GB2135013A

GB2135013A - Variable ratio rack pinion steering gear and method for cutting rack teeth therein

Info

Publication number: GB2135013A
Application number: GB08303736A
Authority: GB
Inventors: Michio Abe
Original assignee: Tokai TRW and Co Ltd
Current assignee: Tokai TRW and Co Ltd
Priority date: 1983-02-10
Filing date: 1983-02-10
Publication date: 1984-08-22
Also published as: GB8303736D0

Abstract

A variable ratio rack pinion steering gear and a method for cutting rack teeth therein are disclosed. A rack bar 1 has variable ratio rack teeth 2 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. <IMAGE>

Description

SPECIFICATION Variable ratio rack pinion steering gear and method for cutting rack teeth therein 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 rackteeth 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 Public Disclosure No.90224/80.

Those disclosed in such printed publications involve defects as discussed hereinunder. British Patent No.609356 has disadvantages 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 ration 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 consideration ofthe strength characteristics of the tooth.A variable ratio steering 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 microvibrationsfrom outside in mesh due to move ment between tooth top and tooth root so that the rack teeth lackstength. 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 includes uniform teeth in mesh with a rackgearto be driven, the rack gear having a variable pitch diameter to obtain a variable ratio therebetween, are such that it is noteasyto manufacturethe rackgearto be driven andthata higher pressure angle should be employed to obtain a lower ratio between the opposite ends of the rack bar. As a result, a great change in efficiency in teeth when they mesh with each other, represents an impulse given to a steering wheel.There is a limitation on selection of a lower ratioto avoid such impulsetotherebyprovide an insufficient transmission gear ratio.It has been found defective in Australian Patent No.241 798 that a steering wheel is caused to be driven over the entire movement region as required lessthantwo(2)turns due to repeated changes in mesh ratio shown whenever it is rotated, thereby lessening the average ratio. The system in accordance with Japanese Patent Public Disclosure No.90224/80 is designed so that uniform pitch rackteeth are formed along a path of rotation ofthe rack bar, defined by a resultant curve consisting of the axial line and the spiral line of the rack bar, and that axial movement of the rack bar at a variable speed with respectto steady rotation of a helical pinion in mesh with the rackteeth to obtain about 50% ofthe gear ratio.The shortcoming derived from such system isthata high-speed rotation is required for the rack barto 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 rackteeth are formed one by one on a cylindrical rack bar by means of a cutter 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 ofthe objects ofthe present invention is to eliminate the aforementioned defects in the prior art by providing anew and an improved variable ratio rack pinion steering gear and a method for cutting the same.

Another object ofthe present invention is to 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 ofthe 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 ofthe rack bar (hereinafter referred to as "rack axis rotation and rack axis movement", respectively).

Another object ofthe 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 ofthe rack bar and the teeth are formed by a cutter, for example, a gear shaper or milling machine which is adapted to make three orfour 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 thatwhen cutting the rackteeth in a rack bar bythe use of a cutter having a variable pitch ratio, the aforementioned rack axis rotation and rack axis movement are continuously imparted to the rack barto allow the cutter to effect the gear cutting at a given helical angle with the axis (hereinafter referred to as "extent of rotation") and the extent of movement in the rack axis (hereinafter referred to as "extent of displacement") are continuouslyvaried 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 mannerthatwhen 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 barto effect a gear cutting at another helical angle so thatthose differentfromthe 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 which 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.

Theforegoing and other objects andfeatures ofthe present inventio will become more apparent upon a consideration of the following description taken in connection with the accompanying drawings wherein: Fig. is a fragmentaryviewshowing a configuration ofthe preferred form of a variable ratio rack pinion steering gear embodying the present invention; Figs. 2,3, and 4 are diagrammatic illustrations of howthe teeth are cut according to the present invention; and Fig. 5 is a viewshowing a cutterforming teeth on a rack bar.

Referring nowto Fig. 1 ,there is 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 2and issupported to rotate about the axis thereof. The pinion 3 is connnectedto a steering wheel (notshown).The rack bar 1 is set to rotate aboutthe axis X ofthe rack along line a orb defined by rotation ofthe rack bar and to move in the direction ofthe axis ofthe 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 a' with the rack axis and may be assumed as a cutter of uniform pitches Po.The rack teeth 2 in engagement with the pinion 3 are generated along a line bof rotation ofthe rack having an inclined angle ss' (ora line a of rotation ofthe rack having an inclined angle ) and includes tooth pitches so varied as to be reduced in the direction of the end.

