Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VARIABLE RATIO RACK PINION
STEERING GEAR A~ID METHOD
FOR CUTTING R~CK TEETH THERE IN
The present invention relates to a rack pinion
steering gear used in the steering system of motor vehicles
and the like, and more specifically, to such a rack pinion
steering gear in which a rack bar has variable ratio rack
teeth formed by a cutter having cutting teeth of uniform
pitches.
Variable ratio rack pinion steering gears of these
classes have been heretofore disclosed in British Patent
Specifications Nos. 609356, 667038, 1356172 and 3267763,
Australian Patent No. 241798, and Japanese Patent ~ublic
Disclosure No. 90224/80.
Those disclosed in such printed publications involve
defects as discussed hereinunder. British Patent No. 609356
has disadvatages in that a sleeve is mounted on a rack, and
therefore capacity and manufacturing cost of the gear
increase and the making of a bearing supporting the sleeve
is difficult. It is disclosed in British Patent No. 667038
that composite teeth and a circular skew required for a
variable ratio rack and pinion may be obtained by a rack
bar in which a helical pinion is used, and that a
variable ratio may be derived from change in a pitch
diameter of the gear, this being applicable to either a
spur gear or a helical gear. Thus, a tolerance limit for
change in the pitch diameter is such that a radius of
curvature of tooth profile should be selected with due
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consideration of the strength characteristics of the tooth.
A variable ratio steerin~ mechanism described in British
Patent No. 1356172 involves disadvantages in that the rack
teeth provide different pressure angles at different points,
and that difficulty is involved in absorbing microvibra-
tions from outside in mesh due to movement between tooth
top and tooth root so that the rack teeth lack strength.
The construction in such British Patent renders a gear
ratio small, say about 20% or so. Defects in British
Patent No. 3267763 in which a circular drive pinion in-
cludes uniform teeth in mesh with a rack gear to be driven,
the rack gear having a variable pitch diameter to obtain
a variable ratio therebetween, are such that it is not
easy to manufacture the rack gear to be driven and that
a higher pressure angle should be employed to obtain a
lower ratio between the opposite ènds of the rack bar.
As a result, a great change in efficiency in teeth when
they mesh with each other, represents an impulse given
~o a steering wheel. There is a limitation on selection
of a lower ratio to avoid such impulse to thereby provide
an insufficient transmission gear ratio. It has been
found defective in Australian Patent No. 241798 that a
steering wheel is caused to be driven over the entire
movement region as required less than two (2) turns due
to repeated changes in mesh ratio shown whenever it is
rotated, thereby lessening the average ratio. The system
in accQrdance with Japanese Patent Public Disclosure
No. 90224/80 is designed so that uniform pitch rack teeth
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are formed along a path of rotation of the rack bar,
defined by a resultant curve consisting of the axial line
and the spiral line of the rack bar, and that axial move-
ment of the rack bar at a variable speed with respect to
steady rotation of a helical pinion in mesh with the rack
teeth to obtain about 50% of the gear ratio. The short-
coming derived from such system is that a high-speed
rotation is required for thé rack bar to thus apply a
high load on a cam track means provided between the rack
bar and a housing of the rack bar for supporting rotation
of the rack bar.
On the other hand, heretofore, in gear cutting by
generating process, rack teeth are formed on a rack bar
by two dimensional control of a cutter having a plurality
of cutting teeth. However, in this process, there is the
disadvantage that the teeth have a complex shape and
; ~ therefore smooth mesh with a pinion is very difficult.
It, also, is easily considered that rack teeth are formed
one by one on a cylindrical rack bar by means of a cutter
ao having a single cutting tooth. However, in this process,
generating type teeth are not able to be made, and the
process is very expensive.
One of the objects of the present invention is to
eliminate the aforementioned defects in the prior art by
providing a new and an improved variable ratio rack pinion
steering gear and a method for cutting the same.
Another object of the present invention is to
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provide a variable ratio rack pinion steering gear and a
method for cutting the same wherein given variable ratio
rack teeth are generated on a rack bar by use of a cutter
having uniform pitch teeth along a given path of rotation
S of the rack bar at a given helical angle and a given
inclined angle while continuously applying rotation about
an axis of the rack bar and axial movement of the rack
bar (hereinafter referred to as "rack axis rotation and
rack axis movement", respectively).
