GB2108595A

GB2108595A - Gas bearings

Info

Publication number: GB2108595A
Application number: GB08224327A
Authority: GB
Inventors: Karl Lechner
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1981-11-03
Filing date: 1982-08-25
Publication date: 1983-05-18
Also published as: IT8249409A0; FR2515754A1; DE3143606A1; JPS58146718A

Abstract

A gas, more particularly an air, bearing, has at least one component which comprises a bearing member 2 which is permeable to gas, which is made of porous material and through which gas is directed into the interior of the bearing. The bearing member is preferably made of a porous, sintered ceramic material of moderate coefficient of thermal expansion, whereas the mating bearing member, is made of a highly compacted ceramic material or of metal. The bearing may support a rotating shaft 7, and radially, or it may support a reciprocating piston. <IMAGE>

Description

SPECIFICATION A gas bearing This invention relates to a gas bearing more particularly an air-operated bearing suitable for use at extremely high temperatures.

The use of air bearings at elevated temperatures and normally present temperature gradients entails changes in tolerances and the attendant notorious problems of seizing, rough running or appreciable reduction in bearing capacity, occur due to the high coefficients of thermal expansion of conventional materials. in order to reduce the thermal expansion or the changes in tolerance, the use of highly-compacted special-analysis ceramic materials is often recommended for the moving part to be borne (e.g. shaft) and the bearing part (e.g. bush) alike.

For conventional air bearings, very small air feed holes and air manifold slots or oucts are required to produce the weight-supporting air cushions in the interior of the bearing. The manufacture of dimensionally accurate holes in a ceramic material, and also of air distribution ducts or manifolds is extremely complex and expensive and by that token, hardly tenable economically.

One object of the present invention is to at least mitigate the disadvantages of conventional gas bearings, and to enable a gas bearing of relatively simple construction to withstand extreme operating conditions.

According to the present invention at least one component of the bearing comprises a permeable bearing member of a porous material, through which gas is directed into the interior of the bearing. The bearing member more particularly is made of a porous, sintered ceramic material of low coefficient of thermal expansion, whereas the counter-member of the bearing may be made of a highly-compacted ceramic material or of metal (or of some other durable material). The use of a porous, sintered ceramic material eliminates the need for producing very small air feed holes and air manifolds as used in the conventional bearings.

Instead, the incoming gas is direct uniformly into the interior of the bearing through the entire porous surface of the bearing, so that in service a non-contact bearing of a predictable clearance results. This not only facilitates the manufacture of the bearing, but it also improves the performance of the bearing: more particularly, the dynamic running properties are substantially improved, and the bearing capacity at low gas consumption is augmented. The bearing lends itself to use at both low and very high temperatures (in excess of 10000C) and at high relative speeds of the components in relative movement one with the other (high bearing speeds of shafts; fast stroke rate of pistons).

In an especially advantageous embodiment, a combined radial and axial bearing has at least one porous bearing bush of a ceramic material, this bush having a circumferentially extending flange to absorb the thrust.

The radial dimensions of oppositely arranged circumferential bearing bush flanges (axial faces of the porous bearing) are preferably made different and/or adjustable to produce a difference between them for the purpose of thrust compensation. An increase or decrease in the size of one of the supporting faces of the bearing bush accordingly serves to prevent unilateral thrust on the rotating part.

Thrust compensation can alternatively be achieved by differently energizing oppositely arranged bearing bush faces with gas. For the purpose, two separate pressure connections are provided.

The axial faces may be formed with radially extending slots for venting the gas cushion of the interior of the bearing, these being less expensive to produce than holes in e.g. a ceramic material.

As a gas, use is preferably made of air. The air is directed into the interior of the bearing at a preselected temperature. Depending on the specific application, use can be made or hot or cold air (gas).

The gas is advantageously fed from an external source of gas but in the case of a compressor bearing, it may be preferred to feed the gas from the compressor to the interior of the bearing, provided the piston-type compressor contains the porous bearing member.