Now, howthe helical rack teeth 2 as above generated 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 Japanese Patent Public Disclosure No.

90224/80, Michio Abe, the inventor of the gear cutting method 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 "cutter"). The points 1,2,3, ..... 14 also serveto show numbers ofthe cutter thereat. Avertical axis Y is indicative of a turn angle formed around the rack axis X.The line defined by rotation ofthe rack bar will pass through the center of each ofthe pitches ofthe rackteeth 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 at a helical angle awith the axis X ofthe rack barto make the cut in the rack in numerical order as numbered from 14to 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, 14 each have uniform pitch Po to reach the amount of racktravel, say TRO- 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 ofa13to a point 131''' at which the perpendicular from the point 13' intersects the horizontal line from the point 14' before cutting a tooth 13' aftercutting teeth 14'. Further, it is necessaryto move rack bar 1 in the direction ofthe axis X ofthe rack (to the left in Fig.

2) by the amount 513 oftravel at a distance to a point P1"' atwhich a line extended at a helical angle a from the point 131"' intersects the horizontal line from the point 14' so that tooth interference may be avoided.

There upon,the turn angle α13 and the travel amount 513 and like are ofthe same value at 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 describing a uniform acceleration curve such as thatwhich may be a circle or a sine curve (from the point assumed as the point 9' to the point I '). Then,the turn angle would be a4, a3, a2, successively, and the travel amount would be 04, 3, 2, successively.

There after, the turn angle α and the travel amount would be Oat a segment describing a horizontal line. It is apparentfrom this thatthetravel amountTR0 of the rack, which is gained by applying no rotationtothe 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 bytheface ofthe rack rod the variable ratio pitch teeth suggested in British Patents Nos.

667038 and 3267763. Figs. 3 and 4 are diagrammatic illustrations ofthe manner in which theteeth are generated in accordance with the present invention.

In orderto cut the variable ratio tooth pitches 21,22, ..... 33,34 on the axis X of the rack, the cutter having teeth 21,22, 33,34 of nonuniform pitches Pa ..... P14 is required to advance atthe helical angle a in the direction of arrow A to give movement to the rack bar 1 bythetravel amount which may be varied to some extent in the axial direction X of the rack thereby obtaining the desired pitches. This will be apparent from the aforementioned British Patents.It is also evident therefrom that the same may be obtained by change in a rotational speed ofthe object two be cut with respect two a rotational speed ofthe cutter if the cutter serves as the gear. In the first procedure according to the present invention, supposing that the cutter has the variable ratio tooth pitches P1, P2 --- P13, P14, this cutter is advanced atthe helical angle a in the direction of arrowAto make the cut on the line a of rack rotation aboutthe rack bar 1 in the same manner as explained with reference to Fig. 2 to obtain the rack teeth 1", 2 ",2" 13", 14" of given variable ratio tooth pitches. At this point, the rack travel is subjected to a reduction from the amountTR1 (Fig.2) to the amount TR2 (Fig. 3). In this connection, it is noted that since the initially prescribed rack travel should have included the travel amountTR1 as given to the pinion 3, the rack travel TR2 is insufficient to this end.

Thus, in the second procedure according to the present invention, a new line b ofthe rack rotation is described to connectthe intersections 1 ',2', 13', 14' atwhich perpendicularsfromthe points 1',2' 13', 14' (Fig. 2) intersect lines which are described at the helical angle from the points 21,22 ..... 33, on the rack axis X, corresponding to the rack teeth 1 2", ....13",14" (Fig.3).This line b is drawn at an inclined angle ss' much smallerthan the inclined angle ss of the rack rotation.More specifically, upon tracing the line b defined by rotation of the rackthrough the rack bar 1, the cutter ofvariable ratio tooth pitches is used to continuously impartthe rack axis rotations a'1, a'2 a'13 (only a'13 is shown) and the rack axis movements '1, a'2 ..--. '13 (only '13 is shown) to make the cut thereby manufacturing a rack of the same variable ratio tooth pitches as in the rack shown in Fig. 2 (or described in Japanese Patent Public Disclosure No.

