numeral 79 denotes an end cap or plug fitted or otherwise 
threaded in an opening 26c to close the same, the opening 26c being formed 
at a lower end of the housing 26. 
FIG. 3A is a side view showing the steering torque detector 3, and FIG. 3B 
is a front elevational view of FIG. 3A. The steering torque detector 
(potentiometer) 3 includes a detector body 21 containing within it a 
resistance element (not shown) and a sliding contact (not shown) slidable 
along the resistance element, and a lever-like actuator 22 pivotally 
movable to move or shift the sliding contact along the resistance element 
inside the detector body 21. 
The detector body 21 is attached by screws (not designated) to an outer 
peripheral surface of a lower end portion of the input shaft 11. The 
lever-like actuator 22 has one end (fixed end) pivotally connected to the 
detector body 21 and an opposite end (free end) engaged in a diagonal 
groove 15a extending in an outer peripheral surface of an upper portion of 
the output shaft 15 at an angle to the axis of the output shaft 15. The 
diagonal groove 15a is skewed or inclined toward the right-hand direction 
in FIG. 3B. The steering torque detector 3 thus constructed can detect a 
relative torsional angle or twist (relative torsional displacement) 
between the input shaft 11 and the output shaft 15. The steering torque 
detector 3 further has a torsion spring 23 acting on the lever-like 
actuator 22 for urging the free end of the lever-like actuator 22 against 
one sidewall of the diagonal groove 15a, thereby eliminating a play or 
clearance between the diagonal groove 15a and the actuator 22 in a 
direction of pivotal movement of the actuator 22. 
Referring back to FIG. 2, the input shaft 11 has a plurality (three in the 
illustrated embodiment) of electric cables 25 wound in plural turns (three 
turns, for example) around a cable reel 24. The electric cables 25 are 
connected at one end to the detector body 21 of the steering torque 
detector 3 and, at the other end, to a connector 27 attached to the 
housing 26. 
FIG. 4 is a cross-sectional view taken along line A--A of FIG. 2, showing a 
detailed cross section of the torque transmitting mechanism 6. The torque 
transmitting mechanism 6 comprises a worm gear mechanism including a worm 
31 coupled to an output shaft 5a of the electric motor 5, and the wheel 32 
rotatably mounted on the output shaft 15. The electric motor 5 is bolted 
to the housing 26. 
By virtue of the torque transmitting mechanism 6, the rack 8 is driven via 
the pinion 9 by a combined torque which is equal in amount to the sum of 
the steering torque in the steering system (input shaft 11.fwdarw.torsion 
bar 13.fwdarw.output shaft 15) and the auxiliary torque from the electric 
motor 5. 
FIG. 5 shows, in exploded perspective, a main portion of the electric power 
steering device. The mechanical clutch 40 includes, as one structural 
component, a generally annular position control means or controller 63 
connected by serration to the lower end of the input shaft 11. The annular 
position controller 63 has downwardly projecting three leg-like position 
control members 64. The leg-like position control members 64 are angularly 
spaced from each other along the circumference of a circle. By virtue of 
the serration connection between the input shaft 11 and the position 
controller 63, the position controlling members 64 are connected to the 
steering wheel 2 shown in FIG. 1. The output shaft 15 is provided with an 
annular output member 34 at an upper end portion thereof. 
FIG. 6 is a cross-sectional view taken along line B--B of FIG. 2 and shows 
a detailed cross section of the mechanical clutch 40, with the housing 26 
being omitted for clarity. 
As shown in FIG. 6, the mechanical clutch 40 comprises the so-called 
one-way clutch assembly and is constructed to transmit an auxiliary torque 
from the electric motor 5 to the steering system only when the acting 
direction of the auxiliary torque agrees with the steering direction of 
the steering system. More specifically, the mechanical clutch 40 is 
composed of two sets of three friction clutch mechanisms, each set being 
operative in response to rotation of the steering wheel 2 (and the 
position controlling members 64 connected thereto) in one particular 
direction. 
In the illustrated embodiment, the three friction clutch mechanisms 41, 41, 
41 in a first set cooperate to transmit the auxiliary torque in the 
counterclockwise direction indicated by the arrow X, while the three 
friction clutch mechanisms 51, 51, 51 cooperate to transmit the auxiliary 
torque in the counterclockwise direction which is opposite the direction 
of the arrow X. The first clutch mechanisms 41 and the second clutch 
mechanisms 51 are disposed alternately on the circumference of a circle. 
