GB2127928A - A tappet for an internal combustion engine - Google Patents

Abstract

A tappet for an internal combustion engine includes a tappet body (10) which has a camface (32) at one end, the camface being positioned for contact with a rotating cam mounted on a conventional camshaft. The tappet body (10) has a closed end and a disc (32) formed essentially of zirconium oxide is adhesively bonded to the closed end of the tappet body to form the camface for the tappet. <IMAGE>

Description

SPECIFICATION A tappet for an internal combustion engine The present invention relates to tappets for internal combustion engines.

According to the invention, a tappet for such an engine comprises a tappet body having, at a closed end thereof, a camface which is positioned for contact with a rotating camshaft, the camface being formed by a disc composed essentially of zirconium oxide which is adhesively bonded to the said closed end of the tappet body.

In one particular tappet in accordance with the invention the closed end of the tappet body defines a recess with the disc of zirconium oxide being positioned within the recess and being secured to the tappet body closed end by an adhesive having epoxy characteristics. Preferably the bonding adhesive completely fills all space between the zirconium oxide disc and the recess holding it. In addition, means can be provided for locating and preventing shear movement between the zirconium disc and the tappet body closed end.

Some examples of tappets in accordance with the invention are ilustrated diagrammatically in the accompanying drawings, wherein Figure 1 is an axial section through an hydraulic tappet illustrating the means for attaching the camface to the tappet body; Figure 2 is an enlarged partial view of a portion of the tappet body and the camface disc showing details of their construction; Figure 3 is an axial section, similar to Fig.

1, but showing a portion of a variant form of tappet body; Figure 4 is a partial axial section showing a modified form of rim used to retain the zirconium oxide disc on the tappet body; Figure 5 is an axial section showing a further modified form of rim used to retain the zirconium oxide disc on the tappet body; Figure 6 is an enlarged axial section showing a modified form of tappet body and specifically including an insert which forms the closed end of the tappet body; Figure 7 is an axial section showing yet another form of tappet body utilising an insert for the closed end thereof; and Figure 8 is an enlarged section of a portion of the insert of Fig. 7.

Automobile and engine manufacturers are directing research and development efforts towards more efficient, powerful engines, but yet which require no increase in either size or weight. Such design requirements of necessity place greater stress on the valve train components. Specifically, in the case of diesel engines which are by nature dirty in terms of the exhaust emissions they produce, Government regulations in many countries have mandated that diesel emissions must be substantially controlled. In order to improve the emission control of diesel engines, it is necessary to change the cam profile so as to provide different valve operating sequences. This again increases the stress on the valve train components and especially on the camfaces of the mechanical or hydraulic tappets therein.

The result of the present design direction of internal combustion engines is that the traditional materials used for the camfaces of such tappets cannot satisfactorily provide the necessary wear and stress characteristics. Accordingly, the emphasis has been on new and improved materials to be utilised at the camface of hydraulic and mechanical tappets, which materials will be sufficiently wear and stress resistant for engines of the type described.

There has accordingly been a shift toward utilisation of tappets having a tungsten carbide wear surface. This type of material, however, has not been wholly satisfactory, principally because the thin tungsten carbide wear element, which is bonded to one end of the tappet body, is susceptible to cracking due to brittleness. If any portion of the tungsten carbide were to break off from the tappet, this element, which would essentially be a cutting device or edge, would then be loose within the engine which would, as practice has shown, destroy the engine. A further disadvantage of tungsten carbide wear surfaces for tappets is that such wear surfaces, while being compatible with steel, are not always compatible with cast iron which is the more conventional material used in the manufacture of camshafts.

The present invention utilises, as a camface, a disc of zirconium oxide, which is commonly called zirconia, and which is essentially zirconium with minor amounts of magnesium oxide.

The product is specifically designated as a partially stabilised zirconia and is of a type as manufactured by Nilsen Sintered Products, a division of the Nilsen Group of Australia. U.S.

Patent No. 4,279,655 describes such a material.

Materials such as tungsten carbide and zirconia or zirconium oxide, which is considered a ceramic, are inherently brittle materials. This is one of the reasons why tungsten carbide has been unsatisfactory. However, it has been determined that zirconia is not as brittle as tungsten carbide and is a satisfactory material for the described use even when it is formed into a wafer or disc. The wear and stress resistant properties of zirconia are of such great advantage that the material has been found to be highly satisfactory for use as the camface of mechanical and hydraulic tappets.

Zirconia has been found to have greater wear resistant characteristics than the most sophisticated and refined chilled iron camface tappet, for example such as shown in U.S. Patent No.

4,153,017. Further, zirconia has been found to have excellent wear and stress resistant characteristics when compared with tungsten carbide.

There are several very specific advantages of zirconia. It has compatibility with both cast iron and steel camshafts which is an unusual characteristic, as normally a material will only be compatible with one or the other. Thus, zirconia has wide use as a tappet camface and can be used with either cast iron or steel camshafts, although cast iron camshafts are more conventionally found in internal combustion engines. The wear on the zirconia camface and on a cast iron camshaft has been found to be essentially the same, which is a distinct advantage in valve train components.

