Note: Descriptions are shown in the official language in which they were submitted.
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ENGINE CRANKSHAFT
The present invention relates to the
construction of engine crankshafts and, more
particularly, to the art of making internal oil passages
within such crankshafts.
It is known to incorporate oil ducts in an
engine crankshaft to enable lubrication of the main
bearings and the big end bearings. Such a construction
is advantageous as it allows oil to bs fed from a single
point to all the bearings of the crankshaft instead of
oil being pumped to each main bearing individually.
As is commercially practiced, a duct is formed
by drilling axially aligned bores in the crankshaft and
plugging the ends of the bores. Radial bores are also
machined to further carry the oil to nscessary
locations. The drilling of the bores in this case is
time consuming, not highly cost effective, and accuracy
is required to ensure that the bores are correctly
aligned to form a continuous duct. Furthermore, there
is a tendency for air bubbles to be trapped at
discontinuities in the ducts or at their juncture giving
rise to inadequate lubrication.
In a further prior art proposal, plastic oil
pipes are placed within a hollow crankshaft and a foamed
plastic material is cast in the crankshaft to hold the
oil pipes in place. In this case, the hollow crankshaft
is weakened, especially at the webs of the cranks.
The invention, therefore, is directed towards
a method of constructing a crankshaft with continuous
axial and radial oil ducts feeding oil to the bearing
surfaces of the crankshaft, which method and resulting
crankshaft do not suffer from the foregoinq
disadvantages.
According to one aspect of the present
invention, there is provided a method of manufacturing a
crankshaft having crank pins, which comprises (1)
casting a hollow metal tube within the crankshaft, and
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(ii) piercing bores by laser in at least the bearing
surfaces of the crank pins of the crankshaft to connect
with the interior of the cast in place metal tube.
In the description which follows, reference is
made to the accompanying drawing, wherein:
The figure is a cantral sectional elevational
view of a crankshaft produced by the method of this
invention.
If a metal tube is placed in a casting mold
prior to the pouring in of the molten metal, then an oil
duct can automatically be formed in the crankshaft; it
does not suffice to drill down into the embedded tube as
there is a serious risk of blocking the oil duct by the
burrs occurring around the ends of the drilled bore.
Deburring cannot readily be carried out and, even if the
burrs are successfully removed from the edges of the
drilled bore, there is a risk of the oil duct being
blocked by the resulting swarf.
Laser piercing permits a bore to be made
without burrs and without generating swarf.
Furthermore, the accuracy of drilling can be greatly
increased, thereby improving the control over the flow
rate of the oil. Laser piercing also avoids the risk of
breakage of drill
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bits of very small diameter.
It is preferable that the e~bedded metal tube be
steel, and should not be too close to the surface of the
crankshaft, in order to avoid weakening of the
crankshaft. The bores to be made to connect with the oil
duct may therefore need to be deep. In this case, it is
not essential that the shole depth o~ the bores should be
laser pierced as it is only at the point of penetration
into the steel tube that the risk of burring presents a
problem. It is poss;ble therefore to drill the first
part of the bores in a conventional manner and to laser
pierce only the last part of the bores. It is not
necessary that the drilled part of the bore should be of
the same diameter as the laser pierced part so that the
need to use small diameter drill bits can still be
avoided.
When molten metal flows over the steel tube in
the casting mold, there is a risk of the steel tube
melting. The tube could be formed of a more refractory
metal different than the cast iron or cast steel o~ the
crankshaft, but this creates a possible problem because
of the different properties of the two materials. Also,
more refractory metals would make the tube and its
bending unduly costly. Low carbon steel tubes cast well
in nodular cast iron, creating a fused metallurgical bond
therebetween.
Certain parts of the steel tube are exposed to
the molten metal for longer than others and it is these
parts which risk melting. In particular, a crankshaft is
normally cast vertically and filled from the lower end in
order to avoid air being trapped. Thus, most of the hot
metal will flow across the lower end of the steel tube to
be cast into the upper regions o~ the crankshaft and thus
the upper end of the steel tube will be exposed to the
molten metal for a shorter time.
