Patent Application
20050217623
This invention relates to cam followers of the type used for example in pushrod combustion engines, and to improved valve trains and engines containing the cam followers.
Cam followers, also known as lifters or tappets, are well known in combustion engines. They are used, for example, in overhead valve engines such as V-8 engines to convert the rotary motion of the camshaft into the reciprocating motion of the pushrod to activate valves.
In a reciprocating piston engine, the rotation of the camshaft is driven by the crankshaft. The camshaft and other parts of the valve train control valve timings, valve durations and valve lift. Lighter cam followers are generally desirable because a lighter reciprocating mass acting on the camshaft permits greater engine power and higher engine speeds. These benefits arise at least in part because less force is required to push the cam follower and therefore more aggressive cam shaft profiles and lighter valve biasing means can be employed.
With reference to
The present invention seeks to provide an improved cam follower, valve train and engine.
According to an aspect of the present invention, there is provided a cam follower having a cylindrical form, with a cam engaging surface to a base of the substantially cylindrical form and a pushrod engaging surface to the other end, the cam follower being hollow and characterised in that the substantially cylindrical form tapers towards the pushrod engaging surface such that the portion of the cam follower bearing the pushrod engaging surface is substantially conicular.
Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and accompanying drawings or may be learned by practice of the invention.
For a better understanding of the present invention, and how the same may be carried into effect, reference will now be made by way of example only to the accompanying drawings in which:
With reference to
Each cam 38 engages a preferred cam follower 40 which, in turn, connects to a pushrod 42 extending between the cam follower 40 and a rocker arm 44. The rocker arm 44 is pivotably mounted such that it actuates a valve 46 against the bias of a spring 48 when the pushrod 42 moves toward the rocker arm 44 as the rotary action of the cam 38 is transferred through the cam follower 40. A skilled person will appreciate that in a V-8 engine four opposed sets of valve train components are located on either side of the cam shaft. Further, a skilled person will appreciate that many other engine designs use cam followers and pushrods in the same or a similar manner.
The overall geometric form of the exemplary cam follower 40 is thus substantially cylindrical with a taper towards the pushrod engaging end 55. This geometric form has particular advantages in that, in use, it efficiently transfers compression forces imported to the cam follower 40 from the pushrod 42 into the walls 62 and 64 of the cam follower 40. This efficient transfer and dissipation of compression forces enables the or each component to be made with thinner walls 62 and 64, thereby saving weight without the need to extend the length of the pushrod and without the need to reinforce any other parts.
In the preferred embodiment, the angle α subtended between a tangential plane through the cylindrical side wall 62 and a corresponding plane through the tapered side wall 64 is 15 degrees plus or minus 2 degrees. The angle α may be anywhere in the range 5 to 40 degrees but is preferably in the range 10 to 20 degrees.
The length Y of the conicular pushrod engaging portion 52 along the longitudinal axis P ought to be greater than 10% of the length X of the cylindrical cam engaging portion 50 in order to efficiently transfer the compression forces incurred in use into the walls 62 of the cylindrical cam engaging portion 50. Preferably, the length Y is greater than 30% of the length X and more preferably still it is greater than 50% of the length X. The length Y of the pushrod engaging portion 52 along the longitudinal axis P is typically between 60 and 75% of the length X of the cylindrical cam engaging portion 50.
The external base diameter Z of the cylindrical cam engaging portion 50 is typically about 25 to 35% of the length X. In one embodiment, the length Y is 68% of the length X and the diameter is 31% of the length X.
One exemplary cam follower which has been tested has dimensions X=35 mm, Y=24 mm and Z=11 mm.
A skilled person will appreciate that a cam follower embodying the invention may be made in any suitable material, such as a metal, alloy or composite, including future developed materials. Further, a skilled person will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing the invention, the invention should not be limited to the specific configurations or method steps disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that the invention has a broad range of applications and that the embodiments may take a wide range of modifications without departing from the inventive concept.
For example, the invention has applications in all manner of combustion engines, for example engines in road vehicles, specialized vehicles, ships and trains, but will have particular application where weight savings are at a premium.
The cam engaging surface 51 may take any suitable form such as a flat, sphere or roller. Further, the pushrod engaging surface or socket 53 of the cam follower 40 may be formed as a cup, ball or any other suitable pushrod engaging means.