A bearing according to the present invention is especially suitable for use as aerostatic or aerodynamic air bearings, wherein the relatively movable parts are made of a porous ceramic material on the one hand of a highly compacted ceramic material on the other. The air bearings lends itself to use at extremely high or extremely low temperatures under constant or transient temperature conditions at maximum speeds and/or oscillating stroke movements. The air or gas is advantageously admitted upstream of a porous bearing bush member through an annulus.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which Fig. 1 illustrates an air bearing for a ceramic shaft; Fig. 2 illustrates a combined radial and axial load bearing of a turbocharger; Fig. 3 is a sectional view of the radial/axial bearing of Fig. 2 taken at line A-A; Fig. 4 illustrates an air bearing for a reciprocating components, e.g. a compressor or engine piston, and Fig. 4a illustrates another air bearing for a compressor or engine piston, and Fig. 5 is a sectional view similar to that of Fig. 4 and illustrates a radial air bearing of a piston.

With reference to Fig. 1 the air bearing is here intended for a ceramic shaft 7 making both rotary and oscillatory movements. The ceramic shaft 7 is subject to a relatively high (radial) supporting force, which is represented by the vertical arrowhead B and which is produced by the outer bearing component 1, the latter taking the shape of a bush or sleeve and being separably connected to the components 1 c by means of screws 11. A porous, permeable ceramic bearing member 2 engages in the bearing component 1 and surrounds the ceramic shaft 7, which is made of a highly-compacted ceramic material, in a snug fit allowing a modest amount of clearance.The air 4 is directed from an external source of compressed air into a annulus 9 defined between the parts 1 and 2, through a radial port of the bearing component 1, said annulus extending axially over practically the entire axial length of the porous bearing member 2. From the annulus 9 the compressed air reaches the interior of the bearing 3 (bearing gap) through the pores of the bearing member 2 and in operation, is allowed to escape to the environment laterally through axial ports 6.

The incoming air produces a supporting air cushion in the interior 3 of the bearing, so that practically no contact exists between the ceramic shaft 7 and the porous bearing member 2. This essentially improves the dynamic running performance of the ceramic shaft 7 as compared with conventional bearings and also improves the load capacity (reduced air consumption).

The highly-compacted ceramic shaft 7 and the porous, sintered ceramic bearing member 2 have essentially the same (moderate) coefficient of thermal expansion to ensure accurate guidance by the bearing at both very low and very high temperatures (to above 1 000 C).

The combined turbocharger radial and axial bearing illustrated in Figs. 2 and 3 comprises a ceramic shaft 7 of conventionally sintered, highlycompacted ceramic material. The bearing component 1 surrounding the ceramic shaft 7 carries two essentially similar porous bearing members 2, each having an external, radially extending flange 8, the flanges forming axial faces of the bearing.

In operation, air 4 is admitted to the annuli 9 through two separate inlets and directed from the annuli into the bearing gap. In either bearing gap a uniformly supporting air cushion is generated also at the faces of the circumferential flanges 8. From the gap the air is allowed to escape to the environment primarily through the gas exits 6 of the bearing components 1. The faces of the circumferential flanges 8 have circumferentially equally spaced (three) radial slots 5, which are shown in more detail in Fig. 3 and through which the bearing is vented.

One of the supporting faces of the porous bearing bush 2 is increased or decreased in size to compensate for unilateral thrust. Unilateral thrust is compensated a so if the air supplied to the separate bearing bushes 2 is under different pressures of preselected levels. This enables the radial supporting air cushions to be different in terms of "hardness" and to be adjusted to suit the amount of unilateral thrust.

Fig. 4 illustrates an air bearing for a reciprocating component, in this example, a piston 7, of a compressor or engine. The piston 7 is made of a highly-compacted ceramic material.

The cylinder of the piston is formed by the bearing components 1 of a highly-compacted ceramic material, the components 1 a and 1 b of the same material, and the component 1 c, which when composed retain by their shape the porous bearing member 2 of a ceramic material (ceramic bearing bush). The porous bearing bush 2 encloses the ceramic piston 8 in a snug fit of modest clearance.

