HYDRAULIC PISTON MACHINE

Abstract: A hydraulic piston machine is disclosed, having arranged in a basic body (1) at least one cylinder (2) in which a bushing made of a plastics material is arranged. Securing the bushing in the basic body occasionally presents problems which render manufacture of the machine more difficult. The present invention is intended to simplify the manufacture of the piston machine. To that end, the external diameter of the bushing (3) and the internal diameter of the cylinder (2) are matched to one another to provide freedom from play and the plastics material has a substantially smaller modulus of elasticity than the basic body (1).

Description: Hydraulic piston machine The invention relates to a hydraulic piston machine having arranged in a basic body at least one cylinder in which a bushing made of a plastics material is arranged. Efforts have been made for some time to replace the synthetic hydraulic oils used as hydraulic fluid in hydraulic piston machine by less toxic fluids. Particular advantages are expected from the use of water as a hydraulic fluid. Unlike oils, however, water has practically no lubricating properties. To reduce the friction between a piston, which is moved back and forth in the bushing, and the bushing, bushings of plastics material have therefore been used; the plastics material has a friction-reducing property. Despite that, friction forces occur between the bushing and the piston during operation which under adverse conditions expel the bushing from the basic body. It is therefore necessary to secure the bushing in the basic body. Securing the bushing is not quite without problems, however. Soldering or adhesively securing the bushing presupposes a suitable material pairing of • bushing and basic body, which is not always the case. From DE 43 01 126 Al it is known to make a groove on the outer wall of the bushing which is brought into register with a corresponding groove on the inner wall of the cylinder. A locking element then engages positively in the groove space formed by the two grooves. DE 43 01 124 Al discloses another method, in which the cylinder bushing is heated at one end and then deformed. The cylinder has a corresponding deformation so that the bushing sits in the basic body like a rivet. Both methods enable the bushing to be held securely in the basic body, but they require a relatively high degree of effort in the manufacturing process. The invention is based on the problem of simplifying manufacture of the hydraulic piston machine. In a hydraulic piston machine of the kind mentioned at the outset, this problem is solved in that the external diameter of the bushing and the internal diameter of the cylinder are matched to one another to provide freedom from play and the plastics material has a substantially smaller modulus of elasticity than the basic body. In this construction, the bushing is inserted essentially only in the basic body, or more accurately, in the cylinder, or, if the bushing is very slightly oversize, it is pressed in. Since the cylinder and the bushing are matched to one another to provide freedom from play, the insertion or pressing of the bushing into the cylinder requires a certain force, because there is a certain friction between bushing and cylinder. At least the same force is also required, however, to expel the bushing from the cylinder during operation. It has been observed that the force that attempts to expel the bushing from the cylinder is dependent on the pressure inside the bushing, in other words, the pressure of the hydraulic fluid. At relatively low pressures, the friction forces between - 3 - piston and bushing are relatively low. They are generally not sufficient to expel the bushing from the cylinder. At higher pressures, the friction forces between piston and bushing increase. Because of the relatively small modulus of elasticity of the bushing, at these higher pressures inside the bushing an outward expansion of the bushing occurs, which in turn increases quite considerably the friction forces between bushing and cylinder. This expansion presents no problems because the bushing and the cylinder are matched to one another so that there is no play between them. No larger gaps appear between the bushing and the piston which could lead to significant leakage of the machine. Basically, the bushing is, merely by the pressures inside it, pressed by virtue of its smaller modulus of elasticity against the inner wall of the cylinder. That force is sufficient to increase the friction between bushing and cylinder to such an extent that the risk of expelling the bushing from the cylinder is virtually excluded. The retention forces which hold the bushing in the cylinder adapt to the forces which are trying to expel the bushing from the cylinder without any additional measures. The bushing is therefore only acted on by force when this is necessary. This protects the bushing and increases its service life. 'The bushing is preferably connected to the basic body by a press fit. A relatively large retention force between the basic body and the bushing is already achieved in this case as the bushing is being inserted, and in operation this retention force can be further increased by the hydraulic pressures occurring inside the bushing. Operational reliability of the machine is therefore further increased. The modulus of elasticity of the plastics material is preferably smaller by the factor ten than that of stainless steel. With such a small modulus of elasticity even relatively small forces are sufficient to press the bushing with the necessary force against the inner wall of the cylinder. The bushing presents no great resistance to this force. The bushing advantageously has a wall thickness that is less than 1.5 mm. The bushing is therefore chosen to be relatively thin. This also contributes to the fact that the pressures inside the bushing, that is, in the working chamber defined by the bushing and piston, can be used more or less completely to press the bushing against the inner wall of the cylinder. The bushing preferably has at one end face a circumferential projection, the front end face of which runs perpendicular to the axis of the bushing. This projection serves essentially for pressing the bushing into the basic body. Even in the case of thin-walled bushings, the necessary area for force to act upon is then available. Because the front end face runs perpendicular to the axis of the bushing, the pressing- in force can be applied by way of this end face without risk that the diameter of the bushing will change or that the bushing will be deformed in an undesirable -manner. The invention is described hereinafter with reference to a preferred embodiment in conjunction with the drawing, in which the single Figure shows a cross- section through a basic body with bushing. Figure 1 shows merely a fragmentary view of a basic body 1 of a hydraulic machine. In a hydraulic axial piston machine, the basic body 1 can be formed, for example, by the cylinder drum. The basic body 1 has at least one cylinder 2 which can be formed, for example, by a continuous bore. The cylinder 2 could, of course, also be produced in a different way as the basic body 1 is being made. In the cylinder 2 there is a bushing 3 in which a piston 4 is movable back and forth. The bushing 3 is formed from a plastics material which co-operates with the material of the piston 4 to produce little friction. Materials from the group of high-strength thermoplastic plastics materials based on polyarylether ketones, in particular polyether ether ketones, polyamides, polyacetals, polyaryl ethers, polyethylene terephthalates, polyphenylene sulphides, polysulphones, polyether sulphones, polyether imides, polyamideimide, polyacrylates, phenol resins, such as novolak resins, or similar substances can be used as the plastics material for the bushing; glass, graphite, polytetrafluoroethylene or carbon, especially in fibre form, can be used as fillers. It should be taken into account here as a further criterion for the selection of the plastics material that it has a relatively small modulus of elasticity. The modulus of elasticity should be smaller than that of stainless steel by about the factor 10. At any rate, it must be substantially smaller than the modulus of elasticity of the basic 'body 1. The wall thickness of the bushing 3 is relatively small. It is less than 1.5 mm. In the present embodiment it has a thickness of about 0.7 mm. At its leading end the bushing has a circumferential projection 5, the front end face 6 of which runs perpendicular to the axis of the bushing 3. The axis corresponds to the direction in which the piston 4 moves back and forth. The external diameter of the bushing 3 and the internal diameter of the cylinder 2 are matched to one another in such a manner that the two parts fit together free from play. An even better retention force is achieved if the bushing 3 and the basic body 1 are connected to one another with a press fit. In that case the external diameter of the bushing 3 is slightly (1/100 - 1/10) larger than the internal diameter of the cylinder 2. In each case the end face 6 of the projection 5 serves as a means whereby even a relatively thin bushing 3 can be pressed into the cylinder 2. In operation, friction forces that arise between the piston 4 and the bushing 3 attempt to push or pull the bushing 3 out of the basic body 1. It has been observed, however, that these friction forces are dependent on the pressure inside the bushing, that is, in a work chamber 8 defined by the bushing 3 and the piston 4 (and possibly an end wall) . At low pressures (the pressures are here intended to be symbolised by the crossed arrows 7) , the friction forces between piston 4 and bushing 3 are relatively small. At greater pressures they increase. Because of the small modulus of elasticity, however, the higher pressures 7 -lead to pressing of the bushing 3 against the basic body 1, or more accurately against the inner wall of the cylinder 2. The friction forces between bushing 3 and basic body 1 are thereby increased commensurate with the friction forces which are attempting to expel the bushing 3 from the basic body 1. This behaviour of the bushing is further promoted by the relatively thin wall thickness. The pressure forces are therefore able to press the bushing against the basic body more or less unhindered. This provides an adequately strong joint. The joint also has the necessary tight seal.

