In prior-art bearing applications used between two surfaces moving relative to each other, primarily mechanical bearing solutions are used, but even hydrostatic and so-called air bearing constructions have been produced. The present in- vention deals with bearing applications mainly relating to linear motors and equivalent. One of the drawbacks of me- chanical solutions is the stringent requirement regarding the smoothness of the bearing surface, because in linear motors the air gap between stator and rotor is relatively small. To meet this smoothness requirement, it is generally necessary to use sturdy structures, which accordingly are heavy and expensive. In addition to the above-mentioned disadvantages, mechanical bearings are noisy. A mechanical solution applied to an elevator drive is presented in US patent number 5,086,881.

The use of hydrostatic bearings requires, in addition to a smooth bearing surface, a closed circulation circuit for the hydraulic liquid. Therefore, it is not reasonable to build very large machines with hydrostatic bearings, and consequently this type of solutions are often used in rela- tively small machines, e. g. machine tools.

The use of traditional air bearings, i. e. aerostatic bear- ings, requires really rigid structures of precise dimen- sions because of the very small bearing clearance. Air bearings, too, are generally intended for small structures, such as machine tools or equivalent. US patent no.

4,798,985 discloses a bearing solution for a machine tool wherein a linearly movable part is provided with both tra- ditional bearings and air bearings. The function of the air bearings is to reduce the load borne by the traditional bearings. The problem with this solution is that it is not usable in applications of a size beyond that of machine tools, due to the high requirements regarding surface pre- cision and to its complicated construction.

US patent no. 5,128,569 discloses a linear motor solution with air bearings for use in conveyors. In this solution, a load pallet mainly movable in a horizontal plane rests on movable supporting elements on concave conveyor tracks. The movable supporting element is constructed to form the sec- ondary circuit of the linear motor, and the primary wind- ings of the linear motor are disposed along the conveyor tracks at regular distances, functioning as stators in this solution. The concave upper surface of the conveyor track is provided with a number of small holes or orifices through which compressed air is blown when the movable supporting element is near said holes, so that the support- ing elements float on an air cushion of a thickness of a few millimeters, and the linear motor construction moves the load. A drawback with this solution is that it is mainly applicable for horizontal operation only. A further problem is that, because of the high requirements regarding the precision of planarity of the small clearance, the construction is implemented using a larger clearance than normally, involving an increased energy consumption. Moreo- ver, in applications of a very large length, additional costs and complexity arise from the supply of air and its sequential application to the appropriate point.

US patent no. 5,668,421 discloses various linear motor drives based on a linear motor with air bearings. High pressure air is supplied into the space between a movable

part and a guide track, either from the movable part or from the guide track. In the former case, the movable part is provided with a compressor for producing compressed air, and the compressed air is passed through orifices into the space between the movable part and the guide track func- tioning as the stator. In this solution, too, the track must have a very smooth surface, especially in applications intended for conveyance of people, in order to achieve traveling properties of acceptable level. This makes the solution very expensive.

The object of the present invention is to eliminate the above-mentioned disadvantages and to achieve a reliable and workable method and system for forming the air gap of an air bearing. The method of the invention is characterized by what is presented in the characterization part of claim 1 and the apparatus of the invention is characterized by what is presented in the characterization part of claim 5.

Other embodiments of the invention are characterized by what is presented in the other claims.

The solution of the invention provides the advantage that, because one of the bearing surfaces of the air bearing consists of a flexible sheet associated with an elastic medium, the rigid bearing surface of the stator is not required to be as precisely planar as normally e. g. in machine tools. The flexible sheet bends and is adapted under the bearing load to the shape of the bearing surface, which may have flaws of shape and deformations caused by temperature variation that are several times larger that the air gap of the bearing, which is of the order of 1/100 mm. As the bearing surface of the stator need not be as precisely shaped as in prior-art air bearing solutions, the solution of the invention can be used in many applications in which prior-art air bearing solutions are not applica- ble. Thus, it is possible to build an air bearing which has

an air gap of 1/100 mm and a length of several meters but does not require more stringent planarity tolerances than e. g. elevator guide rail profiles.

