GB2172691A

GB2172691A - Cast crankshafts

Info

Publication number: GB2172691A
Application number: GB08507103A
Authority: GB
Inventors: John Howard Stringer
Original assignee: Rover Co Ltd
Current assignee: Rover Co Ltd
Priority date: 1985-03-19
Filing date: 1985-03-19
Publication date: 1986-09-24
Also published as: GB8507103D0; IT1188457B; GB2172691B; IT8619789A1; IT8619789A0

Abstract

A crankshaft is cast in a mould which contains an insert 3 which forms part of the cast crankshaft. The insert communicates for example by means of passages 3d, 3e with all the bearing surfaces : thus in use of the crankshaft oil can be forced into openings 3e and oil will be fed from openings 3d. The provision of the insert obviates the need for a subsequent operation to drill oil ways. The insert 5 may have cylindrical cavities 3a, 3b to lighten the crankshaft. The passages, which preferably form location points for the insert during the casting operation, are trimmed off and 3d, 3e are closed during casting and are trimmed off and opened during subsequent finishing operations. In use, when the insert is filled with oil and the engine is running, the oil in the cavities 3b will act as an oil pump and reduce the energy required to drive the main oil pump. <IMAGE>

Description

SPECIFICATION Cast crankshafts This invention relates to a method of and apparatus for casting crankshafts and to crankshafts so made.

Various finishing operations are typically carried out on the castings. For example, internal passages may be formed such as drilled oilways leading from the main bearings to the crankpins and such as drilled apertures through the axes of the crankpins for lightness. It is known to form some of these passages during the casting operation. For example, crankpins have been hollowed out by the use of sand cores: however, this has the disadvantage that great care is needed in knocking out the sand cores since any particles remaining could be dislodged in use of the engine and could cause bearing failure.It is also known in the case of light duty (two cylinder) engines having a short crankshaft (two splash-lubricated crankpins) for the crankshaft to be cast with a hollow running its entire length: this has been done using a sand core or a two-part evaporatable foam pattern split about the centre axis of the hollow.

The invention provides a method of casting a crankshaft wherein the mould contains an insert which after casting forms part of the cast crankshaft, the insert being hollow and extending along the crankshaft in such a way that in the finished component it communicates with the bearing surfaces of the main bearings and crankpins for lubrication thereof.

The method of the invention avoids the need for a subsequent drilling operation to form the lubrication passages by casting these in as a single insert.

The invention also provides apparatus for casting a crankshaft comprising a mould containing an insert so located that after casting the insert forms part of the cast crankshaft, the insert being hollow and extending along the crankshaft in such a way that in the finished component it communicates with the bearing surfaces of the main bearings and crankpins for lubrication thereof.

Advantageously, the insert has a cavity in the region of each main bearing and each crankpin: this reduces the weight of the crankshaft.

The parts of the insert that communicate with the bearing surfaces are advantageously closed before the casting operation, in order to avoid possible contamination which could enter the bearings in use of the engine. The parts may be closed by flattening them, or by means of a plug preferably removable outwardly, such as a threaded insert, or by means of a cap. Advantageously, at least some of the communicating parts are opened after the casting operation while the interior of the insert is pressurised: this may be achieved by forming a recess in one or both ends of the crankshaft and then piercing the insert through the recess; alternatively a plug closing a communicating part may be opened by withdrawing it outwardly.In either case the risk of machining debris entering subsequently opened communicating parts is minimised since the pressurised fluid will tend to flush such debris out, and in both cases the manner of forming the initial opening into the recess is such as to minimise the risk of such debris entering the insert.

The communicating parts preferably extend beyond the mould interior and form location points for the core during the casting operation.

The insert may be formed from metal such as steel pressings, but could be made from ceramics.

The method is suitable for many casting methods, such as green sand, dry sand, gashardened sand, shell, investment or lost pattern, permanent mould (gravity die casting low pressure die casting and pressure die casting).

The method is particularly suited to that kind of investment casting which is known as lost foam casting where an expendable evaporatable pattern typically of polymer foam is used and to so-called investment-shell casting where the pattern is dipped in a ceramic slurry and the pattern evaporated away before casting. Although lost foam casting has been used for casting crankshafts hitherto, as mentioned above, it has been subject to the disadvantage that the pattern was prone to distortion, particularly an investment-shell casting: the insert serves an additional function in the method of the invention of supporting the pattern.

