Sintered Blanks

Abstract

The formation of inner and outer bearing rings is described by roll forming to shape a sintered metal blank. The roll forming is found to densify the ring in the bearing areas where high strength is required and also it is surprisingly found that some flow of the metal to give the required final shape can be achieved without fracture of the blank.

Description

This invention relates to the roll forming of blanks to give inner and outer bearing rings and to the roll formed bearing rings thus produced.

BACKGROUND OF THE INVENTION

Simple shaped articles are often prepared by compacting and sintering a powdered metal. There are various ways of preparing these powder metal articles. According to one method, the powdered metal is compacted in a mould or die and then sintered. The resulting articles are very brittle and not particularly strong or dense. Their strength and density can be increased, however, by subjecting them to further compaction after sintering, optionally followed by further sintering. Alternatively the second compaction can be effected while the article is still hot.

The making of simple powder metal articles by these procedures enables one to make simple regular shaped articles relatively cheaply. One example of a simple regular shape which can be made readily and cheaply is a cylinder.

As noted above these simple shaped powder metal articles are quite brittle and not particularly dense and so they cannot usually be machined or shaped in other ways after compaction.

BRIEF DESCRIPTION OF THE INVENTION

We have now surprisingly found according to the invention that a substantially cylindrical powder metal blank can be roll formed to give an inner or outer bearing ring.

We believe that the roll forming should be effected in such a way that initially the cylindrical powder metal blank is made more dense by further compaction and thereafter the metal is caused to flow to the desired final shape of the bearing ring. We find that, in this way, the expected problems of brittleness are avoided and that the powder metal blanks can be formed without fracture. It appears that the ability of the blank to flow after compaction is largely dependent upon its density and ductility.

We also believe that initially the powder metal blank should have a relatively high density. It appears that the density after compaction and sintering and before roll forming should be at least 96 percent, and preferably about 98 percent, of the density of the solid material.

It apears that the material of the powder metal blank should have a good ductility so as to allow the metal to flow to shape during rolling without fracture. Thus it is preferred that the metal have an elongation factor, i.e., percentage elongation when fracture occurs, of not less than 9 percent, with a value in the region of 12 percent is found to give good results.

An important advantage of roll formed bearing rings prepared according to the invention is that during the roll forming, work hardening and the consequential risk of fracture does not appear to occur, or occur to anything like the same extent, as compared with the similar roll forming of a solid blank of the same metal. For example, we find that cylindrical powder metal blanks of the steel known as EW 31 can be roll formed without fracture, whereas when solid substantially cylindrical blanks of EN 31 are roll formed, they are more susceptible to fracture.

Also the roll formed bearing rings prepared according to the invention are compacted or given greater density and strength in those regions of their surface where greater density and strength are required. Thus the regions of the bearing groove and opposed cylindrical surfaces have greater density and fatigue strength than other regions of the bearing ring.

The substantially cylindrical powder metal blanks for roll forming can be produced, for example, in any of the ways outlined above and will have a very accurate shape and size. They are therefore eminently suitable for use in the roll forming process without further shaping or sizing operations. For example, the powder metal blanks can be cylindrical or cylindrical with the flat annular edges chamfered.

The roll forming of the powder metal blanks is likely to produce a rolled surface of nearly 100 percent density and so the bearing surfaces of the roll formed bearing rings have an excellent polished appearance.

The roll forming of the invention can be effected cold, warm or hot. In addition the roll forming can be effected in any convenient way using known roll forming apparatus. Examples of suitable roll forming apparatus for roll forming the articles according to the invention are shown in United Kingdom Pat. Applications 63607/69 and 5132/71.

Suitable powder metal blanks for roll forming according to the present invention can be made, for example, from those steels having the designations SAE 4600, SAE 8600, SAE 52,100, EN 31, EN 32A and EN 24.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by way of example, with reference to the accompanying drawings, in which:

FIGS. 1A, 1B, 1C and 1D show four stages in the roll forming of a cylindrical sintered metal blank to give an inner bearing ring; and

FIGS. 2A, 2B, 2C and 2D show the equivalent four stages in the roll forming of a cylindrical wrought metal blank.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The cylindrical sintered metal blank 10 is shown mounted on a rotatable mandrel 12 in FIG. 1A and it is to be formed to the shape of an inner bearing ring 14 (FIG. 1D) by means of a suitably profiled forming roll 16. The later has an annular hump 18 on either side of which are cylindrical portions 20 beyond which are inclined flanks 22.

