VALVE ACTUATING MECHANISM FOR INTERNAL COMBUSTION ENGINES

Abstract: A valve actuating mechanism for an internal combustion engine has a rotatable cam which has its camming surface (12) disposed to slide on a cam slipper surface (14) of a rocker arm to thereby open and close an intake valve (or exhaust valve) of the engine by rocking motion of the rocker arm. The dimensions shapes, and relative positions of the rotatable cam and the rocker arm are so designed as to satisfy a condition of V C + V F > 0, where V C represents the velocity of movement of a contact point on the camming surface at which the camming surface slides on the cam slipper surface, and V F the velocity of movement of the contact point on the cam slipper surface at which the cam slipper surface slides on the camming surface.

Description: The present invention relates to a valve actuating mechanism for opening and closing an intake or exhaust valve of an internal combustion engine installed on a vehicle. Conventionally, a valve actuating mechanism for internal combustion engines as shown in Fig. 1 is known in which a rotating cam 1 has its camming surface 2 disposed to slide on a cam slipper surface 4 of a rocker arm 3 to thereby open and close an intake valve 5 (or exhaust valve 6) by rocking motion of the rocker arm 3. Such a valve actuating mechanism is known from GB-A-2 160 922 on which the precharacterising part of claim 1 is based. This type of valve actuating mechanism is required to have so high wear resistance that it is not adversely affected by lubricating conditions which may vary according to the type of lubricating oil used and running conditions of the vehicle, as well as to be light in weight to contribute to upgrading the performance of the engine. However, the conventional valve actuating mechanism shown in Figure 1 has the disadvantage that the camming surface 2 and the cam slipper surface 4 are liable to wear, which makes it impossible to meet the above requirements. Analysis of the cause of the wear has revealed that, in almost all cases, the wear is caused by scuffing due to breakage of the oil film. Breakage of the oil film can cause scuffing and sometimes even seizure even if the pressure or load acting upon the camming surface 2 and/or the cam slipper surface 4 is reduced, which, therefore, cannot completely solve the problem. It is known from Motortechnische Zeitschrift, vol. 27, February 1966, pages 58-61, Stuttgart, DE, to have an arrangement comprising a cam engaging a cam slipper surface wherein, if the velocity of a lubricant film between the engaging parts is zero, there is no supply of lubricant through the point of contact of the cam and the cam slipper surface resulting in that the lubricant can no longer prevent wear by scuffing of the cam and the cam slipper surface. According to the invention there is provided a valve actuating mechanism for an internal combustion engine having at least one intake valve and at least one exhaust valve, including a rotatable cam having a camming surface, and a rocker arm having a cam slipper surface disposed in slidable contact with said camming surface, wherein said intake valve or said exhaust valve is opened and closed by rocking motion of said rocker arm caused by rotation of said rotatable cam, characterised in that said rotatable cam and said rocker arm have dimensions, shapes, and relative positions so designed as to satisfy a condition of V C + V F > O, where V C represents velocity of movement of a contact point on said camming surface of said rotatable cam at which said camming surface slides on said cam slipper surface, and V F represents velocity of movement of the contact point on said cam slipper surface at which said cam slipper surface slides on said camming surface, the condition of V C + V F > O being satisfied by setting r and a such that a r 2.1 is satisfied, where r represents the radius of a base circle of said camming surface, and a represents the radius of curvature of said cam slipper surface. At least in its preferred forms the invention provides a valve actuating mechanism for internal combustion engines which is free from breakage of oil film between the camming surface and the cam slipper surface, and hence has increased wear resistance; and a valve actuating mechanism for internal combustion engines which is reduced in weight. A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:- Fig. 1 is a sectional view of a conventional valve actuating mechanism; Fig. 2 is a sectional view of essential parts of a valve actuating mechanism according to the present invention; Fig. 3 is a diagram showing the dimensional relationships between the essential parts of the valve actuating mechanism shown in Fig. 2; Fig. 4 is a diagram showing the velocity at which lubricating oil passes between the camming surface and the cam slipper surface; and Fig. 5 is a graph showing the relationships between the ratio of the radius of curvature of the cam slipper surface to the radius of the base circle of the camming surface, the velocity at which lubricating oil passes at a contact point between the camming surface and the cam slipper surface, and the weight of the rocker arm; Fig. 6 is a graph showing the relationship between the thickness of oil film between the camming surface and the cam slipper