For a better understanding of the technical problems and so- lutions concerning the mounting of a brake rotor on the hub of the wheel of a motor vehicle, the conventional design of a wheel unit is briefly described herein after, reference being made to figure 1 of the attached drawings.

With reference to figure 1, a wheel 10 forms a radial flange lOa defining an inner edge lOb adapted for resting and cen- tring the wheel on the outer surface of an outwardly axially extending cylindrical portion lla of a hub 11. The hub 11 forms an outwardly extending radial flange llb and a central cylindrical portion llk forming one of the radially inner raceways of a rolling bearing with a dual set of rolling ele- ments 12a, 12b. A separate race element 19 forms the radially inner raceway for the set of rolling elements 12b. The bear- ing further comprises a stationary outer race 13 forming an outwardly extending radial flange 13a in which there are ob- tained axial bores 13b for receiving bolts 14 to fasten the bearing to a suspension standard 17.

The radial flange lib of the hub has an axially outer radial surface llc and a plurality of axial bores lid for receiving bolts 15 for mounting the wheel 10.

A brake rotor 16 forms an inwardly extending radial flange 16a with axial bores 16d aligned with the bores lld of the hubflange 11 and with corresponding axial bores lOd obtained in the wheel 10 to allow the passage of the fastening bolts 15. The tightening of the bolts 15 clamps together the rim flange 10a, the rotor flange 16a and the hub flange llb.

Through the centre of the hub 11 there is formed a splined bore lle adapted to receive a splined shaft 18 driving the wheel for rotation.

According to the prior art, the method of machining and mounting the components constituting the above discussed wheel assembly provides that firstly the complete hub-bearing unit already finished is mounted to the suspension standard 17. Particularly, the finishing operation has to be accurate on the hub flange surface llc which serves as an axial rest for the brake flange 16a. Then, the brake rotor 16 with the opposite sides of its flange 16a and its opposite braking surfaces 161,162 already accurately machined is inserted onto the cylindrical portion lla of the hub. After that, the wheel rim is inserted over the portion lla and finally the bolts 15 are tightened through the aligned bores lOd, 16d and lld.

With this approach, the orientation of the braking surfaces of the brake rotor with respect to the rotation axis of the bearing is affected by manufacturing tolerances of the vari- ous components of the assembly and possible mounting errors.

Therefore, when the wheel assembly is mounted, the braking surfaces are not precisely oriented perpendicular to the ro- tation axis of the bearing. In fact, besides being affected by constructional planarity errors of the facing flanges of the hub and the brake rotor, the planarity of the hub flange and especially of the brake flange is jeopardised by deforma- tion caused by the tightening of the bolts 15, which gener- ates undulations in said radial surfaces.

Planarity errors and non-perpendicular braking surfaces and flange surfaces with respect to the rotation axis of the bearing determine the so-called axial runout of the brake, which is undesirable as it provokes excessive vibration, brake shudder and irregular or premature wear of the brake pads. With present techniques, the axial runout ranges be- tween about 90 and about 100 pm.

U. S. Patent No. 5,430,926 discloses a method of producing a brake rotor and a bearing assembly wherein a brake rotor hav- ing a braking surface and a hub portion is firstly machined at the bore of the hub. A bearing unit is then installed in the bore of the hub portion. The bearing unit includes an outer race which engages the bore and a rotatable inner race.

The assembly comprised of the brake rotor and the bearing unit is then mounted on a machining apparatus supporting the assembly by the inner race of the bearing unit. Using the ma- chining apparatus, the braking surface of the brake rotor is machined.

U. S. Patent No. 5,842,388 proposes a method of machining a wheel hub and a brake component coupled by bolts. The braking surface is machined together with a cylindrical surface of the hub for seating a bearing.

International patent application WO-A-98/38436 proposes to secure a brake disc to the flange of a vehicle wheel hub and mount the assembly comprised of the hub, bearing and brake on a machining apparatus. The machining apparatus is used to ma- chine the braking surfaces of the brake disc so that the these are oriented in a predetermined relationship relative to the outer race of the bearing.

The above mentioned prior art solutions, although providing a braking surface having a minimum axial runout, have a draw- back in that when the original brake rotor, machined with the above processes, has to be replaced with a new brake rotor, the latter has braking surfaces with a considerable axial runout, that may be reduced to acceptable values only if the whole assembly comprising the hub, the bearing and the brake rotor is placed in a machining apparatus to repeat the ma- chining of the braking surfaces.

