The present invention relates to tappets for part i cular ly, though not exclusively, internal com¬ bust i on engi nes .

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 oi l 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 instances more, the power loss may be significant.

It is an object of the present invention to reduce the present levels of friction between the outer cylindrical surface of a tappet and its guide.

GB 2104188 describes the use of raised bearing land portions to generate hydrodynamic o l fi lms to allow pistons in, for example, internal combustion engines, to operate at reduced levels of friction. Such reduced levels of friction are achieved without detriment to the wear between the piston and its associated cylinder wall. In a piston in an internal combustion engine, the loads on the wall of the skirt of the piston are relatively high, especially during the firing or expansion stroke, and are borne on a relatively restricted area of one side of the skirt in each stroke. Thus, as the bearing area is reduced, the oil film thickness is consequently reduced if no steps to counteract this are taken. Therefore, the bearing land portions disclosed in the reference are intended to maximise the oi l film thickness in a very much reduced bearing area.

It has now been discovered that reduced bearing surface areas may be employed in tappets to reduce running friction. It is surprising that this is so since the forces which act on pistons and tappets and the motions performed thereby are very different. A piston experiences the explosive force of the com¬ bustion and is returned by the action of the connect ng rod driven by the crankshaft. The connecting rod imparts sideways forces to the piston which are in different directions in the upstroke and in the down- stroke and are at thei greatest in a middle part of the stroke. A piston also is not generally cylindri¬ cal, being oval in transverse cross-section, and is restrained against rotation about its longitudinal axis by the connection to the connecting rod. A tappet, on the other hand, is subject to a force from a cam on its upper surface which has a sideways component which is always in the same direction and is at its greatest when the tappet is at the top end of its stroke. The springs which return the tappet do not impart a significant sideways force but their force reduces progressively during the upstroke. Tappets are generally cylindrical and they rotate about their longitudinal axes during their motion.

The invention provides a tappet comprising a generally cylindrical body, characterised in that the body has discrete bearing portions formed thereon, each discrete bearing portion having a maximum radial dimension greater than that of the surrounding surface of the body adjacent to the bearing portion, and in that each bearing portion is provided with means to form a convergent gap with a co-operating bore in which the tappet is guided for the formation of a hydrodynamic oil lubrication film. Preferably,

the bearing portions are distributed substantially evenly ci rcumferenti a I ly of the tappet.

The bearing portions may themselves have a geometry such as to form the convergent gap for the generation of hydrodynamic oil lubrication films or may be employed in conjunction with other means to form a convergent gap. In the former case, the whole bearing portion may be domed or curved when a section containing the tappet axis is viewed. In the latter case, the bearing portion may be con¬ stituted by raised lands in the form of plateaux lying substantially in a cylindrical surface and the means to form a convergent gap are provided by inclined ramps leading up to and down from the plateaux, depending upon the direction of travel of the tappet.

Each tappet has a clearance between its outer diameter and associated running guide; t is clearance inevitably results in some axial misalignment between tappet and guide. The entry angle b of the convergent gap forming means to the guide axis must be greater than the angle a, of maximum misalignment between the tappet axis and the guide axis. Preferably, the angle b should be a minimum of 0.1 milliradians greater than angle a. The effect of these angular relationsh ps is to produce a convergent oil film between the raised portion on the tappet and the guide wall, under all conditions of misalignment.

At most normal running clearances between tappet outer diameter and guide wall the angle b may preferably lie within the range from 0.3 to 1.5 milliradians. Greater angles of b than lie within this range may, however, be used since the radial

side load between the tappet and guide wall is relative¬ ly low and hydrodynamic oi l fi lms may be generated over wide ranges of angles.

In one embodiment of the present invention the bearing portions may be in the form of raised lands distributed on the body both axially and cir¬ cumferent al ly. Such tappets may be manufactured by machining of the outer diameter of a blank with a machine tool such as that described in US 4646596.

In a preferred embodiment of the present invent¬ ion, the bearing portions may be disposed such as to be ci rcumferenti a I ly 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 preferred embodiment of a tappet according to the present invention the bearing portions are arranged helically on the tappet body, and more prefei— ably 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 con¬ ventional working processes making the tappets parti¬ cularly economic to produce.

This embodiment of a tappet according to the invention may be by a 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 having associated means for the formation of a convergent gap with an associated guide bore and finishing the outer diameter of the so-formed tappet blank to a desired dimension by metal removal means .

In one embodiment of such a method, the helices are produced on metal tappets by extrusion of individual tappet blanks.

