GB2123921A

GB2123921A - A metal rack bar and a method of manufacturing same

Info

Publication number: GB2123921A
Application number: GB08221108A
Authority: GB
Inventors: Peter Rowland Rogers
Original assignee: TRW Steering Systems Ltd
Current assignee: TRW Steering Systems Ltd
Priority date: 1982-07-21
Filing date: 1982-07-21
Publication date: 1984-02-08
Also published as: GB2123921B; DE3326170A1; IT1162922B; FR2530508A1; IT8367786A0; FR2530508B1

Abstract

A metal rack bar, a method of manufacturing a rack bar and a steering gear which includes such a bar. The rack bar is formed from a stock metal bar having a length less than the rack bar 2 which is to be manufactured. The rack bar 2 has a part length 3 within which is formed an array of rack teeth 4 and a part length 5 of relatively reduced diameter. The parts 3 and 5 merge through shoulder 6. The part length 5 is formed by subjecting the bar to a swaging operation which reduces its diameter and extends it longitudinally to the required length. That swaging operation provides a continuous metal grain in the surface of the workpiece. The end of the part length 5 remote from the rack teeth can be machined to provide a coupling 7 for incorporating the bar in a steering linkage. <IMAGE>

Description

SPECIFICATION A metal rack bar, a method of manufacturing a rack bar and a steering gear which includes such a bar Technical field and background art This invention relates to a metal rack bar, a method of manufacturing a rack bar from a stock metal bar and a steering gear which includes such a rack bar.

Steering gears of the rack and pinion type are well known in the automobile and related industries.

Conventionally such a gear has a pinion rotatably mounted in a housing with the teeth of the pinion engaging an array of rack teeth on a rack bar which extends longitudinally through the housing so that when the pinion is rotated in response to a steering input the rack bar is longitudinally displaced relative to the housing to provide a steering output. In its usual form the rack bar has one end part length on which the rack teeth are formed by machining (such as broaching or by a coining or other pressure moulding technique) while the other end of the rack bar is usualiy plain and provides a convenient extension from the rack portion for connection to the appropriate steering linkage as is well known in the art.Conventionally rack bars are formed from stock metal workpieces and their manufacture involves considerable machining operations where metal is likely to be wasted by removal from the bar, particularly in the finishing of the part length of the bar which extends from the rack portion. The workpiece is cut to a length which corresponds at least to that required of the finished rack bar and the weight, size and cost of a rack bar which is formed therefrom by conventional means is a significant proportion of the weight, size and cost of a steering gear which includes that bar. There is an ever increasing requirement in the steering gear industry that components of a gear should be capable of economic manufacture and permit a reduction in the overall size and weight of the gear without reducing the efficiency, reliability or safety characteristics expected of the gear.It is an object of the present invention to provide an improved rack bar and a method of manufacturing such a rack bar which leads towards satisfying the aforementioned requirements.

Statements ofinvention and advantages According to the present invention there is provided a metal rack bar comprising a first part length having formed therein an array of rack teeth and a second part length which is substantially plain and extends longitudinally from the first part length; the second part length being of substantially constant lateral section the area of which section is less than that of a similar section on the first part length and a peripheral shoulder being provided between said part lengths, and wherein the surface of the bar over the second part length from a position at or adjacent the end thereof remote from the first part length and over a transitional part length of the bar which extends over said shoulder between the first and second part lengths is unbroken to provide a continuous metal grain in that surface.

Further according to the present invention there is provided a method of manufacturing a rack bar from a stock metal bar having a first end part length on which an array of rack teeth are, or are to be, formed which comprises working a second, other end, part length of the bar to reduce the lateral cross sectional area of the second part length relative to the first part length and provide a peripheral shoulder between said part lengths whilst simultaneously increasing the longitudinal extent of the second part length and in said working maintaining an unbroken surface over at least the major extent of the second part length from said shoulder and over a transitional part length of the barwhich extends over said shoulder between the first and second part lengths to provide a continuous metal grain in that surface.

Still further according to the present invention there is provided a steering gear which includes a metal rack bar as specified in the penultimate preceding paragraph or as manufactured by the method specified in the immediately preceding paragraph.

