Valve lifter structure

Abstract: A valve lifter structure having a follower (15) which tilts in a guide groove (13), for a mechanism for varying valve characteristics of a poppet valve by axial displacement of a three-dimensional cam (2) having a profile varying in the axial direction, which is easily machined without degrading a mechanical strength. The guide groove (13) is formed on a top surface of a valve lifter (11) interposed between a cam (2) and a top of a valve stem (4) of a poppet valve (3) to extend perpendicular to the axis of the cam (2). A follower (15) is located in the guide groove (13), which extends in parallel to the guide groove (13) and is capable of tilting in accordance with the change of the cam profile while being brought into slide-contact with the guide groove (13), and is inhibited from displacing in the extending direction by a limiting means (14). The guide groove (13) is formed to have opposite open ends. The limiting means is defined by a stopper (14) located on the guide groove side and an abutment member located on the follower side to abut to the stopper (14). The abutment member is defined by a pair of walls of the follower (15) spaced at a distance opposite to each other in the extending direction thereof.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a valve lifter structure and, particularly to a valve lifter structure in a mechanism for varying valve characteristics, for example, timings of valve opening and valve closing, valve lift and others, using a three-dimensional cam, arranged between the three-dimensional cam and a top of a valve stem of a poppet valve. 2. Description of the Prior Art In a known mechanism for varying valve characteristics, such as disclosed in EP 0 208 663 A1, a three-dimensional cam having a profile varying in the axial direction is displaced in the axial direction for the purpose of changing a valve-opening/closing timing of an intake valve or an exhaust valve, formed by a poppet valve in an internal combustion engine, in accordance with operational conditions. When such a three-dimensional cam is used, the durability of a slanted cam surface and a cam follower can be improved by providing a guide groove on a top surface of a valve lifter arranged between the three-dimensional cam and a top of the valve stem of the poppet valve so that cam and the valve lifter are brought into surface-contact with each other. Japanese Unexamined Patent Publication No. 3-179116 discloses a mechanism wherein a guide groove completely encircled by walls is formed on a top of a valve lifter and a tilting follower is arranged therein, or the guide groove has a raised portion at a center thereof. In this mechanism, a slide-contact surface between the guide groove and the follower is necessarily formed with a high degree of precision so that the follower can be smoothly tilted. However, the former type having the guide groove completely encircled with walls has a drawback in that it is difficult to machine the slide-contact surface to a high degree of precision because the tools usable for such a purpose are limited. On the other hand, the latter type having a raised portion has a drawback in that part of the follower is necessarily thin-walled to receive such a portion, which weakens the follower. EP 0 108 238 A1 discloses a valve lifter structure in accordance with the preamble of claim 1. SUMMARY OF THE INVENTION An object of the present invention is to provide a valve lifter structure for a mechanism for varying valve characteristics using a three-dimensional cam which is easily manufactured and has a sufficient strength. According to the present invention, this object is solved by the valve lifter structure in accordance with claim 1. The present invention will be more fully understood from the description of a preferred embodiment of the invention set forth below, together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is an exploded perspective view of a preferred embodiment of the present invention; Fig. 2 is a side sectional view of an overall structure of the preferred embodiment; Fig. 3 is a top view of a valve lifter of the preferred embodiment; DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is an exploded view of a preferred embodiment of the present invention and Fig. 2 is a side sectional view of an assembly thereof. With reference to Fig. 1, a cam 2 having a three-dimensional cam profile varying in the axial direction is integrated with a cam shaft 1. The cam shaft 1 is driven by a driving means not shown to displace in the axial direction in accordance with the operating conditions. As shown in Fig. 2, a valve stem 4 of a poppet valve 3 is held by a cylinder head 100 via a valve guide 5. A spring retainer 7 is attached to an upper end of the valve stem 4 of the poppet valve 3 via a cotter pin 6. A lower surface of the spring retainer 7 is always applied with an upward bias by a valve spring 8, and, as a result, the poppet valve 3 is also applied with an upward bias, whereby an umbrella portion 9 of the poppet valve 3 is away from a valve seat 10, to form a flow path, only while the poppet valve 3 is pushed down by the cam 2. A valve lifter 11 is placed on a top surface of the valve stem 4 of the poppet valve 3 for receiving a pushing force of the cam 2. With reference again to Fig. 1, on a top surface of the valve lifter 11 is formed a guide block 32 which in turn is provided with a guide groove 33 formed on a top surface thereof. Opposite ends of the guide block 32 project outward from the periphery of the valve lifter 11 as seen in the rotational direction of the cam 2, and the guide groove 33 extends over the length of the guide block 32. Since a follower 35 placed in the guide groove 33 tilts in a sliding manner as described later, it is necessary to machine a slide-contact surface of the guide groove with a high degree of precision. According to this embodiment, this high precision machining is easily achievable without any limitation because the guide groove 33 is provided throughout the length of the guide block 32. According to the preferred embodiment, the guide groove 33 has an enlarged groove section 33a in a central area thereof, and a follower 35 similarly has an enlarged follower section 35a in a central area thereof, so that lateral surfaces of the enlarged groove section 33a and those of the enlarged follower section 35a of the follower 35 abut to each other to inhibit the follower 35 from being displaced in the rotating direction of the cam 2. This structure is advantageous in that the number of parts is minimized because it is unnecessary to prepare separate members as stoppers, and the mechanical strength of the follower increases because the thickness of the enlarged follower section 35 of the follower 35 becomes larger. According to the present invention, a valve lifter for a three-dimensional cam, of a type wherein a follower tilts in a guide groove, is thus provided which can be easily machined without weakening the follower. Fig. 3 is a schematic illustration of the valve lifter 11, thus assembled, viewed from above. A vertical hole 110 is shown by a chain line in Fig. 3, which is bored in the cylinder head 100 so that the valve lifter 11 is movable in parallel to the axis of the valve stem 4 of the poppet valve 3. As shown in Fig. 3, a pair of vertical grooves 120 having recessed cross-sections are formed in the cylinder head 100 so that projections 32a of the guide block 32 are movable. In such a manner, it is possible to prevent the valve lifter 11 from rotating. In this regard, it is also possible to not form a guide block 12 in integral with a valve lifter 11 but to prepare the guide block 12 separately from the valve lifter 11 and fitting the both to each other.