Further, zirconia has essentially the same coefficient of expansion and modulus of elasticity as steel, which is conventionally the material used to form the body of the tappet. Thus, there will be no differential expansion or contraction of the camface relative to the steel tappet body during operation.

Although the tappet body as specifically described herein will be steel, the invention is equally applicable to tappet bodies of other materials, such as various synthetic plastics which are just starting to be used as the material for various internal combustion engine components.

The zirconia disc described herein can have a thickness of 0.05 to 0.2 inch (0.127 to 0.508 cm) and preferably has a minimum thickness of 0.09 inch (0.2286 cm). This thickness will permit the tappet to operate satisfactorily under the above-described engine conditions and to withstand satisfactorily the compression and pressure applied by the cam during normal engine operation. A zirconia insert or disc such as that described herein has a peculiar property of tending to toughen itself as there are stress fractures which propagate through it. In the thickness range of 1.10 to 0.09 inch (2.794 to 0.2286 cm) stress fractures will not propagate completely through the zirconia disc and thus a minimum thickness of substantially 0.09 inch (0.2286 cm) will usually be required.If the thickness of the zirconia disc is substantially greater, economic factors tend to diminish the advantages associated with the use of zirconia.

The zirconia disc is adhesively bonded to the closed end of the tappet barrel. Preferably the adhesive is one having epoxy characteristics and the same viscosity as one normally associates with toothpaste. Such epoxies are organic polymers which are extremely resistant to degradation by engine environments.

Although several different epoxies have been found to be satisfactory, a material manufactured by H.B. Fuller Company designated FE-185 and called "Resi-Weld" has been found to be highly satisfactory.

The manner in which the adhesive is utilised is important. The bonding material should completely cover all surfaces of the tappet recess as well as the opposing surfaces of the zirconia disc. There should be no gaps in the adhesive and the film of bonding material should be coextensive and complete in its coverage. The thickness of the adhesive is important and a thickness range from 0.0005 to 0.0015 inch (0.00127 to 0.0038 cm) has been found to be satisfactory. A thickness less than 0.00127 cm will not provide sufficient strength to retain the disc in the recess. As epoxies of the type described cure a gas is generated and this gas must escape in order for there to be a complete and satisfactory bond between the parts.For this reason, normally pressure is applied to the ceramic disc during the bonding process which will force the gas generated during the curing cycle to be released.

In Fig. 1, a conventional hydraulic tappet (apart from its zirconia disc), for example as illustrated in U.S. Patent No. 3,683,876, is illustrated. The designated patent refers to a powdered metal barrel and the barrel of the present application will be steel or perhaps a synthetic plastics as mentioned above. Regardless of the material forming the barrel of the tappet, the overall construction is the same. The tappet body is indicated at 10 and a hollow plunger 1 2 is reciprocally mounted within the tappet body. A plunger cap 14 closes one end of the plunger and a valve 1 6 closes the opposite end. A small spring 18, held by a retainer 20, urges the valve 1 6 against the mating plunger seat.A larger coil spring 22 is positioned to react oppositely to the plunger and tappet body, as is conventional. An oil passage 24 opens into the chamber defined by the plunger and there is a metering valve 26 closing a small orifice or passage 28 in the plunger cap 14.

A recess 30 is formed in the end of body 10 and a zirconia disc 32 is positioned therein. The recess is defined by an end surface 34 of the tappet body 10 and by an annular upstanding rim 36. Adjacent the junction of the rim 36 and the surface 34 is a shallow annular groove 38. In the process of applying the zirconia disc, the adhesive will be spread completely about the recess to cover all surfaces of the recess which are exposed.

Groove or well 38 functions as a reservoir to retain substantial bonding medium. The zirconia disc is placed within the recess and is subject to pressure to drive out the gas formed during curing, as described above, and to ensure the proper thickness of adhesive film between the two parts. The epoxy film is indicated at 40 in Fig. 2. As particularly shown in Figs. 1 and 2, the rim 36 has its end surface 36a spaced inwardly from the exterior surface 32a of the zirconia disc 32.

The outer surface 32a is quite flat. Because the rim surface 36a is beneath or short of the disc surface 32a, there is no possibility of the engine cam or camshaft contacting other than the zirconia disc face.

Centrally located in the surface 34 of the tappet body 10 is a shallow groove or recess 42 which is essentially a drill point to ensure that the surface supporting the zirconia disc is flat and does not have any upwardly or outwardly-extending metal burrs. The groove 42 must be shallow so as to not undermine the support for the centre of the zirconia disc.

In Fig. 3 there is what is known as a mushroom-type tappet in which a zirconia disc 44 is positioned within a recess 46 formed at the enlarged end of a tappet body 48. Other than the shape of the tappet body, the camface construction of Fig. 3 is the same as that in Figs. 1 and -2.

