Title

POLYGONAL RING FOR DRIVING A PISTON PUMP, PISTON PUMP COMPRISING SUCH POLYGONAL RING AND METHOD FOR MAKING THE POLYGONAL RING

The present invention relates to a polygonal ring for driving a piston pump. In particular, the present invention relates to a polygonal ring comprising an annular body which extends around a given axis; has a cylindrical internal surface which extends around the given axis, at least one planar external surface which is parallel to the given axis and can exchange compressive forces with a piston of the piston pump; and at least one external surface which is parallel to said axis and is adjacent to the planar surface in the circumferential direction.

Piston pumps are widely used for feeding fuel, in particular diesel fuel, to internal combustion engines, especially with the advent of feeding systems which, like the common rail system, are intended to feed fuel at very high pressures, at present above 2000 bar.

Generally, a high-pressure piston pump for feeding diesel fuel to an internal combustion engine comprises a pump body; three pistons which are moveable in respective compression chambers formed in the pump body; and a unit for driving the pistons. In greater detail, the driving unit comprises a shaft, an eccentric part integral with the shaft, the polygonal ring mounted such that it can rotate around the eccentric part on an interposed friction bearing, and, if necessary, three intermediate elements, each of which is arranged between the polygonal ring and a respective piston along a respective planar surface.

The pump body has a chamber which contains the eccentric part, the polygonal ring, the intermediate elements and, in part, the shaft. Some of the fuel is fed to the chamber in order to lubricate and cool those parts of the driving unit in mutual sliding contact because, in the light of the high compression ratio generally associated with piston pumps for feeding fuel, the forces exchanged by the driving unit are very high and it is necessary to dissipate the heat made by the friction.

The structural elements are also subjected to very high stresses from the forces exchanged by the driving unit. For example, the respective pistons subject the polygonal ring to direct compressive forces substantially in the radial direction during the fuel compression phase.

In accordance with the prior art, it is known to make the polygonal ring using a method which involves cutting an annular element from a pipe having a relatively thick wall, finishing, by turning, a cylindrical internal surface of the annular element which extends around a given axis, cutting three planar external surfaces which are parallel to the given axis and distributed uniformly around the given axis; and subjecting the annular element to heat treatment.

One of the disadvantages which arise in the case of polygonal rings made in accordance with the prior art is that of deformation of the polygonal ring during the heat treatment phases.

Another disadvantage consists in the fact that the polygonal rings made by the method described above are relatively expensive.

Moreover, the experimental tests carried out by the applicant have shown that the three planar surfaces bend slightly around the axis owing to deformation when the friction bearing is pressed into the polygonal ring along the cylindrical internal surface. An object of the present invention is to provide a polygonal ring for a piston pump which can minimize the disadvantages of the prior art.

In accordance with the object of the present invention, a polygonal ring for driving a piston pump is provided, the polygonal ring comprising an annular body which extends around a given axis; the annular body having a cylindrical internal surface which extends around said axis, at least one planar external surface which is parallel to said axis and can exchange compressive forces with a piston of said piston pump; and at least one external surface which is adjacent to the planar external surface as seen in the circumferential direction and is shaped so as to define a recessed zone with respect to a hypothetical circumference which is centred on said axis and extends through the ends of said external surface.

In fact, the external surface does not have any specific motion-transmitting functions in the driving unit and, as a result, has no specific constraints governed by the kinematics of the driving unit. In this way, the surface can be selected in such a way as to reduce the mass included between the external surface and the cylindrical internal surface in order to make the structure of the polygonal ring around the axis more uniform. As a result, the variation of the radial deformation during pressing of the friction bearing is more homogeneous along the annular element around the given axis. Moreover, the greater structural homogeneity of the polygonal ring around the axis is particularly advantageous because it also contributes to limiting the permanent deformations of the polygonal ring which result from the heat treatment. According to a particular embodiment of the polygonal ring, the recessed zone extends at least in part between the cylindrical internal surface and a chord of said circumference and connects the ends of said external surface.

This makes it possible to increase the surface to mass ratio and therefore to improve the cooling of the polygonal ring during operation.

Another object of the present invention is to provide a high-pressure piston pump. According to the present invention, a high-pressure piston pump for feeding fuel, in particular diesel fuel, to an internal combustion engine is provided, the piston pump comprising a pump body, pistons which are moveable in respective compression chambers formed in the pump body, and a device for driving the pistons which comprises a polygonal ring which is made according to any one of the preceding claims. A further object of the present invention is to provide a method for making a polygonal ring for driving a piston pump.