It is an object of the present invention to provide a method capable of reducing the axial runout of the outer surface of the hub flange serving as a rest for the brake rotor flange and the wheel. In particular, it is desired to reduce the ax- ial runout to a minimum, regardless of manufacturing toler- ances of the wheel assembly components and independently of errors in mounting the bearing.

Another object of the invention is to provide a method which allows to keep down to a low value the runout of the braking surfaces of the brake discs, both the original ones and those in replacement of the originals.

This object is accomplished, in accordance with the present invention, by a method as defined in claim 1.

It is another object of the present invention to provide a method capable of reducing to a minimum the runout independ- ently of deformation caused by the forced insertion of the wheel mounting studs.

This other object is accomplished, in accordance with the present invention, by a method as defined in claim 2.

Preferred embodiments of the invention are defined in the other dependent claims. The features and the advantages of the invention will appear from the detailed description of the few embodiments thereof, given by way of example, reference being made to the accompa- nying drawings, in which: -FIG. 1 is a partial axial cross sectional view of a wheel assembly manufactured and mounted according to the prior art; -FIG. 2 is an axial cross sectional view showing the ma- chining of a surface of a flange of a hub bearing unit, in accordance with a first embodiment of this invention; and -FIGs. 3 to 6 are axial cross sectional views, similar to that of FIG. 2, showing further embodiments of the invention.

Referring to FIG. 2, there is illustrated a hub bearing unit for a driving wheel, of the type having a stationary outer race 13 as discussed in the introductory part of the descrip- tion.

The general structure of the unit shown in Figure 2 may be considered generally known. Only the elements of specific im- portance and interest for the purposes of the implementation of the present invention will therefore be described in de- tail in the following description. For the construction of the parts and elements not shown in detail, reference may therefore be made to any wheel hub unit of known kind, as for example the one shown in FIG. 1 or those disclosed in the prior art documents cited in the preamble of the description, that are herein incorporated by reference.

The outer race 13 forms an outer radial flange 13a for mount- ing to the suspension (not shown in FIG. 2) of a motor vehi- cle, and the outer raceways for a dual set of rolling ele- ments, rollers or balls, 12a, 12b. The inner raceways are formed by the hub 11 and a pair of separate annular elements 19a, 19b axially locked to the hub for example by cold form- ing an axially inner rim llf of the hub. Cold forming of rim llf is performed applying an axial load to the annular inner elements 19a, 19b in order to obtain an axially preloaded hub-bearing unit.

The radial flange 13a of the outer race 13 provides an axi- ally inner radial surface 13c that in use abuts against a ra- dial surface 17a of the suspension (as shown in FIG. 1).

Proximate to the flange 13a, the outer race 13 provides an outer cylindrical surface 13d adapt for fitting in a seating bore of the suspension, as indicated at 17b in FIG. 1.

The hub 11 forms a tubular central portion llk and an out- wardly extending radial flange llb with an axially outer side lie. The hub further forms an axially outwardly protruding cylindrical portion lla.

The radial surface llc of the hub flange llb serves as an ax- ial resting surface for a radial flange 16a of a brake rotor 16, as shown in FIG. 1. The surface lie must therefore define a surface possibly free of planarity errors, to avoid or at least reduce the inconveniences cited in the introductory part of the description.

Axial bores lld are formed in the hub flange llb for receiv- ing studs 15 (FIG. 1) to mount the hub to the wheel and the brake disc.

The pre-assembled and axially preloaded hub-bearing unit is located on a machining apparatus provided with a plurality of stationary radial jaws 120 and axial locators 121, and a ro- tary collet 122.

The radial jaws 120, only one of which is shown for simplic- ity in FIG. 2, are in this example in number of three, angu- larly spaced 120 degrees therebetween. The radial jaws 120 act against the outer cylindrical surface 13d of the outer race 13 for centring the hub-bearing unit with respect to the axis of rotation of the collet 122. The jaws 120 have axial engaging surfaces 120a for engaging the outer bearing race 13 so as to represent the datum of the cylindrical seat 17b of the vehicle suspension in which the unit will then be mounted.

In the embodiment shown in FIG. 2 there are provided three ax- ial locators 121 (only being shown in the drawing) angularly spaced 120 degrees therebetween with respect to the axis of rotation of the collet 112 and angularly offset about 60 de- grees relative to the radial jaws 120. The axial locators 121 may also be arranged in a number or shape different from what is shown in FIG. 3, as far as they provide several steady ax- ial resting points around the axis x for the axially inner (or inboard) surface of the radial flange 13a of outer bear- ing race 13, so as to represent the datum offered in use by the axial surface 17a of the suspension (FIG. 1).