Another object of the present invention is to provide
a rack gear and a method for cutting the same wherein a
rack bar has continuous generating type teeth on the
surface of the rack bar and the teeth are formed by a
cutter, for example, a gear shaper or milling machine
which is adapted to make three or four dimensional control
by numerical control.
The above and other objects are accomplished by
the two procedures as will be explained hereinafter.
The first procedure is carried out in such a manner
that when cutting the rack teeth in a rack bar by the use
of a cutter having a variable pitch ratio, the afore-
mentioned rack axis rotation and rack axis movement are
continuously imparted to the rack bar to allow the cutter
to effect the gear cutting at a given helical angle with
the axis of rack bar so that the extent of rotation in the
rack axis (hereinafter refe'rred to as l'extent of rotation")
and the extent of movement in the rack axis (hereinafter
'referred to as "extent of displacementl') are continuously
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varied to provide a given inclined angle with the rack
axis and to define a virtual variable ratio pitch on a
line of rack rotation which represents the extent of a
given displacement in association with such a given
inclined angle. The second procedure is followed in such
a manner that when cutting the rack teeth in the rack bar
by the use of a cutter having a uniform ratio pitch, the
rack axis rotation and the rack axis movement are also
continuously transmitted to the rack bar to effect a
gear cutting at another helical angle so that those
different from the aforementioned extent of rotation and
extent of displacement are continuously varied to provide
a given actual variable ratio pitch on a line of different
rack rotation wnich represents the extent of another
inclined angle and the extent of another displacement~
The variable ratio rack pinion steering gear may be
obtained by repeating the aforementioned two procedures.
The foregoing and other objects and features of the
present invention will become more apparent upon a
consideration of the following description taken in
connection with the accompanying drawings wherein:
Fig. 1 is a fragmentary view showing a configura-
tion of the preferred form of a variable ratio rack pinion
steering gear embodying the present invention;
Figs. 2, 3, and 4 are dlagrammatic illustrations of
how the teeth are cut according to the present invention;
and
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1 Fig. 5 i.s a view showing a cutter formi.ng teeth. on
a rack. bar.
Referring now to Fig, 1, there i.s shown a rack
and pinion gear according to the present invention. A rack
comprises a rack bar 1 and rack teeth 2 thereon~ A pinion
3 meshes with the rack. teeth 2 and is supported to rotate
about the axis thereof. Th.e pinion 3 is connected to a
steering wheel ~not shown~ The rack bar 1 is set to rotate
about the axis X.of the rack along line a or b defined by
rotation. of the rack bar and to move in the direction of
the axis of the rack bar as the pinion 3 rotates. A cam
track 10 is adapted to cooperate with guide means (not
shown) to aid the rack bar 1 in its rotation~ The pinion 3
is disposed to form an angle of ~I with the rack axi.s X
and may be assumed as a cutter of uniform pitches Po. The
rack teeth 2 in en~agement with the pinion 3 arè generated
along a line b of rotati.on of the rack having an inclined
angle ~' (or a line a of rotation of the rack having an
inclined angle ~l and includes tooth pitches so varied as
to be reduced in the direction of the end. As will be
understood by a person ski.lled i.n the art the term "pitch"
: as used herein means.the d;stance between adjacent teeth
Now, how the heli.cal rack teeth 2 as above gener-
ated are cut will be explained with reference to the diagrams
shown in Figs. 2, 3 and 4.
Although a rotary rack cutting method has been
disclosed in Japane$e Patent Public Disclosure No. 90224/80,
Michio Abe,.the inventor of the gear cutting method
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according to the present invention of instant application
made an improvement on such rotary rack cutting method
as will be explained by comparison with what is shown in
the aforementioned Japanese Patent Public Disclosure.
The concept of the invention disclosed in the Japanese
Patent Public Disclosure is diagrammatically shown in
Fig. 2, wherein points 1, 2, 3, .. ......14 on the axis X
of the rack are those indicating pitches of a hob cutter
or a gear shaper cutter (hereinafter referred to as
10 "cutter"). The points 1, 2, 3, ... .......14 also serve to
show numbers of the cutter thereat. A vertical axis Y
is indicative of a turn angle formed around the rack axis
X. The line defined by rotation of the rack bar will pass
through the center of each of the pitches of the rack
teeth generated on the face of the rack bar. More
specifically, the cutter is advanced in the direction
(or in a direction opposite thereto) of arrow A at a
helical angle ~ with the axis X of the rack bar to make
the cut in the rack in numerical order as numbered from
14 to 1. In this instance, if neither rack axis rotation
nor rack axis movement is applied to the rack bar, the
standardized teeth 1, 2, ......... 13, la each have uniform
pitch Po to reach the amount of rack travel, say T~o.
