GB2551434A

GB2551434A - Bearing cage

Info

Publication number: GB2551434A
Application number: GB1707011.1A
Authority: GB
Inventors: Munro Grant
Original assignee: ESR Technology Ltd
Current assignee: ESR Technology Ltd
Priority date: 2016-05-03
Filing date: 2017-05-03
Publication date: 2017-12-20
Anticipated expiration: 2037-05-03
Also published as: GB2551434B; GB201607695D0; GB201707011D0

Abstract

A bearing cage (10) of integral structure which holds rolling elements (14), providing dense regions (20, 22) in the portions of the bearing cage (10) that in use come into contact with other items, a dense region around the outer surface of the bearing cage, and providing a low density porous structure (24) linking the dense regions (20, 22). The bearing cage (10) is formed by an additive manufacturing process. This structure may be a three-dimensional open mesh structure. The bearing cage (10) provides a low mass structure, and the significant porosity within the mesh structure (24) may be filled with a lubricant prior to use. The pore size may be between 10µm-2mm and may be smaller towards a surface of the bearing cage.

Description

Bearing Cage

This inventign relates tg a bearing cage, which may be referred tc as a retainer or ball separator, for securing rolling elements in a bearing.

The provision of a bearing cage to locate a number of rolling elements such as balls or rollers within a bearing is widely known, and is typically used to ensure an even spacing between the rolling elements. Such a cage has traditionally been made by processes such as stamping or injection moulding, for large production volumes, or by turning and milling for low production volumes. It has recently been recognised, for example as described in GB 2 528 244 (SKF), that some cases it may be appropriate to make such a bearing cage by 3-D-printing or “additive manufacturing”, as this enables a cage to be produced in a cost efficient manner, and enables the production of a lightweight cage that has the required strength and stiffness, by locating the required material only in those regions where it is necessary to carry the load without significant deformation of the cage. Such an approach, as described in GB 2 521 395 (SKF), may also enable different materials to be provided at different parts of the cage, in particular the use of additive manufacturing would enable specialised materials to be provided in only those portions of the cage that come into contact with other items such as the rolling elements.

According to the present invention there is provided a bearing cage to locate rolling elements, the bearing cage being of integral structure, providing dense regions in those portions of the bearing cage that in use come into contact with other items such as the rolling elements, and providing a lower density porous structure linking the dense regions, the bearing cage being formed by an additive manufacturing process.

The porous structure must be such that a fluid can flow or seep through it, or be retained within it. The porous structure must define linked pores or passages throughout it, so that a fluid could flow or seep through it or be retained within it, and may therefore be referred to as an open structure. The porosity of the porous structure may be at least 25%, but a higher porosity is preferable in providing a larger volume for retaining a fluid, for example a porosity of at least 35%.

The porous structure may for example be a three-dimensional open mesh, for example in the form of a three-dimensional mesh comprising a multiplicity of interlinked strands or wires, formed by the additive manufacturing process. This enables a higher porosity to be provided. For example the mesh structure may have a porosity of at least 50%, more preferably at least 65%, for example 75% or 80%, and the pore sizes (that is to say the width of the gaps between strands forming the mesh) may be between for example 10 pm and 2 mm, more preferably between 40 pm and 1 mm. The pore sizes may be uniform throughout the porous structure, or alternatively may vary, for example being smaller towards a surface of the bearing cage. The portions of the surface of the bearing cage that are adjacent to the dense regions thus consist of the porous structure. In use the rolling elements roll along a circular path around a centre axis. As regards at least one surface of the cage, for example the surface facing the centre axis, substantially the entire surface is therefore constituted by the porous structure.

The dense regions are at least 95% dense, and may be 100% dense, i.e. fully dense. They are typically of thickness less than 1 mm, so that almost all the volume of the bearing cage is constituted by the porous structure. In some cases, for example for a cage of external diameter less than 30 mm, the dense regions may be of thickness less than 0.5 mm. After manufacture by the additive manufacturing process, the dense regions may be subjected to a machining step to ensure a smooth outer surface to that part of the bearing cage.

Prior to use of the bearing cage, the pores of the porous structure may be filled with a lubricant, such as oil or grease or another lubricating fluid, which during operation creeps onto the surfaces of the dense regions within a bearing. Whether or not the pores are filled with lubricant, the porosity in any event reduces the mass of the bearing cage, so high porosity is beneficial in achieving low mass.

The bearing cage comes into contact with the roller elements which are typically of a hard metal such as steel, or of a ceramic, and the bearing cage is typically of a hard metal such as stainless steel or bronze. It may however be of an alternative material, for example a polymer, such as an engineering polymer, for example nylon (polyamides), acrylonitrile butadiene styrene (ABS) or polyether ether ketone (PEEK).

It will thus be appreciated that the bearing cage of the invention is a comparatively lightweight component, as compared to the corresponding structure made of solid material, while providing stiffness and strength. In cases where solid metallic bearing cages are have previously been used, the bearing cage of the invention provides significant mass savings and corresponding performance benefits in dynamic applications.