The continuous metal grain provided in the surface of the rack bar which extends over the shoulder between the first and second part lengths of the bar and therefrom over at least the major extent of the second part length is preferably achieved by subjecting the stock metal bar to a swaging operation whereby the lateral cross sectional area of the bar is progressively reduced. Usually the stock metal bar or workpiece will be cylindrical and of a diameter corresponding to that required for the first part length on which the array of rack teeth are to be formed. The longitudinal extent of the cylindrical bar corresponding to the second part length may then be reduced in diameter by swaging, preferably by a rotary swaging technique where the bar is subjected to radially acting percussive rollers.During the reduction in the diameter (and thereby in the lateral cross sectional area) of the workpiece as aforementioned there will necessarily be an increase in the longitudinal extent of the second part length. This latter increase can be allowed for so that, following the swaging operation, the length of the second part length which results is consistent with the overall length of the rack bar being that which is required.

The swaging technique has the advantages that the decrease in the diameter of the rack bar for a substantial part of its length can provide a significant reduction in the weight of the bar (as compared with a similarly sized cylindrical bar of constant diameter); that the bar can be accurately formed to its required final length; that wastage of material used in the manufacture of the rack bar is alleviated, and that following the swaging operation, finish machining of the surface of the bar over its transitional part length and its second part length is alleviated. These advantages will clearly reflect in reduced manufacturing costs of the rack bar.Furthermore, the relative reduction in the lateral cross sectional area of the second part length of the rack bar relative to its first part length is advantageous as it may permit the use of a smaller than usual housing for the rack bar in a steering gear and the use of smaller than usual bearings within which the second part length of the rack bar may be borne for its longitudinal displacement within the housing (or within a tubular extension of the housing).There is also the possibility that by reducing the lateral dimensions of the second part length, for example by swaging as mentioned above, so that the metal grain in the specified surface of the rack bar is unbroken, greater rigidity may be provided in the rack bar as compared with a similarly sized rack bar in which part of its length corresponding to the second part length is merely reduced in diameter (as compred to its part length which carries the rack teeth) by machining off metal.

The stock metal bar from which the rack bar is manufactured may be solid or tubular. It is frequently desirable that the rack bar has an axially extending passage through which fluid such as air or liquid can flow between axially spaced chambers into which the rack bar may extend in a steering gear. By manufacturing the rack bar from a tubular workpiece the aforementioned axially extending passage can conveniently result from the bore of such a workpiece (even though it is realised that the bore will be reduced in diameter as the lateral cross sectional area of the second part length is reduced).

Conventionally the second part length of the rack bar provides an extension from its rack portion for convenience of coupling the rack bar into a steering linkage, for example through a universal joint to a tie rod orto a servo motor such as a piston and cylinder device for a power assisted steering gear. Whilst the present invention alleviates machining of the workpiece in the sense of removing metal therefrom during the formation of the second part length, it is realised that this second part length may subsequently be subjected to machine cutting as may be required, for example, to provide the free end of the second part length with an internal or external screw thread by which that end can be coupled to another component in the steering linkage.

The array of rack teeth can be formed on the bar prior or subsequent to the formation of the reduced diameter second part length of the bar. The rack teeth may be formed by any convenient means such as by a broaching or other machine cutting technique or by a coining or other forging type technique.

While a steering gear which includes the rack bar of the present invention will usually be of the rack and pinion type it will be realised that the rack bar can be incorporated in othertypes of steering gear where the toothed component which engages with the array of rack teeth is other than in the form of a pinion and may, for example, be a toothed sector.

Drawings One embodiment of a metal rack bar constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings in which: Figure 1 shows a length of cylindrical stock bar from which the rack bar is to be manufactured and Figure 2 shows the rack bar manufactured from the stock workpiece of Figure 1 and is comparable therewith for an appreciation of the increase in length which is achieved during said manufacture.

Detailed description of drawings A solid cylindrical steel bar 1 is cut from stock to a predetermined length which is somewhat less than the length of a rack bar 2 which is to be manufactured from it. The rack bar 2 has a first part length 3 within which is formed an array of rack teeth 4. The part length 3 has a diameter substantially corresponding to that of the workpiece 1. Extending longitudinally from the part length 3 is a relatively reduced diameter plain cylindrical part length 5. The part lengths 3 and 5 are concentric and merge through a transitional part length 6 which forms a shoulder between the parts 3 and 5.

The reduced diameter part length 5 is formed by subjecting the workpiece 1 (other than for the part length of the workpiece 1 which is to correspond to the part length 3 on the finally formed rack bar) to a swaging operation whereby the diameter of the workpiece is progressively reduced and longitudinally extended. The transformation of the workpiece 1 to form the shoulder 6 and extended second part length 5 can be achieved by use of a rapid swaging machine with a radial-action roller percussion system where the workpiece is located in a jaw plate of the machine and within which it is subjected to radial pressure to provide the cylindrical reaction.During this swaging, the surface oftheworkpiece over its length from which the shoulder 6 and second part length extension 5 are formed is unbroken so that such continuous metal grain as was present in the surface oftheworkpiece 1 exists in the shoulder 6 and part length 5 of the rack bar.

