EP0030781B1

EP0030781B1 - Hydraulic tappet for direct-acting valve gear

Info

Publication number: EP0030781B1
Application number: EP19800302755
Authority: EP
Inventors: Stephen Mark Buente; William Thomas Mihalic
Original assignee: Eaton Corp
Current assignee: Eaton Corp
Priority date: 1979-12-05
Filing date: 1980-08-11
Publication date: 1989-12-13
Also published as: BR8005733A; EP0030781A1; DE3072169D1; JPS5683508A; JPH0120286B2

Description

This invention concerns a bucket tappet for the valve gear of an internal combustion engine, which valve gear is of the direct-acting type wherein one end of the tappet is contacted by an engine cam and the other end contacts a stem of a combustion chamber valve.
With reference to DE-A-2941084 the applicant has voluntarily limited the scope of the present application and submitted separate claims for Germany.
In designing valve gear for internal combustion engines operating at speeds in excess of 5,000 rpm, it has been found desirable to employ valve gear of the cam-over-valve type. Valve gear of this type is known as direct-acting valve gear and employs a tappet having one end contacting the engine cam shaft with the other end of the tappet in direct contact with the end of the stem of the combustion chamber valve. Direct acting valve gear offers the advantage of low mass, fewer working parts and higher stiffness due to the elimination of the rocker arm and/or push rods. Low mass and high stiffness result in a high natural resonant frequency which allows the valve gear to attain higher rpm's before valve mis-motion occurs. Direct acting valve gear also permits the use of lighter valve spring loads for a given valve motion and engine speeds as compared with those used in other valve gear arrangements. The low mass and high stiffness of the system also permits valve lift velocities and accelerations which increase the area under the valve lift curve and thus provide increased specific engine output. Although other overhead cam configurations can be made to have comparable lift velocities and accelerations, a direct acting valve gear arrangement offers the additional advantage of permitting rotation of the cam contacting surfaces as the lifter rotates which is not possible with rocker arm type valve gear arrangements. Direct acting valve gear arrangements, therefore, allow higher permissible cam contact stresses.
In addition, the cam profile for other overhead cam valve gear arrangements with high lift accelerations and velocities is more complex than that required for direct acting valve gear. The simpler cam profile requirement of direct acting valve gear results in less manufacturing difficulties and less cost in the valve gear when high velocities and accelerations are desired.
It has, however, been found difficult to provide direct-acting valve gear in engine applications where the height of the engine must be kept to a minimum and consequently the cam shaft located closely adjacent the end of the combustion chamber valve stem. Furthermore, where it is desired to retrofit an hydraulic lash adjusting tappet into the direct-acting valve gear of a production engine, it is often difficult to provide a hydraulic lash adjusting tappet in the space provided between the cam shaft and the end of the valve stem. Since the tappet must be guided in a bore provided in the engine, it is necessary to provide structure intermediate the cam shaft and the end of the valve steminorder to provide the tappet guide bore and the engine height tends to somewhat increase.
Therefore, it has been desired to find an hydraulic lash adjusting tappet with a compact profile height for use in engines having direct acting valve gear with minimum distance between the cam shaft and the end of the valve stem to minimize the mass of engine structure necessary to provide the tappet guides. Furthermore, in designing tappets for direct acting valve gear so as to minimize side loading in the guide for minimizing wear, it is desirable to have the reaction force of the valve stem centered through the tappet at a point as closely adjacent the cam surface as possible. Locating the reaction force near the cam face also permits the tappet to be designed to minimize the mass, which in turn reduces inertia.
A known hydraulic tappet for direct-acting valve gear, such as that shown and described in U.S. Patent 3,509,858 to Scheibe et al, employs a body, or bucket, formed as an integral unit having a reservoir provided by an undercut in the plunger guide bore formed in the bucket. The necessity of the undercutting requires a relatively large plunger guide bore in the body which in turn results in a reduced hydraulic pressure upper operating limit. Furthermore, if the diameter of the plunger guide bore is reduced, the undercut reservoir is reduced in volume and the mass of the tappet is increased, resulting in greater inertia. This tappet design is in any event inherently of relatively high inertia and the undercut annular reservoir is of small volume. Such a tappet is not suitable for use in engines running at over 5,000 RPM.
A further problem encountered in the design of hydraulic bucket tappets has been the necessity of providing precision sliding surfaces on the outer diameter of the plunger and the inner periphery of the plunger guide bore formed in the bucket. Such precision surfaces are required in order to provide control of leakdown from the high pressure hydraulic chamber in the tappet where this sliding interface is employed as the leakdown control surface, as, for example, in the tappet described in German Patent 1,914,693. In tappets having this latter construction, the high pressure hydraulic fluid chamber for effecting lash adjustment is disposed at or near the upper level of the fluid reservoir and consequently is susceptible to retention of trapped air. This requirement for leakdown control has heretofore required extremely tight tolerances on the dimensions of the bucket bore and plunger diameter. The necessity of forming the plunger guide bore in the bucket to tight tolerances has resulted in costly scrap losses if such operations are defectively performed.
DE OS 1,808,000 shows in Figure 2 a design intended to provide a low height and an adequate reservoir volume in a tappet suitable for high-RPM engines. The plunger is fitted over, and is guided and supported by, a cylindrical extension fixed to the underside of the top of the tappet. An annular first reservoir portion surrounds the plunger and a second, central reservoir portion is located within the cylindrical extension and above the high pressure chamber, which is in the lower end of the plunger. The design requires separate forming of the cylindrical extension and the tappet top, and then fixing the two together in the central region of the tappet top such as by welding. This is undesirable since in the same region the tappet top needs to be both fatigue resistant and hardened for wear resistance. Fluid is retained in the tappet by means of a plate 7a and a sealing ring 25 which, together, are merely stated to perform a sealing function.
The invention seeks to provide a bucket tappet for high-RPM engines which not only is relatively low and light but also can be manufactured with reduced scrap losses and without having to weld or otherwise fix components near the centre of the top of the tappet.
According to the invention, there is provided a bucket tappet for use in the valve gear of an internal combustion engine which is capable of operating at speeds in excess of 5,000 RPM, which valve gear is of the direct acting type wherein one end of the tappet is contacted by an engine cam and the other end is in direct contact with a stem of a combustion chamber valve, the tappet comprising:

