GB2088005A - A self adjusting hydraulic tappet for a heat engine particularly a heat engine having valves controlled by means of rockers - Google Patents

Abstract

A self adjusting hydraulic tappet for an overhead or side valve push rod engine comprises a cup-shape body (6) having an end wall (13) contacting a cam (7) of a cam shaft (1) and a side wall (14) slidable in a bore (15) in the engine block, and an inner body (16) slidable within the cup-shape body (6) and defining with it a chamber (17) which can be filled with fluid under pressure notably the engine lubrication oil, via passages (19, 20, 20a) in the two bodies (6, 16). The passages (19) in the inner body (16) each include a check valve comprising a ball (23) housed in a conical valve seat (22) formed at the entrance to the passage (19) where it opens into the chamber (17). The ball valves are positioned to remain open under the action of gravity and to close when the pressure in the chamber (17) exceeds a predetermined value. The balls (23) are retained in their valve seats (22) by a stop member (27) fixed to the inner body (16). <IMAGE>

Description

SPECIFICATION A self adjusting hydraulic tappet for a heat engine particularly a heat engine having valves controlled by means of rockers The present invention relates to a self adjusting hydraulic tappet intended for use on an internal combustion engine, in particular one of the type in which the valves are controlled by a cam shaft and by a transmission including rockers.

In this arrangement, as is known, the transmission of the movement from each cam to the associated valve is obtained by means of a transmission comprising a push rod which is operated by this cam, and a pivoted rocker operated, in turn, by the push rod. In such constructional arrangement the tappet is usually disposed between the cam and the lower end of the push rod. As is known, the members which control the opening and closing of the valves are subjected to wear since they slide under pressure against one another, and in particular the element of the tappet which is in contact with the cam, and which is more readily worn than a cam, is used up during operation thus producing clearances which prevent the complete opening of the valve.

Periodically therefore, it is necessary to effect an adjustment; moreover, the clearance conditions between cam and valve vary with variations in the operating conditions of the motor, this clearance depending on the thermal expansion of the members in the kinematic chain interposed between cam and valve.

There exist, however, mechanisms, usually situated on the tappet, which adjust the clearance in a continuous and automatic manner as soon as it occurs. There are known tappets intended to be used in mechanisms of the type described above, in which the cam shaft is disposed to one side of the cylinders, in which the take-up of the play is effected hydraulically. These substantially comprise a cup-shape body provided with an end wall which can abut on the associated cam and a side wall slidable in a corresponding housing in the engine block, and a movable element, slidable within the said cup-shape body and against which the said push rod abuts; such movable element defines, with the said cup-shape body, a chamber in communication, through a plurality of ducts, with a source of liquid under pressure.Each such duct opens into the said chamber at a flat surface of the said movable element and against this abuts a resiliently deformable plate pressed by a spring a constituting a valve for the duct itself.

When the motor is running liquid under pressure is supplied from the source and this, via the first mentioned duct, can reach the chamber by resiliently deforming the plate, until the chamber itself is fiiled.

When the force exerted by the cam acts on the tappet the oil pressure contained in the chamber increases pressing the plate against the said flat surface and closing the passage towards the ducts.

Such tappets have various disadvantages.

Above all, the oil seal obtained with the said resilient plate is not very good and therefore it allows a certain quantity of oil to escape from the said chamber with the consequence that this does not remain completely full when the force exerted by the associated cam acts on the tappet; consequently the clearance between cam and valve is not entirely eliminated and the characteristic clatter due to the presence of such clearances is not avoided. Moreover, the structure of the tappet is complex and it is therefore expensive and not very reliable.

In fact the assembly constituted by the plate and the spring operable to hold the former in the correct working position, comprises two resilient members which must have very rigorously defined forms and dimensions and which must be positioned in a very precise manner between the cup-shape body and the said movable element. In addition, the said surface, formed on the movable element, on which the said plate rests, and the plate itself, must have a very high surface finish and strict tolerances on shape and dimensions in order to be able to cooperate correctly with one another, and these must therefore be ground.

Finally, the spring and the plate, being cyclically deformed, are subjected to fatigue stresses which could bring about breakage of these.

The object of the present invention is that of providing a hydraulic tappet of the type described, with a very simple structure and of secure operation, which will therefore be free from the above mentioned disadvantages.

According to the present invention there is provided a self adjusting hydraulic tappet for the control of a valve in an internal combustion engine in which the said valve is operated from a cam shaft and by a transmission mechanism including a push rod which contacts the said tappet, the said cam shaft and the said push rod being located to one side of the cylinder of the engine which is provided with the said valve, the said tappet comprising a cup-shape body provided with an end wall which can abut against the said cam and a side wall which can slide in a corresponding seat in the engine block, and a movable element slidable within the said cup-shape body and against which the said push rod abuts and which defines with the said cup-shape body a first chamber in communication, via a duct, with a source of fluid under pressure, there being interposed along the said duct interception members for the said fluid, characterised by the fact that the said interception members comprise at feast one ball operable to cooperate with a corresponding seat formed in the said duct to close the duct itself, the said seat being formed in the said movable element in the region in which the said duct opens into the said first chamber.