surface, and the contact point between the camming surface and the cam slipper surface; and The invention will be described in detail below with reference to Figs. 2 to 6 of the drawings. Fig. 2 shows essential parts of a valve actuating mechanism for an internal combustion engine according to the invention. In the figure, reference numeral 10 designates a cam which is rotatable in the direction indicated by the arrow. The cam 10 is integrally formed on a cam shaft 11. The cam 10 has its camming surface 12 disposed in slidable contact with a cam slipper surface 14 of a rocker arm 13. The rocker arm 13 has a spherical pivot 15 downwardly pendent from an end thereof and fixed to the end by a nut 20 and a bolt 21. The pivot 15 is pivotally fitted in a bearing 16 to thereby support the rocker arm 13 for rocking motion about the pivot 15 and bearing 16 as a fulcrum. The rocker arm 13 also has a stem slipper 17 integrally formed at another end thereof and extending downward therefrom in slidable contact with an upper end face of a stem 18 of an intake valve or an exhaust valve. With rotation of the cam 10, the rocker arm 13 is caused to make a rocking motion, which in turn causes the stem 18 to reciprocate in the directions indicated by the arrows, whereby the intake or exhaust valve is opened and closed. The basic construction of the valve actuating mechanism described above is similar to that of the prior art. Features of the invention which are novel and different from the prior art will be described below. Fig. 3 diagrammatically shows the essential parts of the valve actuating mechanism with numerals and symbols useful for explaining the principle of the invention. In the figure, r represents the radius of the base circle 12a of the camming surface 12, O₁ the axis of the cam shaft 11, O₂ the center of curvature of the cam slipper surface 14 of the rocker arm 13, O₃ the center of curvature of the stem slipper surface 17 of the rocker arm 13, O₄ the fulcrum point of the pivot 15, P a contact point between the camming surface 12 and the cam slipper surface 14, a the radius of curvature of the cam slipper surface 14 of the rocker arm 13, b the distance between the fulcrum point O₄ of the pivot 15 and the center O₂ of curvature of the cam slipper surface 14 of the rocker arm 13, c the distance between the fulcrum point O₄ of the pivot 15 and the axis O₁ of the cam shaft 11, l₁ a straight line passing through the fulcrum point O₄ of the pivot 15 and the center O₃ of curvature of the stem slipper surface 17 of the rocker arm 13, l₂ a straight line passing through the fulcrum point O₄ of the pivot 15 and the axis O₁ of the cam shaft 11, l₃ a straight line passing through the fulcrum point O₄ of the pivot 15 and the center O₂ of curvature of the cam slipper surface 14 of the rocker arm 13, l₄ a straight line passing through the center O₂ of curvature of the cam slipper surface 14 of the rocker arm 13 and the contact point P between the camming surface 12 and the cam slipper surface 14, l₅ a common straight line tangential to the camming surface 12 and the cam slipper surface 14 at the contact point P, y a straight line passing through the axis O₁ of the cam shaft 11 and intersecting with the straight line l₂ at an angle ø thereto, x a straight line passing through the axis O₁ of the cam shaft 11 and intersecting with the straight line y at a right angle thereto, Γ an angle formed by the straight lines l₁ and l₂, λ an angle formed by the straight lines l₁ and l₃, ν an angle formed by the straight lines l₃ and l₄, ψ an angle formed by the common tangent l₅ and the straight line x, and ϑ an angle formed by the straight lines l₃ and x. According to the invention, the following condition is always satisfied throughout the entire angles of the cam 10, i.e. irrespective of the angles assumed by the cam 10: V C + V F > O where V C represents the velocity of movement of a contact point on the camming surface 12 at which the camming surface 12 slides on the cam slipper surface 14, and V F represents the velocity of movement of the contact point on the cam slipper surface 14 of the rocker arm 13 at which the cam slipper surface 14 slides on the camming surface 12. If the valve actuating mechanism is arranged and constructed such that the above condition is satisfied, the velocity at which lubricating oil passes between the camming surface 12 and the cam slipper surface 14 does not become zero, so that breakage of oil film does not occur. The breakage of oil film occurs when the velocity at which the lubricating oil passes between the camming surface 12 and the cam slipper surface 14 is zero. Fig. 4 shows velocities at which the lubricating oil passes between the camming surface and the cam slipper surface. In the figure, supposing that t represents an apparent clearance between the camming surface 12 and the cam slipper surface 14, the breakage of oil film occurs when the velocity component of the lubricating oil at a point of t 2 equals O, i.e. the speed at which the lubricating oil passes is O. If viewed in terms of the velocity of movement of the contact point P on the camming surface 12 at which the camming surface 12 contacts the cam slipper surface 14, the breakage of oil film occurs when V C = - V F . An embodiment of the invention which satisfies the