GB2151742A - A bucket-type tappet - Google Patents

Abstract

A mechanical bucket-type tappet for interposing between a cam and a valve stem or push-rod, comprising a cam-contact portion (1B) in the form of a ceramic disc and a perforated guidance skirt (1A) which may be formed by rolling a strip of sheet steel or by drawing a steel sheet into a die. The cam-contact portion (1B) is attached to the skirt (1A) by crimping the skirt (1A) and/or by means of an adhesive. <IMAGE>

Description

SPECIFICATION A bucket-type tappet This invention relates to a mechanical bucket-type tappet.

In a petrol engine with an overhead camshaft, the valves are operated directly by the camshaft. A tappet (sometimes known as a cam-follower) is usually interposed between the cam and the valve stem to take the scuffing forces of the cam and to avoid side-loading the valve stem. The tappet may be a mechanical bucket-type tappet the closed end of which contacts the cam with the skirt thereof passing over the valve stem and associated spring.

A mechanical tappet differs from a hydraulic tappet in that forces are transferred mechanically, ie directly through the tappet, from the camshaft to the valve stem rather than via a hydraulic mechanism.

The skirt of a bucket-type tappet is usually arranged to be close fit within a bore which guides its reciprocation as the cam shaft rotates. Mechanical bucket-type tappets are usually made of cast iron and machined to precise dimensions.

According to a first aspect of this invention, there is provided a mechanical bucket-type tappet for interposing between a cam and a valve stem comprising: a ceramic cam-contact portion and a metallic guidance skirt portion which have been separately formed, the ceramic cam-contact portion being secured to one end of the metallic guidance skirt portion.

According to a further aspect of this invention there is provided a method of forming a mechanical bucket-type tappet for interposing between a cam and a valve stem, the method comprising the step of securing a ceramic cam-contact portion of the mechanical bucket-type tappet to one end of a metallic guidance skirt portion thereof.

Preferred features of the invention are detailed in the subsidiary claims of this specification.

The invention will now be illustrated, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a mechanical bucket-type tappet in an overhead camshaft engine; Figure 2 is a part side view of a first embodiment of a mechanical bucket-type tappet according to this invention; Figure 3 is a similar view of part of a second embodiment of a mechanical bucket-type tappet according to this invention; Figure 4A and B are part side views of a camcontact and portion and of a guidance skirt respectively of a third embodiment of a mechanical bucket-type tappet according to this invention; and Figure 5 is a part side view of a mechanical bucket-type tappet comprising parts similar to those illustrated in Figures 4A and B. Like reference numerals in the figures refer to like parts.

Figure 1 shows a mechanical bucket-type tappet 1 interposed between an overhead camshaft 2 and a valve stem 3. A skirt 1A of the tappet is a close fit within a bore 4 and passes over the valve stem 3 and a spring 5 around the stem 3. As the camshaft 2 rotates, it causes the tappet 1 to reciprocate within the bore 4. The valve stem 3 is held against the tappet 1 by the spring 5 and thus reciprocates with the tappet 1.

The tappets illustrated in Figures 2-5 comprise a cam-contact disc 1B which is mounted in a generally cylindrical guidance skirt 1A. The disc 1B is formed of a ceramic material such as silicon aluminium oxynitride (eg sialon), silicon carbide, partially stabilized zirconium dioxide (zirconia), or aluminium oxide (alumina). The metallic skirt 1A is preferably made from a low carbon mild steel or another suitable metal or metal alloy.

In the embodiment illustrated in Figure 2, the disc 1B is located and/or supported in the skirt 1A by a shoulder 6 formed in the skirt 1A. A precision washer 7 is provided between the shoulder 6 and the disc 1 B to provide additional support for the disc 1B.

As shown in Figure 2, the disc 1B is secured in place by bending over or crimping one end 8 of the skirt 1A. The bent over or crimped end engage the upper edge of the disc 1 B. The end of the skirt 1A is preferably bent over or crimped around the entire circumference thereof. Alternatively, only a part of the end 8 may be bent over or crimped to secure the disc 1B to the skirt 1A.

