GB2056894A - Manufacture of rack member for rack and pinion assembly - Google Patents

Abstract

A longitudinally extending rack member for a rack and pinion assembly is formed, e.g. by rolling, from a metal bar the grain 11 of which extends longitudinally of the bar and continuously over the length of the bar within which the rack teeth 7a are to be formed. The rack teeth 7a are formed by coin pressing between opposed dies so that the metal grain 11a follows the form of each tooth to be continuous over the array of rack teeth and each tooth 7a is devoid of end grain. Following coin pressing the teeth 7a are not subjected to machining so that the continuous grain is maintained to provide reinforcement against fracture of the teeth. The rack teeth 7a can be coin pressed in a machined recess in the bar which recess has opposed faces 4 and 5 within which faces end grain 12 is exhibited. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the manufacture of rack and pinion assemblies This invention relates to a method of manufacturing a longitudinally extending rack member for a rack and pinion assembly.

Rack and pinion assemblies are well known, particularly in the art of steering gears, whereby the rack member has a longitudinally extending array of rack teeth each of which extends laterally of that member. When incorporated in a rack and pinion assembly, the rack member is longitudinally displaceable in a housing while the rack teeth engage with the teeth of a rotatably mounted pinion, so that upon rotation of the pinion the rack is displaced longitudinally. The teeth of the rack member can take many different profiles to provide the desired engagement with the pinion and a required ratio of drive which is transmitted through the assembly usually from the pinion to the rack bar.In use the rack teeth can be subjected to considerable shear forces, especially in a steering gear installation and consequently the material and profile of the teeth must be of a form and design which is sufficient to withstand such forces and thereby alleviate fracture of the teeth.

In many instances the design of gear ratio which is to be provided between the rack and pinion requires the use of rack teeth having a relatively fragile formation over the crest regions. This may be particularly encountered where the profile of the rack teeth changes for successive teeth in the rack as in the case where the rack is intended to provide a variable ratio during its engagement with the pinion. Examples of such variable ratio rack teeth design are disclosed in U.K. Patent Specification Nos. 977,434 and 1,356,172 where one or more of the teeth in a rack can have a varying profile over its lateral extent to provide a predetermined change in gear ratio as the pinion rotates in engagement with the tooth; this variation in lateral profile of the teeth may necessitate in relatively narrow, poorly supported and thereby frangible crest regions.As discussed in the aforementioned prior Specifications it has hitherto been the generai practice to form the rack teeth wholly by a machining process such as broaching or generating technique or by such a machining technique on a preform of the rack member (which preform may, for example, be hot or cold forged). Machining of the rack member to form the teeth either from a preform or from a plain bar is a lengthy and expensive process, particularly where the teeth have a non-uniform structure as with the aforementioned variable ratio rack members and furthermore it is found that the machining of the teeth can provide regions of weakness where the teeth are cut away to form relatively narrow crests with steeply inclined adjacent flanges.

It is an object of the present invention to provide a method of manufacturing a rack member which is relatively inexpensive, is applicable to a wide variation of tooth structure-this variation may be as between one type of rack and another or as between individual teeth on the same rack where variable ratio steering is involved, it facilitates the manufacture of robust teeth and alleviates the disadvantages of the conventional manufacturing techniques as discussed above.

According to the present invention there is provided a method of manufacturing a longitudinally extending rack member for a rack and pinion assembly said member having a longitudinally extending array of rack teeth each tooth of which extends laterally of the member and which method comprises providing a metal bar the grain of which extends longitudinally thereof and coin pressing the bar to form therein the array of rack teeth so that the grain substantially follows the form of each tooth to be continuous over the array of rack teeth and each tooth is devoid of end grain, the teeth thus formed not being subjected to subsequent machining.

Further according to the present invention there is provided a longitudinally extending rack member when manufactured by the method as specified in the immediately preceding paragraph.

Coin passage is a well known technique for metal shaping whereby the shape of a metal blank is changed without a change in volume by subjecting the blank to pressure between opposed dies so that the metal is displaced into a form determined by the die cavity. By the present invention this technique is applied to the manufacture of a rack member whereby a metal bar workpiece is subjected to pressure between dies causing the rack teeth to be formed with the improved feature that the grain of the metal bar is utilised in a manner which will alleviate fracture of the teeth.In conventional manufacture of rack members where the teeth are subjected to a machining operation, the cutting of the metal workpiece causes end grain to be exposed in the flanks of the teeth with the result that when the teeth are subjected to shear forces during use of the rack member the discontinuous grain can provide lines of weakness along which the teeth can fracture. Coin pressing permits an accurately shaped rack to be formed solely as a result of the pressing operation which avoids the requirement for subsequent machining and by arranging for the grain of the rack member to follow the form of each tooth and to be continuous longitudinally over the array of rack teeth so that each tooth is devoid of end grain, the grain provides continuous longitudinal support throughout the teeth to withstand the aforementioned shear forces.Fur thermore, by appropriate selection of the opposed dies the array of rack teeth can be shaped with widely different profiles, both one rack as compared with another or between different teeth on a common rack as would be the case with a variable ratio rack member.

