WO1995006543A1

WO1995006543A1 - Method of radially pressurising the bore in a crank throw to the yield stress prior to shrink-fitting it to the main bearing pin

Info

Publication number: WO1995006543A1
Application number: PCT/DK1994/000324
Authority: WO
Inventors: Ole Røsler SØRENSEN
Original assignee: Man B & W Diesel A/S
Priority date: 1993-09-03
Filing date: 1994-08-30
Publication date: 1995-03-09
Also published as: DK99593A; KR960703704A; JP3115895B2; DK99593D0; KR100316627B1; JPH09502133A; DK173164B1

Abstract

A crank throw comprises two arms (1) and a crank pin (3). Each arm includes a bore (2) to be shrink-fitted over a pin portion on a main bearing pin. Before assembling the shaft, the bore (2) is subjected to a radial pressure of a level bringing the stress in at least part of the material surrounding the bore up on the yield stress of the material. After stress relaxation compression stresses remain in the material near the bore periphery, which reduces the resulting stress level in the assembled shaft.

Description

METHOD OF RADIALLY PRESSURISING THE BORE IN A CRANK THROW TO THE YIELD STRESS PRIOR TO SHRINK-FITTING IT TO THE MAIN BEARING PIN

The invention relates to a method of manufacturing 5 crankshaft parts for use in a semi-built or fully built crankshaft constructed from several crank throws interconnected through axially extending main bearing pins, which throws each comprise two arms and a connect¬ ing rod pin interconnecting the arms, wherein each arm 0 is connected with the associated main bearing pin by means of a shrink connection comprising a pin portion shrink fitted into a bore, preferably a main bearing pin portion shrink fitted into a bore in the crank arm.

A semi-built crankshaft should be understood to be 5 a shaft in which each crank throw is manufactured integrally contrary to a fully built shaft where the throws are constructed from two arms shrink fitted over a connecting rod pin. In the case of both types of shafts, the number of crank throws corresponding to the 0 number of cylinders of the engine are assembled into a whole shaft by means of the main bearing pins. Usually, an end portion of a main bearing pin is shrink fitted into an associated bore in the crank arm, which is described in Danish patent No. 108,246, among others,

25 but it is also possible, as described in Danish patent No. 128,428, to design the two adjacent arms in respect¬ ive crank throws with an axially projecting pin portion which is shrink fitted into a bore in a joining ring laid around the two neighbouring pin portions.

30 It is a common feature in the known semi-built or fully built crankshafts that the shrink pressure is limited by the condition that after the assembly of the shaft yielding may only occur in the material in the immediate vicinity of the internal surface of the bore, i.e. near the joining surface. If the shrink pressure is higher, and a more extensive yield occurs in the shaft material during operation of the engine, the shaft will not be able to transmit the torque applied without being exposed to a non-allowable deformation. The shrink pressure fixes the individual shaft parts in relation to each other and thus, together with the shrink surface area, determines the magnitude of the torque which can be transmitted by the shaft. As the maximum level of the shrink pressure is limited by the fact, as mentioned, that the shaft must be dimensionally stable during operation, it is necessary in known shafts to increase the shrink surface area and thus the dimension of the shaft in the axial and/or radial direction, if the shaft is to be designed for transmission of a larger torque. This means that both the crankshaft and the adjacent parts of the engine require more space and become heavier, which renders the engine more expensive and reduces its efficiency. The object of the invention is to provide a method which renders it possible to manufacture crankshafts which are able to transmit large torques in relation to the dimensions of the shaft.

With a view to this object, the method according to the invention is characterized in that prior to being shrink fitted over the associated pin portion, the bore is subjected to a radially directed pressure of such a magnitude that the stress in at least part of the material surrounding the bore is increased to the yield stress of the material.

The radially directed pressure applied before shrinking produces yield stresses in the material sur¬ rounding the bore in an annular area, the radial extent of which in the direction away from the bore periphery depends on the magnitude of the pressure applied. When the applied pressure is released, an elastic relaxation of the stresses in the annular area occurs so that compressive stress remain in the area of the material nearest the bore periphery when the pressure applied is fully released. When the pin portion is subsequently shrink fitted into the bore, these compressive stresses¹ counteract the stresses in the material produced by the shrink pressure so that the total stress level in the assembled shaft is reduced for a certain shrink pres- sure.

The method may be used to increase the shrink pressure applied so that with unchanged dimensions the crankshaft may transmit a higher torque, or to reduce the shaft dimensions without any change in its ability to transmit torque, or for a combination of these options. The higher shrink pressure yields an opportun¬ ity for reduction of the shrink area and thus the length of the shaft, but it is also possible to reduce the thickness of the shrinkage reinforcement surrounding the bore as a consequence of the lower stress level.