The mechanical clutch 40, which is composed of two sets of three friction 
clutch mechanisms 41, 51 as described above, has six tapering spaces 61 
arranged in succession along an annular space defined the input member 32b 
and the output member 34. The mechanical clutch 40 further includes six 
locking members 62 each received in a corresponding one of the tapering 
spaces 61 for selectively engaging and disengaging the input member 32b 
and the output member 34, the above-mentioned three position control 
members 64 disposed in the annular space between the input and output 
members 32b, 34 for positioning the locking members 62, and three urging 
members 37 in the form of compression coil springs disposed in the annular 
space for urging the respective locking members 62 toward the adjacent 
position control members 64. 
The output member 34 has a generally isosceles triangular shape in cross 
section, having three sides curved outwardly and three corners truncated 
or rounded. 
The tapering spaces 61 are defined between an inside surface 32c of the 
cylindrical input member 32b and an engagement surface (polygonal outside 
surface) 34b of the output member 34. Each of the tapering spaces 61 has a 
tapered or narrowed circumferential end. The position control members 64 
are rotatable about an axis of the position controller 63 (i.e., the 
common axis of the input and output shafts 11, 15) and they are located on 
the circumference of the same circle at circumferential intervals. 
The thus constructed mechanical clutch 40 operates to selectively engage 
and disengage the input member 32b and the output member 34 in response to 
angular movement or rotation of the position control members 64. 
In each friction clutch mechanism set, a selected one of the clutch 
mechanisms 41, 51 (hereinafter referred to as "selected first clutch 
mechanism 41a" or "selected second clutch mechanism 51a") is so 
constructed as to shift the phase from the engaged state to the disengaged 
state in advance to the other two or remaining clutch mechanisms 41 or 51. 
To this end, a particular one of the position control members 64 
(hereinafter referred to as "selected position control member 64a") which 
is designated to position the locking member 61 of the selected first or 
second clutch mechanism 41a or 51a has a circumferential length L.sub.1 
which is larger than that L.sub.2 of the other two or remaining position 
control members 64. The selected position control member 64a is normally 
aligned with one truncated corner or vertex of the generally isosceles 
triangular output member 34, the remaining position control members 64 
being normally aligned with two other corners of the isosceles triangular 
output member 34 having the same angle. 
The output member 34 is mounted on the output shaft 15 in such a manner 
that the output member 34 and the output shaft 15 are relatively movable 
in a diametrical direction. To this end, the output member 34 has a 
generally oblong or elliptical through-hole 34a, and the output shaft 15 
of circular cross section is fitted in the oblong through-hole 34a. The 
pin 14, used for joining together the torsion bar 13 and the output shaft 
15, extends diametrically across the output member 34 along a major axis 
of the oblong through-hole 34a. A resilient member comprised of a 
belleville washer 35 is loosely fitted around the pin 14 and acts between 
the output member 34 and the output shaft 15 to force an inner peripheral 
wall of the through-hole 34a against an outer peripheral surface of the 
output shaft 15. 
More specifically, the resilient member 35 is disposed between the outer 
peripheral surface of the output shaft 15 and an inner peripheral wall 
portion of the through-hole 34a extending transversely across the major 
axis of the oblong through-hole 34a. The resilient member 35 can generate 
a resilient force acting in a direction parallel to the direction of 
movement of the output member 34 relative to the output shaft 15 (along 
the major axis of the oblong through-hole 34a). Thus, the output member 34 
is forced against the output shaft 15. The output member 34 can be 
displaced toward a center of the width of the selected position control 
member 64a, in a manner described later on. 
To enable the output member 34 to move in the diametrical direction when 
only one clutch mechanism of either set (i.e., the selected first clutch 
mechanism 41a or the selected second clutch mechanism 51a) is disengaged, 
the engagement surface 34b of the output member 34 is profiled such that a 
surface portion which is adapted for engagement with the locking member 62 
of each of the remaining clutch mechanisms 41, 51 has a downslope. With 
the engagement surface 34b thus profiled, when the output member 34 moves 
in a diametrical direction in response to disengagement of the selected 
clutch mechanism 41a or 51a, the locking members 62 roll on the downslope 
surface portions smoothly and do not hinder movement of the output member 
34. As a consequence of this movement of the output member 34, the 
tapering spaces 61 of the remaining clutch mechanisms 41, 51 are slightly 
spread or widened, and so the locking members 61 of the remaining clutch 
mechanisms 41, 51 can be disengaged with a smaller force. In other words, 
all the first or second clutch mechanisms 41, 51 can be disengaged by a 
force substantially equal to the force required to displace only one 
clutch mechanism (i.e., the selected first or second clutch mechanism 41a 
or 51a). 
