The invention relates to a pivot bearing arrangement, in particular for a multiscrew extruder, comprising a rotary shaft which is loadable in the axial direction; a bearing housing in which the shaft is supported radially; at least one pivot bearing fitted in the bearing housing and on the shaft for the purpose of taking an axial load on the shaft; and at least one cylinder/piston unit fitted between the shaft and the bearing housing and connected thereto, and comprising two parts which are movable relative to each other, the first part being connected to the bearing housing and the second part to the shaft, while each cylinder/piston unit is designed to exert such axial force on the shaft, by means of a medium under pressure fed to it, that the cylinder/piston unit takes a part of the axial load, and the pivot bearing takes the remaining axial load not taken by cylinder/piston units.

Such pivot bearing arrangements are known from Britsh patent specification 1 421 751 or German patent application 3 111 834, which show a mechanical bearing arrangement which is relieved by one or more hydraulic cylinder/piston units. Such bearing arrangements are advantageous if the axial load on the pivot bearing is high, while the dimensions of the bearing must remain limited. This is the case in particular in multi-screw extruders, in which the dimensions in the radial direction of the axial bearing arrangement of a shaft not driven directly are limited by the proximity of another shaft, while it must nevertheless be possible to absorb great axial forces which are produced by the kneading with the screw fitted on the shaft and the propelling of the material to be extruded. However, a combination of a mechanical pivot bearing ^' and cylinder/piston units requires a complicated control system for the pressure of the hydraulic fluid or requires a provision to compensate for the counterforce generated by

the bearing arrangements when the axial force on the shaft decreases or ceases. If no measures are taken, rapid wear of the bearing will occur, so that the service life of the bearing is limited and there is a risk of overheating and jamming.

A solution to this problem may be the use of a single hydrostatic, self-adjusting bearing, fed by a pump, as disclosed in German patent application 2 406 705. However, on account of the limited diameter of this bearing, the pressure in the bearing must be very high, which gives rise to problems as a result of the necessary non-standard high- pressure pump and accessories which are difficult to obtain and expensive, and tend to require maintenance. It is known to place several pivot bearing arrangements one after the other on a shaft, in order to reduce the axial load on one pivot bearing. However, very great accuracy of working and fitting of the parts of the bearings is required in order to prevent an uneven load on each individual bearing. If there is a slight deviation, which can occur, for example, through wear, one pivot bearing of the series can receive too great a load to handle and _^ become defective, or it can make too small a contribution to the taking of the axial load, as a result of which the other pivot bearings of the series become defective. Besides, the fitting of different pivot bearing arrangements is a disadvantage from the point of view of cost.

The object of the invention is to provide a pivot bearing arrangement which is reliable, uses standard components^ does not have to be worked and fitted particularly accurately, and can therefore be produced at low cost.

This object is achieved in a pivot bearing arrangement which is characterized in that the pivot bearing is a hydrostatic, self-adjusting bearing whose pressure side is connected to the pressure side of the cylinder/piston units.

The hydrostatic, self-adjusting pivot bearing in the pivot bearing arrangement according to the invention is exposed to a relatively low load during operation, due to the use of the cylinder/piston units. Therefore, a standard pump can be used for supplying hydraulic fluid to the bearing. The self-pressure-adjusting feature of the hydrostatic bearing is used to determine the pressure in the cylinder/piston units, so that with a particular dimensioning of the bearing parts the total axial load is divided with a fixed mutual ratio over the cyliner/piston units and the hydrostatic bearing.

A particularly effective design of the pivot bearing arrangement is obtained by placing the cylinder/piston units one after the other in the axial direction, the cylinders being connected to the bearing housing, and the pistons being connected to the shaft. Seals can be fitted between the bearing housing and the shaft. A very accurate finish or fitting of the parts of the cylinder/piston units is not necessary, which makes the bearing arrangement cheap.

The manufacturing costs of the bearing arrangement can be reduced even further when all cylinder/piston units are the same. Through the use of the same parts, in addition to the use of the same pressure medium and the same pressure in the pivot bearing and in all cylinder/piston units, the design of the bearing arrangement is simple, while only one pump is required.

After each element on the shaft (cylinder/piston unit or pivot bearing) which takes a partial axial load, the remaining axial load is lower, and with the mechanical stress in the shaft remaining constant a smaller diameter of shaft will suffice.

The working surface of each piston connected to the shaft increases with decreasing shaft diameter and thus with the radial dimensions of the bearing arrangement remaining constant a lower pressure of the medium will suffice.