At the start of the roll forming operation shown in FIG. 1A, the rotating forming roll 16 is advanced towards the sintered metal blank 10 and continued advance causes the annular hump 18 to start to form an annular recess 24 in the blank 10. At the stage shown in FIG. 1B the hump 18 causes further densification of the blank with substantially no flow of metal.

As the forming roll 16 continues to advance towards the mandrel 12 (the stage shown in FIG. 1C) the metal in the region of the recess 24 is compressed to a density approaching 100 percent of the density of the solid metal and then a flow of the metal occurs as shown by the arrows 26. Continuation of the advance of the forming roll 16 then causes further metal flow shown by the arrows 28 and 30 so that the blank spreads to conform to the cylindrical portions 20 and flank 22 of the forming roll. At the same time, with width of the blank increases.

The finished bearing ring 14 is shown in FIG. 1D. This has been highly compressed and densified in the region 32 around the annular recess 24 so giving the ring high strength in that region where the bearing ring will be subjected to a large loading when in use. The ring also has a highly polished and excellent finished rolled surface.

The roll forming operation is only shown diagrammatically in FIGS. 1A to 1D. Normally, there will be two diametrically opposed forming rolls 16 which progressively squeeze the blank 10 between them; only one roll has been shown for the sake of simplicity, however.

By way of contrast the steps of roll forming of a wrought cylindrical blank 40 to give an inner bearing ring are shown in FIGS. 2A to 2D, the stages A to D corresponding to the stages A to D of FIG. 1.

As shown in FIG. 2B, as soon as the roll forming commences, metal flow occurs as shown by the arrows 42 since the material is already at its maximum density. With continued advance of the forming roll 16, the metal flow continues and by stage C width growth of the blank 40 (shown by arrows 44) occurs. The finished ring 50 is achieved at stage D (FIG. 2D) and will be appreciated by comparison with FIG. 1, the roll forming of the solid metal cylindrical blank 40 requires a considerable extra flow of metal which of course leads to a greater risk of fracture of the metal during roll forming.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly it is apropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

Claims

What we claim is:

1. A method of preparing an inner or outer bearing ring comprising the steps of:

a. forming a substantially cylindrical sintered powder metal blank having a density of at least 96 percent of the density of the solid metal, and

b. roll forming said blank to the shape of said required inner or outer bearing ring.

2. A method according to claim 1 in which said metal has an elongation factor of not less than 9 percent.

3. A method according to claim 1 wherein said metal has an elongation factor in the region of 12 percent.

4. A method according to claim 1 wherein said blank has a density of about 98 percent of the density of the solid metal.

5. a method according to claim 4 wherein said metal has an elongation factor of not less than 9 percent.

6. A method of preparing a bearing ring comprising:

a. compressing and sintering a powdered metal in a mould to a substantially cylindrical blank to a density of at least 96 percent of the density of the solid metal,

b. squeezing and rolling said cylindrical blank between profiled forming members so as to initially compact parts of said blank and then cause flow of said compacted metal parts to form one or more annular recesses in said blanks so giving said required bearing ring.

7. A method according to claim 6 in which said blank has a density of about 98 percent of the density of the solid metal.

8. A method according to claim 6 in which said metal has an elongation factor of not less than 9 percent.

9. A method according to claim 8 in which said metal has an elongation factor in the region of 12 percent.

10. A method according to claim 7 wherein said metal has an elongation factor of not less than 9 percent.

11. an inner or outer bearing ring manufactured by roll forming a substantially cylindrical blank of sintered powder metal having a density of at least 96 percent of the density of the solid metal, comprising:

a cylindrical ring having an annular load bearing recess located on an inner or outer surface thereof, the region of material around said load bearing recess being compressed and densified to a density approaching 100 percent of the density of the solid metal, thereby providing high strength in said load bearing recess.