GB2438211A

GB2438211A - Cast camshaft with non-circular cross-section shaft portions

Info

Publication number: GB2438211A
Application number: GB0609964A
Authority: GB
Inventors: Stephen Smith; Richard Dunstan; Mark Rawlings
Original assignee: Federal Mogul Camshaft Castings Ltd
Current assignee: Lydmet Ltd
Priority date: 2006-05-19
Filing date: 2006-05-19
Publication date: 2007-11-21
Also published as: GB0609964D0

Abstract

The camshaft (10) is cast as one piece and has a rotational axis (22) along which operational features of the camshaft are arranged. These features include at least one cam or group of cams (12, 14,16 and 18) connected by at least one shaft portion (50). The shaft portion (50) extends along the rotational axis (22) and is formed so that for at least part of its length it has a cross-sectional shape, transversely of the rotational axis (22) which consists of a plurality of radially extending arms separated by recesses.

Description

1 2438211

CAMSHAFTS

This invention is concerned with a camshaft. In particular, this invention is concerned with camshafts of the type which are suitable, for example, for operating valves of internal combustion engines.

The valves of many types of internal combustion engine are operated in the required sequence using a camshaft which has cams which can operate the valves as the camshaft is rotated about a rotational axis thereof. For example, such camshafts are used to operate the poppet valves of an engine operating on the Otto cycle to allow fuel to enter the cylinders of the engine and exhaust gasses to leave the cylinders. Such a camshaft has operational features arranged along its rotational axis, the operational features including the cams which may be arranged in groups, each group being associated with one cylinder of the engine. Other possible operational features include end fixtures etc. These operational features are connected by shaft portions which pass the rotational forces along the shaft.

Camshafts of the type referred to above can be manufactured in various ways. For example, the cams, groups of cams, and other operational features may be made as individual units and assembled on a shaft which is inserted into pre-formed holes through the features. In some cases, the shaft is cylindrical and the features are attached to it by welding or another fixing method. For example, the shaft may be made tubular and expanded in diameter to grip the features. In other cases, the shaft is formed with a non-circular cross-sectional shape which enters complementary holes through the features to provide a driving connection between the shaft and the feature.

Thus, in this type of assembled camshaft it is known to have a shaft which has a non-circular transverse cross-section, examples include hexagonal and splined shafts.

Camshafts are also manufactured as one piece castings usually made of iron, the casting including all the features and shaft portions but usually requiring some finishing operations such as grinding to provide the finished camshaft. It is with camshafts which are cast as one piece with which the present invention is concerned. Conventionally the camshafts of this type have their shaft portions of cylindrical form.

The camshafts which are cast as one piece are relatively heavy and particularly in smaller engines the weight of the camshaft is significant. In particular, a heavier camshaft requires more energy to rotate it. It is known to reduce the weight of a camshaft by casting it around a core which is subsequently removed leaving a hollow space. However, this is a relatively complex process and may weaken the camshaft.

It is an object of the present invention to provide a camshaft which is cast as one piece but which enables a weight reduction to be obtained.

Surprisingly, it has been found that a reduction in the weight of the shaft portions of such a camshaft is possible without compromising the strength of the camshaft.

The invention provides a camshaft which is cast as one piece, the camshaft having a rotational axis along which operational features of the camshaft are arranged, the operational features including at least one cam, the operational features being connected by at least one shaft portion of the camshaft which extends along the rotational axis, wherein said shaft portion is formed so that for at least part of its length it has a cross-sectional shape, transversely of the rotational axis of the camshaft, which comprises a plurality of radially extending arms separated by recesses.

In a camshaft according to the invention, the amount of material used in the shaft portion can be reduced significantly (in some cases by 50 or more percent) in comparison with an equivalent cylindrical shaft portion with a consequent proportional reduction in weight of the shaft portion. Since a conventional camshaft for a four cylinder engine has at least five such shaft portions, the overall saving in weight is significant when the invention is applied to each shaft portion of the camshaft.

In order to simplify the casting process and balance the forces involved, it is preferred that said arms are equally distributed relative to the rotational axis of the camshaft, ie they extend radially in equally distributed directions from the rotational axis. It is also preferred that said arms are of equal length measured from said rotational axis.

A particularly preferred embodiment of the invention has four arms which therefore give the cross-sectional shape of the shaft portion the shape of a cross with equal arms. However, other shapes are possible. Normally the camshaft has at least five shaft portions which preferably have the same orientation relative to the rotational axis, ie each shaft portion has its arms extending in the same directions as the other shaft portions.

It is found to be advantageous if the regions, where the arms meet operational features of the camshaft, are rounded off to avoid corners.

Furthermore, the regions where the arms meet one another are also rounded off and the radially outmost ends of the arms are also rounded off, eg to an arc centred on the rotational axis.

In a camshaft according to the invention, preferably, the material forming said shaft portion occupies between 40 and 60 percent of the volume of a cylinder defined by rotating the shaft portion about said rotational axis. In other words, the envelope defined by the ends of the arms of the shaft portion's cross-section is a cylinder and the cross-section occupies 40 to 60 percent of this volume. This enables the weight of 40 to 60 percent of the material in the shaft portion to be saved.

A camshaft according to the invention is preferably made of cast iron by the well known chill casting method in which the operating surfaces of the cams are solidified against metal chills while the remainder of the shaft is solidified against a sand mould.

There now follows a detailed description, to be read with reference to the accompanying drawings of a camshaft which is illustrative of the invention.