- a tubular outer wall,
- a top, extending transversely of the outer wall at the top end of the tappet, and adapted to contact the engine cam,
- a plunger located within and slidable axially relative to said outer wall, the plunger having at its lower end an end wall for contacting the valve stem,
- an annular first portion of a fluid reservoir located radially in between the tubular outer wall and said plunger and axially between said top and a web which forms the bottom of said annular portion of reservoir, the radial extent of said first portion being substantially the same as that of the web,
- passage means for supplying fluid to said annular portion of reservoir,
- a central second portion of reservoir in fluid communication with said first portion,
- a fluid pressure chamber within the plunger, at least a portion of which chamber is located at a level below said two portions of reservoir,
- a one-way valve operable to permit a flow of fluid therethrough from said central second portion of reservoir into said fluid pressure chamber upon movement of said plunger in a direction tending to expand said chamber and to prevent reverse flow of fluid therethrough from said fluid pressure chamber upon movement of said plunger in a direction tending to diminish said fluid pressure chamber,

said hub being maintained in fixed relation to the top, but with its upper end axially separated from said top, by means of said web which extends between the hub and the tubular outer wall and connects the hub to the tubular outer wall so as to form the sole structural support for the hub.
The construction of the present tappet provides a low profile height for minimizing the distance required between the engine cam and the end of the combustion chamber valve stem, and relatively low mass and inertia. The construction also provides for a relatively large diameter and adequate reservoir of fluid formed by the central reservoir portion within the piston in conjunction with the annular reservoir portion. Supporting the plunger through the web and hub avoids the need to fix any plunger support to the tappet top, and the provision of the leakdown surfaces on the plunger and relatively small diameter piston helps to enable more accurate machining of those surfaces and results in less scrap material due to any machining inaccuracies.
The invention is described further, by way of example, with references to the accompanying drawings, in which:

FIGURE 1 is a cross section of a portion of the direct acting valve gear of an internal combustion engine illustrating the tappet as installed in the engine;
FIGURE 2 is an end view of the plunger end of the hydraulic tappet of the present invention;
FIGURE 3 is a cross section taken along section indicating line 3-3 of Figure 2 and shows the tappet having the one-way valve means in the closed position;
FIGURE 4 is a view similar to Figure 3 showing the one-way valve in the fully open position;
FIGURE 5 is a view similarto Figure 2 showing a portion of another embodiment of the tappet employing an alternate retaining means for the plunger; and
FIGURE 6 is a view similar to Figure 5 showing an additional embodiment of the tappet employing a third plunger retaining means.
FIGURE 7 is a view similar to Figure 3 showing another embodiment of the tappet having an alternate version of the body.

Referring now to Figure 1, the bucket tappet indicated generally as 10 is slidably received in a guide bore 12 provided in the cylinder head H of the engine structure. A cam shaft 14 having a cam lobe 16 contacts the upper end or cam face of the tappet. A typical combustion chamber valve 20 is shown seated on a valve seating surface formed in the cylinder head H with the stem portion 22 of the valve extending substantially vertically upward through a valve guide 24 formed in the cylinder head H, with the upper end 26 of the valve stem contacting the lower end of the tappet. The valve is biased to the closed position by a valve spring 28 having its lower end registering against the exterior of the upper portion of the valve guide 24 and its upper end in contact with a retainer 30 secured to the valve stem adjacent its upper end and retained thereon in a suitable manner as, for example, by the use of a split keeper 32 which is well known in the art.
Referring now to Figures 2, 3, and 4, the presently preferred embodiment of the tappet 10 is shown wherein the body, indicated generally at 40, is shown as formed preferably integrally with an outer tubular wall 42 having a transverse web 44 extending generally radially inwardly from the inner periphery of the outer tubular wall at a location intermediate the ends thereof. The web 44 has formed preferably integrally therewith a tubular hub 46 formed about the inner periphery of the web 44 with the hub 46 extending axially from the web in a downward direction with respect to Figure 3. The hub 46 has the inner periphery thereof extending in generally parallel relationship to the outer periphery of the tubular wall 42. The outer periphery of the tubular wall 42 is sized to be received in the tappet guide bore 12 (see Fig. 1) in a generally closely fitting relationship. Although the outer wall, web and hub have been described as preferably formed integrally, it will be understood that such portions may be formed separately and the body formed by joining of those portions, as for example, by weldment, such as fusion or brazing.
In the presently preferred practice of the invention a cam face member 18 having a relatively thin disc-shaped configuration is joined about the outer periphery thereof with the upper end of the tubular outer wall 42 in a suitable manner, as for example, by laser fusion weldment. A fluid by-pass recess 43 is formed in the underface of member 18, the function of which will be hereinafter described. In the presently preferred practice the tappet cam face member is formed of a suitable alloy steel as, for example, an alloy containing a desired amount of chromium and is suitably hardened for wear resistance. The tubular outer wall in the presently preferred practice is formed of a suitable iron base material as, for example, steel or cast iron. Although in the preferred practice the cam face member is formed of steel having a hardened surface, it will be understood that other metals, for example, nickel alloys may be used, or hardenable cast iron or ceramic materials or cermets may be employed if desired.
The hub 46 has received therein a plunger 49 having the outer periphery 50 thereof in sliding closely fitting relationship with the interior 48 of the hub. The plunger 49 has the transverse face 52 thereof, or lower face with respect to Fig. 3, adapted for driving engagement with the end 26 (see Fig. 1) of the combustion chamber valve stem.
