The invention relates to a gear for a hydraulic piston machine, having at least one gear pairing comprising two gearwheels.

Gears are generally used to convert the output speed of a hydraulic machine so that a specific speed or a specific speed range iε achieved. In this connection it is desirable for the gear to have as long a service life as possible and to cause as few losses as possible. For that reason, gears are normally lubricated, grease or oil frequently being used for that purpose.

With increasing environmental awareness, attempts are being made to replace the mostly toxic hydraulic oils in hydraulic machines by other fluids. Particular advantages are expected of the use of water as a hydraulic fluid.

The hydraulic oils used in the past have the advantage, however, that they serve not only as the hydraulic medium but at the same time have lubricating properties which reduce friction between moving parts of the hydraulic machine, and are compatible with the lubricant used for the gear.

The combination of a gear lubricated with grease or oil with a hydraulic machine operated with water or a similar fluid is not without problems, however. It is virtually impossible to achieve a complete seal. Even slight additions of water to the oil in the gear would

be sufficient to reduce the lubricating properties of the lubricating oil to such an extent that there would be a risk of permanent damage.

US 5 038 523 discloses a motor-driven tool which for underwater use can be operated with sea-water as the hydraulic fluid. In that arrangement an angular gear is provided, which comprises two bevel gears made of a plastics material, namely, Delrin. But because of the limited strength of the plastics material, the torques that can be transmitted with such a gear are restricted. In many cases these transmission properties are inadequate.

The invention is based on the problem of constructing a gear so that it is also able to transmit relatively high forces from a hydraulic machine that is operated with a fluid that has no or only slight lubricating properties, in particular from a machine that is operated with water as the hydraulic fluid.

That problem is solved in a gear of the kind mentioned at the outset in that the first of the two gearwheels is made of a metal and the second of the two gearwheels consists, at least at its εurface, of a friction- reducing plastics material.

Combining the metal with the plastics materials results, on the one hand, in low-friction co-operation of these two parts so that lubrication or cooling to the extent previously required is not necessary. On the contrary, lubrication or cooling is taken over by the plastics material in co-operation with the fluid. Moreover, combining the metal with the plastics material also has the advantage that improved strength of the gearwheel pairing is achieved. One of the two

gearwheelε, to be preciεe, the metal one, has the same strength as before, that is, as in a conventional gear comprising gearwheels that all consist of metal. The second gearwheel having the plastics material has a somewhat lower strength, but in co-operation with the metal gearwheel it fulfils its function satisfactorily.

This is especially so when the second gearwheel advantageously consists entirely of the plastics material. This facilitates manufacture. The risk that a plastics material will become detached from a core, for example, a metal core, no longer exists.

It is alεo preferred for the plaεtics material to be fibre-reinforced, in particular with glasε fibreε and/or carbon fibres and/or organic fibres. Reinforcement with glasε fibreε or carbon fibreε increaεeε the strength of the plaεticε material quite conεiderably, εo that partε that can be loaded to the deεired extent are obtained.

Advantageously, the plastics material has a tensile strength of more than 200 N/mm ² . There is a series of plastics materials having friction-reducing properties which have this tensile strength. Such a tensile strength haε proved satisfactory for very many application purposeε.

Advantageously, the plastics is selected 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 sulphideε, polyεulphones, polyether sulphones, polyether imides, polyamideimide, polyacrylates, phenol resins, such aε novolak resins, or similar substances; glasε, graphite,

polytetrafluoroethylene or carbon, especially in fibre form, can be used aε fillers. Water can be used as the hydraulic fluid when such materials are used. Polyetherether ketones (PEEK) have proved especially suitable, because this plastics material has only a very slight absorptive capacity for water.

The first gearwheel preferably has a smaller diameter than the second gearwheel. In other words, of the two gearwheels of a gearwheel pairing that have different diameters, the εmaller one iε made of metal. The smaller gearwheel generally has to absorb the larger forces or moments. But since the metal gearwheel will generally have a greater strength than the plastics gearwheel this property is meaningfully exploited in thiε combination.

It is also preferred for the first gearwheel to be held by at least three second gearwheels distributed circumferentially. The firεt gearwheel therefore has a rotary bearing which manages without a bearing axle or a journal. No friction is generated in that case between the firεt gearwheel and its journal. Although friction forces do occur between the individual gearwheelε, in particular at the tooth flankε, thiε iε acceptable because, firstly, in these gearwheels the co-operating material pairing is always a metal/plastics material pairing. Moreover, each tooth is only briefly affected by friction. When this tooth has passed through its contact point with the other tooth, it is free again and is able to cool down. The "journal-free" mounting of the first gearwheel therefore contributes to the reduction in friction losses.