The edge 9 of the disc 1 B around the face to be engaged by the cam is chamfered at 450 or 60 to prevent it being chipped as the cam sweeps across it and to aid assembly of the disc 1 B into the skirt 1A.

Figure 3 shows an alternative way of securing the disc 1B to he skirt 1A. In this embodiment the disc 1B has a step 15 around its edge and the end 8 of the skirt 1A is bent over or crimped to engage this step 15. The step 15 is chamfered at 45" or 60 in a manner similar to the edge of the disc illustrated in Figure 2.

Alternatively, or additionally, the disc 1B may be bonded to the skirt by an epoxy or other suitable adhesive. The other end 10 of the skirt 1A is also bent over. This increases the stiffness and circularity of the skirt 1A and gives the end of the skirt 1A a smooth contour to avoid any risk of damaging the bore in which the tappet is to reciprocate.

The skirt 1A is preferably perforated to reduce its weight and hence its inertia. As shown in the figures, elongate holes or perforations 11 are formed in a central portion of the skirt 1A. The holes 11 may together occupy 50 to 90- of the circumference of the skirt 1A and in a preferred embodiment 70 to 80- of the circumference. The holes 11 may occupy 25 to 75- of the length of the skirt 2 and in a preferred embodiment 40 to 60- of the length.

The embodiment illustrated in Figure 2 has about twelve holes 11.

A central portion 12 of the skirt 1A may be indented as shown in the Figure 1 to reduce the area of the outer surface of the skirt 1A which will contact the bore in which the tappet is to reciprocate and thus reduce friction therebetween. Preferably, the holes 11 are formed in the indented portion so that if any sharp edges remain around the periphery of the holes 11 they do not lie on the outer sur face of the skirt 1A which is to contact the bore. It will be appreciated that in order to locate accurately the skirt 1A within a cylindrical bore, and to ensure stable reciprocation therein, each end of the skirt 1A should have a cylindrical outer surface 13, 14 which is a close fit within the bore.

The disc 1 B is formed of a ceramic material as this has a high wear resistance, can support the high Hertzian stresses produced therein by modern designs of cams, and can be formed to operate with a low coefficient of friction, eg u = 0.05.

These properties are of importance since the restricted space available within a cylinder head limits the extent to which the size of conventional tappets can be increased to cope with the high stresses these components are required to bear. In addition, a ceramic disc 1 is lighter than a metallic disc and thus has a lower inertia. Known mechanical bucket-type tappets can weight around 61.sag and the weight of a tappet of the type described above having a perforated metallic skirt 1A and a ceramic disc 1 B can be reduced to around 24 to 309. It will be appreciated that any weight reduction of a moving component of an engine reduces the energy losses of the engine.

Figures 4A and B show a disc 1B and guidance skirt 1A of a third embodiment of a tappet according to this invention before they are secured together. The end 8 of the guidance skirt 1A comprises projections 8A interposed with projections 8B which are slightly shorter than the projections 8A. The illustrated embodiment has around twelve projections of each kind around the end 8.

The edge of the disc 1B has a shoulder 16 formed between a recess 17 and a face 18 which is to be supported by the guidance skirt 1A. The disc 1 B is secured to the skirt 1A by seating face 18 on the ends of the projections 8A and then bending the shorter projections 8B over so that they engage the shoulder 16. Figure 5 shows a guidance skirt and a disc secured together in this way. The skirt and disc of Figure 5 are similar to those of Figures 4A and B but are of slightly different dimensions.The shoulder 16 preferably slopes at an angle of 45" with respect to the edge of the disc 1B. With a smaller angle the disc 1B is not so securely gripped by the projections 8A whereas with a larger angle the projections 8A have to be bent over further to grip the shoulder and this can locally distort the outer surface of the skirt 1A.

The embodiments illustrated in Figures 4 and 5 have the advantage that the face 19 of the disc 1 B which is to engage a camshaft may be as broad as the skirt 1A. Maximum use is thus made of the space available for the tappet.