In the presently preferred practice, the plunger is formed of steel with end face 52 suitably hardened for wear resistance. The outer periphery 50 of the plunger has an annular shoulder 54 formed thereon at the intersection with the end face 52. An annular retainer 56 is received on the end of the hub 46 and engaged therewith, preferably in a groove 58 formed in the outer periphery of hub 46. The inner periphery 60 of the retainer has a diameter intermediate that of the hub interior 48 and the inner diameter of shoulder 54 such that the retainer 56 serves to limit the downward motion of the plunger 49.
The plunger 49 has a precision cylindrical bore 62 formed in the upper end thereof with the lower end of the bore terminating in a shouldered flat bottom. The precision bore 62 has slidably received therein in very closely fitting relationship a piston 64, the outer periphery 66 thereof being of precision diameter and smoothness so as to provide control of the leakdown or passage of pressurized engine lubricant therebetween. In the presently preferred practice the piston 64 is formed of a suitable steel material.
The piston 64 has a fluid passage 68 formed vertically and preferably centrally therethrough with a counterbore 70 formed therein. The bottom end of the passageway 68 has a counterbore 71 provided in the lower end of the piston 64 which counterbore has a flat bottom 73 which intersects the passageway 68 in an annular seat 74. A check-ball 72 rests against the annular seat 74, and is, as known in the art biased thereagainst by a suitable expedient as, for example, a conical check-ball spring 76 to provide a one-way valve. The check-ball 72 is retained by a cage 78 which has an outwardly extending flange 80 received in counterbore 71 and retained therein by suitable means as, for example, a press fit. The cage in the presently preferred practice has a central aperture 75 into which the check-ball 72 is partially received in the valve fully open position as shown in Fig. 4. The subassembly of the check-ball, cage and piston is biased upwardly by a plunger spring 82 having its upper end registering against the flange 80 of the check-ball cage and its lower end contacting the bottom of the plunger.
The area surrounding the plunger 49 above the web 44 and bounded by the under surface of cam face member 18 comprises a first portion 83 of a fluid reservoir which is communicated with the region externally of the outer wall 42 by a passageway 84 provided through the outer tubular wall of the tappet body and the web 44. The by-pass recess 43 functions to maintain a second portion 85 of the reservoir bounded by piston counterbore 70 and the underface of member 18 in continuous fluid communication with the first portion 83 of the reservoir. It will be understood the piston is maintained in the upward extreme position and against the undersurface of member 18, as illustrated in Figures 3 and 4 by spring 82 and the hydraulic pressure in chamber 86 defined between the plunger 49 and the piston 64.
In the presently preferred practice, the fluid retained in the second portion 85 of the fluid reservoir supplies the one-way valve upon engine start-up in the event that during periods of engine inactivity, fluid has drained from the first portion 83 of the reservoir.
The chamber 86 below the check-ball 72 and seat 74 and bounded by the bore of the plunger, and the bottom of the plunger comprises a high pressure fluid chamber for retaining therein fluid entering through passage 68 upon opening of the check-ball 72.
In operation, the combustion chamber valve is biased to a closed position by spring 28 and upon rotation of the cam shaft in timed relationship to the events of the combustion chamber to the position shown in solid outline in Figure 1, the upper surface of the tappet is registered against the base circle portion of the cam with the lobe 16 oriented so as not to contact the cam face member 18 of the tappet. Upon rotation of the cam shaft 14 to the position shown in dashed outline in Figure 1, the cam load contacts the cam face member 18 of the tappet, causing the tappet to move downwardly thereby opening the combustion chamber valve. Upon subsequent rotation of the cam shaft to return to the solid outline position of Figure 1, the valve event is complete and the valve is reseated on the valve seat.