Prior to or following the formation of the part length 5, the array of rack teeth 4 can be formed, conveniently by conventional means such as broaching or other means such as swaging or otherwise cold forming.

In a typical rack and pinion steering gear of which the rack bar 2 may form part, the ends of the rack bar will be coupled to tie rods (not shown) to form part of the steering linkage. For this coupling the ends of this rack bar 2 may be subjected to a machine cutting operation, for example, to provide screw threaded holes 7 in those ends.

By comparing Figures 1 and 2 it will be seen that the rack bar 2 is considerably longer than the workpiece 1 so that there is a substantial saving in stock material as compared with a rack bar manufacturing technique in which the part length 5 is formed by cutting metal from a workpiece the length of which corresponds to the length of the rack bar.

Furthermore, the continuous grain over the shoulder 6 and part length 5 of the rack bar alleviates the likelihood of the rack bar being unnecessarily weakened in the region of its shoulder 6 (as may occur by the aforementioned machine cutting of the part length 5). As will be appreciated from the aforegoing description the length of the workpiece 1 can be determined without difficulty from knowing the length required of the rack bar (particularly that required of the part length 5 and the diameter which is required for that part length).

Claims

CLAIMS (Filed on 20/6/83)

1. A metal rack bar comprising a first part length having formed therein an array of rack teeth and a second part length which is substantially plain and extends longitudinally from the first part length; the second part length being of substantially constant lateral section the area of which section is less than that of a similar section on the first part length and a peripheral shoulder being provided between said part lengths, and wherein the surface of the bar over the second part length from a position at or adjacent the end thereof remote from the first part length and over a transitional part length of the bar which extends over said shoulder between the first and second part lengths is unbroken to provide a continuous metal grain in that surface.

2. A rack bar as claimed in claim 1 in which the first and second part lengths comprise tubular portions which together form a longitudinally extending passage within the bar.

3. A rack bar as claimed in either claim 1 or claim 2 in which the second part length is substantially cylindrical.

4. A rack bar as claimed in any one of the preceding claims in which the second part length extends longitudinally from the peripheral shoulder to coupling means by which the rack bar is intended to be coupled into a steering gear.

5. A rack bar as claimed in claim 4 in which the coupling means comprises a machined region of the bar.

6. A rack bar as claimed in any one of the preceding claims in which, substantially, the second part length is an end part length of the rack bar.

7. A metal rack bar as claimed in claim 1 and substantially as herein described.

8. A method of manufacturing a rack bar from a stock metal bar having a first part length on which an array of rack teeth are, or are to be, formed which comprises working a second part length of the bar to reduce the lateral cross sectional area of the second part length relative to the first part length and provide a peripheral shoulder between said part lengths whilst simultaneously increasing the longitudinal extent of the second part length and in said working maintaining an unbroken surface over at least the major extent of the second part length from said shoulder and over a transitional part length of the bar which extends over said shoulder between the first and second part lengths to provide a continuous metal grain in that surface.

9. A method as claimed in claim 8 in which,.

substantially, the first part length is a first end part and the second part length is the other end part of the bar.

10. A method as claimed in either claim 8 or claim 9 which comprises working the second part length of the bar by swaging to reduce the lateral cross sectional area of that part length and provide the peripheral shoulder.

11. A method as claimed in any one of claims 8 to 9 which comprises manufacturing the rack bar from a tubular stock metal bar.

12. A method as claimed in claim 11 which comprises working the tubular bar to maintain at last part length of the bar therein, said part length providing a longitudinally extending passage within the bar.

13. A method as claimed in any one of claims 8 to 12 which comprises forming on the bar at the end of the second part length remote from the first part length coupling means by which the rack bar is intended to be coupled to a steering linkage.

14. A method as claimed in claim 13 which comprises machining the bar to form said coupling means.

15. A method as claimed in either claim 13 or claim 14when appendant to claim 9 which comprises forming coupling means at the end of the bar which end is substantially at the end of the second part length of the bar remote from the first part length.

16. A method as claimed in any one of claims 8 to 15 which comprises providing the array of rack teeth in the first part length of the bar subsequent to reducing the lateral cross sectional area of the second part length to that which is required.

17. A method of manufacturing a rack bar as claimed in claim 8 and substantially as herein described.

18. A rack bar when manufactured bythe method as claimed in any one of claims 8 to 17.

19. A steering gearwhich includes a metal rack bar as claimed in any one of claims 1 to 7 or claim 18.