The use of the mesh structure to retain lubricant is beneficial in extending operating life of bearings in situations where maintenance is difficult or impossible, or where there are restrictions on performing maintenance, for example where maintenance can only be performed at very long intervals, and has the benefit that the lubricant is located immediately adjacent to the surfaces where it is required, so the lubricant need only migrate a short distance to be effective. The invention enables more effective lubrication of bearing components to be provided over an extended duration, and is particularly useful in demanding environments such as in space satellites, where there may also be additional constraints on mass and/or structural integrity.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a bearing cage of the invention;

Figure 2 shows a perspective view, partly broken away, showing the bearing cage of figure 1 forming part of a bearing assembly;

Figure 3 shows a perspective view at a larger scale, of part of the bearing cage of figure 1; and

Figure 4 shows a perspective view of an alternative bearing cage of the invention.

Referring to figure 1, a bearing cage 10 is in the form of a ring which defines several apertures 12 to locate the rolling elements, the apertures 12 being arranged to ensure the rolling elements are equally spaced around the ring. In this example the bearing cage 10 defines twelve such apertures 12, and the rolling elements are spherical balls 14 (shown in figure 2).

Referring to figure 2, a bearing assembly 15 consists of the bearing cage 10 including the spherical balls 14 between an inner ring 16 and an outer ring 18. The inner ring 16 and the outer ring 18 may define a track or groove to locate the balls 14, so in use the balls 14 roll around the track or groove. The outer ring 18 is partly broken away in figure 2, to show the balls 14 more clearly; and in this case it is evident that the outer ring 18 is asymmetric, with a radial flange on one side but not on the other - this asymmetry is a conventional feature on an angular contact bearing, as the asymmetric outer ring 18 aids assembly.

Referring now to figures 1 and 3, the bearing cage 10 is a single integral item made of metal by an additive manufacturing process, for example of stainless steel or bronze. Surrounding each aperture 12 is a wall portion 20 in which the metal is non-porous and so is fully dense, and around one end face of the bearing cage 10 is a portion 22 in which the metal is non-porous and so is fully dense. As indicated schematically by crosshatching in figure 1, and as shown in more detail in figure 3, all the remaining portions of the bearing cage 10 are in the form of a three-dimensional mesh 24 consisting of a multiplicity of wire or strand-like elements 25 in an approximately hexagonal or tetrahedral array, leaving pores or apertures 26 between the elements 25. In this example the three-dimensional mesh 24 extends to the end face shown at the bottom of figures 1 and 3.

Thus the bearing cage 10 is constructed so as to have fully dense portions 20 and 22 in those places where the bearing cage 10 may come into contact with other items during use of the bearing assembly 15, and consists of the three-dimensional mesh 24 in all other portions. This significantly reduces the mass of the bearing cage 10 as compared to a bearing cage of the same dimensions but of solid metal, without significantly decreasing the stiffness and resilience of the bearing cage 10. By way of example the three-dimensional mesh 24 may have a porosity of about 75%, so its bulk density is only 25% that of dense material; taking into account the dense portions 20 and 22, the mass of the bearing cage 10 may for example be between about 30% and 35% that of a bearing cage of the same dimensions but of solid metal.

If the bearing assembly 15 or its application is such that neither of the end faces of the bearing cage will come into contact with other items during use, then there is no requirement for the dense portion 22 at one end face, so that in a modification the three-dimensional mesh 24 may extend to both the end faces. On the other hand, if the bearing assembly 15 or its application are such that both of the end faces of the bearing cage may come into contact with other items during use, then as an alternative modification there may be a dense portion 22 provided at both the end faces. In either case the portions of the surface of the bearing cage 10 that are adjacent to the dense portions 22 (which in use come into contact with other items) thus consist of the three-dimensional mesh 24.

The bearing cage 10 is described as accommodating rolling elements in the form of spherical balls. In an alternative, a bearing cage may define apertures to locate cylindrical rollers (not shown) as the rolling elements.

As indicated above one benefit of the structure of the bearing cage 10 is the reduction in mass. A further potential advantage is that the three-dimensional mesh 24 may be packed with a lubricant such as grease in all the pores 26, before use. During operation it is found that lubricant tends to migrate from the three-dimensional mesh 24 across the adjacent surfaces and so to creep onto the surfaces of the dense portions 20 and 22 where lubrication is required. Hence the bearing cage 10 enables such lubrication to be provided over a prolonged period, without requiring any attention from maintenance staff.

