GB2141792A - Foil bearings - Google Patents

Abstract

A method of manufacturing a foil bearing includes the steps of supporting a metal foil or foils (22) by a plurality of support assemblies (13, 14, 15) equally spaced from each other around a mandrel (20), placing the foil/s under tension, heat treating the foil/s to effect plastic deformation of the foil/s and cooling the foil/s to form a bearing surface of an exact contoured copy of the outer surface of the mandrel. The mandrel is of a larger diameter than a rotary shaft to be positioned in the bearing. Consequently, a predetermined clearance between the bearing surface and the shaft can be obtained. <IMAGE>

Description

SPECIFICATION Foil bearings The invention relates to foil bearings and to methods of their manufacture.

One known type of foil bearing is a dynamic pressure type radial gas bearing, in which a single sheet of metal foil or a plurality of foils is arranged to form a surface surrounding a rotating shaft.

By virtue of the inherent flexibility of the metal foil, such a foil bearing has many advantageous features, including superior resilient supporting capacity, and high resistance to thermal deformation, which are not obtainable with other conventional types of such bearings, or with sealed bearings.

There are two kinds of foil bearings, one of which utilizes axial tension applied to a metal foil or foils and the other relies on the rigidity or inflexibility of a foil or foils.

Afoil bearing of tension type is composed of a plurality of support members disposed axially around a rotary shaft and at a suitable distance from each other, for guiding and retaining the metal foil, a sheet of metal foil passed around the rotary shaft through said support members and stretched under suitable tension between its opposite axial ends, to provide a bearing surface of high accuracy with clearance to allow a very effective gas layer.

By virtue of this construction, the rotating torque required is fairly low, but a high value of starting torque is required due to the fact that the metal foil tightly wraps or grips the rotary shaft when it is stationary.

On the other hand, several types of stiff foil flexible bearings such as, multi-leaved type, hydrocele (multi-pad) type, multi loop types and the like are being used, all composed of a plurality of press formed metal foils assembled and fixed to the housing of a bearing to receive and support a shaft due to the rigidity and inflexibility of the assembly. This construction ensures that their required starting torque is considerably less than that of the aforesaid tension types, but there are several drawbacks attributable to independent functioning of the respective leaves of the assembly during running.In other words, it is difficult to ensure a bearing surface which maintains the highly accurate bearing clearance that is necessary to ensure the formation of an effective layer of gas, and this results in such high contact pressure between the metal foil and the shaft that an undesirably high level of starting torque is required, and to a deteriorated characteristic during high speed rotation.

One object of the present invention is to provide a foil bearing which avoids such drawbacks, and yet is able to permit high speed rotation of the shaft with minimum torque both when starting and during normal running.

Another object of the invention is to provide a method of manufacturing the foil bearing using a novel manner of heat treatment to form a bearing surface.

According to one aspect the present invention provides a foil bearing comprising a flexible metal foil or foils supported buy a plurality of support assemblies equally spaced around a central axis and equally spaced therefrom, said or each foil having been formed by heat treatment to provide discrete segments of a bearing surface having a diameter larger than a rotary shaft positioned in said bearing and substantially on said axis so as to retain a predetermined bearing clearance between said rotary shaft and each said bearing surface segment of said metal foil or foils which surrounds said rotary shaft.

Thus, by retaining a sheet of flexible metal foil stretched to surround a mandrel using a plurality of axially extending support members at a suitable distance from each other around the outer periphery, the metal foil can be formed by heat treatment to provide segments of a bearing surface having an inside diameter slightly larger than the rotary shaft, to give a predetermined bearing clearance, the thus formed metal foil then being assembled about the mating shaft while at least one axial end is fixed to one of the aforesaid support members.

The mandrel is formed with an outer surface having a diameter slightly larger than the shaft that is to be used, to give a predetermined bearing clearance, and a suitable tensional force is imparted by stressing means so that the foil is tightly wrapped about the mandrel, subsequently the metal foil and the mandrel are kept in wrapped state and together fed into a heat treatment furnace, to be heated and then slowly cooled.

By these steps, the metal foil is formed to have an internal bearing surface exactly conforming to the outer surface of the mandrel.

Since the metal foil is formed by heat treatment to have a bearing surface whose diameter is slightly larger than that of the rotary shaft, sufficient to give a required bearing clearance, the metal foil can then be assembled to the rotary shaft so that the bearing surface faces the shaft, by fixing at least one end of the foil to the support member.