Fig. 4 illustrates a variant form of mushroom-type tappet with the difference between the Figs. 3 and 4 constructions being in the nature of the rim which defines the disc recess. In the Fig. 3 construction the rim is integral with the tappet body as it is in the structure of Figs. 1 and 2. In Fig. 4 the rim for the disc recess is defined by a sleeve 50 which is contoured to match the enlarged head of the tappet body and extend a distance beyond the end of the tappet body to define and form the same type of recess as described above. The sleeve 50, which may be formed of a suitable metallic material, is brazed, welded or otherwise attached to the head of the tappet body.

In Fig. 5 the tappet construction is similar to that illustrated in Figs. 1 and 2; however, in this case, the tappet body 52 has an annular shoulder 54 upon which is seated a metal sleeve 56. Sleeve 56 and the end surface of the tappet body 52 combine to form a recess similar to that shown in Figs. 1 and 2. Again, the sleeve may be welded, brazed or otherwise suitably attached to the end of the tappet body.

In the construction of Fig. 6, the tappet body is indicated at 58 and does not have a closed end in the same manner as the constructions of Figs. 1-5, but, rather, the closed end is provided by an insert 60 which has a portion 62 extending within the tappet body and an annular shoulder 64 which is seated upon the end of the tappet body. Insert 60 may have a recess 66 and an associated rim, as illustrated in the Figs. 1-3 constructions, and a zirconia disc 68 is adhesively bonded therein. The principal difference in the Fig. 6 construction, over that shown previously, is the use of a separate insert rather than an integral wall to close the end of the tappet body.

In the construction of Figs. 7 and 8, the tappet body is indicated at 70 and again has an open end which is closed by an insert 72.

In this case the insert differs slightly from that of Fig. 6 in which the insert may itself be attached by an epoxy adhesive to the body. In the Figs. 7 and 8 constructions, as particularly illustrated in Fig. 8, the exterior wall of insert 72 in that portion which will extend within the body, has a series of annular grooves and adjoinig and intermediate projections on steps, indicated at 74, which surfaces have an exterior diameter slightly greater than the interior diameter of the tappet body bore. Thus, when the insert is forced within the tappet body bore, there may be a slight swaging or cold forming of the metal in the adjoining parts so that the effect of the assembly process is to firmly seat and attach the insert into the tappet body. The recess for the zirconia disc and the attachment of the disc will be in the manner described above.

In all the examples described above, the zirconia disc has a thickness of between 0.127 and 0.508 cm. and, preferably, a minimum thickness of substantially 0.2286 cm. Similarly, the adhesive film has a thickness of between 0.00127 and 0.0038 cm.

Claims (18)

1. A tappet for an internal combustion engine, comprising a tappet body having, at a closed end thereof, a camface which is positioned for contact with a rotating camshaft, the camface being formed by a disc composed essentially of zirconium oxide which is adhesively bonded to the said closed end of the tappet body.

2. A tappet according to claim 1 having means for locating the disc on the tappet body closed end and for preventing relative shear movement between them.

3. A tappet according to claim 2, in which the disc-locating and shear movement preventing means include a peripheral rim extending outwardly from the tappet body and defining a recess with the tappet body closed end, the disc being positioned within the recess and being confined by the said rim.

4. A tappet according to claim 3, in which the rim is integral with the tappet body.

5. A tappet according to claim 3, in which the rim is formed by a sleeve attached to the tappet body.

6. A tappet according to claim 3, in which the tappet body is in the form of a cylinder with the closed end of the cylinder being formed by an insert, the rim being integral with the insert.

7. A tappet according to claim 6, in which the insert is pressfitted into the tappet cylinder with an exterior portion of the insert having adjacent annular portions of varying diameter in contact with the interior of the cylinder.

8. A tappet according to claim 3 or any claim dependent thereon having a shallow groove within the recess and generally centrally located therein.

9. A tappet according to claim 3 or any claim dependent thereon, in which the disc is adhesively bonded in the recess by an adhe sive having epoxy characteristics, the adhesive forming a thin film completely over the side and bottom of the recess.

10. A tappet according to claim 9, in which the adhesive film has a thickness of between 0.0005 to 0.0015 inch (0.00127 to 0.0038 cm) and extends completely over and between all facing surfaces of the disc and recess.

11. A tappet according to claim 3 or any claim dependent thereon, in which the disc has at least a portion thereof positioned outwardly beyond the rim.

12. A tappet according to claim 11, in which the disc has a generally spherical outer surface.

1 3. A tappet according to claim 11, in which the outer surface of the disc is essentially flat and is positioned outwardly from the rim.

14. A tappet according to claim 3 or any claim dependent thereon, in which an annular groove is formed in the recess at the junction of the tappet body closed end and the rim, the groove providing a reservoir for the adhesive bonding the disc to the tappet body.

1 5. A tappet according to any preceding claim, in which the zirconium oxide disc has a minimum thickness of 0.09 inch (0.2286 cm).

1 6. A tappet according to any preceding claim, in which the tappet body is metallic.

1 7. A tappet for an internal combustion engine substantially as described herein with reference to the accompanying drawing.

18. An internal combustion engine incorporating a tappet as claimed in any preceding claim.