In operation, with the engine cam lobe 16 in the position shown in Fig. 1, the plunger spring 82, aided by hydraulic pressure, maintains the upper end of piston 64 in contact with the undersurface of cam face member 18 and urges the plunger 49 in the downward direction until the end face 52 thereof contacts the upper face 26 of the valve stem 22 thereby eliminating lash in the valve gear. This causes expansion of chamber 86 which draws open the check-ball 72 to the position shown in Fig. 4 permitting fluid to flow into chamber 86. Upon cessation of the expansion of chamber 86, the check-ball 72 closes under the biasing spring 76. Upon subsequent rotation of the cam lobe 16, the ramp of the cam lobe begins to exert a downward force on the cam face member 18 of the tappet tending to compress the piston 64 into bore 62 in the plunger, which compression is resisted by fluid trapped in chamber 86. The fluid trapped in the chamber 86 prevents substantial movement of the piston 64 relative to plunger 49 and transmits the motion through the end face 52 of plunger 49 onto the top of the valve stem 26. It will be understood by those having ordinary skill in the art that a minor movement of the plunger with respect to the piston occurs, the magnitude of which is controlled by the amount of fluid permitted to pass through the aforesaid leakdown surfaces provided by plunger bore 62 and piston outer periphery 66. The piston 64 and plunger 49 thus act as a rigid member transmitting further lifts of cam lobe 16 for opening the valve to the position shown by dashed line in Fig. 1.
Referring now to Fig. 5, an alternate embodiment of the tappet is shown generally at 100 as employing a tubular outer wall 102 having an inwardly extending web 104 integrally formed therewith and a central tubular hub 106 formed integrally with the web 104. A plunger 108 is slidably received in the hub 106. The plunger has a hydraulic piston and one-way valve similar to that of embodiment of Fig. 3, and the outer tubular wall 102 has the one end thereof closed by a cam face member 107. The plunger 108 in the embodiment of Fig. 5 has a circumferential groove 110 formed about the periphery thereof with a radially compressible-expandable generally C-shaped snap-ring 112 received therein.
In assembling the embodiment of Fig. 5, the plunger assembly with the ring 112 received thereon is piloted into the inner periphery of the hub 106, the ring 112 is then compressed such that its outer circumference is less than the inner periphery of the hub. The plunger is then moved upward with respect to Fig. 5 until the snap ring 112 passes through the hub 106 and then is free to expand slightly so as to retain the plunger assembly from removal from the hub. It will be recognized that the embodiment of Fig. 5 eliminates the need for the forming of a groove, such as groove 58, in the wall of the hub 46 (see Fig. 3) which results in difficult manufacturing operations in forming such groove particularly when the outer diameter of the body is less than 35 mm. The embodiment of Fig. 5 however, results in a tappet assembly in which the plunger subassembly is potentially more difficult to remove.
Referring now to Fig. 6, a further alternate embodiment of the invention is illustrated wherein portions of the tappet indicated generally at 120 are shown. The body of the tappet of Fig. 6 has an outer tubular wall 122 and a transverse web 124 having integrally formed therewith a central tubular hub 126 extending axially downwardly from the web 124. The inner periphery of the hub 126 has a circumferential groove 128 formed therein adjacent the lower end thereof with a retaining means in the form of a snap ring 130 received therein for retaining the plunger 132 in the hub.