The helices may be divided into discrete bearing portions by cutting or grinding means. Inclined ramp portions in the axial direction may be produced simultaneously during this operation.

The outer diameter may be finished by a centre¬ less grinding operation due to the overlapping nature of the bearing portions, pro iding effectively a cylindrical surface.

In a further embodiment of a tappet in accordance with the invention, the discrete bearing portions comprise two ci rcumferent i a I ly extending portions adjacent opposite ends of the body. These portions may be of cylindrical form.

The raised portions may be disposed at or near each axial end of the body, the only proviso being that the maximum diameters of the raised portions are not completely expelled from the guide at each axial extreme of the reciprocating stroke of the tappet .

In order that the present invention may be more fully understood, examples will now be described by way of illustration only ith reference to the accompanying drawings, of which:

Figure 1 shows a view in elevation of a first embodiment of a tappet according to the present invention;

Figure 2 shows a schematic view of the tappet shown in Figure 1 in a guide;

Figure 3 shows a view in elevation of a second embodiment of a tappet according to the present invention;

Figure 4 shows alternative sectional shapes of convergent gap forming means for the generation of hydrodynamic oi l lubrication fi lms;

Figure 5 shows a view in elevation of a blank for producing the first, the second or a third embodi¬ ment of a tappet according to the present invention;

Figure 6 shows the blank of Figure 5 after a first processing stage in producing the third embodi¬ ment;

Figure 7 shows the blank of Figure 6 after a second processing stage;

Figure 8 shows a section through a grinding tool used to produce the tappet shape in Figure 7;

Figure 9 shows a perspective view of the finished tappet of Figure 7;

Figure 10 shows a view in elevation of a fourth embodiment of a tappet according to the present invention;

Figure 11 shows the tappet of Figure 10 in an associated guide;

Figure 12 shows the area -(_' of Figure 11 in greater detai I;

Figure 13 shows a section through the tappet of Figure 10; and

Figure 14 shows a view in elevation of a fifth embodiment of a tappet according to the present invention,

Referring now to Figures 1 and 2 where a tappet is shown generally at 10 and comprises an essentially cylindrical body member 12 having thereon raised plateaux or lands 14. The body is hollow having an internal cavity 16 (Figure 5) and an end closed with a face member 18 which provides a contacting surface for a cam 20. The inner surface 22 of the face member 18 acts against the end of a valve stem, shown in part at 24. The lands 14 lie on a cylindrical surface which has a greater diameter than that of the body 12. Each land 14 is provided with ramp portion 8,26,28 in the axial direction for the generation of hydrodynamic oi l films to reduce friction between the tappet 10 and the bore 30 of its associated guide. It should be noted that the radial height of the lands 14 above the general diameter of the body 12 and the ramp 26,28 angles shown in the figures are grossly exaggerated for the sake of clarity and ease of explanation. The tappet lands 14 may be produced by a machining process and on apparatus simi lar to that described in US Patent No. 4646596. The final surface finish to the lands 14 may be provided by a centreless grinding operation due to the three rings of lands overlapping in the circumferential

direction. Where the tappet is produced by a method and apparatus such as described in US 4646596, ramps 32,34 may also be produced in the circumferential direction; these ramps, however, have little or no influence on the formation of the hydrodynamic oi l fi lm and are merely present as a result of the machining method. In operation the tappet reciprocates in the bore 30 of its associated guide 36, being moved in a downwardly direction 'A' under the influence of the cam 20 and in the upwardly direction -B' by the valve stem 24 under the influence of a coil spring (not shown) . The tappet 10 slides in the bore 30 and has a clearance 38 (shown greatly exaggerated) therebetween. The effect of the clearance 38 is to cause the tappet, under the influence of the cam 30, to become misaligned in the bore, the axis 40 of the tappet being at a maximum angle a, of misalign¬ ment with the bore axis 42 under the worst condition. The direction and degree of misalignment will depend upon the combination of forces acting upon the tappet at any instant. In order to ensure the formation of a hydrodynamic oil fi lm, it is necessary that the entry angle b of the ramp is greater than the maximum misalignment a of the tappet axis 40 with the bore axis 42 in both directions of tappet travel.

Figure 3 shows an alternative form of tappet 43, which is simi lar to the tappet 10, but in this instance the rings of lands 14 do not overlap in the circumferential direction. Full machining of the tappet including the surface finish of the lands 14 may be produced such as described in US Patent No. 4646596.

Figure 4 shows four examples of raised land portions and associated convergent gap forming means.

Figure 4(i) shows a land 14 with ramps 26 and 28 as described with reference to the preceding figures.

