GB2212595A

GB2212595A - Cams with hardened lobes

Info

Publication number: GB2212595A
Application number: GB8826580A
Authority: GB
Inventors: Helmut Riemscheid; Karl Weiss; Herbert Frielingsdorf; Engelbert Schwarz; Heribert Grewe; Klaus Greulich
Original assignee: Emitec Gesellschaft fuer Emissionstechnologie mbH
Current assignee: Vitesco Technologies Lohmar Verwaltungs GmbH
Priority date: 1987-11-15
Filing date: 1988-11-14
Publication date: 1989-07-26
Anticipated expiration: 2008-11-14
Also published as: IT8805234A0; GB8826580D0; IT1226976B; FR2623266B1; DE3738809A1; ES2009115A6; GB2212595B; FR2623266A1; DE3738809C2

Description

2 2 12 5 9:5) 1 BT/87052GB1 CAMS WITH HARDENED LOBES This invention

relates to a cam having a cam lobe and a bore whereby a plurality of such cams may be assembled on a hollow tubular shaft passing through the cam bores and secured thereto by radially outward expansion of the shaft to form a camshaft. Such a camshaft may also conveniently have other elements such as pinions and bearing sleeves secured on the shaft by radial expansion- thereof.

Such radial expansion can conveniently be carried out hydraulically as is known in the prior art and, in such an hydraulic expansion process, it is necessary to produce a connection between the cams and the shaft which is sufficiently secure to ensure that the, in use, torque applied to the cams and other elements can be accommodated without slippage relative to the shaft. Steel cams have been proposed for use in such a camshaft, and for use in conventional single piece integral camshafts, but, with modern internal combustion engines subjected to high loads, especially with those engines comprising multi-valve cylinder heads, certain operating conditions can cause damage to occur to the working faces of the cams i.e. at the radially outer surfaces of the cam lobe regions.

It has therefore been preferred by certain manufacturers to produce camshafts of chilled cast iron wherein the working surfaces of the cam lobe regions will have a higher hardness value than that of steel. However, a disadvantage of a cast iron camshaft is in its weight especially when compared with the types of 2 assembled cam and radially expanded shaft construction referred to above.

It has been difficult to produce individual cams from a cast material and secure them by radial expansion to a hollow tube as such cams have insufficient elastic properties and tend to fracture during expansion of the shaft.

It is an object of the present invention to provide individual cams which meet the strength requirements of a camshaft produced by radial expansion of a hollow tubular shaft into the bores of the cams.

In accordance with the invention there is provided a cam having a bore therethrough whereby it may be mounted on a hollow tubular shaft and secured thereto by expansion of said shaft beyond its elastic limit wherein at least in the cam lobe region of the cam there is provided a surface layer having a hardness in excess of 56 HRC (Rockwell hardness) and wherein the hardness gradient of the cam taken in a radial direction from the said surface layer does not exceed 150HV/mm and wherein the tensile strength of the cam in the region of the said bore is at least 20t higher than that of said shaft.

Such a cam is suitable for the prior art hydraulic expansion process in production of a camshaft and results in a high quality product capable of meeting all operational requirements. Hardening of the at least cam lobe regions of the cams can take place prior to securing of the cams to the shaft, with finish-grinding taking place subsequent to securing of the cams on the shaft.

The cams may comprise a hardenable material such as T.

L, 1 c 3 steel or a cast or sintered material and may be hardened in packages prior to being secured to the shaft by radial expansion. The hardening may comprise induction or laser hardening for example.

A secure connection of the cams to the shaft is enhanced if the tensile strength of each cam in the region of the said bore is up to 50 higher than that of the said shaft. With these values of tensile strength,_ the expansion pressures applied to the shaft may be very high without any damage occurring to the cams at their hardened and less flexible surface layer.

Preferably, at the apex of the said cam lobe region, the said surface layer is twice as deep as that of the said surface layer at the flanks of the cam lobe region.

Also, it is preferable that the said surface layer has a maximum depth of 6mm, and preferably a maximum depth of 4mm, to prevent the necessary resilience in the region of the cam bore from being adversely affected.