In a preferred method, in case of a bore in a crank arm, the pressure used is of such a magnitude that the yield point is reached in the material at the outer side of the arm in the area where the outer side has the shortest distance to the bore. By increasing the pressure applied in this manner until the material is plasticized over the full thickness of the arm, the maximum obtainable compressive stress at the pressure release occurs in the material near the bore periphery. This full exploitation of the effect of the invention may, at an unchanged geometry of the crankshaft, render it possible to increase the shrink pressure by up to 85 per cent in relation to previously known shafts, which renders the shaft capable of transmitting a torque which is about 85 per cent higher. The radially directed pressure may be applied in different manners. It is possible to apply the pressure in a purely mechanical manner by inserting an expandable mandrel in the bore, but preferably, the pressure is applied by arranging a sealing cover on each side of the arm abreast of the bore, filling the cavity located between the covers with a pressure fluid, increasing the pressure of the fluid to the desired level, releasing the pressure and removing the covers. The use of a pressure fluid presents the advantage that the bore periphery is influenced by a completely uniform pressure over its full area, which can hardly be achieved in the purely mechanical manner, because the mandrel must be formed in several pieces which can be pressed radially outwards, and during the pressing-out, these pieces will be separated slightly from each other, and the pressure loading on the bore periphery will vary in the transi¬ tional areas between the pieces. Furthermore, it is very simple to adjust the level of the pressure applied by controlling the pressure in the fluid let in.

The invention also relates to a device for use in the above methods, which device is characterized by comprising a pin having a smaller diameter than the bore, two covers having larger diameters than the bore, at least one tightening member for mounting the two covers on either side of the bore with the pin posi¬ tioned substantially coaxially in the bore, and a fluid inlet which, in the mounted position of the device, communicates with the cavity between the pin and the internal side of the bore. The device is of a simple design and can be rapidly mounted, used, and dismounted, so that the use of the method according to the invention does not involve substantial cost increases, but on the contrary large savings in the amount of material spent for the manufacture of a shaft capable of transmitting a predetermined torque. As the pin must have a smaller diameter than the bore in the crank arm or the joining ring, one and the same device may be used for shafts of different dimensions. In a preferred embodiment, on the side facing the arm, each cover has two annular sealing means which are in sealing contact with the side surface of the crank arm and the end surface of the pin, respectively, in the mounted position of the device. The sealing means cause the fluid pressure to act only on the annular cover area located between the sealing means, which substantially reduces the forces acting on the cover so that the size of the tightening member as well as the thickness of the covers may be reduced, so that the device becomes lighter and thus more manoeuvrable.

The invention will now be explained in further detail below with reference to the very schematic drawing, in which

Fig. 1 is a side view of an arm of a crank throw for a semi-built shaft,

Figs. 2-4 show diagrams of the state of stress in the arm material in a prior art assembled shaft, in an arm manufactured according to the invention before shrinking, and in the arm in Fig. 3 after the shrink assembly, respectively,

Fig. 5 is a view corresponding to that of Fig. 1 of a crank arm, where the invention has been utilized to reduce the amount of material in the shaft, and

Fig. 6 shows a cross-sectional view through a device according to the invention, shown mounted on a crank arm.

Fig. 1 shows a crank arm 1 with a bore 2 for a pin portion on a main bearing pin, not shown, having a central bearing section and two end sections which are shrink fitted into the associated bore 2 of a respective crank throw, when the shaft is assembled. The crank throw is intended for a semi-built shaft and has a connecting rod pin 3 formed integrally with the two arms

1 of the throw. On the side of the bore facing away from the pin 3, the bore 2 is surrounded by annular arm material 4 called the shrinkage reinforcement.

If the pin portion is shrink fitted into the bore

2 in the known manner, the stresses shown in Fig. 2 will occur in the shrinkage reinforcement after the assembly of the shaft. The axis of abscissas here indicates the distance from the centre of the bore. The peripheral surface of the bore has a distance a from the centre, and b indicates the distance to the outer side of the arm in the area 5, where the outer side has the shortest distance from the bore, viz., the radially outer end of the shrink material. The radial thickness of the shrinkage reinforcement is thus b-a (see Fig. 6).

The shrink pressure induces both radial stresses σ_R and tangential stresses σ_τ in the arm material, and the total state of stress, the reference stress, is found to be σ_ref = σ_R + σ_τ. The radial stress σ_R corre¬ sponds to the shrink pressure at the bore periphery and is σ_R = 0 at the outer side of the arm. On the ordi- nate axis, the stresses are standardized with the yield stress σ_f of the material. It can be seen from Fig. 2 that the shrink pressure is so high that the stress in the material near the bore periphery is at the yield point.