According to the present invention, a method for making a polygonal ring for driving a piston pump is provided; the method comprising the steps of forging an annular body which extends around a given axis and comprises a cylindrical internal surface which extends around said axis, a planar external surface which is parallel to said axis and can exchange compressive forces with a piston of said piston pump; and at least one external surface which is adjacent to the planar external wall as seen in the circumferential direction and is shaped so as to define a recessed zone with respect to a hypothetical circumference which is centred on said axis and extends through the ends of said external surface.

The production of the polygonal ring from a semi-finished product obtained by forging makes it possible to substantially reduce costs. This is because forging makes it possible to produce semi-finished products having relatively complex shapes without having to carry out an excessive number of chip removal steps or without having to remove a large amount of material by means of mechanical chip removal steps. The cy- lindrical internal surface, the planar external surface and the external surface are produced directly by forging. As a result, the cylindrical internal surface and the planar external surface only require finishing operations while the external surface does not require any additional work.

Other features and advantages of the present invention will become clear from the following description of a non-limiting exemplary embodiment thereof, with reference to the accompanying drawings in which:

- Figure 1 is a schematic cross-sectional view, with parts removed for clarity, of a piston pump for feeding fuel to an internal combustion engine which is made according to the present invention;

- Figure 2 is a front elevation view, with parts removed for clarity, of a polygonal ring made according to the prior art;

- Figure 3 is a front elevation view, with parts removed for clarity, of a polygonal ring made according to the present invention;

- Figure 4 is a view of a polar coordinate diagram showing the deformations found on a friction bearing during the pressing of the latter into the polygonal ring shown in Figure 2; and

- Figure 5 is a view of a polar coordinate diagram showing the deformations found on a friction bearing during the pressing of the latter into the polygonal ring shown in Figure 3.

In Figure 1, the reference numeral 1 denotes a piston pump for feeding fuel, preferably diesel fuel, to an internal combustion engine (not illustrated in the accompanying drawings). The piston pump 1 has a high compression ratio and is able to raise the fuel to pressures above 2000 bar.

The piston pump 1 comprises a pump body 2, three pistons 3 and an driving unit 4 for moving the pistons 3 simultaneously with respect to the pump body 2. The pump body 2 is the supporting structure of the piston pump 1 and in this case comprises a central body 5 and three heads 6 assembled on the main body 5. Each head 6 has a respective compression chamber 7 in which a respective piston 3 can move. Ducts for feeding and delivering the fuel (not illustrated in the accompanying drawings) are formed in the pump body in order to carry the fuel to and from, respectively, each compression chamber 7. Moreover, feed and delivery valves (not illustrated in the accompanying drawings), which are generally formed in the manner of non-return valves, are arranged along said feed and delivery ducts. The central body contains a chamber H which, in part, houses the driving unit 4, said driving unit comprising a shaft 8, an eccentric part 9 integral with the shaft 8, a polygonal ring 10 which rotates around the eccentric part 9, a friction bearing 11 arranged between the eccentric part 9 and the polygonal ring 10, three intermediate elements 12, each of which is arranged between the polygonal ring 10 and a respective piston 3; and three springs 13, each of which is compressed between the pump body 2 and a respective piston 3 for the intake stroke of the respective piston 3. The friction bearing 11 is preassembled on the polygonal ring 10 by press fitting.

In the case shown in Figure 1, each intermediate element 12 is defined by a small cup which has a bottom wall in contact with the polygonal ring 10 and a side wall which acts as a guide element for the piston 3.

The shaft 8 is supported at two opposite ends by the pump body 2 in a known configuration (not illustrated in the accompanying drawings) and by means of respective friction bearings (not illustrated in the accompanying drawing).

As can be seen more clearly from the illustration in Figure 3, the polygonal ring 10 comprises an annular body 14 which extends around a given axis A, has a cylindrical internal surface 15 which extends around the axis A, three planar external surfaces 16, each of which is parallel to the axis A and can exchange compressive forces with a respective piston 3 of said piston pump 1 (Figure 1); and three external surfaces 17 parallel to the axis A.

