CA1210615A - Variable ratio rack pinion steering gear and method for cutting rack teeth therein - Google Patents

Abstract

VARIABLE RATIO RACK PINION
STEERING GEAR AND METHOD
FOR CUTTING RACK TEETH THEREIN

Abstract of the Disclosure A variable ratio rack pinion steering gear and a method for cutting rack teeth therein are disclosed herein. A rack bar has variable ratio rack teeth which are formed along a given line on the rack bar. The pitches of the teeth are gradually varied from the center of the rack bar to one end thereof. The teeth are of the continuous generating type and are formed by a numerical-control cutter having a plurality of cutting teeth.

Description

121~i5 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 .....

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 12~ S
~ -6-1 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 B

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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 .. . . . . .. .. . . . . . . .

~Z~ 5 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 ' 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-.,,~, .. . . . . . . . . .

-- ~z~ s 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 ` ~21~S

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, .21~r~5 - 12 ~
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-. , . ~ .
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 .
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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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a variable ratio rack pinion steering gear comprising a pinion rotatable by being subjected to a steering torque derived from a steering wheel, and a rack bar being movable axially along and rotatable about the longitudinal axis thereof by driving engagement with said pinion, the rack bar having a series of rack teeth which are formed directly on the rack member along a given line curving at least part way around the rack bar, said rack bar being characterized in that the distance between adja-cent teeth is gradually varied from the center of said rack bar to one end thereof.

2. A variable ratio rack pinion steering gear as claimed in Claim 1 wherein the pinion includes teeth of uniform pitch.

3. A variable ratio rack pinion steering gear as claimed in Claim 1 wherein the aforementioned rotation and movement are different from those effected when the cutter having teeth of uniform pitches is used to form rack teeth on a different path defined by rotation of said rack.

4. A variable ratio rack pinion steering gear as claimed in Claim 2 wherein the rack travel of said rack bar

Claim 4 cont.

travel initially set for the cutter having teeth of uni-form pitches.

5. A method for rack cutting on a variable ratio rack pinion steering gear characterized by being composed of disposing a cutter of standardized tooth pitches with respect to the axis of a rack bar to form a given helical angle, disposing a cutter of standardized tooth pitches with respect to the axis of a rack bar to form a given helical angle, obtaining a variable ratio having a given inclined angle and virtual path of rack rotation with the rack travel being such that said rack bar is rotated about the axis thereof at a given turn angle and is moved in a given amount of movement in the axial direction of said rack shaft to make the cut by means of said cutter, placing the cutter of said variable ratio tooth pitches on said virtual path of rack rotation at a said given helical angle, obtaining another variable ratio with another inclined angle and another virtual path of rack rotation based on another rack travel in such a manner that the cut is made by applying another turn angle and another amount of movement to said rack bar, positioning the cutter of said standardized tooth pitches on said other virtual path of rack rotation at another helical angle, and providing not only a variable ratio substantially identical with said given variable ratio but also initially set rack travel by applying another turn angle and another amount of movement to said rack bar.