In a conventional design of rack member the teeth are located in a longitudinally extending recess. The method may therefore comprise forming such a longitudinally extending recess in the bar so that the longitudinally extending base of the recess is devoid of end grain and coin pressing the material of the teeth of the bar within the extent of the recess. The bar will likely be formed by a rolling or extrusion process and will generally be substantially cylindrical. The aforementioned recess may be formed in the bar by forging (which has the advantage that the grain of the bar is maintained uninterrupted) but usually a forging process is insufficiently accurate and some machine cutting operation may be required to provide an accurately shaped recess of required dimensions.It is envisaged therefore that the recess will usually be formed by a machine cutting operation in which case the longitudinally exposed side faces of the recess will have end grain, however this is unlikely to be detrimental to the strength of the rack member since, although end grain may be present in the side faces of the recess, the bulk of the bar material on which the side faces are formed will be sufficiently robust to alleviate the likelihood of the rack member fracturing over these regions.

Bearing in mind that with coin pressing the shape of the bar is changed over its longitudinal extent which corresponds to the rack teeth without changing the volume of the bar which is subjected to the pressing operation, the dimensions of the aforementioned recess may be determined so that the teeth formed by the coin pressing operation have a laterally peripheral profile which generally corresponds with the peripheral profile of the bar over the non recessed part lengths thereof adjacent to the recess.

If required the bar can be heated, say at least over the part length of the bar within which the rack teeth are to be formed, to facilitate flow of the metal during coin pressing. Conventional rack members are usually machined from high carbon steel which is then hardened whereby the surface of the member is heated and is then cooled rapidly by quenching. High carbon steel is expensive in comparison with low carbon steel and the possibility is envisaged that the metal bar from which the rack member of the present invention is to be formed may be of a low carbon steel which, by subjection to the coin pressing operation becomes work hardened.

By arranging for the grain of the rack member to extend continuously through the rack teeth and by having each tooth devoid of end grain it is possible that the strength which this affords to the rack teeth will alleviate the requirement for a hard surface to be provided on the teeth by use of expensive high carbon steel and that the less expensive lower carbon steel may be suitable as aforementioned.

The internal profile of the opposed dies, particularly the part of the profile which is complementary to the rack teeth, can be formed under computer control. This is particularly advantageous where the rack teeth are, for example, to be non-uniform as in the case of a variable ratio rack and pinion gear which incorporates a standard uniformly toothed pinion. With this in mind the form of the proposed rack teeth may be determined mathematically and a computer programme may be produced in accordance with such theoretical calculation; this programme may then be utilised to computer control an appropriate tool for machining the desired profile on the die with which die the rack teeth are to be coin pressed.

One embodiment of a method of manufacturing a rack member for a rack and pinion assembly and 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 is a perspective view of a longitudinally extending metal bar from which the rack member is to be formed; Figure 2 is a side elevation of the opposed dies of a coining press and diagramatically illustrates the manner in which the bar of Fig.

1 is subjected to deformation by coin pressing; Figure 3 is an end view of the arrangement shown in Fig. 2, and Figure 4 is a side elevation of part of a rack member formed by a coin pressing technique in accordance with the invention and from a bar as shown in Fig. 1 and diagrammatically illustrates the grain in the rack member.

The longitudinally extending metal bar 1 shown in Fig. 1 may be of low carbon steel, is generally cylindrical and is provided with male screw threads 2 at each end to provide a convenient means of coupling linkage members to the rack member which is to be formed from the bar and when incorporated in a rack and pinion assembly (as for example in a steering gear installation). The bar 1 is formed, for example by a rolling process, so that the grain thereof extends longitudinally and continuously throughout the length of the bar. This bar which is initially formed with a uniform cylindrical profile has machined in its side a longitudinally extending recess 3 with opposed side faces 4 and 5 and a substantially flat base 6. The base 6 extends substantially parallel with the axis of the bar and is machined so that it is substantially devoid of end grain while the cutting of the recess 3 exposes end grain in the opposed faces 4 and 5.