That part of the cam which is diametrically opposed to the cam lobe region is of uniform wall thickness and may or may not be provided with a said surface layer but, if it is so provided with a surface layer, it is preferred that such surface layer has a maximum depth of 0.3mm. The said surface layer at the uniform wall thickness part of the cam at the cam bore region preferably will not exceed 50% and preferably will not exceed 30%, of said wall thickness. These parameters enable higher radial expansion pressures to be applied to the shaft without the danger of cracking the cam especially in its said hardened surface layer. Said surface layer should be of uniform radial depth between the end faces of the cam.

4 When hardening the surface layer it is necessary not only to observe the depth requirements but also certain hardness gradients, i.e. large differences in hardness which might lead to internal stresses have to be avoided, and thus it is preferable that the hardness gradient in the radial direction from the said surface layer does not exceed 120 HV/mm.

In order to further reduce the weight of the cams it is advantageous for each cam, in its cam lobe region, to be recessed axially inwardly from each of its ehd faces.

In such an arrangement, radially outwardly of said recesses, that part of the cam lobe region and its said surface layer preferably extend axially over the overall axial dimension of the cam.

If the cam is made of steel then it is preferable to utilise a steel composition having a maximum grain size of 8 ASTN. The said surface layer in such a steel cam preferably comprises fine grained martensite having a maximum grain size of 6 ASTN.

Instead of steel cams, the cams of the invention may be formed of a hardenable cast material such as black malleable cast iron or may be formed of a hardenable mixture of sintered metal containing carbide forming agents.

Also in accordance with the invention there is provided a method of forming a steel cam as described above wherein heat is applied to the radially outer surface of at least the cam lobe region and wherein, during cooling, martensite transformation occurs in the said surface layer and a structural transformation occurs in the remainder of the cam material underlying said Y 4 t t 17 surface layer. As mentioned above, the hardening may be carried out by induction or laser heating and the said structural transformation in the underlying layer occurs as a result of heat conducted from said underlying material to the said surface layer during said cooling. It is preferable that, during said application of heat, at least the cam lobe region of the cam reaches a temperature of approximately 8500C to a depth of at least lmm from the radially outer surface of said region.

Also in accordance with the invention there is provided a camshaft comprising a plurality of cams of the type described above which are secured on a hollow tubular shaft by radial expansion of the shaft.

Other features of the invention will become apparent from the following description given herein solely by way of example with reference to the accompanying drawings wherein:

Figure 1 is an end view of a cam formed in accordance with the invention; Figure 2 is a side cross sectional view through the cam of Figure 1; Figure 3 is an end view of a modified construction of cam formed in accordance with the invention; and Figure 4 is a side cross sectional view through the cam of Figure 3.

The cam illustrated in Figures 1 and 2 of the drawings is of conventional external profile and is provided with a circular bore 10 therethrough. The cam 6 has a raised cam lobe region having an apex 12 and two sloping flanks 14 whereas the remainder 16 of the cam is of uniform wall thickness around the bore. The dotted line 18 shown in Figures 1 and 2 indicates the boundary between the outer hardened surface layer 20 and the unhardened base material of the cam.

The hardened surface layer 20, in accordance with the invention, has a hardness in excess of 56 HRC (Rockwell hardness) and, in the region of the cam bore 10, the radial resistance to deformation of the-cam is higher than that of the shaft (not illustrated) to which the cam is secured by radially outward expansion of the shaft. As will be seen from Figure 2, the hardened surface layer 20 is of uniform depth and extends axially across the full width of the cam.

A modified construction of cam in accordance with the invention is illustrated in Figures 3 and 4 wherein, for the purposes of saving in weight, the cam is recessed axially inwardly at 22 from each of its end faces in the cam lobe region. The dotted line 18 in Figures 3 and 4 again illustrates the boundary between the hardened surface layer 20 and the unhardened base material of the cam and, in this embodiment, the hardened surface layer 20 extends over the full cam lobe region i.e. over the flanks 14 and the apex 12 but does not extend around the uniform wall thickness part 16 of the cam or into the web portion 24 which supports the apex area of the cam. Again, as will be seen from Figure 4, the hardened surface layer 20 is of uniform depth and extends across the full width of the cam.

