GB2157396A

GB2157396A - Piston/connecting rod joint for hydrostatic displacement machine

Info

Publication number: GB2157396A
Application number: GB08508598A
Authority: GB
Inventors: Konrad Christ; Hans-Dieter Schulze
Original assignee: ORSTA HYDRAULIK VEB K
Current assignee: ORSTA HYDRAULIK VEB K
Priority date: 1984-04-05
Filing date: 1985-04-02
Publication date: 1985-10-23
Also published as: DD224371A1; GB8508598D0; DE3512495A1; IT8547937A1; GB2157396B; IT1181632B; IT8547937A0

Abstract

A hydrostatic displacement machine of the eccentric shaft kind, comprises a ball joint 2 connection between the piston and the connecting rod 3, in which the piston 4 which is formed as a hollow cylinder disposed between the bore of the cylinder 1 and the ball head portion 2, the ball is always in contact with the whole of the inner surface of the socket throughout the entire reciprocation cycle and the area F of the working surface of the piston is 2 or 3 times the projected area Fp of the inner surface of the socket. <IMAGE>

Description

SPECIFICATION Piston for a hydrostatic displacement machine This invention relates to a piston for a hydrostatic displacement machine, in particular for a low-speed radial piston machine of the eccentric shaft type, with a ball joint connection between the piston and the connecting rod.

In order to provide an advantageous powerweight ratio or to save material, attempts have been made to construct hydrostatic low-speed radial piston machines of the eccentric shaft type, with a small outside diameter and thus a compact design.

In order to attain that aim, substantiai importance is attributed to the design configuration of the piston with its adjoining components such as cylinder, connecting rod or slipper. Various designs of that structural group which transmits the hydraulic force pressure X piston areas-to the eccentric of the main shaft, are known.

The construction which is most frequently used in radial piston machines of the eccentric shaft type involves the piston with an internal cup seat in which the connecting rod is positively connected, by a ball head portion. The design of that piston structural unit has long met with success, in terms of function, but it suffers from the disadvantage that it makes the dimensions of the machine relatively substantial in a radial direction. The same also applies to the construction wherein the piston with ball joint connecting rod is replaced by a piston with a ball pin.

German patent specification No 1 728 244 discloses a construction in which the piston unit comprises oscillating telescopic cylinders which on the one hand bear against the spherical surface of an eccentric which is fixedly connected to the rotating shaft, and on the other hand against the spherical portions which are fixedly connected to the housing.

The transmission of force is effected exclusively hydraulically so that friction and wear are substantially reduced. In comparison, the technological expenditure and the hydraulic losses are of considerable magnitude, due to the three sealing means provided.

German published specification (DE-AS) No 2244 920 discloses a further design configuration of the piston unit. The piston is directly connected to the slide shoe or slipper and is guided in spherical bearings, thereby providing a compact construction. However, the spherical bearing arrangement involves an increase in the cylinder diameter and thus an increase in the cylinder chamber so that high hydraulic forces must be carried, which make it necessary to have a massive construction.

The invention is based on the problem of providing for hydrostatic displacement machines, a working piston which, while retaining the service-proved ball joint for the pistonconnecting rod connection, reduces the radial dimensions of the piston unit.

According to the present invention there is provided a piston for a hydrostatic displacement machine, in particular for a low-speed radial piston machine of eccentric shaft type, with a ball joint connection between the piston and the connecting rod, comprising an intermediate piston which in use is disposed between the bore of the cylinder and the ball head portion provided on the connecting rod, said intermediate piston being formed as a hollow cylinder through which the ball head portion projects, the length (L) of the intermediate piston being equal to or less than the diameter of the ball head portion and being so dimensioned that, with the maximum deflection of the connecting rod, the full projected surface area (Fp) is in engagement and the annular surface area (F) of the intermediate piston is about two to three times the projected surface area (Fp).

An embodiment of the invention will now be described, by way of an example, with reference to the accompanying drawing, in which the single Figure shows a view in section through the piston unit.

An intermediate piston 4 which is in the form of a hollow cylinder is guided in the cylinder bore of the cylinder 1. The intermediate piston 4 is positively connected in known fashion by means of a split holding ring 6 and a retaining ring 7 to a ball head portion 2 which is provided on the end of a connecting rod 3 and which is provided with a flat end portion 5. A sealing ring 8 is provided for sealing between the cylinder bore and the intermediate piston 4. The length of the intermediate piston 4 is equal to or less than the diameter of the ball head portion 2 and is of such a dimension that, with the maximum deflection of the connecting rod 3, the full projected surface Fp of the ball head and the cup seat is in engagement.

The outside diameter of the intermediate piston 4 determines the effective area for the piston force which is formed by the surface F0 of the ball head portion 2, to which pressure is applied, and the annular surface F of the intermediate piston 4, to which pressure is also applied.

The major part of the piston force is applied by the ball head portion surface F0 to which pressure is applied, which surface however is of no significance in relation to the contact pressure as between the ball head portion 2 and the intermediate piston 4. The annular surface F of the intermediate piston 4, to which pressure is applied, produces the smaller part of the piston force. As the intermediate piston 4 is supported on the ball head portion 2, the projected surface Fp must transmit that part of the piston force. In order to govern the admissible pressure applied to the projected surface Fp a surface area ratio F/Fp of about two to three, is to be observed.

Due to the flattened end portion 5 on the ball head portion 2 and the intermediate piston 4 which extends through the arrangement, the above-described piston unit is of a particularly compact construction so that the outside diameter of the cylinder 1 can be kept very small.

Claims

1. A piston for a hydrostatic displacement machine, in particular for a low-speed radial piston machine of eccentric shaft type, with a ball joint connection between the piston and the connecting rod, comprising an intermediate piston which in use is disposed between the bore of the cylinder and the ball head portion provided on the connecting rod, said intermediate piston being formed as a hollow cylinder through which the ball head portion projects, the length (L) of the intermediate piston being equal to or less than the diameter of the ball head portion and being so dimensioned that, with the maximum deflection of the connecting rod, the full projected surface area (Fp) is in engagement and the annular surface area (F) of the intermediate piston is about two to three times the projected surface area (Fp).

2. A piston for a hydrostatic displacement machine, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

3. A hydrostatic displacement machine provided with a piston as claimed in claim 1 or claim 2.