The skirts 1A described above may be made by rolling a strip of sheet metal into a cylindrical form and butt-welding the ends thereof together. The strip may be perforated before rolling and may be shaped to have a cross-section as illustrated in the drawings. The skirt 1A is preferably de-fraised and case-hardened. The skirt may also be coated with polytetrafluoroethylene (PTFE) or phosphated to enhance its wear characteristics. The end 8 of the skirt 1A is bent over during the rolling process to hold the disk 1 B in position. It may be necessary to soften the end 6 of the skirt 1A to facilitate this.

Alternatively, the skirt 1A may be formed by drawing a sheet of mild steel into a die to form a cylindrical tube. The shoulder 6 may be formed during this drawing process. The perforations 11 may then be formed by piercing to reduce the weight of the skirt 1A. It will be appreciated that mechanical bucket-type tappets of the type described above are much lighter than conventional mechanical bucket-type tappets and are less expensive to manufacture.

This invention also relates to a cylinder head provided with a mechanical bucket-type tappet of the type described above and to an engine provided with such a cylinder head.

Claims (20)

1. A mechanical bucket-type tappet for interposing between a cam and a valve stem comprising: a ceramic cam-contact portion and a metallic guidance skirt portion which have been separately formed, the ceramic cam- contact portion being secured to one end of the metallic guidance skirt portion.

2. A tappet as claimed in claim 1, in which the ceramic is sialon, zirconia or silicon carbide.

3. A tappet as claimed in claim 1 or 2 in which the metallic guidance skirt portion is perforated.

4. A tappet as claimed in any preceding claim in which an outer surface of the metallic guidance skirt portion comprises two cylindrical portions and an indented portion joining the two cylindrical portions.

5. A tappet as claimed in any preceding claim in which the ceramic cam-contact portion is supported on a first set of extensions provided at the said one end of the metallic guidance skirt portion and is secured thereto by a second set of extensions interposed with the first set and bent over to engage a shoulder on an edge of the ceramic cam contact portion.

6. A tappet as claimed in claim 5 in which the second set of extensions are bent over by substantially 45" to engage the shoulder.

7. A tappet as claimed in claim 5 or 6 in which the face of the ceramic cam-contact portion to be engaged by a cam is at least as broad as the said one end of the metallic guidance skirt portion.

8. A tappet as claimed in any of claims 1 to 4 in which the metallic guidance skirt portion is provided with a shoulder for locating the ceramic cam-contact portion.

9. A tappet as claimed in claim 8 in which a washer is provided between the shoulder and the ceramic cam-contact portion.

10. A tappet as claimed in claim 8 or 9 in which at least a portion of the said one end of the metallic guidance skirt portion is bent over to hold the ceramic cam-contact portion in place.

11. A tappet as claimed in any preceding claim in which the face of the cam-contact portion to be engaged by the cam has chamfered edges.

12. A mechanical bucket-type tappet substan tially as herein described with reference to the accompanying drawings.

13. A cylinder head provided with a mechanical bucket-type tappet as claimed in any preceding claim.

14. An engine provided with a cylinder head as claimed in claim 13.

15. A method of forming a mechanical buckettype tappet for interposing between a cam and a valve stem, the method comprising the step of securing a ceramic cam-contact portion of the mechanical bucket-type tappet to one end of a metallic guidance skirt portion thereof.

16. A method as claimed in claim 15 in which the guidance skirt portion is produced by rolling a strip of sheet metal or metallic alloy to a substantially cylindrical form.

17. A method as claimed in claim 16 in which the strip is perforated before it is rolled.

18. A method as claimed in claim 16 or 17 in which the ends of the strip are welded together after it has been rolled to a substantially cylindrical form.

19. A method as claimed in claim 15 in which the guidance skirt portion is produced by drawing a sheet of metal or metallic alloy to a substantially cylindrical form.

20. A method of forming a mechanical buckettype tappet substantially as herein described with reference to the accompanying drawings.