The invention also provides a crankshaft made according to any of the methods described above.

Methods of, and apparatus for, casting crankshafts will now be described, by way for example with reference to the accompanying drawings in which: Figure 1 is a plan view of a part of one half of a shell mould for a crankshaft with the insert located therein.

Figure 2 is an axial cross-section through an expanded polystyrene foam pattern formed around an insert; Figure 3 is an axial cross-section through the crankshaft made according to the methods of Figs. 1 and 2; and Figure 4 shows on an enlarged scale a part of an expanded polystyrene foam pattern formed around an alternative insert.

Referring to Fig. 1, the shell mould half 1 has a recess 2 for casting a crankshaft with five main bearings (journals) and four pins.

The other shell mould half (not shown) is a mirror image of the illustrated half. The two halves are clamped together to enable casting to take place in the usual way.

The illustrated shell mould half is manufac tured in the usual way. A pattern plate has a raised portion corresponding to the shape of half the crankshaft extending from a plate Dry sand mixed with thermosetting resin is dropped onto the heated pattern plate. A shell is formed, excess sand is removed and the pattern and shell are heated in an oven to cure the shell which is then removed from the pattern plate. The other half of the shell mould is made in the same way.

In accordance with the invention, the insert indicated generally by the reference number 3 is placed between the two halves of the shell when they are brought together for casting.

As will be apparent hereinafter, the insert and mould interlock for accurate location of the insert.

The insert 3 has cylindrical cavities 3a in the region of the main bearings, cylindrical cavities 3b in the region of the crankpins, intermediate passages 3c joining the regions 3a and 3b and, extending from all the cylindrical cavities communicating passages 3d, 3e which in the finished component connect the interior of the insert and the respective bearings surfaces. One communicating passage 3e only extends from each of the outermost cavities 3a (these are where oil is fed to the cavity in use of the crankshaft). Four passages 3d, equally spaced in a circumferential direction, extend from the other journal cavities 3a.A similar arrangement of passages 3d extends from the crankpin cavities 3b, except that the passage 3d which is uppermost when the respective piston is at T.D.C. is omitted in order to provide an unbroken bearing surface over the region of the bearing that is subject to the greatest stress.

The passages 3d, 3e that lie in the plane of the drawing are closed by short metal caps 3f, 3g, respectively, each cap being cylindrical and closed at one end. The shell mould halves have recesses shaped to accommodate the stepped cylindrical profile thus formed. The passages 3d that project perpendicularly, upwardly and downwardly, relative to the plane of the drawing, are received into corresponding recesses in the shell mould halves. Those passages are covered by deeper metal caps (not shown) which extend to the position where the crankshaft surface will be formed: if short caps 3f, 3g were used, molten metal would run into the hollow cylindrical space between the caps and the crankshaft defined by the cylindrical recess in the mould (equal to the outer diameter of the caps) and the outer diameter of the passages 3d, and would need to be machined away.However, this could be done if it was desired to use short caps.

The insert 3 is made of two presings of mild steel joined at the plane in which the section of Fig. 1 is taken. The long caps (not shown) are placed on the passages 3d extending upwardly and downwardly, and the two pressings are joined together simply by placing the short caps 3f, 3g on the passages 3d, 3e. The passages 3d extending at right angles to the plane of the page may be integrally formed during the pressing operation.

The insert 3 is then inserted in the mould halves which are clamped together and surrounded by material to maintain a uniform pressure on both shells.

Spheroidal graphite iron is then inserted into the mould, the mould halves are separted when the casting has set, and the crankshaft with cast-in insert removed.

Although a shell mould has been described with a recess to form one crankshaft, it would be usual for the mould to have recesses for more than one crankshaft, for example, three recesses.

Tapered recesses 6, 7 (Fig. 3) are now made in each end of the crankshaft and, with a special tool, the recesses are extended into the crankshaft sufficiently far to cut part way through the end walls of the insert, but without piercing them. After washing out the machining debris, the thus weakened end walls are then pierced with a pointed tool to form apertures 12, 13. This ensures that no swarf and grindings enters the insert.