Claims: A valve lifter structure for a mechanism, for varying valve characteristics of a poppet valve (3) by the axial displacement of a three-dimensional cam (2) having a profile varying in the axial direction, comprising: a guide groove (33) formed on a top surface of a valve lifter (11) interposed between the cam (2) and a top of a valve stem (4) of the poppet valve (3), while extending perpendicular to the axis of the cam (2) and being formed to have opposite open ends as seen in the extending direction thereof, a follower (35) extending in parallel to the guide groove (33) and capable of tilting in accordance with the change of the cam profile, while being brought into slide-contact with the guide groove (33), and means for limiting the displacement of the follower (35) in the extending direction thereof, comprising a stopper which is located on the guide groove side, and an abutment member which is located on the follower side to abut to the stopper and is comprised of a pair of walls arranged opposite to each other at a space in the extending direction of the follower (35), characterized in that the stopper is defined by a pair of walls of an enlarged guide groove section (33a) opposite to each other in the extending direction of the guide groove (33), the enlarged guide section being formed in a central area of the guide groove (33) as seen in the extending direction of the guide groove (33), and the abutment member is defined by a pair of walls of an enlarged follower section (35a) as seen in the extending direction of the follower (35), the enlarged follower section (35a) being formed to be brought into slide-contact with the enlarged guide groove section (33a) in a freely tilting manner. A valve lifter structure as defined by claim 1, wherein the guide groove (33) is formed in a guide block (32) protruded from a top surface of the valve lifter (11), and the guide block (32) has opposite ends projected outside from the outer periphery of the valve lifter (11), to define the anti-rotation member.

Inventors: HASEGAWA TADAO, MORIYA YOSHIHITO, SUGIMOTO KIYOSHI, HASEGAWA, TADAO, MORIYA, YOSHIHITO, SUGIMOTO, KIYOSHI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1998
Main classification: F01L1
IPC codes: F01L13, F01L1