The operation of the mechanical clutch 40 will be described in further 
detail with reference to FIGS. 1 and 6 through 8. 
When the steering wheel 2 (FIG. 1) is not actuated, the steering torque 
detector 3 generates no detection signal and, hence, the control unit 4 
sends no assist command signal to the electric motor 5. The electric motor 
5 is, therefore, kept in the stationary or inoperative condition. Thus, 
all the clutch mechanisms 41, 51 are in the disengaged state (neutral 
position), as shown in FIG. 6. 
In cases where a steering torque on the steering wheel 2 is small and the 
electric motor 5 generates no auxiliary torque, the relative position or 
phase between the position control members 64 connected to the input shaft 
11 (FIG. 2) and the output member 34 does not show any material change. 
Stated in other words, the position control members 64 are slightly turned 
in one direction (counterclockwise direction indicated by the arrow X 
shown in FIG. 6, for example). But the degree or extent of angular 
movement of the position control members 64 is still insufficient to 
engage the first clutch mechanisms 41. Thus, the output member 34 is free 
from the effect of a friction and an inertial force produced by the 
electric motor 5 and is turned or rotated by a steering torque in the 
steering system (input shaft 11.fwdarw.torsion bar 13.fwdarw.output shaft 
15), thereby driving the output shaft 15. 
Alternatively, when a steering torque on the steering wheel 2 is large and 
the electric motor 5 is generating an auxiliary torque, the phase between 
the position control members 64 and the output member 34 is changed 
greatly. In this instance, the position control members 64 (including the 
selected position control member 64a) are greatly displaced in one 
direction (counterclockwise direction indicated by the arrow X shown in 
FIG. 7, for example). With this angular displacement of the position 
control members 64, the locking members 62 of the first clutch mechanisms 
41 are displaced by the force of the urging members 65 toward the narrowed 
circumferential ends of the associated tapering spaces 61 and then wedge 
between the input member 32b and the output member 34 to thereby connect 
them together. All the first clutch mechanisms 41 are thus brought to the 
engaged state. 
In this instance, since the electric motor 5 is rotating, the input member 
32b is rotated in the direction of the arrow X. Thus, the auxiliary torque 
generated from electric motor 5 is transmitted from the input member 32b 
to the output member 34 via the first clutch mechanisms 41. The output 
member 34 is rotated in the direction of the arrow X to thereby drive the 
output shaft 15 by a combined torque which is equal to the sum of the 
steering torque in the steering system (input shaft 11.fwdarw.torsion bar 
13.fwdarw.output shaft 15) and the auxiliary torque from the electric 
motor 5. 
Thereafter, when the need arises due to some reasons, the first clutch 
mechanisms 41 are disengaged while transmission of the auxiliary torque 
from the electric motor 5 is still continuing. In this instance, the 
electric power steering device operates as follows. 
The steering wheel 2 (FIG. 1) is steered or turned in the opposite 
direction whereupon the position control members 64 turn in a direction 
(indicated by the arrow Y in FIG. 9) which is opposite to the rotating 
direction of the input member 34b. During that time, the selected position 
control member 64a having a longer circumferential length than the other 
two position control members 64 comes into contact with the right-hand 
adjacent locking member (designated by 62a for expediency) before the 
other two or remaining position control members 64, 64 reach the 
associated locking members 62. The selected position control member 64a 
then forces the locking member 62a in the direction of the arrow Y against 
the force of the urging member 65 and a frictional force acting between 
the input member 32b and the output member 34. 
With this displacement of the locking member 62a, the selected first clutch 
mechanism 41 is disengaged. In this instance, however, since the remaining 
position control members 64, 64 are still distant from the right-hand 
adjacent locking members 62, 62, the forces applied from the respective 
locking members 62 onto the output member 34 are ill-balanced. More 
particularly, the output member 34 is subjected to an unbalanced load or 
force (indicated by vector Z.sub.3 shown in FIG. 8) resulting from two 
component forces (indicated by vectors Z.sub.1, Z.sub.2) applied from the 
two other or remaining locking members 62. By the effect of this 
unbalanced load, the output member 34 is slightly displaced toward the 
selected position control member 64a against the force of the resilient 
member 35 while it is guided by the pin 14. With this displacement of the 
output member 34, the tapering spaces 61 receiving therein the remaining 
locking members 62 are slightly spread or enlarged and, consequently, the 
wedging force applied from the remaining locking members 62 to the input 
and output members 32b, 34 is reduced. 
