GB2245327A

GB2245327A - Tappets

Info

Publication number: GB2245327A
Application number: GB9110524A
Authority: GB
Inventors: Roger Harvey Slee
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1990-05-17
Filing date: 1991-05-15
Publication date: 1992-01-02
Anticipated expiration: 2011-05-15
Also published as: GB9011125D0; GB2245327B; GB9110524D0

Abstract

Tappets particularly, though not exclusively, for use in internal combustion engines have their bearing surfaces provided with raised lands (26) to reduce running friction. A method of manufacture is described in which helical lands are produced by extrusion and then are divided into the discrete lands (26) by a grinding operation. <IMAGE>

Description

TAPPETS The present invention relates to tappets for particularly, though not exclusively, internal combustion engines and to a method for the manufacture thereof.

A disadvantage of the type of tappet, generally known as a "bucket" tappet, is its inherently high friction due to viscous drag of the lubricating oil as it reciprocates in its associated guide. The power loss due to friction is cumulative and, therefore, with modern designs of engine which are tending towards valve layouts of four per cylinder, and in some instance s more, the power loss may be significant.

It has now been discovered that reduced bearing surface areas may be employed in tappets to reduce friction caused by viscous drag.

According to a first aspect of the present invention there is provided a tappet for use in, for example, an internal combustion engine, the tappet comprising a generally cylindrical body having discrete bearing portions formed thereon, each discrete bearing portion having a maximum radial dimension greater than that of the surrounding surface immediately adjacent the bearing portion.

The bearing portions may lie with their outer surfaces on a cylindrical surface. Tappets are subjected to relatively low side forces and may be provided with bearing portions distributed about their entire cylindrical surface since they are truly cylindrical and, in operation, tend to rotate about their axis.

Therefore, with a tappet, there is scope to reduce the bearing area without such a great danger of too low a resulting oil film thickness as exists, for example, with a piston which experiences high side forces.

In one embodiment of the present invention the bearing portions may be raised pads disposed around the tappet body in both the circumferential and the axial directions.

In a preferred embodiment of the present invention the bearing portions may be disposed such as to be circumferentially overlapping when viewed in the axial direction. This feature allows the outer diameter to be finished by centreless grinding thus making the manufacturing process more economic.

Tappet blanks may be produced in metal, ceramic or in plastics material by known methods such as, for example, investment casting, powder metallurgy techniques, injection moulding.

In a more preferred embodiment of a tappet according to the present invention the bearing portions may be disposed in a generally helical form about the outer cylindrical surface and more preferably in a form where the bearing portions appear to be in a form analagous to a multi-start thread having a coarse pitch. A significant advantage of this type of geometry for the disposition of the bearing portions is that they may be formed by conventional working processes making the tappets particularly economic to produce.

According to a second aspect of the present invention there is provided a method of making a tappet, the method comprising the steps of forming a generally cylindrical tappet blank, producing raised helices on the outer surface of the cylindrical tappet blank by metal forming means, forming the raised helices into discrete bearing portions and finishing the outer diameter of the so-formed tappet blank to a desired dimension by metal removal means.

In one embodiment of a method according to the present invention, the helices may be produced on metal tappets by extrusion of individual tappet blanks.

The helices may be divided into discrete bearing portions by cutting or grinding means.

The outer diameter may be finished by a centreless grinding operation due to the overlapping nature of the bearing portions, providing effectively a cylindrical surface.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a view in elevation of a blank for producing one preferred embodiment of a tappet according to the present invention; Figure 2 shows the blank of Figure 1 after a first processing stage; Figure 3 shows the blank of Figure 2 after a second processing stage; Figure 4 shows a section through a grinding tool used to produce the relieved areas in the tappet of Figure 3; and Figure 5 shows a perspective view of the finished tappet of Figure 3.

The figures show a tappet and the manufacturing steps employed for its production. Referring now to the drawings, a blank for a tappet is shown at 10. The blank is hollow, having a cavity 12, open at the lower end 14, and closed with a face member 16 at the upper end. The face member 16 provides a reaction surface 18 for a co-operating tappet actuating member, such as a cam (not shown), for example.

The tappet blank may be a casting or be produced by powder pressing and sintering. The blank 10 is subjected to an extrusion step in known apparatus (not shown) to produce helical, raised lands 20, similar in appearance to a multi-start thread. The helical lands may form an angle, typically of between 150 and 250 with the axis 22. The raised lands 20 are circumferentially spaced apart by a distance which may correspond approximately to their own circumferential length. The extruded blank is then subjected to a grinding operation by a form grinding tool 24, shown in Figure 3, which splits the helical, raised lands 20 into discrete facets or bearing portions 26. The facets 26 may be provided with bevelled edges 28 at their axial extremes. The facets 26 are amenable to a surface finishing operation by centreless grinding. This is due to the facets overlapping each other in the circumferential direction allowing an economic diametral finishing operation to be employed. The facets 26 are separated by relieved areas 30 which permit oil to enter and leave the tappet skirt area. In a typical embodiment the relieved areas 30 may lie at a radial dimension of between 0.05 and lmm below that of the facets 26.

Claims

1. A tappet, the tappet comprising a generally cylindrical body having discrete bearing portions formed thereon, each discrete portion having a maximum radial dimension greater than that of the surrounding surface adjacent the bearing portion.

2. A tappet according to Claim 1, wherein the bearing portions are raised pads disposed around the tappet body in both the circumferentially and axial directions.

3. A tappet according to Claim 2 wherein the raised pads are circumferentially overlapping when viewed in the axial direction.

4. A tappet according to either Claim 2 or Claim 3 wherein the raised pads are disposed in helical form on the tappet body.

5. A tappet according to Claim 4 wherein the helices form an angle of between 150 and 250 with the tappet body axis.

6. A tappet according to any one preceding claim wherein each discrete bearing portion has a radial dimension of between 0.05mm and lmm greater than the surrounding surface.

7. A method of making a tappet, the method comprising the steps of forming a generally cylindrical tappet blank, producing raised helices on the outer surface of the cylindrical tappet blank by metal forming means, forming the raised helices into discrete bearing portions and finishing the outer diameter of the so-formed tappet blank to a desired dimension by metal removal means.

8. A method according to Claim 7 wherein the raised helices are formed by extrusion of the tappet blank.

9. A method according to either Claim 7 or Claim 8 wherein the helices are divided into discrete bearing portions by a form grinding tool.

10. A method according to any one of preceding Claims 7 to 9 wherein the outer diameter of the tappet is finished by centreless grinding.

11. A tappet substantially as hereinbefore described with reference to the accompanying specification and Figures 3 and 5 of the drawings.

12. A method substantially as hereinbefore described with reference to the accompanying specification and drawings.