Fig. 6 shows a device for pressure loading of the material surrounding the bore 2, before it is shrink fitted over the pin portion. First, a pin 6 having a smaller diameter than the bore 2 is positioned inside the latter, and covers 7 and 8 are fastened on the pin by means of fastening means, such as bolts 9, and tightened for sealing contact with the sides 10, 11 of the arm, so that the ends of the bore are closed and the pin is surrounded by an annular cavity 12. A supply source for pressure fluid is connected with a fluid inlet 13 in the form of a threaded bore with a conduit leading into the cavity 12. The inlet pressure of the fluid may be controlled, for example by the supply source having a higher pressure than required, and by the supply conduit comprising an adjustable pressure reducing valve. Alternatively, the fluid may be supplied from a pressure pump with an adjustable supply pressure. The fluid used is normally a liquid, such as oil or water.

When the cavity 12 is filled with fluid, and the pressure rises, the stress in the material at the bore periphery will first be increased to the yield stress σ_f of the material, and at continued increase of pres¬ sure in the cavity, the material is plasticized at an increasing distance from the bore periphery. When the desired extent of the plastification has been obtained, the pressure in the cavity is released, which yields a pure elastic stress relaxation in the material, and the device is removed from the bore 2, which is subsequently ready to be shrink fitted over a pin portion in the quite usual manner. After the pressure release, compressive stresses will be present in the part of the material closest to the bore periphery, as illustrated in Fig. 3 by σ_ref, while near the outer side of the arm 1 or the joining ring, tensile stresses will exist. Fig. 4 shows the state of stress in the assembled shaft in the case where the shrink pressure is of the same magnitude as in Fig. 2. The remaining stresses appear as the sum of the residual stresses shown in Fig. 3 and the stresses produced by the shrink pressure, and it may be seen that σ_ref in the area near the bore periphery is substantially lower than the yield stress σ_f of the material, and that σ_ref in the area near the outer side of the arm is higher than in Fig. 2. By a suitable choice of the maximum value of the fluid pressure, the assembled shaft may achieve a largely constant stress σ_ref in the material positioned between a and b and thus an optimum use of the strength of the material.

The lower stress level in the assembled shaft may be used to increase the shrink pressure so that at an unchanged shrink area, the shaft may transmit a higher torque. Alternatively, it is possible to reduce the shrink area by giving the bore a smaller diameter and/or reducing the axial width of the arm. It is also possible to reduce the weight of the shaft by reducing the thickness of the shrinkage reinforcement 4. Fig. 5 shows a crank arm where the invention has been utilized to reduce the weight of the shaft. The shaft may transmit the same torque as the shaft shown in Fig. 1, and it may be seen that the material saving is considerable. If the shaft is fully built, the bore for the connecting rod pin may be preloaded in the same manner as the bore 2 for the main bearing pin.

Claims

P A T E N T C L A I M S

1. A method of manufacturing crankshaft parts for use in a semi-built or fully built crankshaft con¬ structed from several crank throws interconnected through axially extending main bearing pins, which throws each comprise two arms and a connecting rod pin interconnecting the arms, wherein each arm is connected with the associated main bearing pin by means of a shrink connection comprising a pin portion shrink fitted into a bore, preferably a main bearing pin portion shrink fitted into a bore in the crank arm, c h a r ¬ a c t e r i z e d in that prior to being shrink fitted over the associated pin portion, the bore is subjected to a radially directed pressure of such a magnitude that the stress in at least part of the material surrounding the bore is increased to the yield stress of the material.

2. A method according to claim 1, c h a r a c¬ t e r i z e d in that in case of a bore in a crank arm, the pressure used is of such a magnitude that the yield point is reached in the material at the outer side of the arm in the area where the outer side has the shortest distance to the bore. ^*

3. A method according to claim 1 or 2, c h a r- a c t e r i z e d in that a sealing cover is arranged on each side of the arm in alignment with the bore, that the cavity located between the covers is filled with a pressure fluid, that the pressure of the fluid is increased to the desired level, and that the pressure is released, and the covers are removed.

4. A device for use in the method according to any one of claims 1.-3, c h a r a c t e r i z e d by comprising a pin having a smaller diameter than the bore, two covers having larger diameters than the bore, at least one tightening member for mounting the two covers on either side of the bore with the pin posi¬ tioned substantially coaxially in the bore, and a fluid inlet which, in the mounted position of the device, communicates with the cavity between the pin and the internal side of the bore.

5. A device according to claim 4, c h a r a c¬ t e r i z e d in that on the side facing the arm, each cover has two annular sealing means which are in sealing contact with the side surface of the crank arm and the end surface of the pin, respectively, in the mounted position of the device.