In the case shown in Figure 3, the planar external surfaces 16 are distributed uniformly around the axis A and are spaced apart by the external surfaces 17. Each external surface 17 defines a recessed zone with respect to a circumference Cl which is centred on the axis A and extends through the ends of the external surface 17. In the case shown in Figure 3, the recessed zone extends in part between a chord C2 which extends between the ends of the external surface 17 itself and the cylindrical internal surface 15. In particular, each external surface 17 is defined by two adjacent concave faces 18 which have a curvature of a sign opposite to the curvature of the cylindrical internal surface 15. The two concave faces 18 are joined together by a convex face 19 arranged between the two concave surfaces 18.

Each external surface 17 extends radially, in part, outside of the chord C2 and, in part, between the chord C2 and the cylindrical internal surface 15. To sum up, the concave faces 18 are substantially arranged between the cylindrical internal surface 15 and the chord C2, while the convex face 19 is arranged radially outside of the chord C2. Moreover, in the embodiment described, the external surfaces 17 are parallel to the axis A.

With reference to Figure 2, the reference numeral 20 denotes a polygonal ring which is made in accordance with the prior art and comprises an annular body 21 which extends around a given axis Al, has a cylindrical internal surface 22 which extends around said axis Al; three planar external surfaces 23, each of which is parallel to the axis Al and can exchange compressive forces with a piston 3 of said piston pump 1 (Figure 1); and three convex external surfaces 24 which are, in particular, cylindrical surfaces concentric with the cylindrical internal surface 22.

The shape of the polygonal ring 20 is determined by the features required by the driving unit to which it belongs; by the method adopted for making it; and, in part, by the production costs. To sum up, and with reference to the prior art method, the shape of the polygonal ring 20 is determined by the shape of a pipe (not illustrated in the accompanying drawings) and by the subsequent mechanical chip removal steps carried out on said pipe. The characteristics of the material are optimized by means of heat treatment processes.

The applicant has carried out experimental tests to measure and compute, using FEM calculations, the deformations of the friction bearing 11 during the pressing of the friction bearing 11 into the polygonal ring 10 (Figure 1) and during the pressing of a friction bearing into the polygonal ring 20 shown in Figure 2.

These tests have shown that, in the case of the polygonal ring 20 shown in Figure 3, the radial deformations of the friction bearing vary very considerably with respect to the maximum deformation DMAXl, by about 40% of the deformation DMAXl as shown in the polar coordinate diagram shown in Figure 4. This large variation along the annular body 21 is attributable to a non-uniform distribution of the masses around the axis Al caused by the levelling of the planar external surfaces 23.

By contrast, the friction bearing 11 mounted in the polygonal ring 10 (Figure 1) shows a smaller variation in the deformation in the circumferential direction, as shown in the polar coordinate diagram in Figure 5. Here, although there is a slight increase in the maximum deformation DMAX2, the deformation of the bearing 11 is contained within an interval equal to 10% of the maximum deformation DMAX2. This implies that the polygonal ring 10 allows fairly uniform deformation during the pressing of the friction bearing 11 and therefore said ring deforms in a fairly uniform manner. The uniform deformation of the polygonal ring 10 around the axis A has a minor influence on the shape of the polygonal ring 10, in particular of the planar surfaces 16, and causes only a small dimensional variation while noticeably reducing the effect of the bending of the planar surfaces 16. Moreover, the greater structural uniformity of the polygonal ring 10 reduces the permanent deformations which arise from the heat treatment processes to which the polygonal ring 10 is subjected.

Moreover, the shape of the polygonal ring 10 (Figure 3) increases the surface to mass ratio and promotes the dissipation of heat from the polygonal ring 20 as compared with the polygonal ring shown in Figure 2.

In order to limit production costs, the polygonal ring 10 shown in Figure 3 is made by a process which includes a forging phase, this phase providing a semi-finished product which substantially has the shape of the polygonal ring 10 shown in Figure 3. The mechanical chip removal steps are limited to steps for finishing the cylindrical internal surface 15 and the planar external surfaces 16. The external surfaces 17 do not require additional work and are obtained directly in the forging phase. The advantages of the present invention are clearly apparent from the above description.

The present description refers explicitly to a polygonal ring provided with three planar surfaces, but the present invention can of course be used in polygonal rings provided with any number of planar surfaces.

It is also evident that variations can be made to the present invention without, however, departing from the scope of protection defined in the accompanying claims.