The bar 1 is subjected to a coin pressing operation as shown in Figs. 2 and 3 whereby the metal of the bar over the longitudinal extent of the recess 3 is deformed to provide a longitudinally extending array of rack teeth 7 (see Fig. 4) each tooth 7a of which will extend laterally of the bar 1.

As shown in Figs. 2 and 3 the bar 1 is located between the upper and lower dies 8 and 9 of a coin press of which the cavity 10 in the upper die 8 is of complementary shape to the array of rack teeth 7 which are to be formed over the recessed part length of the bar 1. The dies 8 and 9 are closed under pressure to cause the material of the bar 1 over the length of the recess 3 to be displaced and flow into the toothed regions of the cavity 10 and thereby mould the teeth 7a to such accuracy that subsequent machining of the rack is avoided. It is believed that the considerable pressure to which the material of the bar 1 is subjected during coin pressing will work harden the surface of the rack teeth 7a sufficiently to withstand the wear to which they are likely to be subjected in use even though the bar metal is of low carbon steel.It will of course be appreciated that the bar can alternatively be of high carbon steel. If required the bar 1 can be heated prior to the coin pressing operation to facilitate the metal displacement between the dies 8 and 9.

The cavity 10 in the die 8 is of course accurately shaped to correspond with the rack teeth 7a irrespective of whether all of the teeth are of the same profile and dimensions for a standard rack or whether the teeth differ one from the other as in the case of a variable ratio rack. Furthermore, the dies 8 and 9 serve to change the shape of the metal of the bar 1 over the region where the rack 7 is to be formed without changing the volume of the metal in that region and as indicated by the shape of the cavity 10 in Fig. 3, the teeth 7a formed by the coin pressing operation have a lateral peripheral profile of part circular configuration which substantially corresponds with the cylindrical profile of the non-recessed part lengths of the bar 1.

During displacement of the metal in the coin pressing operation to form the teeth 7a, the grain of the bar which was continuous over the length of the bar underlying the recess 6 maintains its continuity as indicated by the chain lines 11 in Fig. 4 but is displaced as indicated at 11 a to substantially follow the form of each tooth. Since the teeth 7a are formed with such accuracy that their subsequent machining is avoided, each tooth 7a is devoid of end grain and the continuous grain throughout the array of rack teeth provides reinforcement which serves to alleviate fracture of the teeth, particularly over their crests. Although end grain, as indicated by the chain lines 12 in Fig. 4, is present in the regions corresponding to the opposed side faces 4 and 5 on the rack member, these regions are formed on parts of the rack member the bulk of which is sufficient to alleviate fracture over the end grain.

Claims (11)

1. A method of manufacturing a longitudinally extending rack member for a rack and pinion assembly, said member having a longitudinally extending array of rack teeth each tooth of which extends laterally of the member and which method comprises providing a metal bar the grain of which extends longitudinally thereof and coin pressing the bar to form therein the array of rack teeth so that the grain substantially follows the form of each tooth to be continuous over the array of rack teeth and each tooth is devoid of end grain, the teeth thus formed not being subjected to subsequent machining.

2. A method as claimed in claim 1 which comprises forming a longitudinally extending recess in the bar so that the longitudinally extending base of the recess is devoid of end grain and coin pressing the rack teeth in the material of the bar within the extent of the recess.

3. A method as claimed in claim 2 which comprises forming the recess by machining so that the longitudinally opposed side faces of the recesses have end grain.

4. A method as claimed in either claim 2 or claim 3 in which the longitudinally extending base of the recess is substantially flat.

5. A method as claimed in any one of claims 2 to 4 which comprises coin pressing the rack teeth within the recess so that the teeth thus formed have a lateral peripheral profile which substantially corresponds with the peripheral profile of the bar over the nonrecessed part lengths thereof adjacent to the recess.

6. A method as claimed in any one of the preceding claims in which the bar is substantially cylindrical.

7. A method as claimed in any one of the preceding claims which comprises heating the bar, at least over the part length thereof within which the rack teeth are to be formed, to facilitate flow of the metal during coin pressing.

8. A method as claimed in any one of the preceding claims which comprises work hardening the surface of the rack teeth by said coin pressing.

9. A method as claimed in any one of the preceding claims which comprises determining mathematically the form of the rack teeth which are to be provided for the rack member; producing a computer programme in accordance with said mathematical determination; utilising said programme to computer control a tool to machine on a coin pressing die component a profile which is complemen tary to said rack teeth and applying the so machined die to coin press the rack teeth in the metal bar.

1 0. A method of manufacturing a longitudinally extending rack member for a rack and pinion assembly substantially as herein described with reference to the accompanying illustrative drawings.

11. A rack member when manufactured by the method as claimed in any one of the preceding claims.