Therefore, a linear motor with aerostatic bearings accord- ing to the invention is well suited for use e. g. as the hoisting machine of an elevator, in which case the stator, which has a length approximately corresponding to the hoisting height, needs no separate grinding to achieve an extreme surface smoothness; instead, a solution with normal surface treatment will be both economical and technically sufficient.

A further advantage in an elevator application is that it is thus possible to build advantageous elevators without machine room that need no pulleys or hoisting ropes and associated fittings in the elevator shaft. As compared with previously known elevator applications employing a linear motor, the invention provides a simple, noiseless and eco- nomical solution which, containing a small number of parts subject to wear, does not require much maintenance.

In the following, the invention will be described by the aid of examples of embodiments with reference to the at- tached drawings, wherein Fig. l presents a solution according to the invention in a partial and simplified side view as a section taken along line I-I in Fig. 4, Fig. 2 presents a solution according to the invention in a partial and simplified side view as a section taken along line II-II in Fig. 4, Fig. 3 presents a detail of the solution of the inven- tion in simplified form, as a section along line III-III in Fig. 4, the bearing clearance and deformations of the stator bearing surface being shown in exaggerated magnification, and

Fig. 4 presents a detail of the bearing solution of the invention in a partial and simplified top view not showing the movable part of the linear motor.

The solution of the invention can be used in various bear- ing applications involving two surfaces which move in rela- tion to each other in close proximity to each other. In the following, a bearing application in a linear motor designed especially for an elevator drive will be described, though in a simplified form.

The apparatus comprises a fixed stator part 2 with a bear- ing surface 4 against a movable part 1. The surface preci- sion of the bearing surface has been chosen to suit the purpose so that the surface may have deformations larger than the clearance of the air bearing of the linear motor, said clearance being here denoted as an air gap 9 between a bearing surface 4 and the movable part 1. In addition, between the movable part and the stator part there is the air gap of the motor, which has been adjusted to be larger than the air gap 9 of the air bearing. The other bearing surface of the air bearing is a flexible sheet 3 enveloping the movable part 1, which moves substantially in the same plane, said sheet forming a closed frame of rectangular form as seen from above, which is hollow inside and has sides and end faces of a width equal to the width of the sheet 3. The flexible sheet 3 is provided with thin through holes 6 placed at regular distances and functioning as orifices for the supply of compressed air. The holes are located e. g. along the center line of the flexible sheet 3 as seen from above, but, depending on the structure, they may also be placed elsewhere than on the center line. The flexible sheet 3 is made of non-ferromagnetic material, and preferably of a material having a low electrical conductiv- ity in order to eliminate the adverse effects of eddy cur-

rents. A sheet material applicable for this purpose is a composite material.

Fitted in the back surface of the flexible sheet 3 as seen from the bearing surface of the air bearing is a channel 7 for compressed air which runs continuously along the center line of the sheet throughout its length, covering all holes 6. The compressed air channel may be e. g. a trough-like metal plate attached to the back surface of the flexible sheet with its convex side upward. The holes 6 form an outlet through which the compressed air leaves the com- pressed air channel 7. In addition, the flexible sheet is connected by its back surface to the body of the movable part 1 by a plurality of elastic cushions 5 filled with a liquid. To enable the cushions 5 to be filled, each cushion is provided with a separate filling valve 12, through which each cushion can be filled separately with the cushions fastened to the movable part 1. The filling valves 12 are placed e. g. in holes going through the movable part 1. The elastic cushions act as an elastic transmission element according to the invention. The material chosen for the wall of the cushions is such that it withstands repeated motion, is inextensible and very flexible and thus presents no resistance to the motion of the liquid inside the cush- ions. The front surface of the cushions is fastened to the back surface of the flexible sheet, and the back surface of the cushions is correspondingly fastened to the body of the movable part 1. The fastening is implemented either by gluing, vulcanizing or by mechanical means, depending on the construction. The cushions 5 are disposed in succession with a small gap 11 between them, and in the lateral direc- tion along the center line of the flexible sheet 3 so that the flexible sheet assumes a position parallel to the bear- ing surface 4 in the widthwise direction, thus forming an air gap 9 as uniform in thickness as possible across the entire width of the flexible sheet.