In a preferred embodiment the pivot bearing is fitted at one end of the shaft and the cylinder/piston units directly in front of that.

The pivot bearing arrangement according to the invention is particularly intended for use in a multi-screw extruder, where it must be possible to take a great axial load with small radial dimensions of the bearing arrangement. The invention therefore also relates to an extruder which is provided with at least one pivot bearing arrangement according to the invention.

The invention is explained with reference to the drawing, which shows schematically in a partial lon¬ gitudinal section a preferred embodiment of the pivot bearing arrangement according to the invention. Mounted in a bearing housing 2 is a rotary shaft 6 which is loadable axially in the direction of arrow 4, and which is at one end hydrostatically axially supported in a known manner and is radially supported in a conventional manner not shown in any further detail. The hydrostatic bearing arrangement comprises a bearing disc 8, a bearing cup 10 which is fixed to the bearing housing 2 and has a hollow core, and a movable intermediate bearing disc 12 with a hollow core fitted between the bearing disc 8 and the bearing cup 10. The intermediate bearing disc has a partially convex side which fits into a partially cup- shaped side of the bearing cup 10 and is movable therein. The other side of the intermediate bearing disc 12 is essentially flat, as is the side of the bearing disc 8 facing it. Both of the above-mentioned flat sides have an axial collar at the edge, between which collars a gap 14 can be produced on movement of the shaft 6 opposite to the direction of the arrow 4.

Through the infeed of a medium under pressure, for example oil, through feed aperture 16 to the hydrostatic bearing, the bearing disc 8 is pressed, with a force which depends on ^' the pressure of the medium and the working surface of the bearing disc, away from the intermediate

bearing disc 12 lying against the bearing cup 10, as a result of which the gap 14 increases and the pressure medium fed in can flow away to a discharge aperture 18. The pressure in the hydrostatic bearing decreases again as a result. This force on the bearing disc 8 is offset by the axial force which is exerted on the bearing arrangement in the direction of the arrow 4. A forces equilibrium is produced in this case. This forces equilibrium is achieved at a specific gap width, depending on the pressure/volume characteristic of the pump 19 used for placing the pressure medium under pressure and the viscosity of the oil.

An actual gap width occurring in an extruder in operation is very small, namely the size of approximately 30 micrometres. The design of the bearing arrangement is such that this width can increase to a maximum of 40 micrometres when the extruder is out of operation.

A number of discs 20 are fixed on the shaft 6, which discs at the outside edge are virtually sealed against the inside wall of the bearing housing 2 by means of sealing means which are not shown in any further detail. Each disc 20 can move in the axial and in the tangential direction in a cylindrical space inside the bearing housing 2. Each cylindrical space is virtually sealed at the two axial ends against the shaft by means of sealing means not shown in any further detail. Feed apertures 22 for a pressure medium are provided in the bearing housing 2, which apertures open out at one side of each disc. The space at the other side of each disc is connected to a discharge aperture 24, so that any pressure medium leaked in this space can be discharged pressureless. Through this arrangement, each disc 20 can transmit a force to the shaft 6.

The discharge apertures 18 and 24 are in open communication by means of a line 25 to a pressureless tank 27 from which the pump 19 takes pressure medium. The force exerted on the shaft jointly by the hydrostatic bearing and the cylinder/piston units for a good bearing arrangement must be in equilibrium with the

axial force operating in the direction of arrow 4. In the embodiment shown in the drawing this is achieved in a simple manner by connecting all feed apertures 22 by means of a line 23 to the pressure medium feed aperture 16 for the hydrostatic bearing, as a result of which the gap 14 determines the pressure of the pressure medium in the whole bearing arrangement. Consequently, the axial load is absorbed in a design-determined ratio by the hydrostatic bearing and the cylinder/piston units interacting with it. It is, however, also possible to feed the cylin¬ der/piston units with a medium at a fixed or variable pressure, separated from the pressure medium feed of the hydrostatic bearing.

Placing a particular number of cylinder/piston units one after the other on the axially loaded shaft makes it possible with a limited diameter and a limited maximum pump pressure to construct a pivot bearing arrangement which can take a great axial load.

A pump with a maximum pressure of 22 MPa which is commercially available as a standard hydraulic pump can make a pivot bearing arrangement with three discs 20 take the same load as a pump with a maximum pressure of 60 MPa in the absence of the discs 20. The latter pump is not standard and is made only to order. The mutual positioning of the pivot bearing and the cylinder/piston units on the shaft is not of essential importance; it is also possible to place one of the cylinder/piston units at the end of the shaft, instead of the pivot bearing.