Preferably, the εecond gearwheel iε rotatably mounted on a journal, a groove being formed between the gearwheel and the journal. Even if the journal conεiεts of metal, the material pairing in this case iε again metal/plaεticε. Theεe two aterialε work together with little friction. Nevertheleεε, friction loεses cannot be completely avoided here. The groove between the gearwheel and the journal therefore serveε to allow fluid to penetrate into the region between gearwheel and journal. Thiε fluid can then eaεily diεεipate the heat caused by friction. The friction then has no adverse effects on the service life of the gear. The groove, which can also be referred to as a channel, can be provided on the circumferential surface of the journal or alternatively in the inner εurface of the gearwheel that surrounds the journal. Of course, it would also be posεible to provide εuch a groove in both εurfaces.

It is here especially preferred for the groove to run helically. As the gearwheel rotates on the journal, a kind of pumping effect then occurε, by which the fluid iε conveyed into the region between the gearwheel and the journal.

It iε especially advantageous for the second gearwheel to have at leaεt one channel running circumferentially. The channel interrupts the teeth for a part of their axial length, but this has the advantage that fluid iε able to enter the gaps between the teeth more eaεily, to be precise, into the region in which the second gearwheel meεheε with the first gearwheel. Any heat occurring can therefore be disεipated immediately and directly.

It iε especially preferred for the channel to have a depth corresponding to the depth of the teeth. Even in the regions in which the two gearwheels engage with one another, a permanent exchange of fluid that is not hampered by the engagement is then posεible.

The second gearwheel is in that caεe preferably formed from two gearwheel parts which are εeparated from one another by a spacer washer. The two gearwheel parts and the spacer washer then lie axially on top of each other. Such a procedure facilitates manufacture.

It is also advantageous if the internal diameter of the second gearwheel is enlarged at the end face in the region of the journal. Such an enlargement facilitates pressure equalization across the gearwheel. Occasionally, differences in pressure are unavoidable. The risk that these differenceε in preεsure will lead to uneven loading and thus to an increaεe in the friction of the εecond gearwheel on itε journal thus becomes less.

Preferably, the gearwheels are arranged in a houεing which iε filled with water. Water may have no or only slight lubricating propertieε, but it doeε serve to disεipate heat that occurs from the places affected by friction. With appropriate dimensions, this water cooling is εufficient to ensure a satiεfactory operation combined with an acceptable εervice life. The water is alεo able to flow through the gear. The gearwheels can be in direct contact with the water.

Preferably, the gear is connected to a connection for leakage fluid of the hydraulic machine. With moεt machineε it iε virtually impoεεible to prevent the eεcape of a certain amount of leakage fluid. Thiε

leakage fluid iε now advantageously used to supply the gear and thus to cool it. In the hydraulic machine the water εerveε aε hydraulic fluid.

In an eεpecially preferred practical form, the first gearwheel iε provided on one end face with a layer of friction-reducing plaεticε material. Thiε layer does not change the strength of the first gearwheel. It does, however, allow the first gearwheel to be εupported axially to a certain extent.

Advantageouεly, the gear iε in the form of a planetary gear, the firεt gearwheel forming the εun wheel and at leaεt three εecond gearwheelε being arranged in a planet wheel carrier. Using such a planetary gear, favourable transmission ratios can be achieved with a small overall size. The second gearwheels in the planet wheel carrier are then virtually the only gearwheelε that have to be journalled in order to be able to rotate.

That iε the caεe in particular when the firεt gearwheel is non-rotatably connected to the shaft of the hydraulic machine and the planet wheel carrier is non- rotatably connected to a driven shaft of the gear. In that case, the firεt gearwheel iε held by the εecond gearwheelε in the planet wheel carrier. The only frictional engagement to take place is then between the first and the second gearwheels through the metal/plastics material pairing. The second gearwheels can rotate freely on the planet wheel carrier because they are mounted therein likewise with a plastics material/metal combination. The second gearwheels in the planet wheel carrier in turn co-operate with an internally toothed ring. This toothed ring can also be made of metal. For example, the first gearwheel can

consist of steel, especially stainless steel. The toothed ring can consist of the same or a different metal, for example, it can alεo consist of aluminium.

The invention is explained hereinafter with reference to a preferred embodiment in combination with the drawingε, in which:

Fig. 1 is a plan view of an opened gear and Fig. 2 is a longitudinal section through a gear assembled with a radial piston motor.

Fig. 1 showε a plan view onto an open gear 1, which iε illuεtrated in Fig. 2 coupled up to a motor 2. The motor iε here in the form of a radial piston motor known per se. Explanation thereof is therefore unnecessary. The view according to Fig. 1 is obtained when the motor 2 is removed from the gear 1 and the observer looks from the right (in Fig. 2) at the gear. Instead, of course, an axial piεton motor can be used.

The gear 1 has a housing 3 in which a toothed ring 4 having an internal toothing 5 is non-rotatably mounted.

In the centre of the housing 3 there is arranged a gearwheel 6, which shall be referred to hereinafter as the first gearwheel and haε the function of a sun wheel in the gear 1. The first gearwheel 6 has an external toothing 7 and an internal toothing 8, the latter meshing with a complementary external toothing of a driven shaft 9 of the motor 2. The firεt gearwheel 6 is therefore non-rotatably connected to the driven shaft 9. Certain axial displacementε are allowed, however.