90224/80). In the continuation ofthe second procedure, according to the present invention, as previously described in connection with Fig. 4, the cutter of uniform pitch Po is actuated in the manneras explained with reference to Fig. 2 or 3 to make the cut along a line which is described at a helical angle a' defined by connecting the points 1',2',....13',14' (the same as in Fig. 3) on the line b (the same as in Fig. 3) to the points 1,2, 13,14(the same as in Fig. 2) on the rack axisXwhile continuously applying the rack axis rotations a"1, a"2, .... a''13 (only a"13 is shown) and the rack axis movements #"1,#"2.... 5"13 (only a"13 is shown) so that the cutter of uniform pitch Po may be used atthe helical angle ss' (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 racktravel TR1 (the same as in Fig. 2). Atthis time, the helical angle a' shown in Fig. 4 is less than the helical angle a shown in Fig. 3, and the inclined angle ss shown in Fig. 4 is equal to the inclined angle ss shown in Fig. 3 but is less than the inclined angle ss shown in Fig. 2.

In the embodiment ofthe invention described above, the rack may have 50% of the gear ratio. A narrow inclined angle ss' formed with the line b of rotation ofthe 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, sincethe 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 trackfor rotation and other parts for rack rotation.

In orderthatthe invention may be better understood, a preferred form thereof where a gear cutting is effected by the use of a gear shaper is illustrated in Fig.

5wherein a gear cutter 101 is a pinion of 5 teeth and a standard pitch line 12 is made as in the standard pinion. Avariable pitch line 103 is, however, scribed to form rackteeth ofvariable pitches. For the sake of simplification of explanation, a circle defined by the gear cutter is divided by 5 to obtain arc lengths P1, P2, P3, P4 P72 (corresponding to one revolution ofthe cutter - 360"'5C Pn, of sectors defined by radii and the variable pitch lines intersecting therewith, that is, P1 > P2 > P3 > P4 > P43 ( =PO) > P72 > P. On the other hand, each of the arc lengths of 5" sectors defined by radii and the standard pitch lines is PO = constant.

The gear cutter 101 is so driven by a slidable movement of its shaft in a axial direction so asto approach from a point P1 upstream of a center line 104 along which the center is moved,to a rack bar 1 mounted in parallel with the center line 104to effect a predetermined depth of cut on the rack bar. Whereupon, the gear cutter is rotated (a cut is shown as made by intermittent rotation) by every 5 at a constant rate simultaneously with its movements such as P1, P2, P3, ..... Pn at every rotation by 5 along the center line 104 to thus obtain the rack teeth of variable pitches P1 P1.....

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 rackteeth ofvariable pitches may be readily generated by the same machining operations as those four a conventional gear shaper exceptthatfine axial movementandfine rotation as aforementioned are 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 systemfora standard gearshaper.

Because the pitches of the rackteeth are gradually varied from the centerofthe rack barto one end thereof, asthesteering wheel is turned to the right or left fro 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 ofthe present invention.

Claims

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 having rackteeth engageable with said pinion and formed in a given line to rotate about the axis of the rack bar and move in the axial direction ofthe rack bar, said variable ratio rack pinion steering gear being characterized in that said rack teeth have variable ratio pitches and said pitches being gradually varied from the center of said rack bart one end thereof.

2. Avariable 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. Avariable ratio rack pinion steering gear as claimed in Claim 2 wherein a turn angle defined by said rotation and an amount of said movement are of the samevalue at a segment where said given line of rack rotation is made linear but is varied with respect tothefollowing curves by minute changes in volume at a segment where said line describes a uniform acceleration curve or other curves whereas they would be Oat a segment describing a horizontal line.

5. Avariable ratio rack pinion steering gear as claimed in Claim 2 wherein the racktravel of said rack bar due to said rotation and movement is the same as the rack travel initially setforthe cutter having teeth of uniform pitches.

6. 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 racktravel 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 rackshaftto 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 anothervariable ratiowith another inclined angle and anothervirtual path of rack rotation based on another rack travel in such a mannerthatthe cut is made by applying anotherturn angle and another amount of movement to said rack bar, positioning the cutter of said standardized tooth pitches on said othervirtual path of rack rotation at another helical angle, and providing not only a variable ratio substantially indentical with said given variable ratio but also initially set rack travel by applying anotherturn angle and another amount of movement to said rack bar.

7. A method for rack cutting on a variable ratio rack pinion steering gear as claimed in Claim 5 characterized in that said otherturn angle, said other amount of movement, said other inclined angle, said other variable ratio, and said other path of rack rotation are all predetermined values.

8. Avariable ratio rack pinion steering gear substantially as herein described with reference to, and as illustrated in, the accompanying drawings.

9. A method for rack cutting on a variable ratio rack pinion steering gear substantially as herein described with reference to, and as illustrated in, the accom- panying drawings.