Assembly of cylinder and piston, and piston for use therein

Firstly, the invention relates to an assembly of cylinder and piston, wherein the piston comprises a circumferential edge for engaging an inner surface of the cylinder. For an assembly of cylinder and piston it is essential that the engagement between the circumferential edge of the piston and the inner surface of the cylinder occurs in a fluid-tight manner. For obtaining such a fluid-tight engagement the dimensions of the piston and cylinder should be such, that a tight fit between the piston and cylinder is obtained. However, a tight fit not only may be achieved by such a close correspondence between the dimensions of the piston and cylinder, but also by means of the provision of specially designed sealing members positioned between the cylinder and piston.

Regardless the manner, in which a fluid-tight engagement between the piston and cylinder will be obtained, such an assembly of cylinder and piston is rather expensive in its manufacture. Moreover, during use wear of the piston and/or cylinder will negatively influence the necessary fluid-tight engagement between the piston and cylinder. Further, the desired tight fit renders assembling a piston- cylinder assembly difficult, because it is difficult to introduce the piston into the cylinder without causing damage. The disadvantages mentioned before are highlighted even more when the cylinder, and thus the piston, does not have a circular cross-section.

It is an object of the present invention to provide an improved assembly of cylinder and piston of the type referred to above.

Thus, in accordance with the present invention, an assembly of cylinder and piston of the type referred to

above is characterised in that the circumferential edge of the piston defines a three-dimensional curve.

Whereas in a state of the art assembly of cylinder and piston, the circumferential edge of the piston basically extends in a two-dimensional manner (i.e. in a flat plane) , in accordance with the present invention the circumferential edge now extends three-dimensionally. As a result, the shape of the circumferential edge of the piston is more readily adaptable to the shape of the inner surface of the cylinder, thus mitigating the above-mentioned disadvantages.

Preferably, the piston is defined by a cup-shaped hollow body with an open top, wherein recesses connecting to the open top are provided in the wall of the body. Due to its shape as a cup-shaped hollow body such a piston has an optimised adaptability of its circumferential edge to the shape of the inner surface of the cylinder.

The adaptability of the piston may be improved even more when, in accordance with yet another preferred embodiment of the assembly, the body is manufactured of a re- silient material, for example a resilient plastic material. Preferably, the body tapers from the circumferential edge in a direction away from the open top. This means, that, as seen in a cross-sectional manner, the body is widest at its top (i.e. at the circumferential edge), and gets smaller in a direction away from said open top. As a result, it is guaranteed that under all circumstances only a line contact occurs between the circumferential edge of the piston and the inner surface of the cylinder, which leads to very reproducible results in terms of fluid-tightness. Fur- ther, such an embodiment will reduce the friction (and thus wear) between the piston and cylinder.

The special shape of the circumferential edge of the piston in accordance with the present invention especially is advantageous, when the cylinder and piston both have a non-circular cross-section. Because of the non-

uniform engagement between the inner surface of the cylinder and the circumferential edge of the piston when the cylinder and piston both have a non-circular cross-section, the three-dimensional shape of the circumferential edge of the piston still will provide the desired proper engagement along the entire circumferential edge, thus assuring a correct fluid-tight engagement between the cylinder and piston. Specifically, it is possible that the cylinder and piston both have an oval cross-section defined by two oppo- site curved ends and two opposite and substantially parallel sides connecting said curved ends. In such an embodiment the piston obtains an oblong shape which is very effective in providing the desired fluid-tight engagement.

Preferably, in such an embodiment with oval cylin- der and oval piston, in each of the wall portions of the body defining said parallel sides a single recess is provided. Specifically, in such a case, such a recess preferably would have a U-shaped or V-shaped outline.

In a second aspect, the present invention relates to a piston presenting all the features of the piston disclosed in the present invention and being thus constructed and evidently intended for use in the assembly in accordance with the present invention.

Hereinafter the invention will be elucidated while referring to the drawing, in which an embodiment of an assembly of cylinder and piston in accordance with the present invention is illustrated.

Figure 1 shows a perspective view of the assembly of cylinder and piston, in a disassembled state; figure 2 shows a side-elevational view of the piston according to II in figure 1 ; figure 3 shows a side-elevational view of the piston according to III in figure 1, and figure 4 shows a frontal view of the piston ac- cording to IV in figure 1.

Firstly referring to figure 1, an assembly of cylinder 1 and piston 2 is illustrated in a disassembled state. One can clearly see, that the cylinder 1 has an oval cross- section which is defined by two opposite curved ends 3 and two opposite and substantially parallel sides 4 connecting said curved ends 3.

Correspondingly the piston 2 also has an oval cross-section defined by two opposite curved ends 5 and two imaginary opposite and substantially parallel sides 6 con- necting said curved ends 5.

The piston 2 is defined by a cup-shaped hollow body 7 with an open top 8. It is noted, that "open top" is dependent upon the specific orientation of the piston 2 within the cylinder 1, and thus such an open top could face in any direction, for example downwards, sidewards etcetera.

In each of the wall portions of the body 7 defining the (imaginary) parallel sides 6 a single recess 9 is provided. In the illustrated embodiment, these recesses 9 substantially have a U-shaped outline. As a result of the shape of the piston 2 as illustrated in figure 1, such a piston 2 comprises a circumferential edge 10 for engaging an inner surface 11 of the cylinder 1, which circumferential edge 10 defines a three- dimensional curve. Specifically, such a circumferential edge 10 extends along the upper edge of a first curved end 5, along the downwardly and upwardly extending slopes of a first recess 9, along the upper edge of the second curved end 5 and, finally, along the downwardly and upwardly extending slopes of the second recess 9. From figures 2 and 3 it appears, that the body 7 of the piston 2 tapers form the circumferential edge in a direction away from the open top 8.

In a preferred embodiment, the body 7 of the piston 2 is manufactured of a resilient plastic material. Be- low such a resilient body 7 an attachment member 12 may be

provided (which may be rigid) for connecting thereto any exterior means not shown in detail here. For example, such means could comprise driving means for the piston 2.

The invention is not limited to the embodiment de- scribed before, which may be varied widely within the scope of the invention as defined by the appending claims. Thus, for example, any cross-sectional shape of the cylinder 1 with corresponding cross-sectional shape of the piston 2 could be envisaged, for example a triangular, square, rec- tangular, or other polygonal shape with or without rounded corners. Such a shape could be regular or irregular, with a corresponding shape of the piston.