Figure 4(ii) shows a facetted raised portion comprising five distinct lands 50,52,54,56,58, the bands 50 and 58 being ramp portions, the bands 52 and 56 being ramp portions which are less steep than the port ons

50 and 58, and the Land 54 being a plateau parallel to the axis 40.

Figure 4( ii) shows a raised portion in the form of a smooth curve 60 with little or no different¬ iation between the convergent gap forming means and a plateau portion such as 54 in Figure 4(ii) . Figure 4(iv) has a raised portion which comprises an inclined ramp 70, and a plateau portion 72 which then drops away in a smooth curve 74 from the point 76. It should be stressed that the examples shown in Figure 4 are i llustrative only and by no means intended to be exhaustive of the forms which may be employed.

Figures 5 to 9 show a tappet and the manufactur¬ ing steps employed for its production; features which are the same as in preceding figures have common reference numerals. Referring now to the drawings, a blank for a tappet is shown at 80. The blank is hollow, having a cavity 16, open at the lower end 82 and closed with a face member 18 at the upper end. The blank 80 is subjected to an extrusion step in known apparatus (not shown) to produce helical, raised lands 84, simi lar in appearance to a multi- start thread. The helical Lands form an angle, typical¬ ly of 15° to 25° with the axis 40. The raised lands 84 are ci rcumferenti a I Ly spaced by a distance which corresponds approximately to their own circumferential length. The extruded blank is then subjected to

a grinding operation by a form grinding tool 88 (Figure 8), which splits the helical, raised lands 84 into discrete facets 90. The facets 90 as shown in Figure 9 are provided with axially directed ramps 92 which may be imparted by the tool 88. More importantly, however, the facets 90 are amenable to a surface finishing operation by centreless grinding. This is due to the facets 90 overlapping each other in the circumferential direction allowing an economic finishing operation to be employed.

Referring now to Figures 10 to 13, the fourth embodiment of the invention is shown in the form of a tappet 100, like reference numerals being used in relation to the tappet 100 for like parts of the tappet 10.

The tappet 100 comprises a generally cylindrical body 12. The body is hollow, having an internal cavity 16 (Figure 13) and closed at one end with a face member 18, the outer surface 1 of which forms a contacting face with an actuating member, which in this case is a cam 20. The inner face 22 of the member 18 acts against the end of a valve stem, shown only in part at 24. The outer surface of the body 12 is provided with two bearing lands 126,128 which have a greater diameter than the general body surface diameter 130. Each land 126,128 is provided with inclined ramp portions 132,134 and 136,138 respectively. It should be noted that the radial height of the Lands 126,128 and the angles of the inclined ramps 132,134 and 136,138 are shown greatly exaggerated for the purpose of clarity and ease of explanation. Oi l drainage and evacuation holes 140 are provided in the body between the two ramps 134,136. The tappet 100 reciprocates in an associated guide having a

bore 30, and is moved in a downwardly di rection 'A' under the influence of the cam 20 and in the upwardly direction 'B' by the valve 24 under the influence of a valve spring (not shown) . The tappet 100 slides in the bore 30 and has a clearance 38 (shown greatly exaggerated) therebetween. The effect of the clearance 38 is to cause the tappet, under the influence of the cam 20, to become misaligned in the bore, the axis 40 of the tappet being at a maximum angle a with the bore axis 42 under the worst condition. The direction and degree of misalignment wi ll depend upon the combination of forces acting upon the tappet at any i nstant .

In order to ensure the formation of a hydro¬ dynamic oil fi lm 50 (Figure 12), it is necessary that the entry angle b of the ramp is greater than the maximum misalignment angle a of the tappet axis 40 with the bore axis 42 in both directions of tappet travel .

The conditions shown in Figure 12 are. also repeated at the Lower end 152 of the tappet as shown in Figure 11. The difference at this position, however, is that it is the raised land 128 itself which is helping to generate the hydrodynamic oi l fi lm. Under conditions of travel in the downwardly direction 'A*, the lubrication regime at 'C ^* and 152 are reversed.

Figure 14 shows a tappet 160 which is provided with three raised land portions 162,164,166 which again have inclined ramp portions to each Land. Oi l drainage holes 168 are again pro ided to reduce viscous loss.

The lands 126,128 and 162,164 and 166 can have any of the profiles shown in Figure 4.

Although Figure 11 shows a tappet having a grossly exaggerated clearance and consequent misalign¬ ment for the purpose of explanation, it is believed that tappets according to the present invention will produce hydrodynamic oil lubrication films around virtually their entire circumference.