All the portions of the bearing cage 10, that is to say the dense portions 20 and 22 and the three-dimensional mesh portions 24, are integral with each other, and are made of metal as a near net shape by an additive manufacturing process such as selective laser melting or selective laser sintering, to form the near-final shape. The production process may be described as a three-dimensional printing process from a powder. The powder is a metal powder. In one example the bearing cage 10 is produced by direct metal laser sintering. This process involves fusing together of very fine layers of metal powder using a focused laser beam. The powder may be deposited in layers of between 20 and 60 pm, and each layer is then scanned with a high intensity laser beam so that the particles within the layer fuse together, and the layer fuses to the layer below. This direct metal laser sintering process involves gradually and repeatedly lowering a piston or support plate on which is repeatedly placed a thin layer of a powder of material, and scanning with a laser those portions of the layer of powder which are to be sintered together. After scanning the selected parts of each layer, the powder bed is lowered by one layer thickness, and the process repeated. Preferably the entire bed of powder is warmed up to slightly below the temperature required for sintering, so that the power required by the laser is reduced.

This process is substantially identical to those referred to as selective laser metal sintering, or selective laser metal melting. An equivalent process, which may also be suitable, is electron beam melting.

After performing the sintering as described above, the bearing cage is separated from the remaining powder. The resulting bearing cage 10 is single integral structure, with no joints. The manufacturing process may then include a machining step to achieve any necessary tolerances and surface finishes. Before the final machining operation the bearing cage 10 may be subjected to a heat treatment to ensure a particular material property. During manufacture it may be found advantageous to incorporate temporary additional features, which are subsequently removed before use.

As mentioned above the bearing cage 10 is constructed to have dense regions 20, 22 in those portions that in use come into contact with other items such as the rolling elements, and providing a low density three-dimensional open mesh structure 24 linking the dense regions. In some cases it may also be desirable to provide additional dense regions. For example in the case of a bearing cage for use in a high-speed bearing, where the bearing cage is to be loaded with lubricant prior to use, it may also be desirable to provide a dense region (not shown) around the entire outer surface of the bearing cage 10, so as to prevent outflow of the lubricant through that outer surface as the bearing cage rotates rapidly.

In the example described above, the bearing cage 10 provides apertures 12 that surround the ball bearings 14, but the asymmetrical structure of the outer ring 18 enables assembly. In deep groove radial bearings, the inner and outer ring and the rolling elements must be assembled before the cage is fitted. This requires a somewhat different type of cage, as shown in figure 4, to which reference is now made.

As shown in figure 4, a bearing cage 40 for a deep groove radial bearing is in the form of a ring which defines several apertures 42 to locate the rolling elements, the apertures 42 being arranged to ensure the rolling elements are equally spaced around the ring. In this example the bearing cage 40 defines twelve such apertures 42, and the rolling elements are spherical balls 14 (not shown in figure 4). In this case each aperture 42 is approximately keyhole-shaped, with a circular part 43 in which the ball 14 fits during operation with significant clearance, and a slot 44 of slightly narrower width that extends to an adjacent edge of the bearing cage 40, the slot 44 being a tight fit to the spherical ball 14.

The bearing cage 40 is a single integral item made by an additive manufacturing process, for example of a metal such as stainless steel or bronze. Surrounding both the circular part 43 and the slot 44 of each aperture 42 is a wall portion 45 in which the metal is non-porous and so is fully dense, and around both end faces of the bearing cage 40 are end portions 46 and 47 in which the metal is non-porous and so is fully dense. As indicated schematically by crosshatching in figure 4, all the remaining portions of the bearing cage 40 are in the form of a three-dimensional mesh 24 of the same structure as described above in relation to figure 3 consisting of a multiplicity of wire or strand-like elements 25 in an approximately hexagonal or tetrahedral array, leaving pores or apertures 26 between the elements 25. Thus the bearing cage 40, like the bearing cage 10, has fully dense portions 45, 46 and 47 in those places where the bearing cage 40 may come into contact with other items during use, and consists of the three-dimensional mesh 24 in all other portions. Hence the portions of the surface of the bearing cage 40 that are adjacent to the dense portions 45, 46, 47 (which in use come into contact with other items) consist of the three-dimensional mesh 24.

Claims

1. A bearing cage to locate rolling elements, the bearing cage being of integral structure, providing dense regions in those portions of the bearing cage that in use come into contact with other items such as the rolling elements, and optionally providing a dense region around the entire outer surface of the bearing cage, and providing a lower density porous structure linking the dense regions, so that the bearing cage consists of the porous structure in all portions apart from the dense regions, the bearing cage being formed by an additive manufacturing process.

2. A bearing cage as claimed in claim 1 wherein the porous structure is a three-dimensional mesh structure.

3. A bearing cage as claimed in claim 2 wherein the mesh structure has a porosity of at least 50%.

4. A bearing cage as claimed in claim 3 wherein the mesh structure has a porosity of at least 65%.

5. A bearing cage as claimed in any one of the preceding claims wherein the pore sizes are between 10 pm and 2 mm.

6. A bearing cage as claimed in any one of the preceding claims when dependent on claim 2, wherein the pore sizes, that is to say the gaps between elements that form the mesh, vary through the mesh, being smaller towards a surface of the bearing cage.

7. A bearing cage as claimed in any one of the preceding claims wherein the pores of the porous structure are filled with a lubricant prior to use of the bearing cage.