Now, when the rack bar 1 is rotated and moved to cut the
variable ratio pitch teeth therein, the rack bar is
required to be rotated by a turn angle of ~13 to a point
131 " ' at which the perpendicular from the point 13'
intersects the horizontal line from the point 14' before
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cutting a tooth 13' after cutting teeth 14 ?, Further, it
is necessary to move rack bar 1 in the direction of the
axis X of the rack (to the left in Fig. 2) by the amount
~13 of travel at a distance to a point Pl''' at which a
line extended at a helical angle ~ from the point 131'''
intersects the horizontal line from the point 14' so that
tooth interference may be avoided.
There upon, the turn angle al3 and the travel
amount ~13 and like are of the same value ~t a segment
where the line a is made linear (from the points 14' and
13' to a point assumed as the point 9' unshown). They
are varied by minute change in volume with a segment de-
scribing a uniform acceleration curve such as that which
may be a circle or a sine curve (from the point assumed
as the point 9' to the point 1'). Then, the turn angle
would be a4, a3 a2, successively, and the travel amount
would be ~4, ~3~ ~2' successively. Thereafter, the turn
angle ~ and the travel amount ~ would be 0 at a segment
describing a horizontal line. It is apparent from this
that the travel amount TRo of the rack, which is gained
by applying no rotation to the rack is reduced to the
initially set amount of travel of the rack) as best shown
in Fig. 2.
The present invention is intended to generate on
a curve defined by the face of the rack rod the variable
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ratio pitch teeth suggested in ~ritish Patents Nos. 667038
and 3267763. Figs. 3 and 4 are diagrammatic illustrations
of the manner in which the teeth are generated in accord-
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ance with the present invention.
In order to cut the variable ratio tooth pitches
21, 22, .. .....33, 34 on the axis X of the rack, the cutter
having teeth 21, 22, ...~. 33, 3~ of nonuniform pitches
5 Pl ........ P14 is required to advance at the helical angle
~ in the direction of arrow A to give movement to the rack
bar 1 by the travel amount which may be varied to some ex-
tent in the axial direction X of the rack thereby obtain-
ing the desired pitches. This will be apparent from the
aforementioned British Patents. It is also evident there-
from that the same may be obtained by change in a rota-
tional speed of the object to be cut with respect to a
rotational speed of the cutter if the cutter serves as
the gear. In the first procedure according to the present
invention, supposing that the cutter has the variable
ratlo tooth pitches Pl, P2 ...... P13, P14,
advanced at the helical angle ~ in the direction of arrow
A to make the cut on the line a of rack rotation about
the rack bar 1 in the same manner as explained with
2~ reference to Fig. 2 to obtain the rack teeth 1'', 2'' .....
13'', 14'' of given variable ratio tooth pitches. At
this point, the rack travel is subjected to a reduction
from the amount TRl (Fig. 2) to the amount TR2 ~Fig. 3).
In this connectionl it is noted that since the initially
prescribed rack travel should have included the travel
amount TRl as given to the pinion 3, the rack travel TR2
is insufficient to this end.
Thus, in the second procedure according to the
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present invention~ a neW line b of the rack rotation is
described to connect the intersections 1', 2', ...... 13',
- 14' at which perpendiculars from the points 1', 2', .....
13', 14' (Fig. 2) intersect lines which are described at
the helical angle a from the points 21, 22 .... 33, 34 on
the rack axis X, corresponding to the rack teeth 1'', 2 " ,
.... 13'', 14'' (Fig. 3). This line b is drawn at an in-
` clined angle ~' much smaller than the inclined angle ~ of
the rack rotation. More specifically, upon tracing the
, 10 line b defined by rotation of the rack through the rack bar
1, the cutter of variable ratio tooth pitches is used to
; ~ continuously impart the rack axis rotations a'l, a'2,
a'l3 (only '13 is shown) and the rack axis movements ~
'2' ~'13 ~only ~'13 is shown) to make the cut there-
lS by manufacturing a rack of the same variable ratio tooth
pitches as in the rack shown in Fig. 2 (or described in
j, ~ . .