Between the firεt gearwheel 6 and the toothed ring 4

there are arranged three gearwheelε 10, to be referred to hereinafter aε second gearwheels 10. The second gearwheels 10 are arranged on a common carrier 11 and have the function of planet wheels. The number of teeth or the diameter of the first and second gearwheels 6, 10 and of the toothed ring 4 determine the transmiεsion ratio of the gear.

For mounting the second gearwheels 10, the carrier 11 has a respective journal 12 for each second gearwheel 10, on which the εecond gearwheel 10 iε able to rotate.

The second gearwheel 10 is εecured on its journal 12 by mean of a spring ring 13. With its other side the gearwheel 10 lies against the carrier 11, if desired, through the intermediary of a bearing washer 14.

On the journal 12, or more accurately, in a circumferential surface, a helical circumferential groove or channel 15 is provided; thiε groove is so arranged that it covers the entire region between the second gearwheel 10 and its journal 12 both circumferentially and axially. Each εecond gearwheel 10 iε provided at itε end, but still in the region of the journal 12, with a circumferential recesε 19 which enlargeε the internal diameter here. Thiε receεε 19 facilitateε, firεtly, the entry of fluid into the circumferential helical groove 15. The receεε 19 alεo improves the opportunities for presεure equalization across the second gearwheel 12, so that there is less risk of skewing.

The inside of the housing 3 is filled with fluid, in particular with water, so that on rotation of the second gearwheel 10 on the journal 12 the fluid is conveyed through the groove 15 and iε able to diεεipate

any heat generated in the region between journal 12 and εecond gearwheel 10.

As is eεpecially clear from Fig, 2, the εecond gearwheel 10 consists of two gearwheel parts 10a, 10b which are separated from one another by a spacer washer 10b. A channel 16 is consequently formed circumferentially in the toothing of the second gearwheel 10; the channel 16 is at least as deep aε the teeth of the second gearwheel 10. Even when the second gearwheel 10 engages with the first gearwheel 6 or with the toothed ring 4, it is posεible for fluid to penetrate aε far aε the portion which is engaging at that moment in the respective other gearwheel.

The first gearwheel 6 consists of stainlesε steel. The second gearwheels 10 consist of a plastics material, especially of polyether ether ketone (PEEK) . For example, PEEK of the type 450CA30 can be used for that purpose, because this material has relatively high resistance to pressure and tensile strength. Its tensile strength iε more than 210 N/mm ² . A εecond gearwheel 10 made of this material can therefore absorb a load that is about 90% of that of a εteel gearwheel. Such a load capability iε εufficient, however. Aε is especially clear from Fig. 1, the first gearwheel 6 has a smaller diameter than the second gearwheelε 10. The largeεt forceε therefore occur here, but can be abεorbed without difficulty because the first gearwheel 6 consists of stainless steel. The load on the second gearwheels 10 iε diεtributed over three gearwheelε of larger diameter so that this loading is correspondingly smaller there.

The first gearwheel 6 iε held in the gear 1 excluεively by the three εecond gearwheelε 10. These gearwheels 10

centre the firεt gearwheel 6. The firεt gearwheel 6 is therefore mounted without a journal. Although a certain friction does also occur between the first gearwheel 6 and the second gearwheels 10, this friction is also relatively uncritical, because it occurs only briefly at individual teeth. The tooth in question then becomes free again and is able to cool down in the surrounding fluid until the next engagement.

The first gearwheel 6 iε provided only in the region of its end face remote from the motor 2 with a thin layer 17 of a friction-reducing plastics material, in particular PEEK. Relatively small axial forces are therefore absorbed. Friction between the εecond gearwheel 6 and the carrier 11 can be kept small.

The fluid supply inside the houεing 3 iε continuously supplemented by leakage fluid from the motor 2. The leakage fluid flows in that case along the driven shaft

9 between the driven shaft 9 and a seal 18. This joint can in any case be εealed only with great difficulty. The leakage fluid iε therefore used to cool the attached gear 1.

The carrier 11 is connected non-rotatably to a driven shaft 20 of the gear 1. If the driven εhaft 9 of the motor 2 iε now turned, the firεt gearwheel 6 is also caused to rotate. It mesheε with the εecond gearwheelε

10 which correspondingly in turn roll in the internal toothing 5 of the toothed ring 4. Since the toothed ring 4 in the housing 3 is stationary, this rolling movement inevitably leadε to a rotational movement of the carrier 11, which movement continueε onto the driven εhaft 20.

In this mode of operation, it is clear that, disregarding the bearing of the driven shaft 20, all moving parts alwayε lie adjacent to each other with a metal/plastics material pairing. Friction iε thus kept low. It iε alεo clear, however, that in each of these material pairingε meaεures are provided to dissipate immediately, using fluid, the heat that arises as a result of the inevitable friction. In this manner satisfactory operational behaviour can be ensured over a relatively long service life.