Cast iron camshaft for internal combustion engines

A cast iron camshaft (1) for an internal combustion engine, has at one end a toothed pulley (3) engaging a toothed belt, and an internal tensile member including a screw threaded bolt (5). The bolt engages part of the camshaft (1) adjacent the end where the toothed pulley (3) is fixed, and extends along a longitudinal bore (6), and has a screw threaded end (8) which engages an internal thread (9) positioned adjacent the point where the bending moment exerted on the camshaft by the toothed belt tension P becomes zero. As a result, the alternating bending loads which would otherwise occur in the vicinity of the cross-section X--X which is in danger, are converted into a continuous compressive load, which can be sustained by the cast iron material more effectively than an alternating bending load.

In modern internal combustion engines a very high specific surface pressure 
occurs between the cams of the camshaft and the tappets, rocker arms or 
rocker levers which co-operate therewith. On account of its good wear 
properties cast iron (grey cast iron, spheroidal-graphite cast iron or 
malleable cast iron) is particularly suitable as a material for camshafts, 
but a serious disadvantage of cast iron is its low alternating bending 
strength, which can cause fractures since the camshaft is subject to 
considerable fluctuations and reversal of bending moments during 
operation. Such camshaft failures mainly occur in the zone adjacent to or 
slightly beyond the point of support which is nearest to the end of the 
camshaft at which the drive pulley or sprocket is fixed, since a high 
bending moment is exerted on the camshaft by the driving chain or belt, 
which moment results in high alternating bending loads on account of the 
rotation of the camshaft. 
Casting materials with a higher alternating bending strength are in fact 
known, but these generally have a lower permissible specific surface 
pressure, and therefore inferior wear properties. 
The problem underlying the invention is t provide a cast iron camshaft, 
which not only permits high specific surface pressures and has favourable 
wear characteristics but with the danger of failures due to bending 
vibrations is reduced to a minimum. 
Broadly stated the invention consists in a cast iron camshaft for an 
internal combustion engine, having at one end a drive pulley or pinion for 
an endless tension member and including means for creating in the camshaft 
a longitudinal compressive stress in at least part of the zone extending 
from the said end to a point at which the bending moment exerted by the 
tension member on the camshaft becomes zero. 
The invention is based on the appreciation that cast materials in question 
can sustain a continuous compressive load which may be up to six times as 
great as the permitted alternating bending load. As a result of the 
invention compressive stresses are created in the cast iron which 
superimpose the alternating bending loads which are converted into 
continuous non-reversing -loading when the shaft rotates. As a result the 
camshaft can be more highly loaded continuously, so that failures are 
effectively avoided with a camshaft formed of a material with a relatively 
small alternating handing strength, but with good wear resistant 
properties. 
The said compressive stress may be created for example by a screw threaded 
member which bears longitudinally on part of the camshaft adjacent the 
said end, and extends along an internal bore in the camshaft, and has a 
threaded part engaging an internal screw thread in the bore, positioned 
close to the point at which the bending moment becomes zero. 
According to a preferred feature of the invention the screw threaded member 
also serves for axial location of the drive pulley or pinion. 
Alternatively, the camshaft may have a longitudinal bore through which 
extends a tie member which is axially fixed at positions adjacent both 
ends of the camshaft. 
Thus the compressive stress can be created in the cast material by 
tightening a screw or the like at one or both ends of the tie member. 
In another form of the invention compressive stresses can be created in the 
cast material when the camshaft is cast, by providing a tie member which 
is actually included and cast into the casting material and which may 
consist of a tube or a bar formed of a material whose linear temperature 
coefficient of expansion is greater than that of the main camshaft casting 
material, means being provided for absorbing the longitudinal stresses 
between the tie rod and the cast material which occur when the cast 
material cools. When the cast assembly cools, tensile stresses are built 
up in the tie rod and compressive stresses in the cast material, on 
account of the different linear expansion coefficients. The shearing 
stresses which may occur in the boundary layer can be absorbed by suitable 
superficial roughness or by profiling of the surface of the tie rod or 
even by a collar at each of the two ends of the tie rod. To avoid any 
undesirable chilling effect en the cast material of the tie rod, the 
latter may be heated up beforehand, or during the casting operation may be 
held at a desired temperature by passing an electric current through it. 
Undesirable gas bubbles in the casting can also be avoided by this means. 
In the case of a grey cast iron material with a linear expansion 
coefficient .alpha.=9-10, one of the following materials may for example, 
be used for the tie rod: 
______________________________________ 
10 NiCrAlTi 32,20 
.alpha. = 15 
2 CrNiSi 18,25 
.alpha. = 16.5 
15 CrNiSi 20,12 
.alpha. = 16.5 
normal steel 
.alpha. = 12. 
______________________________________