Japanese Patent Public Disclosure-No. 90224/80). In the
continuation of the second procedure, according to the
present invention, as previously described in connection
20 ~with~Fig. 4, thé cutter of uniform pitch Po is actuated in
~ the manner~as explained with reference to Fig. 2 or 3 to
s ~ make the cut along a line which is described at a helical
angle~' defined by connecting the points 1', 2', .... 13',
14' (the same as in Fig. 3) on the line _ (the same as in
25;~Fig. 3)~to the points 1, 2, .... 13, 14 (the same as in
Fig. 2) on the rack axis X while continuously applying the
rack axis rotations a 1~ ~ 2~ .......... 13 13
shown) and the rack axis movements 8 1~ ~ 2~ ~ 13
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(only ~''13 is shown) so that the cutter of uniform pitch
Po may be used at the helical angle ~' (the same as in Fig.
3) to obtain a rack of 'the same variable ratio tooth pitch
as in the rack shown in Fig. 3 having the rack travel TRl
(the same as in Fig. 2). At this time, the helical angle
~' shown in Fig. 4 is less than the helical angle a shown
in Fig. 3, and the'inclined angle ~' shown in Fig. 4 is
- equal to the inclined angle ~' shown in Fig. 3 but is less
than the inclined angle ~ shown in Fig. 2.
10In the embodiment of the invention described above,
the rack may have 50~ of the gear ratio. A narrow inclined
angle ~' formed with the line b of rotation of the rack
bar results in the considerable increase in torque'needed
for rotation of the rack bar, making possible load on the
component'extremely lighter. Further, since the teeth
are those generated by continuous cutting of the gear
cutter, a self rotational force due' to its meshing with
the rack requires no cam track for rotation and other
parts for rack rotation.
~0In order that the invention may be better under-
stood, a preferred form thereof where a gear cutting is
effected by the use of a gear shaper is illustrated in
Fig. 5 wherein a gear cutter 101 is a pinion o~ 5 teeth
and a standard pitch line 12 is made as in the standard
pinion. A variable pitch line 103 is, however, scribed
to form rack teeth of variable pitches. For the sake of
simplification of explanation, a circle defined by the
gear cutter is divided by 5 to obtain arc lengths Pl,
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P2' P3~ P4 ~ P72 (CrresPonding to one revolution of
the cutter - 3~0/5) ...... Pn, of sectors defined by
radii and the variable pitch lines intersecting therewith,
that is, Pl P2 P3 P4 P43 ( Po) ---- P72 ----
>Pn. On the other hand, each of the arc lengths of 5
sectors defined by radii and the standard pitch lines is
P0 - constant.
The gear cutter 101 is so dri~en by a slidable
movement of its shaft in an axial direction so as to
approach from a point Pl upstream of a center~line 104
along which the center is moved, to a rack bar 1 mounted
in parallel with the center line 104 to effect a pre-
determined depth of cut on the rack bar. Whereupon, the
gear cutter is rotated ~a cut is shown as made by inter-
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mittent rotation) by every 5 at a constant rate simulta-
neously with its movements such as Pl, P2, P3, ..... Pn
at every rotation by 5 along the center line 104 to thus
r~obtain the racX teeth of variable pitches Pl ...... P5 on
the rack bar 1. Tooth forms thus obtained may be plotted
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as variable tooth curves as diagrammatically shown.
.
In practically machining the gear shaper, the axis
of the gear cutter is made stationary whereas the rack
bar is moved in ~an axial direction thereof.
As is apparent from the foregoing, according to the
present invention, the rack teeth of variable pitches may
;~ ~ be readily generated by the same machining operations as
those for a conventional gear shaper except that fine
~; axial movement and fine rotation as aforementioned are
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given to the rack bar by a numerical control or a cam.
Accordingly, a straight tooth type rack, and a helical
rack of widely variable pitches may be obtained by only
conversion of rack movement system for a standard gear
shaper.
Because the pitches of the rack teeth are gradually
varied from the center of the rack bar to one end thereof,
as the steering wheel is turned to the right or left from
its neutral position, a very lightened steering force is
obtained.
While the preferred embodiments have been described~
many modifications in design can be made without departure
from the spirit and scope of the present invention.
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