The crankshaft is then mounted at its end on a combined sawing and grinding machine which is also designed to force filtered cutting oil into the insert through the apertures 12, 13. The passages 3d, 3e are then sawn off, and the bearing surfaces are rough ground, finish ground, and the openings 3d, 3e formed are chamfered. The bearing surfaces are then superfinished. All these operations take place with filtered cutting oil forced into the ends of the insert under pressure, thus ensuring that machining debris does not enter the insert.

As an added precaution against machining debris entering the bearings during service, the insert may be flushed with a fixative or congealing agent so that any foreign particles which are inside the insert are permanently secured to the interior surface.

The open ends of the insert are each blocked off with a steel plug 8, 9. The final appearance of the crankshaft is as shown in Fig. 3.

Among the advantages of the crankshaft so manufactured are the avoidance of the need for an operation to drill oil passages, lightness without the need for drilling or sand coring, greater torsional stiffness than if the crankpins and journals are hollowed (wholly or partially) as is sometimes done. Also the oil contained in operation in the hollow cavities will reduce transmission noise when compared with a solid or semi-hollow crankshaft.

In operation of the crankshaft, lubricating oil is fed from the block into openings 3e whereupon it is fed out of openings 3d. The interior of the insert will fill with oil, and its flow into the bearings will be assisted by the centrifugal force of the oil in the crankpin cavi ties, which acts as an oil pump when the engine is running and reduces the energy re quired to drive the main engine oil pump.

The invention is also suited to the so-called lost foam type of casting which will be de scribed with reference to Fig. 2.

A polystyrene foam pattern 4 surrounds an insert 5 which is identical to the insert 3 de scribed with reference to Fig. 1, and is closed by caps in the same way: the reference numerals 5a to 5g denote identical parts to 3a to 3g shown in Fig. 1 and 3.

The pattern is made by blowing expanded polystyrene beads into a tool the interior shape of which corresponds with that of the desired crankshaft and which engages the communicating passages 5d, 5e to locate the insert 5 therein. Steam is then passed through the tool so that the beads expand further and form the desired pattern around the insert.

After cooling the pattern 4 surrounding insert 5 is removed.

The pattern 4 is then buried in fine sand which is vibrated to accurately cover the pat stern. An evacuating pump is then applied to one end of the body of sand while the other is blocked with an air-impermeable membrane such as polythene sheet, in order to consoli date the sand. Metal such as spheroidal graphite iron is then poured in, burning the pattern away to define the mould cavity.

When the casting has set, the evacuating pump is then switched off.

The casting is then removed and is finished in the manner described above in relation to the shell mould casting with reference to Fig.

1, and the final appearance of the finished crankshaft is again as shown in Fig. 3.

In addition to the advantages noted above in relation to the shell mould casting, the lost foam method has the advantage that more complicated crankshaft shapes, such as with throws equally spaced by 120 , can be easily accommodated. Also the insert 5 provides support for the pattern 4.

Various modifications to the above de scribed method are possible. Thus, instead of the so-called full mould lost foam process de scribed above, the ceramic shell lost foam process can be used; in this process, the ex panded polystyrene pattern 4 is dipped in a slurry and then coated with a granular refrac tory for example in a fluidised bed until a thin shell is built upon the pattern. The ceramic shell-coated pattern is then fired for a few minutes to remove the polystyrene foam while retaining the shell shape with its internal per manent core. The shell is then inserted in sand in the same way that the pattern 4 was as described above, and casting undertaken while evacuation takes place. A further advan tage of the insert in this case is that it sup ports the pattern while it is being dipped in the slurry where it would otherwise be prone to distortion.Also, other foam materials could be used for the pattern in place at polystyrene beads.

Various modifications to the above described methods are possible without departing from the scope of the invention. Thus, instead of shell moulding or lost foam moulding, the moulding method could be green sand, dry sand, gas-hardened sand or a die cast in a permanent mould.

Instead of spheroidal graphite iron, cast iron, steel or light alloy or plastics material could be used for the material to be cast.