When the piston type compressor operates, gas 4 is fed to the interior 3 of the bearing via the annulus 9 and through the porous bearing bush 2, or is fed from 10 (compressed gas) to the annulus 9 through the hole 14, where a supporting air cushion is formed (radial air bearing). The air is allowed to escape at, e.g. zero pressure at a point below the piston 7. Owing to the restricting effect of the porous ceramic material, the air consumption is rather moderate, so that the pressure drop suffered in the annulus 9 is of secondary importance. When the piston 7 is moved downwards as in Fig. 4, expansion of the air, at zero pressure at first, is prevented. The very small volume of air in the pores of the porous bearing bush 2 causes the air pressure to build rapidly, thus permitting a very high frequency of the axial stroke movement of the piston-type compressor.

The axial ports 12, 1 3 indicate the compressor inet and outlet.

In the compressor shown in Fig. 4a, the compressed air is directed from the compressor space 10 into the annulus through holes 4b in the porous bearing member 2.

The embodiment of Fig. 5 essentially corresponds to that of Fig. 4. Equivalent parts are indicated by the same numerals. Unlike the previous embodiment the radial air bearing of Fig.

5 has a bearing bush 7 of conventionally sintered, highly-compacted ceramic material, whereas the compressor piston 14, the main body of which is highly-compacted ceramic, comprises a porous, sintered cylindrical bearing member 2 which, together with the bearing bush 7, defines the interior 3 of the bearing.

In operation -- as shown in the right-hand half of the drawing of Fig. 5 - air 4 supplied from an external source (not shown on the drawing) is directed to the annulus 9 through a crankshaft connection 1 5 of the compressor piston 14. and is forwarded from there into the interior 3 of the bearing through the porous bearing member 2, where a uniformly supporting air cushion is formed on the cylindrical circumference of the compressor piston 14.

In an alternative arrangement -- as shown in the left-hand half of Fig. 5 - direct pressurisation from the compressor space 10 is achieved through the hole 4a, so that the supply with hot or cold air under pressure from an external source can advantageously be omitted.

Claims

1. A gas bearing comprising a bearing member which is made of a porous material, which is permeable to gas, and through which gas is directed into the interior of the bearing.

2. A bearing according to claim 1, wherein the gas permeable bearing member is made of a porous, sintered material having a low coefficient of thermal expansion.

3. A bearing according to claim 1 or 2, wherein the countermember of the bearing is made of a highly-compacted ceramic material or of metal.

4. A bearing according to any one of claims 1 to 3, which is a combined radial and axial bearing and has at least one porous, ceramic bearing bush with a circumferential flange to absorb axial loads.

5. A bearing according to claim 4, wherein the radial dimensions of oppositely arranged circumferential bearing bush flanges (axial faces of the porous bearing) are made different and/or variable to be different one from the other for the purpose of compensating thrust.

6. A bearing according to claim 4 or 5, wherein thrust is compensated by pressurisation with gas of oppositely arranged faces of the bearing bush(es) at different pressure levels.

7. A bearing according to any one of claims 4 to 6, wherein the faces have radially extending slots.

8. A bearing according to any one of claims 1 to 7, wherein the gas is air.

9. A bearing according to claim 8, with an aerostatic air bearing.

1 0. A bearing according to claim 8, which is an aerodynamic air bearing parts of the bearing which are in relatively moveable being made of a porous ceramic material on the one hand and of a highly-compacted ceramic material on the other.

11. A bearing according to any one of claims 1 to 10, wherein the gas is directed to the interior of the bearing at a preselected temperature.

12. A bearing according to any one claims claim 1 to 11, wherein the gas is supplied from an external source of gas.

13. A bearing according to any one claims 1 to 11, wherein in a bearing for a compressor, the gas is directed from the compressor space to the interior of the bearing through holes in the compressor piston or through holes in the porous member of the bearing.

14. A gas bearing constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.