above expression (1) will be described below. According to the embodiment, the radius r of the base circle 12a and the radius a of the curvature of the cam slipper surface 14 are set at such values as to satisfy the following expression (2): a r ≦ 2.1 Fig. 5 shows the relationships between the ratio a r of the radius a of curvature of the slipper surface 14 to the radius r of the base circle of the camming surface 12, the velocity at which the lubricating oil passes at the contact point between the camming surface 12 and the cam slipper surface 14, and the weight of the rocker arm 13. In the figure, a curve (I) indicates the velocity V C + V F of the lubricating oil, and a curve (II) indicates the weight of the rocker arm 13. The curve (II) has been obtained by varying the radius r of the base circle 12a while the radius a of curvature of the cam slipper surface 14 is kept at a constant value. As is clear from Fig. 5, the velocity V C + V F of the lubricating oil passing through the contact point between the camming surface 12 and the cam slipper surface 14 becomes zero when a r exceeds 2.1. Further, a range A of O < a r ≦ 1.8 indicates an optimum zone in which the weight of the rocker arm 13 can be reduced by an amount of 5% or more as compared with that of the conventional rocker arm, and the velocity of the lubricating oil becomes so high that the formability of lubricating oil film between the camming surface 12 and the cam slipper surface 14 is improved to a large degree. A range B of 1.8 < a r ≦ 2.0 indicates a zone in which the weight of the rocker arm 13 can be reduced by an amount of less than 5%, and at the same time the velocity of the lubricating oil is a little increased so that the formability of lubricating oil film between the camming surface 12 and the cam slipper surface 14 is improved to some degree. A range C of 2.0 < a r ≦ 2.1 indicates a critical zone in which the velocity of the lubricating oil is not equal to O, i.e. no breakage of oil film occurs, but above which the lubricating oil velocity is equal to O to cause breakage of oil film. A range D of a r > 2.1 indicates a zone in which, as described above, the lubricating oil velocity is equal to O to thereby cause breakage of oil film. In the case of a r > 2.1, the oil film is broken at two points on the cam slipper surface 14, as shown in Fig. 6. Fig. 6 shows the relationship between the thickness of oil film between the camming surface 12 and the cam slipper surface 14, and the contact point between the camming surface 12 and the cam slipper surface 14. In the figure, (a) indicates a point at which the high of the camming surface 12 starts to slide on the cam slipper surface 14, and (b) indicates a point at which the high of the camming surface 12 finishes sliding on the cam slipper surface 14. In the figure, a curve A is obtained in the case of a r > 2.1, where the thickness of oil film becomes O, i.e. the oil film is broken at two points (c) and (d). Further, curves B, C, and D are obtained in the cases of a r ≦ 2.1, a r 2.0, and a r ≦ 1.8, respectively. In all these cases, the thickness of the oil film does not become 0, and therefore the oil film is not broken. Therefore, the valve actuating mechanism according to the present invention is free from breakage of the oil film between the camming surface 12 and the cam slipper surface 14, and therefore has greatly improved wear resistance. Further, it is possible to reduce the weight of the rocker arm 13 since the length of the cam slipper surface 14 thereof can be reduced by setting the values of a r to 2.0 or less.

Claims: 1. A valve actuating mechanism for an internal combustion engine having at least one intake valve and at least one exhaust valve, including a rotatable cam (10) having a camming surface (12), and a rocker arm (13) having a cam slipper surface (14) disposed in slidable contact with said camming surface (12), wherein said intake valve or said exhaust valve is opened and closed by rocking motion of said rocker arm (13) caused by rotation of said rotatable cam (10), characterised in that said rotatable cam (10) and said rocker arm (13) have dimensions, shapes, and relative positions so designed as to satisfy a condition of V C + V F > O, where V C represents velocity of movement of a contact point on said camming surface (12) of said rotatable cam (10) at which said camming surface (12) slides on said cam slipper surface (14), and V F represents velocity of movement of the contact point on said cam slipper surface (14) at which said cam slipper surface (14) slides on said camming surface (12), the condition of V C + V F > O being satisfied by setting r and a such that a r 2.1 is satisfied, where r represents the radius of a base circle (12a) of said camming surface (12), and a represents the radius of curvature of said cam slipper surface (14).

Inventors: HAMAMOTO, TOSHIKAZU C/O HONDA R&D CO., LTD., IZAWA, SHINICHIRO C/O HONDA R&D CO., LTD., SANADA, MAKOTO C/O HONDA R&D CO., LTD., YAMASHITA,HIROSHI C/O HONDA R&D CO., LTD., Hamamoto, Toshikazu c/o Honda R&amp;D Co., Ltd., Izawa, Shinichiro c/o Honda R&amp;D Co., Ltd., Sanada, Makoto c/o Honda R&amp;D Co., Ltd., Yamashita,Hiroshi c/o Honda R&amp;D Co., Ltd.
Applicants: HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: F01L1
IPC codes: F01L1