For a better understanding of the present invention one embodiment thereof will now be described by way of example with reference to the attached drawings, in which: Figure 1 is a schematic view of the valve control mechanism of an internal combustion engine of which the tappet of the invention forms part; Figure 2 is an axial section of the tappet of the present invention; Figure 3 and 4 are axial sections of the tappet of the invention having a slightly different structure from that of Figure 2; and Figure 5 is a plan view of a stop ring which can form part of the tappet of the invention.

The valve control mechanism on which the tappet of the invention can be used is of the type including a rocker and in which the cam shaft 1 is disposed to one side of the longitudinal axis of the cylinders 2. When such constructional arrangement is adopted each valve, which can be overhead valve as shown in Figure 1 , or a side valve, is operated by a transmission including the tappet 6 which abuts directly against a cam 7, a push rod 8 one end of which abuts against the tappet whilst the other contacts one arm of a rocker 9; the valve 3 is normally held in the closed position by a helical spring 1 0.

The tappet of the invention comprises a cupshape body 12 (Figure 2) provided with an end wall 13 which can abut against a cam 7 and a side wall 14 slidable in a corresponding seat 1 5 formed in the engine block, as well as an element 1 6 axially movable within the body 12 and defining, with the side and end walls of this, a chamber 17.

The movable element 16 has a substantially cylindrical form with an annular cavity 1 8 which is in communication, via a series of holes 19, with the chamber 17; the said holes are conveniently disposed on the outer peripherai part of the element 1 6 as can be clearly seen in Figures 2 3 and 4. The annular cavity 18 is, moreover, in communication, via at least one hole 20a and an annular groove 20 formed in the side wall 14 of the cup-shape body, with a hole 21 through which oil under pressure can arrive from any suitable source.

The end of each hole 1 9 facing the chamber 1 7 is formed with a seat 22 for a ball 23 which can constitute an interception member to the passage of oil along the hole itself. The seat for this valve can have any suitable form allowing a suitable coupling with the associated ball for the purpose of preventing the passage of oil; conveniently this seat comprises a conical surface the generatrices of which lie at a predetermined angle with respect to the axis of the surface itself; such angle, which is conveniently between 100 and 200, must not be too small in order to avoid the ball 23 jamming within the seat.

Rigidly connected to the movable elements 16 are stop means for the balls 23 which allow a predetermined displacement of these and prevent them from falling out of the chamber 17. Such means can be constituted by a member of annular form 25 illustrated in Figures 2 and 5 and comprising a pair of rings 26 and 27 (Figure 5) connected by spokes 28. The said member is made from a resiliently deformabie meterial, for example a plastics material, and can be fixed to an axially projecting part 29 of the movable element 16: for this purpose an annular groove is formed on it, in which groove the ring 26 of the said member can be snap engaged.Alternatively, the stop means for the balls 23 can be constituted by pins 30 each of which is inserted in a corresponding radial hole in the projecting part 29 of the movable element 1 6 beneath a corresponding ball, as can be seen in Figure 3.

The push rod 8 (Figure 1, 2 and 3) of the valve control mechanism has a spherical lower end which can abut against a corresponding spherical surface 32 of the movable element 1 6.

Conveniently, the axial length of the projecting part 29 of the movable element 16 is chosen in such a way that its end surface 29a abuts against the corresponding surface of the end wall 13 when the element itself is in its lower end-of-path position, a condition which occurs when the oil in the interior of the chamber 1 7 is not under pressure. A resilient ring 33, inserted in a corresponding annulargroove in the side wall 14 of the cup-shape body 12, constitutes an upper end-of-path stop for the relative movement of the movable element 1 6 with respect to the cupshape body 12.

Conveniently, for purposes which will be indicated below, the lower end of the axially projecting part 29 of the movable element 1 6 can be housed in a cavity 34 (Figure 4) formed in the end wall 13 of the cup-shape body 12.

The operation of the tappet described is as follows.

When the tappet is in the rest condition there is no fluid in the chamber 1 7 (or it is at atmospheric pressure) and therefore the movable element 1 6 is in its lowermost end-of-path position in which the surface 29a of this element is in contact with the end wall 13 of the cup-shape element 12.As soon as the motor is started, oil under pressure arrives at the hole 21 of the engine block and from there, through the annular groove 20, the hole 20a and the annular cavity 18, passes to the seats 22 (from which, due to their weight, the balls are spaced and rest on the annular member 25 (Figures 2 and 4) or on the pins 30 (Figure 3)), and from these into the chamber 1 7. The quantity of oil which enters this chamber is that necessary axially to space the movable element 1 6 from the cup-shape body 1 2 by an amount sufficient to eliminate any play in the transmission between the cam 7 and rocker 9 (Figure 1).