Claims: Patent Claims 1. Hydraulic piston machine having arranged in a basic body at least one cylinder in which a bushing made of a plastics material is arranged, characterized in that the external diameter of the bushing (3) and the internal diameter of the cylinder (2) are matched to one another to provide freedom from play and the plastics material has a substantially smaller modulus of elasticity than the basic body (1) . 2. Machine according to claim 1, characterized in that the bushing (3) is connected to the basic body (1) by a press fit. 3. Machine according to claim 1 or 2, characterized in that the modulus of elasticity of the plastics material is smaller than that of stainless steel approximately by the factor ten. 4. Machine according to one of claims 1 to 3, characterized in that the bushing (3) has a wall thickness that is less than 1.5 mm. 5. Machine according to one of claims 1 to 4, characterized in that the bushing (3) has at one end ' face a circumferential projection (5) , the front end face (6) of which runs perpendicular to the axis of the bushing.

Inventors: HANSEN OVE THORBOEL, MARTENSEN LARS, HANSEN, OVE, THORBOEL, MARTENSEN, LARS
Applicants: DANFOSS AS, HANSEN OVE THORBOEL, MARTENSEN LARS, DANFOSS A/S, HANSEN, OVE, THORBOEL, MARTENSEN, LARS
Application year: 1996
Main classification: F04B1
IPC codes: F04B53