Expansible chamber devices – With lubricating means – Lubricant passage extends axially through articulated piston...
Reexamination Certificate
1999-06-29
2001-05-29
Look, Edward K. (Department: 3745)
Expansible chamber devices
With lubricating means
Lubricant passage extends axially through articulated piston...
Reexamination Certificate
active
06237467
ABSTRACT:
The invention relates to a piston for a hydrostatic machine, in particular an axial piston machine, and to a process for producing a piston of this kind.
The solid pistons usually used in axial piston machines set limits to operations at relatively high rotational speeds. High rotational speeds give rise to problems of strength for the cylinders on account of the high centrifugal forces and for the piston retaining device on account of the high inertial forces, as well as thermal problems at the contact surfaces between pistons and cylinders on account of the frictional forces resulting from the centrifugal forces. Hollow pistons are therefore used to operate axial piston machines with a high rotational speed.
Hollow pistons are already known in various designs and are generally produced by means of a machining manufacturing process. Constructions with a cavity which is open towards the working cylinder have the disadvantage of the cavity being filled with pressure fluid with each piston stroke. This region of the volume is consequently compressed and expanded again with each piston stroke, the result of which is a deterioration in efficiency. It is therefore advantageous to seal the cavity of the hollow pistons. The usual procedure until now has been to close the hollow pistons with a cover connected to the main body of the piston by spin welding, electron-beam welding or laser welding. Hollow pistons of this kind are known, e.g. from DE-OS 23 64 725. Hollow pistons produced by means of electron-beam or laser welding are found in, e.g. DE 36 02 651 A1 and U.S. Pat. No. 3,319,575.
A feature common to these known hollow pistons is the necessity of prefabricating the main body and the cover in a complex machining process before the cover is welded to the main body. It is then necessary to rework the outer contour of the piston in a further machining step and make the central bore for supplying the pressure fluid to the pressure pockets of the sliding blocks connected to the ball heads. The overall result where the known hollow pistons are concerned is therefore a relatively complex and cost-intensive manufacturing process. A relatively large material inventory is also required.
The starting point of the invention is therefore a piston according to the preamble of claim
1
produced from a tubular blank and known from DE 34 06 782 C2. The process for producing these pistons is relatively inexpensive, as, instead of machining steps being required, the piston is obtained particularly easily by rolling from a tubular blank. However it has not until now been possible to produce hollow pistons with a cavity which is sealed towards the working cylinder of the hydrostatic machine by means of this known process.
It remains to be pointed out that it is known from DE 37 32 20 648 C2 to insert a tubular sleeve in a hollow piston produced by means of a machining manufacturing process and open towards the working cylinder of the axial piston machine and to fill the cavity between the sleeve and the hollow piston by means of a light material. However the inner tube does not in this case extend into the region of the ball head, and the ball head is formed by a solid body produced in a complex machining process rather than by a shaped outer tube.
The object of the invention is to provide a hollow piston for a hydrostatic machine having at least one sealed cavity which is of a particularly simple type, and to indicate a process for producing a hollow piston of this kind which can be carried out with cost-saving process steps and a low material requirement.
The object is achieved with respect to the piston according to the invention by an inner tube which extends inside the outer tube over the entire length of the latter, and which is fixed with a sealing effect to the outer tube at the common end on the skit part side, and wherein the outside diameter of the inner tube and the inside diameter of the outer tube are dimensioned so as to form a cavity between the inner tube and the outer tube, at least in the region of the skirt part. These characterizing features are considered in conjunction with the features constituting the type and with regard to the production process by introducing a blank for the inner tube into a blank for the outer tube, and shaping the blank of the outer tube to form the skirt part, the ball joint part and the neck part, so that the outer tube lies against the inner tube at the common end on the ball joint part side and the common end on the skirt part side.
The invention is based on the recognition that the known shaping process for forming a piston from a tubular blank for producing a hollow piston can be developed by using a second tubular blank which is inserted before shaping as an inner tube in the first blank functioning as an outer tube. The outer tube is then shaped until it lies against the inner tube, at least in the end regions. Further shaping produces in the inner tube a trough or a shoulder which axially fixes the inner tube to the outer tube. The result is a piston with a cavity which is formed between the inner tube and the outer tube and sealed towards the outside. The piston can be produced in a rotary swaging process without any machining manufacturing step. This production process is particularly cost-saving, especially in the series manufacture of large batches. A further result - in contrast with a machining manufacturing process - is the particularly good utilisation of material, which is especially important in particular when using relatively expensive alloys. The use of the tubular inner blank obviates the necessity of providing a central bore to feed the pressure fluid to the sliding blocks. Another advantage lies in the possibility of using a relatively inexpensive material for the inner tube, as it is subjected to smaller loads than the outer tube. This enables further manufacturing costs to be saved.
A further cavity may also be formed to particular advantage between the inner tube and the outer tube in the region of the ball joint part. The weight of the piston according to the invention is further reduced by this measure. The outer tube can lie flush against the inner tube in the region of the neck part connecting the ball joint part to the skirt part. The inner tube then defines the shaped diameter of the neck part when the outer tube is shaped.
The inner tube may comprise at the end on the ball head side and/or at the end on the skirt side, but also in the region of the neck part, a trough and/or a shoulder which is/are formed when the outer tube and the inner tube are shaped, so that the outer tube lies closely against the inner tube in this region. The inner tube is thus axially fixed in the outer tube.
The opening cross section of the inner tube may be constricted in the region of the trough and/or the shoulder to an extent such that a flow throttle is produced. The opening cross section of the flow throttle can be set by the shaping of the inner tube.
The outer tube may be shaped such that it also lies against the inner tube in the region of the neck part. This further improves the strength and flexural rigidity of the piston according to the invention.
The inner tube is shaped, this may advantageously be shaped to an extent such that a flow throttle with a predeterminable opening cross section is formed in the region of the trough and/or shoulder used for axial fixing purposes. The opening cross section of the flow throttle can be determined by introducing a fixing body into the inner tube before the latter is shaped, this body being removed again after the inner tube has been shaped.
REFERENCES:
patent: 3068563 (1962-12-01), Reverman
patent: 3319575 (1967-05-01), Havens
patent: 5076148 (1991-12-01), Adler
patent: 5265331 (1993-11-01), Engel et al.
patent: 1 593 775 (1970-06-01), None
patent: 23 64 725 (1975-07-01), None
patent: 26 53 867 (1978-06-01), None
patent: 3204264 A1 (1983-08-01), None
patent: 3406782 C2 (1984-08-01), None
patent: 3609892 A1 (1986-10-01), None
patent: 3602651 A1 (1987-07-01), None
patent: 3804424 C1 (198
Brueninghaus Hydromatik GmbH
Lazo Thomas E.
Look Edward K.
Scully Scott Murphy & Presser
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