Accordingly, the metal foil will not tightly grip the rotary shaft, even when it stops rotating, and this results in the need for a very small starting torque, and ensures a bearing surface having high accuracy with respect to bearing clearance.

An ordinary type heat treatment furnace can be used, and relies on generally known arts of heat treatment.

The invention will now be described with reference to the drawings, in which Figure 1 is a perspective view of one exempiary embodiment of a foil bearing constructed in accordance with the present invention; Figure 2 is a cross-section of the embodiment shown in Figure 1, on the line 2-2; Figure 3 is a perspective view of a further exem plaryembodiment; and Figure 4 is an end view of the embodiment shown in Figure 3.

In the embodiment shown in Figure 1,a mounting assembly 10 for stretching and forming a metal foil is composed of a pair of opposed retaining discs 11, joined bythree support assemblies 13, 14 and 15.

Each retaining disc 11 has a central opening 12, and the two openings 12 are aligned on the same central axis when assembled.

The three support assemblies 13, 14 and 15 are disposed axially, mutually parallel, and equally spaced around the central axis at positions equally spaced from the axis. Each of the support assemblies consists of a pair of guide pins 16 positioned closely adjacent each other and at an equal distance from the axis, together with a support pin 17 located radially outward at an equal distance from each of the pair of guide pins.

The three support pins 17 all have the same diameter and their centres lie on a circle about the axis of the support discs 11.

A mandrel 20 having an outer diameter smaller than the opening 12 but larger than the shaft which is to be used, is inserted into the space defined by the aforesaid two opposing openings 12, the diameter giving a predetermined bearing clearance in the finished product. The peripheral configuration is not necessarily a round cylinder, as depicted in Figure 1.

A metal foil 22 which has been cut from a sheet of stainless steel foil (10 Itm thickness) to have an axial length slightly shorter than the distance between the two support discs 11 is then threaded through passages defined between the mandrel 20 and the support assemblies 13, 14 and 15 successively, for example, starting from the first support assembly 13, next through the second support assembly 14, further through the third support assembly 15 and returning again to the first support assembly 13.The path is shown in detail in Figure 2, taking a snaked path bent along the radially inner surface of the guide pin 16 from the support pin 17 in the first support assembly 13, outer surface of the mandrel 20, inner surface of one guide pin 16 of the second support assembly 14, outer face ofthe associated support pin 17, inner face of the other guide pin 16 of the second support assembly 14, and again along the outer surface of the mandrel 20, and further repeating the same sequence. Ends 23 and 24 of the metal foil 22 are fixed lengthwise to the guide pin 17 of the first support assembly 13 by any suitable means, such as welding, whilst the foil 22 is held under a desired stress by a tensioning means, not shown, so as to tightly enclose the mandrel 20 and maintain the metal foil 22 under tensile stress.After the metal foil has been welded to the support pin 17 at both ends 23 and 24, any surplus portions of the metal foil 22, outside the weld zone are cut away.

The mounting assembly 10 together with the metal foil thus stretched around the mandrel 20 are then introduced into a heat treatment furnace and heated for a predetermined period of time to a temperature sufficiently close to the melting point to enable plastic deformation to take place, which depends upon the kind of steel to be used. The heated member is then slowly cooled.

It is preferred that the heating step is carried out either in a vacuum, or in a neutral or inert gas atmosphere, such as nitrogen gas.

The heat-treated metal foil 22 can now be released from the applied stress by removal of the mandrel, and is plastically deformed due to the fact that it has been heated at a temperature near to the melting temperature of the material, so the metal foil 22 has a set of bearing surface segments 25 obtained by exactly copying the outer surface of the mandrel 20.

After having finished the heat treatment and removing the mandrel 20, the resultant foil bearing obtained consists of the mounting assembly 10 and the metal foil 22.

The foil bearing made in the above-mentioned manner is assembled to a rotary shaft having a diameter smaller than the mandrel 20 by an amount sufficient to form a required bearing clearance.

In the embodiment explained above, both ends of the metal foil were fixed to a support pin of a support assembly prior to the heat treatment, but the metal foil can be heat-treated during the period when the ends are being held bythetension-imparting means, after which the ends are then fixed to the support pin.

The metal foil in this embodiment is a single sheet, but a plurality of sheets of foil can be used by fixing the ends of each foil to a support pin and/or a plurality of foil strips can be used to form respective bearing surface segments 25.