The camshaft 1 illustrated in FIG. 1 is supported at the points A,B and C 
in an engine cylinder head (not illustrated) and provided with cams 2 for 
actuating inlet and/or exhaust valves (also not illustrated). At the left 
hand end of the camshaft 1 a driving wheel 3, (in this particular 
embodiment a toothed pulley), is fixed non-rotatably by means of a key 4. 
The tension of the belt engaging the toothed pulley which is illustrated by 
the arrow P, exerts on the camshaft 1 a bending moment whose value varies 
as illustrated by the graph of FIG. 2. This bending moment, when the 
camshaft 1 rotates, creates alternating bending load which can lead to 
failure in the cross-section X--X if a cast iron with good wear properties 
but a low alternating bending strength is used for the camshaft. So as to 
make use of the good wear properties of this material and at the same time 
reduce the danger of failure to a reasonable dimension, a compressive 
stress is created and superimposed on the alternating bending stress in 
the endangered cross section X--X illustrated in FIG. 3, so as to convert 
it into a purely compressive stress as shown in FIG. 4: this is applied 
more particularly in the zone of the camshaft which extends from the point 
of application of the driving force P to the point Z at which the bending 
moment becomes zero (see FIG. 2). In the example illustrated in FIG. 1, 
this compressive stress is created by a bolt 5, which extends into a 
longitudinal bore 6 in the crankshaft and is located at its left hand end 
by the bolt head 7 engaging the toothed pulley 3, while at its right hand 
end a threaded section B engages in an internal thread 9 in the bore 6, 
this thread 9 being positioned at the zero point Z of the bending moment 
When the bolt 5 is tightened, the aforementioned compressive stress is 
therefore built up in the zone of the camshaft 1 which runs from the left 
hand end of the camshaft to the zero point Z of the bending moment, so 
that the stress pattern illustrated in FIG. 4 is provided in the 
endangered cross section X--X. 
The example illustrated in FIG. 5 differs from that of FIG. 1 mainly in 
that the camshaft 1 is formed with a longitudinal bore 10 running its 
whole length through which extends a tie rod 11 having a bolt head 12 
engaging the toothed pulley 3' at the left hand end of the camshaft 1', 
and a threaded section 13 at the other end, onto which is screwed a nut 14 
which bears on the right hand end of the camshaft 1'. By tightening the 
nut 14, there is built up in the camshaft 1' a superimposed compressive 
stress by means of which the alternating bending load which would 
otherwise occur as shown in FIG. 3, is converted into a continuous 
compressive load at each cross-section of the camshaft. The tie rod 11 may 
be formed for example of a heavy bar or even a tube. 
In the further example illustrated in FIG. 6, the required longitudinal 
compressive stress is created in the camshaft 1" when it is actually cast. 
To this end, there is cast in the camshaft 1" a tubular tie rod 15, which 
extends the entire length of the camshaft and is formed of a material 
having a linear thermal coefficient of expansion which is greater than 
that of the cast material. When the cast assembly cools, tensile stresses 
are built up in the tie rod 15 and compressive stresses in the cast 
material, on account of the different linear expansion coefficients. In 
order to absorb the shearing stresses which occur in the boundary layer 
between the tie rod 15 and the cast material when the cast material cools, 
the tie rod 15 is provided at each of its ends with a collar 16 or 17, 
against which the cast material is axially supported. The collar 16 has an 
internal thread 18 for accepting a bolt 19 by which the toothed pulley 3" 
is fastened in position. In addition to the collars 16 and 17, or instead 
of the latter, the surface of the tie rod 15 may be roughened or profiled 
to absorb the shearing stresses, as indicated diagrammatically by the 
thick lines 20. A heavy bar may also be cast into the camshaft instead of 
a tubular tie rod. 
The function of the cast tie rod as shown in FIG. 6 is basically the same 
as that of the continuous tie rod shown in FIG. 5. 
The advantages of the invention are naturally also evident if the camshaft 
drive is produced not by a toothed belt, but by a chain, as in both cases 
the alternating bending load on the camshaft is converted into a 
continuous compressive load.