Tilting pad thrust bearing arrangement

Details Tilting pad thrust bearing arrangement Tilting pad thrust bearing arrangement

1999-10-27

2001-02-06

Hannon, Thomas R. (Department: 3682)

Bearings

Rotary bearing

Plain bearing

Reexamination Certificate

active

06183138

ABSTRACT:

This invention relates to tilting pad thrust bearing arrangements for apparatus having a shaft that is rotatable about an axis and particularly, but not exclusively, relates to such bearing arrangements in which the bearing pads are made from a hard, but brittle, ceramic material.
Tilting pad thrust bearings are known, for example, from GB-A-1535165, GB-A-1241098 and U.S. Pat. No. 3,937,534, the common constructional principle being that a plurality of bearing pads, disposed in a circular array about the shaft axis, each have an axially facing supporting face that is adapted to support (by way of an intervening fluid film), a radially extending part of the shaft and, opposite thereto, a supported face by which the pad itself is supported with respect to a carrier body on a pivot edge, hereinafter referred to as a ‘tilt pivot’, about which it can undertake a limited degree of tilting motion.
It will be appreciated that such thrust bearing pads are generally mounted loosely with respect to the carrier body in order to accommodate misalignments and tilting by the action of hydrodynamic films of the fluid when supporting the rotating shaft, said loose mounting being confined principally to retaining the pads within the arrangement when not in operation.
Various materials are known for the bearing pads, as are mounting methods which may be more or less suitable in dependence upon the nature of the pads. One particular pad form with which this invention is particularly concerned is that where each pad comprises a block of hard, but somewhat brittle, ceramic material, such as reaction bonded silicon carbide, for use with a low viscosity lubricant such as water.
Whereas pads made from such material are hard and able to function with relatively poor lubrication, the material is difficult to work (in terms of machining operations) and susceptible to damage by impacts and shock forces applied locally.
By way of example, FIGS.
1
(
a
) and
1
(
b
) show respectively somewhat schematic plan and sectional elevation views of a known form of tilting pad thrust bearing arrangement
10
in which the individual pads are made of a silicon carbide ceramic material. The bearing arrangement comprises an annular body carrier
11
extending circumferentially about axis
12
, although only an arcuate part of the body is shown. The body has an axially facing carrier surface
13
on which is supported individual ceramic bearing pads
15
1
,
15
2
, . . . . Each bearing pad
15
i
is a monolithic block of ceramic material formed with a substantially flat supporting face
16
and opposite thereto a supported face
17
i
which is profiled to provide a radially extending tilt pivot ridge
18
i
. Each pad is supported on the face
13
of the carrier body by way of intervening supporting means
20
, comprising an annular spring member
21
(of thin resilient steel) supported above the face
13
by spring deflection pivot pieces
22
1
,
22
2
, . . . formed by washers that are located on axially extending pins
23
1
,
23
2
, . . . . The pins and washers are disposed in a circumferential direction such that each pin and washer is disposed between the tilt pivot ridges of adjacent bearing pads.
The pins extend through the annular spring member and serve to locate the spring member and each pad against significant circumferential displacement whilst permitting relative freedom with regard to taking up a position determined by the borne load. To prevent the pads from falling from the carrier during and/or after assembly of the arrangement
10
, a loosely fitting carrier projection or pin
24
co-operates with a groove
25
formed in at least one edge of the pad.
In operation, when the supporting face
16
of each pad is subjected to axially directed loads, the spring member
21
serves to absorb axial loading on the bearing pads by deflecting about the deflection pivot washers.
However, there are practical disadvantages to such structure. Firstly, the loose mounting which permits ready alignment under load also provides potential for the pads to impact on the pins, particularly in conditions of vibration, where damage may be caused to the pad material. Secondly, the groove
25
(or the like) required for retaining the pad in the arrangement requires at least one extra manufacturing step, which may not be trivial in ceramic materials, and assembly step. Thirdly, the simple spring has a substantially linear stiffness response (deflection vs. load characteristic) so that is must be carefully chosen in relation to anticipated axial loads to provide displacement of the pad but without ‘bottoming’ against the carrier body and leaving the pad vulnerable to shock loads. It will be appreciated that to minimise the chances of such ‘bottoming’ of the supporting means the spring member will have a stiffness or spring rate which is high and provided by a relatively thick component, and the deflection pivot washers
22
1
, . . . will be relatively thick, typically of the order of 2 mm and similar to the thickness of the spring member.
In particularly heavily loaded bearing arrangements the thickness required of the spring member may be such that it is vulnerable to cracking about the edge of a pivot washer and/or any stress points caused by the pins and/or their apertures, again leaving the pad open to experience shock loads. Furthermore, the provision of a spring stiffness higher than is required to support normally encountered loads, but deemed necessary for emergencies, may result in an unacceptable low damping characteristic.
The problems of axial resilience and accommodating vibrational modes associated therewith has been addressed in the aforementioned U.S. Pat. No. 3,937,534, wherein the annular spring member supporting means takes the form of a stack of thinner, identical annular spring members each supported with respect to the next by an axially raised deflection pivot about which it can undergo deflection in response to axial loading, said deflection pivots of adjacent members being offset circumferentially with respect to each other so that the pivots of alternate members are axially in line. This is illustrated in simplified form in FIG.
1
(
c
), which corresponds to the sectional elevation of
FIG. 1
(
b
) described above and shows bearing pads
15
′
1
, and
15
′
2
on spring members
21
′
A
,
21
′
B
and
21
′
C
separated by pivot members
22
′
A
etc. Essentially, when an axial load is applied by way of the bearing pad, at a point between two deflection pivots, the load is transmitted by way of the pivots to the next spring member and so on, deflecting all of the annular spring members simultaneously and effectively providing a spring with a constant, low spring stiffness and low resonant frequency.
However, whilst such structure of supporting means is inherently less susceptible to damage to the spring members, it does not address the dichotomy of accommodating both acceptable
ormal axial loads and exceptional loads of high magnitude, should they occur and be particularly damaging to the pads by virtue of pad materials.
Notwithstanding the particular problems of brittleness associated with ceramic materials or bearing pads, it may also be disadvantageous if the ability to absorb axial loads progressively and without risk of component failure is not available to more conventional, metal bearing pads.
In accordance with the present invention a tilting pad thrust bearing arrangement for apparatus having a shaft that is rotatable about an axis and comprising an annular body and an array of discrete bearing pads supported on the body by way of a tilt pivot and intervening annular pad supporting spring means, and in which the supporting spring means is in the form of a stack of at least two annular, axially deflectable spring members each being spaced from an adjacent annular surface by an associated circular array of deflection pivots disposed one pivot each between the tilt pivots of adjacent pads and acting as a fulcrum for spring deflection, is characterised by correspon

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