Immediately thereafter, the remaining position control members 64, 64 come 
into contact with the remaining locking members 62, respectively, and then 
force them toward their original neutral position shown in FIG. 6. Thus, 
the other two or remaining first clutch mechanisms 41 are disengaged. In 
immediate response to this disengagement of the remaining clutch 
mechanisms 41, the output member 34 automatically returns to its original 
neutral position by the resilient force of the resilient member 35. 
The second clutch mechanisms 51 operate in the same manner as, but in the 
opposite direction to, the first clutch mechanisms 41. Accordingly, in 
response to rotation of the steering wheel 2 (FIG. 1), the second clutch 
mechanisms 51 can be also selectively engaged and disengaged in like 
manner as described above with reference to FIGS. 6 to 8. 
FIGS. 9A to 9C illustrate the principle of operation based on which a 
neutral position adjustment of the steering torque detector 3 is 
performed. 
For purposes of illustration, the steering torque detector 3 having a 
detector body 21 attached to the input shaft 11 and a lever-like actuator 
22 engaged at its free end with the diagonal groove 15a in the output 
shaft 15 is initially disposed in the position shown in FIG. 9A. 
When the input shaft 11 is moved upwards, as shown in FIG. 9B, the 
lever-like actuator 22 turns in the counterclockwise direction about its 
distal end pivoted to the detector body 21 because the free end of the 
actuator 22 is guided to ascend the diagonal groove 15a inclined 
rightwards in FIG. 9A. Counterclockwise movement of the actuator 22 causes 
the sliding contact (not shown but contained within the detector body 21) 
to slide along the resistance element (also not shown but contained within 
the detector body 21) in one direction. 
Conversely, when the input shaft 11 is lowered, as shown in FIG. 9C, the 
lever-like actuator 22 turns in the clockwise direction relative to the 
detector body 21 as the free end of the actuator 22 is guided to slide 
down the diagonal groove 15a. With this clockwise movement of the actuator 
22, the sliding contact contained within the detector body 21 moves in the 
opposite direction along the resistance element. 
Since the actuator 22, in response to the axial movement of the input 
shaft, turns to move the sliding contact along the resistance element 
(both contained within the detector body 21), it is possible to adjust the 
neutral position of the steering torque easily and accurately. 
Referring now to FIGS. 10A-10D, there is shown the manner in which the 
neutral position of the steering torque detector 3 and the neutral 
position of the mechanical clutch 40 (friction clutch mechanisms) are 
adjusted. 
(1) At first, as shown in FIG. 10A, the torsion bar 13 is inserted in the 
tubular input shaft 11. The input shaft has a pin receiving hole 11a at 
its upper end portion. Then, part of the torque transmitting mechanism 6, 
the input shaft 11, the torsion bar 13, the output shaft 15, and the 
mechanical clutch 40 are assembled together. 
(2) Subsequently, the detector body 21 of the steering torque detector 3 is 
attached to an outer peripheral surface of the input shaft 11, and the 
lever-like actuator 22 of the steering torque detector 3 is engaged in the 
diagonal groove 15a in the outer peripheral surface of the output shaft 
15. Thus, the input shaft 11, the output shaft 15, the torsion bar 13, the 
steering torque detector 3, the torque transmission mechanism 6, and the 
mechanical clutch 40 (clutch mechanisms) are assembled together to form a 
single semi-finished steering assembly 80. 
(3) Thereafter, the input shaft 11, which is still kept rotatable relative 
to the torsion bar 3, is turned about its own axis until an exact neutral 
position of the mechanical clutch 40 is found or established. Stated in 
other words, the input shaft 11 is turned in a manner as indicated by the 
arrows shown in FIG. 10A so that the position control members 64 are 
placed in their neutral position, as shown in FIG. 10B. This position 
adjustment work or operation is very simple but is enough to perform the 
neutral position adjustment of the mechanical clutch 40 with improved 
accuracy. 
