HYDRAULIC TAPPET

Abstract: A hydraulic tappet to provide more rapid compensation for play between a cam and a valve stem, comprises a diagonal duct (46) by-passing a ball-valve (44) when the tappet has collapsed, eg. after a period of non-use, so that the inner oil reservoir (16) can communicate directly with the high pressure chamber (18). In one embodiment, provision is made for a diaphragm (38) having a bore (36) to be placed on top of the inner reservoir (16) to stop all the oil draining away from the inner reservoir. According to another embodiment, the latter is provided with horizontal ducts (54) in lieu of the diagonal duct (46) to ensure direct communication with a blow-by region or leakdown path (48).

Description: The present invention refers to a hydraulic tappet for taking up play between a driving cam and the stem of a valve, as soon as possible after a startup, even after a certain period of inactivity of an internal combustion engine. Known hydraulic tappets comprise essentially a first, outer part which engages the driving cam and which has an inner reservoir where the oil collects and an outer reservoir connected to the inner one via an opening; and a second, inner part which slides telescopically inside the first part and contacts the valve stem, such telescopic assembly of the first part inside the second part providing a high pressure chamber of variable capacity which communicates in unidirectional manner with the inner reservoir, eg. via a ball-valve. One of the drawbacks of this type of tappet is the fact that, when the engine has been switched off for some time, the oil drains from the high pressure chamber to the outer reservoir passing through a leakdown region leading to collapse of the telescopic assembly. Hence, when the engine is restarted, due to expansion, the amount of oil reaching the high pressure chamber can be insufficient. This is caused by the fact that, although a one way ball-valve is provided between the inner reservoir and the high pressure chamber, oil seeps away from the high pressure chamber when the engine is switched off, through a leakdown region between the outer surface of the inner piston which is in contact with the outer reservoir, and the inner surface of the guide wall provided in the sliding sleeve, leading to collapse together of the two telescoping parts of the tappet. As a result, there can be play between the tappet and the cam and between the tappet and the valve stem, such play causing noise when the engine is switched on and the noise lasting until the play is taken up by the high pressure chamber again becoming filled with oil. Known hydraulic tappets have a further drawback resulting from the previous one. In fact, the oil of the lubrication circuit, which collects first in the outer reservoir and then in the inner reservoir, may contain air bubbles which pass on with the oil into the high pressure chamber when the pressure in the latter is reduced. Therefore the oil presence is reduced in the high pressure chamber even when the engine is active again, and therefore undesirable play within the tappet may continue. DE-A-3 418 707, the state of the art, discloses in Fig.1 thereof a duct between a leakdown region and an inner reservoir. Its function is not described, but this duct could not contribute to solving the above problems. DE-A-1 808 000, Fig.1 and description, suggests no inner or central reservoir as such, but the normal feed path of the hydraulic fluid is inwards from an outer annular reservoir, across a leakdown path, into an annular feed groove (not an inner reservoir), and thence via an axial duct to a high pressure chamber without anywhere for releasing air bubbles. According to this invention, the state of the art DE-A-3 418 707 is characterized as set out in Claim 1. Thus in order to minimize the drawback of air bubbles being present in the high pressure chamber and to provide rapid adjustment of the play of the tappet by filling the high pressure chamber with oil after startup, according to a main feature of the present invention, a diagonal duct is provided between the inner reservoir and the blow-by annulus, which is able to bypass the ball-valve but only when the two telescopic parts of the tappet are telescoped together with the high pressure chamber collapsed, such duct being nearly closed under normal operating conditions with the high pressure chamber not collapsed. When the diagonal duct is open, the oil can quickly reach the inner reservoir, rather than the outer one, and equally quickly the air can come out, thus minimising the length of time during which the engine is noisy once it is switched on after having been off for a long time, or after a large number of repeated starts. According to an option, there is also provided a shielding diaphragm positioned at the inlet or on the top of the inner reservoir, which has a small central bore to allow the oil to pass through. Such diaphragm stops all the oil draining from the inner reservoir, when the oil pump is inoperative for some time and the car or the engine has an inclined aspect. The present invention will now be described in detail, with reference to the attached drawing, wherein Fig. 1 shows a cross-section of the hydraulic tappet of the present invention. A known hydraulic tappet comprises an outer part 10, an inner part 12 and an inner reservoir 16 inserted in the inner part 12. Between such parts 10 and 12 there is defined an outer reservoir 14. Between said inner part 12 and said inner reservoir 16 there is defined a high pressure chamber 18. Lubricating oil originating from the lubricating circuit enters the outer reservoir 14 via an opening 20. From the outer reservoir 14 the oil moves into the inner reservoir 16 via an aperture 36 provided in a diaphragm 38 or by flowing directly into the inner reservoir 16 if diaphragm 38 is not provided. The function of the optional diaphragm 38 will be explained hereinafter. From the inner reservoir 16 the oil flows into the high pressure chamber 18 as a result of a relative movement between the inner part 12 and the inner reservoir 16, i.e. between a wall 40 of the inner part 12 and a wall 42 of the inner reservoir 16, whereby a one-way check valve or ball-valve 44 opens as known in the prior art. In known hydraulic tappets, when the engine is switched off for a certain length of time, the oil in the high pressure chamber 18 tends to seep away through a blow-by or leakdown region 48 formed between the inner surface of the wall 40 of the inner part 12 and the outer surface of the wall 42 of the inner reservoir 16. In time, or after repeated starts, the inner reservoir 16 loses enough oil by the pumping action to chamber 18, and then out to chamber 14, to become insufficiently filled for its correct feeding of the chamber 18. This ultimately causes air ingestion, and play between the cam and an outer surface 50 of the outer part 10 and/or between the lower surface of the inner part 12 and the upper end of the stem of the valve of the internal combustion engine. When the engine is switched on, play continues to exist for a certain length of time, and therefore the tappet is fairly noisy until the expanded high pressure chamber 18 is again adequately filled with oil. In order to overcome this drawback, in preferred embodiments of the present invention, a diagonal duct 46 is provided in the thickness of the floor of the inner reservoir 16, such duct 46 normally having one end opening into the oil leakdown region 48 and having the other end opening into the inner reservoir 16. In a preferred embodiment, the duct 46 is positioned at an angle alpha of approx 35 degrees in relation to the vertical plane. Any air which is undesirably present in the oil of the high pressure chamber can normally flow through this duct to a non-critical area, making it easier for adequate oil with a minimum of air bubbles to reach and occupy the high pressure chamber 18. An even more important advantage is that, thanks to such duct 46, when the engine is switched on under a condition whereby the inner reservoir 16 has collapsed under valve spring pressure, the lower end of the duct 46 is located so as no longer to be in communication with the leakdown region 48 but instead communicates with an annular space provided by a rebate 52 in the inner wall of the outer part 12. Hence, after a collapse, the oil can flow directly and quickly from the inner reservoir 16 to the chamber 18, bypassing continuously for a period the ball-valve 44, which only opens and closes cyclically, and can relatively rapidly fill the chamber 18. Consequently, the inner reservoir 16 rises and so does the outer surface 50 of the outer part 10 which soon maintains perfect contact with the cam, hence the play of the cam is rapidly adjusted out. Another important advantage is the fact that the oil which has drained from the high pressure chamber during the collapsed phase can return directly to the inner reservoir instead of leaking first via 48 to the outer reservoir. Once the collapsed phase is over, the outlet of the diagonal duct 46 ceases to communicate with the annular space provided by the rebate 52, and therefore the oil can flow normally from the inner reservoir 16 to the chamber 18 only via the ball-valve 44, except for a slight leakdown, as is conventional practice, but keeps being partially recirculated from chamber 18 to 16 rather than completely to chamber 14. Moreover under certain conditions or designs, eg. when a vehicle with an engine is temporarily in a non horizontal position, for instance with the vehicle on a slope or with one side at a different level from the other, or with an inclined engine mounting, the oil tends to quickly drain from the inner reservoir, in a prior art tappet. The optional provision of said diaphragm 38, positioned on the open top of the upper reservoir 16, slows down and prevents all the oil from draining away, whereas its opening 36 allows the oil from the outer reservoir to flow into the inner reservoir 16 under pressure. The diaphragm thus counters or inhibits the collapse of the telescoped parts caused by oil draining away.