During the opening of the valve, because of the force applied to the tappet by the cam 7, the pressure of the oil within the chamber 17 increases with the consequence that the balls 23 are pressed against the associated seats 22 preventing the escape of oil from the chamber; in this phase therfore, the oil contained in the chamber acts as a proper hydraulic bearing able to maintain the two cut-shape bodies in their correct relative axial positions.

It has been found that, with the tappet described, during operation of the engine, play in the transmission between cam and valve is completely eliminated and the rattling consequent on the presence of such play is completely suppressed.

The time required for such play to be taken up is very short and much lower than that necessary to obtain the same effect in prior art tappets of similar type: moreover, a complete filling of this chamber is obtained even with very low oil pressures such as exist when the engine is running slowly, and which would not be sufficient to fill the chamber of prior art tappets described above. This favourable behaviour is probably due to the improved sealing operation obtained in all conditions of use by the interception member devised for the tappet of the invention, by the absence of springs which require not inconsiderable force for the opening of such members, and by the small influence exerted by the inertia of such members.

It is evident, then, that the structure of the tappet described is very simple, allowing resilient members to be completely dispensed with; moreover, the construction of its parts does not present technological difficulties, the regions of these parts which must be worked with any significant precision being of very small extent: in particular, the most sensity region from this point of view, which is constituted by the seats 22 for the balls 23, is of very small extent, is easily accessible, and is of a simple and well defined goemetric form.

Because of its very simple structure the operation of the tappet is certain even over long periods of use, there-being no members which deform resiliently during the operation and which could therefore give rise to breakages by fatigue.

The embodiment of Figure 4, in which embodiment the lower end of the projecting part 29 of the movable element is always housed in the cavity 34 may be preferable whenever it is required that, in any operating conditions, and in particular upon starting of the motor at a very low temperature, the force which the oil contained in the chamber 17 exerts on the movable element 16 should not be very high, and lower than a predetermined value.As is known, the oil in the engine can achieve abnormal, even very high pressures when the motor is started in very low ambient temperature conditions; with the constructional arrangement of Figure 4 the area on which this abnormal pressure is exerted is very small (this area is that of the annular ring the outer and inner diameters of which are equal, respectively, to those of the chamber 1 7 and the projecting part 29, and therefore the force which it transmits to the push rod 8 is very small, which force, if it were excessively high, could overcome that exerted by the spring 10 (Figure 1) and make the valve 3 open. In fact, during the initial starting period (during which the said abnormal excess pressures can be produced), because of the difficulty of leakage between the facing surfaces of the projecting part 29 and the cavity 34, the pressure within this latter is, during the cited period, substantially equal to the pressure which exists when the motor is stationary, that is to say atmospheric.

Finally, it is clear that the described embodiments of the present invention can be modified and varied, both as to the form and the arrangement of the various parts, without departing from the scope of the invention itself.

Claims (6)

1. A self adjusting hydraulic tappet for the control of a valve in an internal combustion engine in which the said valve is operated by a cam shaft and a transmission mechanism including a push rod which abuts against the said tappet, and a pivoted rocker operated by the said push rod, the said tappet including a cup-shape body provided with an end wall which can abut on the said cam and a side wall siidable in a corresponding seat in the engine block, and a movable element slidable within the said cup-shape body and against which the said push rod abuts, which defines with the said cup-shape body a first chamber in communication, via a duct, with a source of fluid under pressure, there being interposed along the said duct interception members for the said fluid, characterised by the fact that the said interception members comprise at least one ball (23) able to cooperate with a corresponding seat (22) formed in the said duct (19) to close the duct itself, the said seat 22 being formed on the said movable element (16) in the region thereof in which the said duct (19) opens into the said first chamber (17).

2. A tappet according to Claim 1, in which the said movable element has an annular cavity defining, with the said side wall of the said cupshape body, a second chamber of annular shape forming part of the said duct, and at least one substantially axial hole which puts the second chamber in communication with the said first chamber, characterised by the fact that the said seat (22) is formed in the region in which the said hole (19) opens into the said first chamber (17).

3. A tappet according to Claim 1 or Claim 2, characterised by the fact that it includes stop means (26,27) for the said balls (23), operable to limit the displacement of these with respect to the associated seats (22) by a predetermined amount, the said means (26,27) including at least one stop member (27) fixed to an axial projection (29) of the said movable element (16).

4. A tappet according to any preceding Claim, characterised by the fact that the said axial projection (29) of the said movable element (16) has a predetermined length such as to constitute a stop to the movement of the said movable element (16) towards the end wall (13) of the said cup-shape body (16).

5. A tappet according to Claim 6, characterised by the fact that in the said end wall (13) of the said cup-shape body (6) there is formed a cavity (34) able to house the end of the said axial projection (29).

6. A self adjusting hydraulic tappet substantially as described and illustrated in the attached drawings.