Referring now to Figure 7, an alternate embodiment of the tappet 140 is illustrated having an outer tubular wall 141 closed at the upper end thereof by a transverse end wall 142 providing a cam face member and having the upper surface thereof hardened for contacting a cam. The transverse wall 142 is preferably formed integrally with the outer wall 141 and of a suitable iron base material such as a readily formable and hardenable steel. The outer wall 141 has a preferably annular rib 144 extending from the inner surface thereof intermediate the ends of the outer wall and has a circumferential groove 147 formed in its inner periphery and spaced vertically downward in Figure 7 from the rib 144. An annular member 146 formed of a suitable iron base material as, for example, an unhardened carbon steel, is received in the inner periphery of the outer wall 141 with the upper surface of annular member 146 registering against the lower surface of annular rib 144. The annular member 146 forms a central web as in the previous embodiments and has a downwardly extending annular hub 148 formed integrally therewith and centrally disposed with respect thereto which hub 148 has received therein a suitable hydraulic lash-adjusting assembly indicated generally at 150 and similar to the assembly shown and described hereinabove with respect to Figures 3, 4 and 5. The hydraulic lash-adjusting assembly 150 is retained in the inner periphery of hub 148 by deformation of the lower edge of the hub inwardly to form a lip 152.
The outer periphery of annular member 146 has an annular wall 154 extending downwardly with respect to Figure 7 from the outer periphery of the annular member 146 and along the inner periphery of the outer wall 141 of the tappet body. The lower edge of wall 154 is deformed outwardly into the groove 147 to retain the annular member 146 registered against the undersurface of rib 144 and to prevent removal of the annular member 146 from the tappet body. The annular member 146 thus defines the first portion 156 of the hydraulic fluid reservoir similar to the portion 83 of the embodiment of Figure 3. The function of the hydraulic lash-adjusting assembly 150 in tappet 140 is otherwise identical to that described hereinabove with respect to the embodiments of Figures 2 through 5. The embodiment of Figure 7 provides a simplified structure for the bucket or tappet body; however, a separate insert is required to provide for the first portion of the fluid reservoir and the supporting hub for the lash-adjusting assembly.
The present invention, as described and illustrated in the foregoing embodiments, provides a unique bucket tappet for use in direct acting valve gear of internal combustion engines and it is particularly suitable for operating speeds in excess of 5,000 rpm. The present tappet employ a hardened steel member attached over one end of a tubular outer wall of the tappet body, for providing the cam face of the tappet. The tubular outer wall has, preferably formed integrally therewith, an inwardly extending web disposed intermediate the ends of the tubular outer wall, with the web having formed integrally therewith a centrally disposed tubular hub for receiving the hydraulic lash adjusting assembly. The plunger assembly is retained preferably by retaining means engaging the tubular hub or alternatively, by a snap ring provided in the outer periphery of the plunger. The described tappet provides the lash adjustment by a precision fit of a piston in a bore formed in the plunger slidably received in the hub, and thus eliminates the need for precision fitting leakdown control surfaces on the interior of the tappet hub. The area surrounding the plunger between the web and tubular outer wall of the body and the cam face member provides a first portion of a reservoir and a cavity in the piston provides a second portion of the reservoir for fluid to supply the one-way valve of the hydraulic lash adjusting assembly. The external retaining means in the preferred embodiment permits ease of manufacturing and ready removal of the hydraulic plunger assembly for cleaning and/or parts replacement.
In all the embodiments, the fluid pressure chamber is located near or below the lowest level of the two-part fluid reservoir to provide a convenient escape path for any trapped air.