(4) Then, as shown in FIG. 10C, the input shaft 11 is moved in the axial 
direction until an exact neutral position of the steering torque detector 
3 is found or established. Stated in further detail, the input shaft 11 is 
moved up and down as indicated by the arrows shown in FIG. 10C so that the 
neutral position of the steering torque detector 3 is set in accordance 
with the principle described above with reference to FIGS. 9A-9C. This 
adjustment work is also simple but is enough to perform the neutral 
position adjustment of the steering torque detector 3 with improved 
accuracy. 
(5) Finally, as shown in FIG. 10D, the torsion bar 13 is machined or 
otherwise drilled to form a pin receiving hole 11a in registry with the 
pin receiving hole 11a in the input shaft 11. Then, the pin 12 is 
press-fitted in the pin receiving holes 11a, 13a, thereby joining the 
input shaft 11 and the torsion bar 13. 
As described above, the neutral position of the mechanical clutch 40 is 
adjusted first and then the adjustment of the neutral position of the 
steering torque detector 3 is performed. Thus, the neutral position of the 
mechanical clutch 40 and the neutral position of the steering torque 
detector 3 are adjusted separately and precisely. 
Now, a method of the present invention for assembling an electric power 
steering device will be described below with reference to FIGS. 11A and 
11B. 
(1) At first, as shown in FIG. 11, electric cables 25 and a connector 27 
are attached to a semi-finished steering assembly 80 (identical to one 
shown in FIG. 10A). A bearing 77 is mounted on a lower end portion of the 
housing 26. Then, the semi-finished steering assembly 80 is inserted into 
the housing 26 through an opening 26a at an upper end of the housing 26 
(as indicated by the arrow 1 in circle) such that part of the input shaft 
11, the output shaft 15, the steering torque detector 3, the torque 
transmission mechanism 6 and the mechanical clutch 40 (clutch mechanisms) 
are received in the housing 26. 
(2) Subsequently, as shown in FIG. 11B, a nut 78 is inserted into an 
opening 26c formed at the lower end of the housing 26 and then threaded 
over an externally threaded lower end portion of the output shaft 15 to 
thereby set or fasten the output shaft 15 with respect to the bearing 77 
(as indicated by the arrow 2 in circle). In this condition, the output 
shaft 15 is rotatably mounted on the housing 27 by means of two bearings 
77, 75, the bearing 75 being mounted in advance on the semi-finished 
steering assembly 80. Then, the opening 26c at the lower end of the 
housing 26 is closed by an end cap 79 (as indicated by the arrow 3 in 
circle). 
(3) Thereafter, a lid 71 with a bearing 74 mounted thereon is fitted on the 
input shaft 11 (as indicated by the arrow 4 in circle). The lid 71 is then 
secured by screws 72 (only one being shown) to the upper end of the 
housing 26 to thereby close the opening 26a. 
(4) Finally, a dust cover 75 is fitted on the input shaft 11 (as indicated 
by the arrow 5 in circle) such that a bearing retainer portion 71a of the 
lid 71a is covered by the dust cover 75. 
The worm 31 of the torque transmitting mechanism 6 may be assembled in the 
housing 26 either before or after the semi-finished steering assembly 80 
is inserted in the housing 26. 
Thus, by inserting the semi-finished steering assembly 80 into the housing 
26 through the opening 26a, the torque transmitting mechanism 6 and the 
mechanical clutch 40 (first and second friction clutch mechanisms 41, 51) 
are automatically and simultaneously received in the housing 26. 
One preferred structural embodiment of the present invention has been 
described and disclosed. It is to be noted however that various changes 
and modifications of the present invention are possible in the light of 
the above teaching. 
For instance, the resilient member 13 should by no means be limited to the 
torsion bar shown in the illustrated embodiments but may include any other 
resilient member on condition that the member is able to create a relative 
torsional displacement between the input shaft 11 and the output shaft 15 
which is proportional in amount to the steering torque. 
The diagonal groove 15a may be formed in an outer peripheral surface of the 
input shaft 11 for receiving and guiding the free end of the lever-like 
actuator 22 of the steering torque detector 3. In this instance, the 
detector body 21 is attached to an outer peripheral surface of the output 
shaft 15. 
The number of the first clutch mechanisms 41 or the second clutch 
mechanisms 51 should by no means be limited to three in the illustrated 
embodiments but may be determined in option. 