It should be noticed that in the example of FIG. 2, the inner raceways are formed by the pair of inner race elements 19a, 19b. It is to be understood that the present invention, in all its possible embodiments, can be equally applied both to hub-bearing units in which one of the radially inner raceways is obtained directly on the hub (as shown in FIGs. 3 and 5) and to units in which the inner raceways are formed by race elements produced separately and then fitted onto the hub (as shown in FIG. 4).

The rotating collet 122 serves to drive the hub 11 and the inner race elements 19a, 19b for rotation with respect to the outer race 13 so that a cutting tool 23 can finish the axi- ally outer surface llc of the hub flange llb.

To impart rotary motion to the rotatable parts of the hub- bearing unit, the collet 122 provides a lower head portion 122a for engaging the hub, in this example by engaging the annular protruding hub portion lla. The engaging head 122a may also be shaped differently form the embodiment here shown. In any case, the collet head 122a should grip the hub with a moderate force or anyway in such manner so as not to generate stresses in the hub which provoke appreciable elas- tic deformation of the flange llb. Otherwise, the cutting tool 23 would finish the hub flange side llc with a surface that would not result planar once the head 122a is removed from the unit being machined.

So, the surface lie is machined by rotating the hub about the rotation axis defined by the position of the outer race 13, whereby this surface, once finished, is perpendicular with respect to the geometrical axis about which the hub will ro- tate in use.

In order that the collet 122 will not transmit onto the flanged hub 11 considerable stress components in the direc- tion of a geometrical axis not being coincident with the ro- tation axis x, preferably the rotation axis of the collet is floating, for example by mounting the collet on a flexible coupling or a ball joint, schematically designated 122b. As an alternative or in addition, the collet may be made of a yielding material.

It will be appreciated that the accuracy with which the sur- face llc is machined is independent of the manufacturing and mounting tolerances of the components constituting the hub- bearing unit. The axial runout of the surfaces llc can so be kept down to values not exceeding 10 urn, therefore much lower with respect to the prior art.

Furthermore, it will be appreciated that when it is necessary to replace the original brake rotor with a new one, the lat- ter will also have its braking surfaces oriented perpendicu- lar with respect to the rotation axis of the bearing, as it will rest against a radial surface (surface llc) having an extremely low axial runout, i. e. perfectly perpendicular with respect to the rotation axis defined by the outer race of the bearing.

Another embodiment of the method according to the invention is shown in FIG. 3. In this variant, the radial surface llc is machined with the studs 15 already forcefully inserted in the bores lld of the flange llb. As a result, the planarity of the finished surface lie will not be affected by deformation provoked by the forced insertion of the studs. To machine the surface llc, in such variant there is used a cutting tool 23a of elongated and thin shape to reach also that portion of the surface llc comprised between the zone of the bores lld and the cylindrical portion lla.

It is understood that the cutting tool 23a, although being thin, cannot easily reach the surface llc up to the zone where the bores lld open onto the surface llc when the studs 15 are inserted, to avoid leaving zones of the surface llc which are not machined and protrude from the rest of the sur- face, the flange llb is formed with a recessed zone llm in the shape of a circular annulus embracing the zone where the bores lld rise to the surface llc. The cutting tool 23a ma- chines the portions of surface llc externally and internally adjacent to the recessed annulus llm, as indicated at llc' and llc''. These portions of machined surface are those against which the brake rotor will rest axially.

Generally, the invention is applicable to any kind of hub bearing unit with rotating flanged annular element. In FIGs. 4 and 5 the method of the invention is carried out on hub bear- ing units in which the stationary outer race 13 is not flanged and therefore is locked only by radial jaws 120. For these and other applications where the outer cylindrical sur- face 13d of race 13 is to be mounted forcefully with radial interference in a seat of the suspension, during the machin- ing it is preferable that a radial preloading is applied to the outer race, simulating the aforesaid forced mounting.

Such a radial load may for example be exerted by the same ra- dial jaws 120.

Similarly, the invention is equally applicable to hub bearing units of the type illustrated in FIG. 6, where the rotating flanged race 11 forms the outer raceways and the inner race- ways are formed by a pair of axially adjacent stationary bearing rings 13. In this later case, and generally for all those applications where the hub bearing units are retained on the machining apparatus in a non-preloaded condition, it is advantageous to apply an axial preloading P during machin- ing, so as to nullify axial clearance between the inner and the outer raceways.