Alternative methods of closing the passages 3d, 3e, 5d, 5e are possible. For example, it is possible that the upwardly and downwardly extending tubes can be formed with an integrally closed end during the pressing operation. Equally, the passages could be closed by squashing them. Instead of three passages communicating with the crankpin cavities, four equally spaced ones could be provided if desired. Also, the passages need not be circular but- could be elongate in the direction of the circumference of the bearing: this enables the same bearing area to be lubricated by an opening with a smaller axial dimension. It may be found possible to provide a single oil feed at one end of the crankshaft only, in which case one of the wider passages 5e could be made as narrow as a passage 5d and operate as an oil outlet.

The two halves of the insert 3, 5 may be secured together instead of by means of the caps 3f, 3g, 5f, 5g by welding the two halves together. Alternatively the halves may be flanged and welded or mechanically connected by turning one rim over the other. Equally, they may be secured by adhesive to each other. As a further alternative, the insert could be made of ceramics instead of metal.

Referring to Fig. 4, an alternative method is described of producing an opening in the insert for pressurising it to allow the passages 3d, 5d to be opened without risk of contamination of the interior. Thus, an insert 10 in a polystyrene pattern 13 (although this is applicable to any casting method) identical to the inserts 3, 5 has a grub screw 11 located in a screw-threaded passage 10e set below the surface of the pattern. A similar grub screw 11 is located in the screw-threaded passage at the other end of the insert. Instead of applying a tool to the ends of the crankshaft and piercing a hole in the ends of the insert, after the casting has been made, the ends of the passages 10e are cut off, and all machining operations on the associated bearings (the outer two main bearings) are carried out.The grub screws are then unscrewed by means of an Allen key-way recessed in the centre of the grub screws, and the crankshafts are clamped in a grinding machine which applies cutting oil under pressure to the end two main bearings. The passages 10d are then sawn off, and the other bearings of the crankshaft are ground and finished in the way described above with reference to Figs. 1 to 3 for when pressure was applied to the ends of the insert.

Other alternatives are possible to grub screws: thus, for example, a cup-shaped insert with the open end facing the interior of the insert and set below the pattern surface could replace the grub screw. The passage 10e would again be cut off and the bearing would again be finished before the cup-shaped insert was removed: when this was done, the procedure would be the same as when grub screws were used.

If desired, all the passages on the insert could have removable plugs.

Finally, as another alternative to opening the passages 3d, 3e, 5d, 5e while pressure is applied, the passages may all be closed beneath the surface of the crankshaft and the openings may be made by piercing them after all machining operations have been completed and after subsequent washing to reduce the risk of machining debris entering the insert.

Claims

1. A method of casting a crankshaft wherein the mould contains an insert which after casting forms part of the cast crankshaft, the insert being hollow and extending along the crankshaft in such a way that in the finished component it communicates with the bearing surfaces of the main bearings and crankpins for lubrication thereof.

2. A method as claimed in claim 1, wherein the parts of the insert that communicate with the bearing surfaces are closed before the casting operation.

3. A method as claimed in claim 2, wherein at least some of the communicating parts are opened after the casting operation while the interior of the insert is pressurised.

4. A method as claimed in claim 3, wherein the insert is pressurised via an opening made in one end of the insert by forming a recess in the respective end of the crankshaft and then piercing the insert through the recess.

5. A method as claimed in claim 3, wherein the insert is pressurised via a communicating part which is opened by withdrawing outwardly a plug closing that communicating part.

6. A method as claimed in claim 5, wherein the plug is in threaded engagement with the communicating part and has a keyway.

7. A method as claimed in any one of claims 2 to 6, wherein the communicating parts extend beyond the mould interior and form location points for the core.

8. A method as claimed in any one of claims 1 to 7, wherein the insert is made of metal pressings.

9. A method as claimed in any one of claims 1 to 8, wherein the insert has a cavity in the region of each main bearing and crankpin.

10. A method as claimed in any one of claims 1 to 9, wherein the casting is by the green sand, shell moulding or die casting method.

11. A method as claimed in any one of claims 1 to 9, wherein the insert is surrounded by a polymer foam pattern and the casting is by the lost foam casting methods

12. A method substantially as hereinbefore described with reference to the accompanying drawings.

13. A crankshaft made by the method of any one of claims 1 to 12.

14. Apparatus for casting a crankshaft comprising a mould containing an insert so located that after casting the insert forms part of the cast crankshaft, the insert being hollow and extending along the crankshaft in such a way that in the finished component it communicates with the bearing surfaces of the main bearings and crankpins for lubrication thereof.