The output member 34 should be displaceable in the diametrical direction 
when it is forced by the remaining locking elements 62 upon disengagement 
of the selected locking element 62a. However, there is no limitation about 
the direction of displacement of the output member 34 relative to a 
peripheral element. This is because a frictional engagement force acting 
between the remaining locking members 62 and the output member 34 can be 
reduced automatically when the output member 34 is displaced in the 
diametrical direction by the force exerted by the same locking members 62. 
The guide member used in combination with the output member 34 for guiding 
the latter is not limited to the pin 14 shown in the illustrated 
embodiments. Similarly, the resilient member used for urging the output 
member 34 against the output shaft 15 should by no means be limited to the 
belleville spring 35 but may be a compression spring (compression coil 
spring). 
The tapering spaces 61 should preferably be defined between the inside 
surface 32c of the inside member 32 and the outside surface 34b of the 
output member 34. The shape of the outside surface (engagement surface) 
34b of the output member 34 is not limited to one shown in FIG. 6. The 
input member 32b may have a polygonal inside surface in which instance the 
output member 34 preferably has a cylindrical outside surface. 
The locking members 62 may be spherical other than cylindrical provided 
that they are selectively engageable and disengageable with the tapered or 
narrowed circumferential ends of the mating tapering spaces 61 to engage 
and disengage the input member 32b and the output member 34. 
The urging members incorporated in the mechanical clutch 40 should by no 
means be limited to the compression springs (compression coil springs) 65 
but may include a rigid rubber member or a plate spring. 
The first and second clutch mechanisms 41, 51 should preferably be composed 
of a frictional engagement clutch and hence may include a known sprag 
clutch described below. 
The sprag clutch includes a hollow cylindrical outer member or race 
(corresponding to the input member 32b) having a cylindrical inside 
engagement surface, a hollow cylindrical inner member or race 
(corresponding to the outside member 34) disposed concentrically with the 
outer race and having a cylindrical outside engagement surface, a 
plurality of sprags (cams having a wedging action) placed between the 
inner and outer races, a member (corresponding to the position control 
members 64) connected with the steering handle and positioning the sprags, 
and springs urging the respective sprags in a direction to wedge them 
between the inside and outside engagement surfaces (see the clutch device 
shown in Japanese Patent Laid-open Publication No. HEI 1-188727). 
The neutral position of the electric power steering device may be adjusted 
in such a manner as described below: At first, the input shaft 11 or the 
output shaft 15 which is kept rotatable relative to the resilient member 
13 is turned until the neutral position of the mechanical clutch is 
obtained. Then, the input shaft 11 or the output shaft 15 is displaced in 
the axial direction until the neutral position of the steering torque 
detector 3 is obtained. Thereafter, the input shaft 11 or the output shaft 
15 is fixed to the resilient member 13. 
The electric power steering device of the present invention and the method 
of assembling the same offer various advantages described below. 
Since a detector body of a steering torque detector is directly attached to 
one of the input shaft and the output shaft, and an actuator of the 
steering torque detector is engaged with the other shaft, an adjustment of 
the neutral position of steering torque detector can be performed after 
the input shaft, the output shaft, the steering torque detector, and a 
torque transmitting mechanism are assembled together to form a single 
semi-finished steering assembly, and before the semi-finished steering 
assembly is received in a housing. More particularly, since the neutral 
position adjustment is effected in such a condition that the steering 
torque detector is already assembled with the semi-finished steering 
assembly but the semi-finished steering assembly is still standing alone 
before it is assembled in the housing, the neutral position of the 
steering torque detector can be obtained with excellent accuracy but 
through a simple adjustment work or operation. 
In addition, since the detector body and the actuator of the steering 
torque detector are received at one time in the housing automatically when 
the semi-finished steering assembly is set or installed in the housing, 
the efficiency of the assembling operation (of the detector and of the 
power steering device) is extremely high. The steering torque detecting 
means of the foregoing construction is easy to replace. 
Furthermore, since a clutch mechanism is disposed between the torque 
transmitting mechanism and the pinion shaft (output shaft) and is received 
in the housing together with the torque transmitting mechanism and the 
pinion shaft, since the pinion shaft is rotatably mounted only on the 
housing, and since a wheel interconnecting the clutch mechanism and the 
torque transmitting mechanism is rotatably mounted only on the pinion 
shaft, it is possible to perform an adjustment of the neutral position of 
the clutch mechanism after the pinion shaft, the torque transmitting 
mechanism, and the clutch mechanism are assembled together to form a 
semi-finished steering assembly and before the semi-finished steering 
assembly is set or installed in the housing. More particularly, since the 
neutral position adjustment is effected in such a condition that the 
clutch mechanism is already assembled with the semi-finished steering 
assembly but the semi-finished steering assembly is still standing alone 
before it is assembled in the housing, the neutral position of the clutch 
mechanism can be obtained with high accuracy but through a simple 
adjustment work. 