In the drawings: Figure 1 is a side elevational view of the illustrative camshaft; Figure 2 is a perspective view on a larger scale than Figure 1 of a portion of the illustrative camshaft; Figures 3, 4 and 5 are cross-sectional views, on a larger scale, taken, respectively on the lines X-X, Y-Y and Z-Z in Figure 1; and Figure 6 is a view similar to Figure 5 but also showing in broken line the outline of a conventional camshaft at the line Z-Z.

The illustrative camshaft 10 is cast as one-piece from cast iron by a conventional chill-casting process. The camshaft 10 is intended for use in operating poppet-type valves in an internal combustion engine having four cylinders. The camshaft 10 has a rotational axis 22 along which operational features of the camshaft are arranged, the operational features being connected by shaft portions 50 which extend along the axis 22.

The operational features of the camshaft 10 include four groups of cams 12, 14, 16 and 18. Each group has two cams, one for operating an inlet valve and the other for operating an outlet valve of a cylinder of the engine.

Each cam has a lobe 20 which projects furthest from the rotational axis 22 of the camshaft 10. Each lobe 20 is formed with a hardened surface for operating the associated valve, On the opposite side of the cam to the lobe 20, each cam is reduced in width as is conventional.

The cams of the groups 12, 14, 16 and 18 are cast as integral parts of the camshaft 10 so that the lobes 20 of the two cams of each group project in the same direction, relative to the axis 22. However, the cams of each group project in a different direction. Specifically, as illustrated in Figure 1, the cams of the group 12 which is closest to the left-hand end of the camshaft 10 (as shown) have their lobes 20 projecting upwardly, the cams of the next group 14 project towards the viewer of Figure 1 at 90 degrees to the cams of the group 12, the lobes of the cams of the next group 16 project away from the viewer of Figure 1, and the lobes of the cams of the group of cams 18 closest to the right-hand end of the camshaft 10 project downwardly. Thus, the groups of cams 12, 14, 16 and 18 have their lobes 20 projecting each in one of four directions which are arranged at 90 degree intervals.

The groups of cams 12, 14, 16 and 18 of the camshaft 10 also comprise four cylindrical bearing journals 32, 34, 36 and 38 which are cast integrally with the remainder of the camshaft. These journal bearings are located so that one is between the two cams of each group of cams, ie the journal bearing 32 is located between the cams of the group 12, the journal bearing 34 is located between the cams of the group 14 and so on. These journal bearings are for use in supporting the camshaft 10 for rotation about the axis 22.

The camshaft 10 also comprises integrally cast end features 40 and 42 of conventional form. These features include journal bearings which co-operate with the journal bearings 32, 34, 36 and 38.

The aforementioned shaft portions 50 of the camshaft 10 join the end feature 40 to the group of cams 12, the group of cams 12 to the group of cams 14, the group of cams 14 to the group of cams 16, the group of cams 16 to the group of cams 18, and the group of cams 18 to the feature 42.

These five shaft portions 50 are all of the same form except that in the region illustrated in Figure 4 there is a feature 52 formed on the shaft portion 50 adjacent one of the cams of the group 12. This feature 52 is a location feature for use in locating the camshaft 10 for grinding to its final form.

Each of the shaft portions 50 is formed so that, for at least part of its length, it has a cross-sectional shape, transversely of the axis 22, which comprises four radially extending arms which are separated by recesses in the camshaft 10. These arms are equally distributed relative to the axis 22 and are of equal length measured from the axis 22. As can be seen from Figure 2 in perspective and from Figure 5 in cross-section, the cross-sectional shape of the shaft portion 50 is that of a cross of four identical arms which project each in one of four directions which are arranged at 90 degree intervals. The end of each of the arms is rounded off by a circular arc 51 centred on the axis 22. Also, the regions where the arms meet each other are rounded off.

As shown at 54, the junctions at the ends of each shaft portion 50 with one of the groups of cams or other features are rounded off outwardly arcuately to avoid corners.

Figure 6 shows the cross-sectional view of the shaft portion 50 shown in Figure 5 but superimposed on the outline of a cylindrical shaft portion 60 of a conventional camshaft which is similar to the camshaft 10 except in the form of the shaft portions. The cylindrical outer surface of the shaft portion 60 of the conventional camshaft is co-incident with the surface of a cylinder defined by the ends of the arms of the shaft portion 50 as the camshaft 10 is rotated about the axis 22. It can be seen from Figure 6 that the cast iron forming the shaft portion 50 occupies approximately 50 percent of the volume of the cylinder aforementioned.

Claims

CLAIMS

1 A camshaft which is cast as one piece, the camshaft having a rotational axis along which operational features of the camshaft are arranged, the operational features including at least one cam or group of cams, the operational features being connected by at least one shaft portion of the camshaft which extends along the rotational axis, wherein said shaft portion is formed so that for at least part of its length it has a cross-sectional shape, transversely of the rotational axis of the camshaft, which consists of a plurality of radially extending arms separated by recesses.

2 A camshaft according to claim 1 wherein said arms are equally distributed relative to the rotational axis.

3 A camshaft according to claim 1 or claim 2, wherein said arms are of equal length measured from said rotational axis.

4 A camshaft according to any one of claims 1 to 3, wherein there are four arms so that said cross-sectional shape is that of a cross.

A camshaft according to any one of claims 1 to 4, wherein the material forming said shaft portion occupies between 40 and 60 percent of the volume of a cylinder defined by rotating the shaft portion about said rotational axis.

6 A camshaft according to any one of claims 1 to 5, wherein the camshaft is formed of cast iron.

7 A camshaft as substantially hereinbefore described with reference to and as shown in the accompanying drawings.