Claims: A hydraulic tappet comprising an outer part (10) in contact with a driving cam, an inner part (12) which is in contact with the stem of a valve of an internal combustion engine and arranged within the outer part (10), the two parts together forming an outer oil reservoir (14), an inner oil reservoir (16) slidable in the inner part (12) and normally abutting the outer part (10) axially, the inner reservoir (16) and the inner part (12) sliding together forming between them axially a high pressure chamber (18) of variable axial length which can communicate with the inner reservoir (16) via a unidirectional path (44), in order to minimize axial free play, the sliding parts forming between them radially a leak-down or blow-by communication annulus region (48) between the pressure chamber (18) and the outer reservoir (14), and comprising a duct (46) forming a fluid pathway between the inner reservoir (16) and a point in the leakdown annulus (48), characterized in that, for more rapid recovery from the effects of a collapse of the high pressure chamber (18) and a consequent non-abutment between the inner reservoir (16) and the outer part (10), the duct (46) is arranged to open directly into the high pressure chamber (18) under the collapsed condition of this chamber. A hydraulic tappet according to Claim 1 characterized in that, from an inlet adjacent the floor of the inner reservoir (16) and passing generally obliquely through the thickness of such floor, to an outlet position where it meets the exterior of its boundary wall (42), which outlet position is thus said to point in the leakdown annulus, there is formed a duct (46) sloping by an angle of inclination in relation to said boundary wall (42) of the inner reservoir (16), said angle of inclination being preferably around 35°, the outlet position of this diagonal duct (46) being at the leakdown region (48) when the high pressure chamber is not in a collapsed condition, but opening onto an annular space, obtained by means of a rebate (52) provided on the inner surface of an interior wall of the inner part (12) of the tappet, only when said collapse occurs as a result of the high pressure chamber (18) becoming inadequately filled with oil. A tappet according to Claim 1 or 2 characterized in that a diaphragm (38) having an aperture (36) is located substantially at a position corresponding to the top of the inner reservoir (16) with consequent tendency for less rapid loss of fluid from this reservoir and hence a more rapid recovery from a collapsed state of the high pressure chamber (18).

Inventors: RIGAMONTI, FLAVIO
Applicants: EATON AUTOMOTIVE SPA.
Application year: 1990
Main classification: F01L1
IPC codes: F01L1