T 7 A

Claims

1. A cam having a bore therethrough whereby it may be mounted on a hollow tubular shaft and secured thereto by expansion of said shaft beyond its elastic limit wherein at least in the cam lobe region of the cam there is provided a surface layer having a hardness in excess of 56 HRC (Rockwell hardness) and wherein the hardness. gradient of the cam taken in a radial direction from the said surface layer does not exceed 150HV/mm and wherein the tensile strength of the cam in the region of the said bore is at least 20% higher than that of said shaft.

2. A cam as claimed in Claim 1 wherein its tensile strength in the region of the said bore is up to 50% higher than that of said shaft.

3. A cam as claimed in either one of Claims 1 or 2 wherein, at the apex of said cam lobe region, the said surface layer is twice as deep as the said surface layer at the flanks of the cam lobe region.

4. A cam as claimed in any one of Claims 1 to 3 wherein the said.surface layer has a maximum depth of 6mm and preferably does not exceed 4mm.

5. A cam as claimed in any one of Claims 1 to 4 wherein that part thereof which is diametrically opposed to the cam lobe region is of uniform wall thickness and is provided with a surface layer having a maximum depth of 0.3mm.

6. A cam as claimed in any one of the preceding claims wherein the said surface layer at the uniform wall thickness part of the cam at the cam bore region does not 8 exceed 50, and preferably does not exceed 30%, of said wall thickness.

7. A cam as claimed in any one of Claims 1 to 4 wherein that part thereof which is diametrically opposed to the cam lobe region is of uniform wall thickness and is not provided with a said surface layer.

8. A cam as claimed in any one of the preceding claims wherein the said surface layer is of uniform radial depth between the axial end faces of the cam.

9. A cam as claimed in any one of the preceding claims wherein the hardness gradient thereof taken in a radial sense from the said surface layer does not exceed 120 HV/mm.

10. A cam as claimed in any one of the preceding claims wherein, in the cam lobe region, the cam is recessed axially inwardly from each of its axial end faces.

11. A cam as claimed in Claim 10 wherein, radially outwardly of said recesses, that part of the cam lobe region and its said surface layer extend axially over the 20 overall axial dimension of the cam.

12. A cam as claimed in any one of the preceding claims formed of a hardenable steel having a maximum grain size of 8 ASTN.

13. A cam as claimed in Claim 12 wherein the said 25 surface layer comprises fine grained martensite.

14. A cam as claimed in either one of Claims 12 or 13 wherein the said surface layer has a maximum grain size 9 1 4 of 6 ASTN.

15. A cam as claimed in any one of Claims 1 to 11 formed of a hardenable cast material comprising black malleable cast iron.

16. A cam as claimed in any one of Claims,l to 11 formed of a hardenable mixtu ' re of sintered metal containing carbide forming agents.

17. A method of forming a steel cam as claimed'in any one of Claims 1 to 14 wherein heat is applied to the radially outer surface of at least the cam lobe region and wherein, during cooling, martensite transformation occurs in the said surface layer and a structural transformation occurs in the remainder of the cam material underlying said surface layer._

18. A method according to Claim 17 wherein the said structural transformation occurs as a result of heat conducted from said underlying material to the said surface layer during said cooling.

19. A method of forming a cam substantially as hereinbefore described with reference to the accompanying drawings.

20. A cam constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

21. A camshaft comprising a plurality of individual cam elements each constructed as claimed in any one of Claims 1 to 16 or 20 secured on a hollow tubular shaft by expansion of said shaft.

Published 1989 at The Patent Office, State House, 86171 High Holborn, London WCIR4TP. Further copies maybe obtainedfrom. The PatentOInce, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. prmted by Multiplex techniques ltd. St Mary Cray. Kent, Con. 1/87