Figures 3 and 4 illustrate another embodiment, wherein a metal foil is retained at three positions equally spaced around the wall of an outer cylinder 30.

Three slits 31 passing through the wall of the outer cylinder 30, each of equal length and extending axially from one end of the cylinder to a point near to the other end of the cylinder to correspond to the support assemblies 13, 14 and 15 of the embodiment in Figure 1.

Each of these support assemblies in this embodiment is composed of two identical L-shaped pieces, one arm "A" of each piece being symmetrically placed in back-to-back relation, while the inner face of the other arm "B" of each piece contacts the outer face of the outer cylinder 30 near the margin of the respective slit, and is secured.

As can be seen from Figure 4, a mandrel 20 is inserted into the opening of the outer cylinder 30 and three strips of metal foil are respectively disposed between adjacent slits to sealably surround the mandrel 20.

The opposite ends of each sheet of foil are inserted between the two mating "A" sides of the associated support assemblies and seam welded at the portion 32 of the outer face of the arm "B" of each L-shaped member.

The tensioning and heating step for forming the required bearing surface segments 25 is conducted in a manner similarto that explained forthe above-mentioned embodiment.

After having formed the bearing surface segments 25, the mandrel 20 is pulled away, to leave a completed foil bearing as shown in Figure 3.

The foil bearing of the present invention is constructed to have a bearing surface in the metal foil formed by thermally copying the outer surface of a mandrel having an outside diameter larger than that of the rotary shaft, so as to retain a predetermined bearing clearance. In addition, both ends of the foil are attached to the support members and are ready for assembly on a desired rotary shaft.

Consequently, since the metal foil incorporated in the foil bearing of this invention does not grip the rotary shaft when it is stopped, the starting torque becomes very low, and also a bearing surface of high accuracy is obtained, so giving a foil bearing of superior performance than has heretofore been possible.

As ordinary heat treatment furnaces can be used without requiring any particular equipment, and therefore foil bearings having varous unexpected advantages over conventional types can be manufactured economically.

Advantageous constructions to provide enhanced safe-guards against the foil or foils contacting the rotating shaft at the foil ends during high speed operation are described and claimed in our copending United Kingdom Patent Application of even date, Serial No (Our Ref Y781).

Claims (7)

1. A foil bearing comprising a flexible metal foil or foils supported by a plurality of support assemblies equally spaced around a central axis and equally spaced therefrom, said or each foil having been formed by heat treatment to provide discrete segments of a bearing surface having a diameter larger than a rotary shaft positioned in said bearing and substantially on said axis so as to retain a predetermined bearing clearance between said rotary shaft and each said bearing surface segment of said metal foil orfoilswhich surround said rotary shaft.

2. Afoil bearing as claimed in Claim 1, in which each said support assembly is composed of a pair of L-shaped pieces held in a respective slit formed axially through the wall of an outer cylinder, and a said metal foil is fixed between associated ones of said L-shaped members in two adjacent support assemblies, so that a plurality of segments surrounds the rotary shaft within said outer cylinder.

3. Afoil bearing as claimed in Claim 1, in which said support assemblies are formed by respective support and guide pins secured to axially opposed retaining discs to form a mounting assembly.

4. Afoil bearing as described with reference to any one of Figures 1 to 4.

5. A method of manufacturing a foil bearing as claimed in Claim 1, including the steps of supporting a flexible metal foil or foils by a plurality of support assemblies equally spaced around said shaft at an equal distance from the rotary axis of said shaft; inserting a mandrel of larger diameterto be such rounded by said foil or foils, the outer surface of said mandrel that is to be mated with the bearing surface of said bearing being larger in diameter than said rotary shaft by an extent sufficient to maintain a predetermined bearing clearance; imparting a tensile force to said foil or foils by a suitable means; maintaining the support assemblies and said metal foil or foils in said tensioned state and heating them in a heat treatment furnace to effect plastic deformation of the or each foil; cooling the heated members to form said bearing surface whilst the surface of said metal foil is forcibly contacted with said mandrel to form an exactly contoured copy of the outer surface of said mandrel; allowing said heated components to cool; and then removing said mandrel.

6. A method of manufacturing a foil bearing as claimed in Claim 5, wherein said metal foil is a stainless steel, and said heating is carried out in vacuum or an inert gas atmosphere at a temperature near the melting point temperature of said stainless steel.

7. A method as claimed in Claim 5 or Claim 6, substantially as described with reference to any of Figures 1 to 4.