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It is possible to avoid the risk of melting of
the steel tube by increasing the wall thickness of the
tube either throughout its length or only in the regions
at risk of melting. This may be achie~ed, for example,
by a cladding on the steel tube.
A particularly simple solution is to fill the
tube, at least partially, prior to casting of the molten
metal into the mold, with a material, such as sand or
wax, that increases the thermal capacity. If the
material has a lower melting point than the tube, it can
flow out during the casting process.
An alternative possible approach is to allow the
tu~e to melt partially, but to prevent its collapse by
filling it with a powder such as fine sand. The sand
will also increase the thermal capacity in order to
resist melting and can be blown out after completion of
the casting.
The invention will now be described further, by
way of example, with reference to the accompanying
drawing which is a section through a crankshaft.
The crankshaft 10 has four main bearings 12 and
four cranks 14. Between the two middle cranks, a
balancing weight 16 is formed. The whole of the
crankshaft is of cast iron construction and a steel tube
18 is cast in situ in the cran~shaft 10 to define an oil
duct 20 for lubricating the meain bearings 12 and the big
end bearings on the cranks 14.
The steel tube 18 is o~ mild steel which is
preformed to a desired shape prior to being inserted in
the casting mold. As can be seen, the tube 18 is
encapsulated entirely within the crankshaft 10 except for
a portion 22 which is opposite the balance weight 16.
The tube 18 is preformed to have sufficient length so
that portion 22 will lay outside the mold during casting
of the cast material into the casting cavity. In the
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case of a five main bearing crankshaft, the tube 18
would be totally contained within the crankshaft 10.
It is important to be able to position the
tube 18 accurately during the casting process, and the
portion 22 is very advantageous in this respect as it
can be gripped directly to determine the angular
position of the tube 18 within the mold. In the case of
a crankshaft in which the tube 18 is totally
encapsulated, it is possible to ~orm the crankshaft with
hollow crank pins. The bores oE the crank pins are
formed by cores in the mold and these cores may be
provided with locating recesses for the tube 18.
During casting, the mold is vertical and Eills
from the lower end. A cladding, consisting o~ a second
layer of mild steel, may be formed around the tube 18 at
the lower end to prevent its melting during the
casting. Alternatively, the tube may be filled prior to
the molten metal being introduced into the mold with a
material to increase its thermal capacity, such as sand,
oil or wax.
After cooling of the crankshaft, it is
necessary to form bores to enable oil in the oil duct 20
to reach the bearing surfaces. These are formed by
conventional drilling of blind bores 30 in the bearing
portions of the crankshaft to approach the steel tube 18
but not penetrate it; such drilling may employ tool
steel rotary drill bits to define the bores. These
blind bores may typically be 2.5-3.5mm in diameter.
Next, these blind bores 30 are deepened by laser
piercing to connect with the oil duct 20, the diameter
of the laser pierced part 32 of the bores 30 being
typically between 0.5mm and l.Omm in diameter. Laser
piercing may be carried out in accordance with the
teaching oE U.K. patent 1,088,510.
Laser piercing avoids burrs which could
cause blockage of the oil duct 20. ~urthermore, it
enables a
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small diameter bore to be drilled without the use of a
fragil~ drill bit of slender diameter, such drill bits
being frequently repsonsible for stoppages in the course
of mass production of drilled components.
It is not essential to form the bores in two
parts and the predrilling may be unnecessary if safety
regulations allow the use of a laser having sufficient
power to drill a hole of the desired diameter and depth
within an acceptable time. For smaller engines, a bore
of typically 15mm depth is required, and lasers currently
deemed to be safe can pierce such a hole in about two
seconds.
It is the big end bearings which present the
greater problem in lubrication and the oil duct should
feed oil at least to these bearings. Main bearing
lubrication could be carried out separately, but it is
preferred that the oil duct should feed the main bearings
as well as the big end bearings.
While particular embodiments of the invention
have been illustrated and described, it will be obvious
to those skilled in the art that various changes and
modifications may be made without departing from the
invention, and it is intended to cover in the appended
claims all such modifications and equivalents as fall
within the true spirit and scope of the invention.