Claims

1. A bucket tappet (10) for use in the valve gear of an internal combustion engine which is capable of operating at speeds in excess of 5000 RPM, which valve gear is of the direct acting type wherein one end of the tappet (10) is contacted by an engine cam (16) and the other end is in direct contact with a stem (22) of a combustion chamber valve (20), the tappet (10) comprising:

- a tubular outer wall (42; 102; 122; 141),

- a top (18; 107; 142), extending transversely of the outer wall at the top end of the tappet, and adapted to contact the engine cam,

- a plunger (49; 108; 132) located within and slidable axially relative to said outer wall, the plunger having at its lower end an end wall (52) contacting the valve stem,

- an annular first portion (83) of a fluid reservoir located radially in between the tubular outer wall (42; 102; 122; 141) and said plunger (49; 108; 132) and axially between said top (18; 107; 142) and a web (44; 104; 124; 146) which forms the bottom of said annular portion of reservoir, the radial extent of said first portion being substantially the same as that of the web,

- passage means (84) for supplying fluid to said annular portion of reservoir,

- a central second portion (85) of reservoir in fluid communication with said first portion,

- a fluid pressure chamber (86) within the plunger, which chamber is located near or below the lowest level of said two portions of reservoir,

- a one-way valve (72, 74) operable to permit a flow of fluid therethrough from said central second portion (85) of reservoir into said fluid pressure chamber (86) upon movement of said plunger (49; 108; 132) in a direction tending to expand said chamber (86) and to prevent reverse flow of fluid therethrough from said fluid pressure chamber (86) upon movement of said plunger (49; 108; 132) in a direction tending to diminish said fluid pressure chamber,

characterised in that a piston (64) unsecured to the top (18; 107; 142) is slidably received in said plunger (49; 108; 132) and cooperates with said plunger to define said fluid pressure chamber (86), biasing means (82) being arranged between said plunger and said piston for biasing said plunger and said piston in a direction away from one another, in that said piston (64) fits within said plunger (49; 108; 132) in a manner adapted to provide controlled leakdown from said fluid pressure chamber (86) upon the application of an axial load on said piston (64) tending to urge said piston (64) in a direction to compress fluid in said fluid pressure chamber (86), in that said central portion of reservoir is located within the piston (64), between the piston and said top (18; 107; 142), and in that the plunger is received in a generally tubular hub (46; 106; 126; 148) supported within the outer wall and extending generally parallel thereto,

said hub being maintained in fixed relation to the top (18; 107; 142), but with its upper end axially separated from said top, by means of said web which extends between the hub and the tubular outer wall and connects the hub to the tubular outer wall so as to form the sole structural support for the hub.

2. A tappet as claimed in claim 1 characterised in that said hub (46; 106; 126; 148) is formed separately from said top (18; 107; 142).

3. A tappet as claimed in claim 1 or claim 2 characterised in that said web (44; 104; 124; 146) extends inwardly from said outer wall (42; 102; 122; 141) intermediate the ends of said outer wall (42; 102; 122; 141).

4. A tappet as claimed in any of claims 1 to 3, characterised in that said web (44; 104; 124) is formed integrally with said outer wall (42; 102; 122) and said hub (46; 106; 126) is formed integrally with said web (44; 104; 124).

5. A tappet as claimed in any of claims 1 to 3, characterised in that said web (146) and said hub (148) are provided by an annular member mounted within said outer wall (141).

6. A tappet according to claim 1 or 2, characterised in that said top (18; 107) comprises a disc shaped member disposed across one end of said outer wall (42; 102; 122) and secured thereto about the periphery thereof.

7. A tappet as claimed in any of claims 1 to 3, or 5, characterised in that said top (142) is formed integrally with said outer wall (141).

8. A tappet as claimed in any preceding claim, characterised by retaining means (56; 112; 130; 152) for retaining said plunger assembly (49, 64) in said hub (46; 106; 126; 148).

9. A tappet as claimed in claim 8, characterised in that said retaining means comprises an annular member (56) engaging the outer periphery of said hub (46).

10. A tappet as claimed in claim 8 characterised in that said retaining means comprises an annular member (56) received over the end of said hub (46) remote from said top (18).

11. A tappet as claimed in claim 8 characterised in that said hub (46) includes a circumferential groove (58) formed in the outer periphery thereof and in that said retaining means comprises an annular spring clip (56) engaging said circumferential groove (58).

12. A tappet as claimed in claim 8, characterised in that said plunger (40) has a circumferential recess (110) and in that said retaining means comprises an annular member (112) received in said recess (110).

13. A tappet as claimed in claim 8, characterised in that said hub (148) is formed with a lip (152) providing said retaining means.

14. A tappet as claimed in any preceding claim, characterised in that said passage means comprises a fluid passage (84) through said outer wall (42; 102; 122; 141) communicating the outer periphery of said outer wall (42; 102; 122; 141) with said fluid reservoir (83; 85; 156; 176).