Desmodromic valve

Abstract: A valve train comprises a cylinder head and poppet valve thereon, a valve-actuating rocker platen pivotally mounted on the cylinder head by means of a pivot member so that the platen may pivot between positions on either side of the member, a valve connected to one end of the rocker platen for movement of the valve simultaneously with movement of the rocker platen between the rocker platen positions, and actuator means for moving the rocker platen between the positions thereby positively moving the valve between its open position and its closed position, the actuator means comprising a rotary cam means and cooperating follower pivotally mounted and spring biased into engagement with the rocker platen and engaged with the cam means for pivoting the rocker arm upon rotation of the cam means, the follower comprising a pivotally mounted casing, spring biased plunger means on the casing, and biased to a roller bearing on and adapted to ride over the rocker platen, thereby moving the platen between the positions, the roller rotatably attached to the pivoted plunger means at the roller center rotation. An improvement to this valve train is provided wherein the platen is pivotally mounted at a point above the surface on which the roller rides and further, the radius of the roller equals the vertical distance from the platen to the point of pivotal mounting so that the center of rotation of the roller passes through the axis of the pivot point of the platen. The valve-actuating rocker platen may comprise two intersecting arc-shaped sections, the platen so constructed that each of the sections has the pivot point of the pivot casing as its center of curvature when the spring-biased plunger means is engaged with the respective section. This invention relates to a valve gear, and particularly to an improved valve gear for use in an internal combustion engine. Generally, internal combustion engines are provided with a cam shaft which is operated from the engine's crankshaft which in turn drives a series of intake and exhaust valves. In the past the cam shaft has been located longitudinally and below the engine; the shaft will have connecting means between a series of cams and the intake and exhaust valves. In order to provide a more efficient valve operating mechanism, engine designers have changed the mounting of the cam shaft of some engines onto the top of the cylinder head. This position reduces the mass of the engine and the valve-actuating mechanisms by eliminating the pushrods and their associated parts. Another design advance has been to utilize desmodromic valve mechanisms whereby a double cam provides both positive and closing of the valve. Previous problems associated with this type of valve arrangement have included high costs which are the results of the extremely high tolerances required to insure complete closure of the valves. Further, proposed lay-outs of desmodromic valve-actuating mechanisms have seriously limited the serviceability of the engine and have rendered such designs unsuited for wide commercial usage. Ashley, U.S. Pat. No. 3,626,469, relates to an improved positivelyoperated valve gear that is designed to provide a valve train that does not require frequent adjustment and may be built with relatively loose tolerances. The Ashley invention provides a valve train for an overhead camshaft engine that utilizes rocker arms for transmitting motion between the cam and the valve for both opening and closing the valves. As pointed out in that patent, such an arrangement permits a greater latitude in valve placement. In many conventional types of valve gear arrangement, a keeper is carried by the upper end of the stem of the poppet valves. In a conventional engine these keepers transmit an opening or closing force to the valve. The valve closing spring operates through these keepers so that the valve is held in its closed position. The Ashley patent points out that the use of rocker arms for transmitting motion to the valves eliminates the problems of assembly and disassembly of such prior conventional systems. Finally, Ashley points out that with the use of rocker arms it may be possible that the required pieces may be stamped rather than cast or forged, thus reducing the cost of the components and in many instances permitting a weight savings. The Ashley invention is described as a positive valve gear for an internal combustion engine, comprising a valve, a rocker arm, a pivot member, a pair of spaced oppositelyextending bearing portions, means for positively connecting the rocker arm to the valve, and operating means for moving the rocker arms between positions. The pivot member supports the rocker arm and has a shank portion adapted to be affixed to an associated component of the engine. The bearing portions extend substantially perpendicular to the shank portion. The rocker arm defining bearing surfaces are in engagement with the pivot member bearing portions so that the rocker arm is supported upon the pivot member for pivotal movement between a valve opening position and a valve closing position. The bearing surfaces of the rocker arm engage the pair of spaced oppositely-extending bearing portions. The operating means comprises a rotary cam and a cooperating follower carried by the rocker arm and engaged with the cam for pivoting the rocker arm upon rotation of the cam. At least one of the follower means is spring-biased into an engagement with the cam and is fixed to the rocker arm and engages the pivot member bearing portions opposite to the side engaged by the rocker arm bearing surfaces. One object of the present invention is to provide an improved valve mechanism of this general type. It has been found that an improved valve mechanism is provided having structural features as will be described in detail below.