The function of the cushions 5 is to transmit the load from the body of the movable part 1 to the flexible sheet 3 so that the flexible sheet 3 can adapt to the shape of the bearing surface 4 of the stator part 2. The cushion 5 is filled with a medium suitably selected so that its volume will practically not change, but when the cushion yields at some point so that its thickness is reduced, its thickness will be increased correspondingly at some other point. In other words, the mean thickness of the cushion remains the same throughout the length of the cushion. In addition, the changes in thickness must take place very quickly. The composition of the medium inside the cushion is important for the operation of the solution of the invention. To enable the magnitude of the air gap of the air bearing to be sufficiently well controlled and kept sufficiently small, e. g. at about 1/100 mm, the medium has to be icom- pressible, i. e. in practice a liquid. In addition, by ap- propriate choice of viscosity of the medium, it is possible to suppress vibrations between the parts. Moreover, the relative widths of the cushion 5 and the flexible sheet 3 are optimally so chosen that the flexible sheet 3 will not bend into a trough-like shape in the transverse direction although it is so thin that it will bend with changes in the shape of the bearing surface 4. Fig. 3 visualizes the interaction between the cushion 5 and the flexible sheet 3.

In this figure, the dimensions have been changed and the deformations of the bearing surface 4 are considerably exaggerated in order to show as clearly as possible how the cushion 5 forces the flexible sheet 3 to adapt to the pro- file of the bearing surface 4 in the longitudinal direction of the sheet.

If desirable, the gaps 11 between the cushions 5 can be sealed off using e. g. sealing elements 8 placed at the outer edges of the cushions, thus producing inside the

frame formed by the flexible sheet acting as a bearing an overpressure that bears part of the load. Placed at appro- priate points between the cushions 5 are compressed air supply channels 10 for supplying compressed air via the compressed air channel 7 of the air bearing and further through the compressed air orifices 6 while the movable part 1 of the linear motor is moving along the bearing surface 4 of the stator 2. The supply channels 10 are placed in holes going through the movable part 1 and imple- mented in an elastic manner using e. g. a bellows or equiva- lent so that they will not form an obstruction to the flexible sheet 3 as it is bending to the shape of the bear- ing surface 4.

The apparatus of the invention is installed and brought into working order e. g. by placing between the movable part 1 and the stator 2 a filler plate having a thickness equal to the desired air gap of the linear motor and by pressing the movable part into contact with the stator. After this, the cushions 5 are filled by supplying a medium selected for this purpose via the filling valves 12 placed in the holes going through the movable part 1 until a selected pre-charge pressure is reached so that finally each cushion is a separate closed unit. Now, when the filler plate is removed from between the movable part 1 and the stator 2, the air gap of both the air bearing and of the linear motor will have the right magnitude when the linear motor is moving.

It is obvious to the person skilled in the art that the invention is not limited to the example described above, but that it may be varied within the scope of the claims presented below. Thus, for example, the construction of the apparatus may differ from that described above in that, instead of a flexible sheet 3, small separate rigid air bearings placed side by side and loaded by a common liquid

cushion are used. Such a separate bearing element should be so small that the planarity tolerance of the bearing sur- face in the area corresponding to its size is clearly smaller than the air gap of the air bearing. Likewise, instead of a liquid-filled cushion 5, it is possible to use a gas-filled cushion, which may also form the compressed air channel of the air bearing. However, a gas-filled cush- ion will not readily give the structure the same rigidity of suspension as a cushion filled with a substantially incompressible liquid does.

In addition, the air bearing of the invention can be used in many other applications besides an elevator drive system provided with a linear motor as mentioned in the descrip- tion of the present invention. The bearing of the invention can be generally used to form axial and radial bearings in all rotating motors or machines. It is also applicable for use in the guideways of machine tools and in various appli- cations for the conveyance of goods or people, in which the load is lightened or kept clear of its bearing surface by providing air bearings between the load and the guideway.