In addition, since the clutch mechanism is automatically received when the 
semi-finished steering assembly is set in the housing, assembling 
efficiency of the electric power steering device increases greatly. 
Furthermore, since the pinion shaft is rotatably mounted only on the 
housing, and since the wheel of the torque transmitting mechanism is 
rotatably mounted only on the pinion shaft, it is possible to fit the 
pinion shaft directly with the housing. By virtue of this supporting 
structure, the assembling accuracy of the electric power steering device 
is greatly improved. 
In one preferred form, the steering torque detector is composed of a 
detector body directly attached to one of the input shaft and the output 
shaft, and an actuator engaged with the other of the input shaft and the 
output shaft for activating the detector body. The clutch mechanism is 
disposed between the torque transmitting mechanism and the output shaft. 
The torque transmitting mechanism includes a wheel for connecting together 
the torque transmitting means and the clutch mechanism, the wheel being 
rotatably supported solely on the output shaft. And, the steering torque 
detecting means and the clutch mechanism are received in the housing 
together with the input shaft and the output shaft. With this 
construction, an adjustment of the neutral position of the clutch 
mechanism and an adjustment of the neutral position of the steering torque 
detector can be performed after the input shaft, the output shaft, the 
steering torque detector, the torque transmitting mechanism, and the 
clutch mechanism are assembled together to form a single semi-finished 
steering assembly, and before the semi-finished steering assembly is 
received in the housing. More particularly, since both adjustments are 
effected in such a condition that the clutch mechanism and the steering 
torque detector are already assembled with the semi-finished steering 
assembly but the semi-finished steering assembly is still standing alone 
before it is assembled into the housing, the neutral position of the 
clutch mechanism and the steering torque detector can be obtained with 
excellent accuracy but through a simple adjustment work. 
In addition, since the clutch mechanism and the steering torque detector 
are received at one time in the housing automatically when the 
semi-finished steering assembly is set or installed in the housing, the 
electric power steering device can be assembled with improved efficiency. 
Furthermore, replacement of the steering torque detector is easy to 
perform. Since the output shaft is rotatably mounted only on the housing, 
and since the wheel of the torque transmitting mechanism is rotatably 
mounted only on the output shaft, the output shaft can be directly fitted 
with the housing. By virtue of this supporting structure, the assembling 
accuracy of the electric power steering device is greatly improved. 
The electric power steering device assembling method of the present 
invention is characterized by the steps of: assembling together an input 
shaft adapted to be connected to a steering wheel, an output shaft adapted 
to be connected to steered wheels, a resilient member for connecting 
together the input shaft and the output shaft while allowing them to 
rotate relatively to each other, a steering torque detecting means for 
detecting relative rotation between the input shaft and the output shaft, 
a torque transmitting means for transmitting an auxiliary torque generated 
by an electric motor to the output shaft, and a clutch mechanism thereby 
to form a semi-finished steering assembly; then, inserting the 
semi-finished steering assembly into a housing through an opening at one 
end of the housing such that part of the input shaft, the output shaft, 
the steering torque detecting means, the torque transmitting means, and 
the clutch mechanism are all received at one time in the housing; and 
thereafter, attaching a lid to the one end of the housing to close the 
opening. The method may further include, between the assembling step and 
the inserting step, a step of adjusting the neutral position of the clutch 
mechanism, and a step of adjusting the neutral position of the steering 
torque detecting means. The step of adjusting the neutral position of the 
steering torque detecting means and the step of adjusting the neutral 
position of the clutch mechanism are performed separately from each other. 
Since both adjustment processes are effected in such a condition that the 
clutch mechanism and the steering torque detecting means are already 
assembled with the semi-finished steering assembly but the semi-finished 
steering assembly is still standing alone before it is assembled in the 
housing, the neutral position can be obtained with excellent accuracy but 
through a simple adjustment work. 
In addition, since the clutch mechanism and the steering torque detector 
are received in the housing automatically when the semi-finished steering 
assembly is set or installed in the housing, the electric power steering 
device can be assembled with improved efficiency. 
Obviously, various minor changes and modifications of the present invention 
are possible in the light of the above teaching. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.