GB2140877A - Centrifugal oil weirs - Google Patents

Abstract

A centrifugal oil weir 98 transfers lubricating oil between, e.g. a rotating shaft 36 and an oil catcher ring 102 which is also rotating, but at a different speed from shaft 36. Normally, oil, held in chambers 94 between weir 98 and shaft 36 by centrifugal force, enters flow channels 108 in the weir surface 112 via inflow edge 110 and is centrifugally dispensed to catcher ring 102 via outflow edge 114 and oil flinger lip 115. There is also normally an axial clearance between lip 115 and a bearing race (Figure 2) with which catcher ring 102 is associated, but in the event of this clearance being reduced sufficiently to cause blockage of the outflow edge 114, that oil which would normally be dispensed from the outflow edge is dispensed instead from apertures in the form of scallops 106 in the outflow edge. In this way, the supply of oil to the bearing is maintained. <IMAGE>

Description

SPECIFICATION Centrifugal oil weirs This invention relates to centrifugal oil weirs and finds application in the supply of lubricating oil to rolling element bearings, particularly - but not exclusively - in the supply of oil to bearings located between relatively rotating power transmission shafts in gas turbine engines.

In the context of the gas turbine aeroengine industry, it is well known to provide rolling element bearings between two coaxial power transmission shafts having relative rotation with respect to each other, the bearings acting to locate the shafts axially with respect to each other.

It is also well known to supply lubricating oil to such a bearing from oil feed passages associated with one of the shafts via a centrifugal. oil weir attached to that shaft. Such an oil weir generally includes a cylindrical portion which extends axially from the shaft to which the oil weir is attached towards the side of the bearing inner race, the cylindrical portion having a radially inner weir surface whose axially opposed edges comprise an inflow edge in communication with the oil feed passages, and an outflow edge which dispenses the oil to the bearing; the oil is held to the weir surface by centrifugal force and flows thereover, being then flung off the outflow edge by the centrifugal action and intercepted by a catcher ring of larger radius on the bearing cage.

One difficulty experienced by designers of aeroengines is how to design them to "fail-safe" insofar as is possible. Thus, in the present case, if the rolling elements of the bearing, or the bearing tracks, begin to fail and break up, the positive axial location of one shaft with respect to the other through the bearing will cease and the necessary axial clearance between the outflow edge of the weir surface and the side of the bearing will be lost. This may lead to blockage of the outflow edge of the weir surface due to contact between the outflow edge and the side of the bearing inner race, thereby cutting off the oil supply to the bearing. This further accelerates deterioration of the bearing and increases frictional heating, leading to fire hazards and catastrophic failure.

An object of the present invention is to maintain the supply of oil to a failing bearing for as long as possible.

According to the present invention, a centrifugal oil weir suitable for supplying oil from oil feed means to a rolling element bearing coaxial therewith, but having an axial clearance therefrom, has an axially extending cylindrical portion with a radially inner weir surface, said weir surface having an inflow edge for receiving oil from said oil feed means and an outflow edge for dispensing oil therefrom, an alternative flow path being provided forthe oil, said alternative flow path comprising apterture means extending through said cylindrical portion from said weir surface, the arrangement being such that in the event of reduction in said axial clearance sufficient to cause blockage of the outflow edge of said weir surface, that oil which would normally be dispensed from the outflow edge is dispensed through the aperture means instead.

It is very much preferred that the weir surface be provided with flow channels arranged such that during normal operation the oil does not flow through the aperture means, but if the outflow edge of the weir surface is blocked, the oil overflows the channels and is dispensed through the aperture means.

It is also preferred that the weir surface slopes radially outwards from its inflow edge to its outlfow edge to assist the flow of oil.

The aperture means conveniently comprises circumferentially spaced-apart scallops in the outflow edge of the weir, but notches or slots in said edge, or even rows of holes at various distances from the outflow edge, could also be utilised.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which Figure lisa diagrammatic view of a turbofan aeroengine which is partly sectioned to show where a centrifugal oil weir according to the present invention can be situated; Figure 2 is a more detailed enlarged view of part of Figure 1, showing the centrifugal oil weir and adjacent engine structure; Figure 3 is an enlarged view of parts of the centrifugal oil weir and adjoining structure seen in Figure 2; Figure 4 is a partial view of the centrifugal oil weir as seen on arrow A in Figure 2; Figure 5 is a partial view of the centrifugal oil weir as seen on arrow B in Figure 2; and Figure 6 is a partial view of the centrifugal oil weir as seen on arrow C in Figure 5.

The drawings are not to scale.

Referring to Figure 1, a turbofan aeroengine 2 is encased by an outer cowling 4. The main features of the engine 2 hidden by the cowling 4 are outlined in dashed lines and in addition a portion of the cowling is "broken away" to reveal a section through part of the engine in a vertical plane through the engine's axis of rotation.

Engine 2 represents a known design to which the invention is applied, the engine having an engine core 6, a bypass duct 8 defined between bypass duct casing 10 and the outer casing 12 of engine core 6, and an exhaust system including an exhaust bullet 14 at the rear of core engine 6, a core exhaust nozzle 16, and a final propulsion nozzle 18.

The bypass duct 8 is supplied with bypass air from front fan 20 acting as a low pressure compressor which also supplies the engine core 6. Air supplied to the engine core 6 passes along the flowpath outlined therein, passing through intermediate and high pressure compressor sections 22 and 24 respectively, combustion zone 26, and high, intermediate and low pressure turbine sections 28,30 and 32 respectively. Fan 20 is driven from low pressure turbine section 32 via a "low pressure" (L.P.) power transmission shaft 34, intermediate pressure compressor 22 is driven from the intermediate pressure turbine section 30 via an "intermediate pressure" (I.P.) shaft 36, and the high pressure compressor section 24 is driven from the high pressure turbine section 28 via a "high pressure" (H.P.) shaft (not shown).

Engine core 6 is suspended within cowling 4 and bypass duct casing 10 by means of fan outlet guide vanes 38 and suspension struts and linkages (not shown) within a suspension fairing 40 which extends across the top centre sector of the bypass duct 8. The L.P., I.P. and H.P. shaft mentioned above in turn suspended concentrically within the engine core 6 by means including internal casing structure 42, guide vanes/struts 44, web 46 and rolling element bearings 48 and 50.

Bearings 48 and 50 with some of their associated structure are shown in more detail in Figure 2.

As will be seen, l.P. shaft 36 is supported from, interalia, frusto-conical support web 46 by means of bearing 48. The forward end of the l.P. shaft 36 is substantially "bell-shaped", having a radially extending flange 52 by means of which it is joined to a frusto-conical drive web 54 of the l.P. compressor drum (see also Figure 1). Also secured to flange 52 is the outer race 55 of bearing 50 whose inner race 56 is secured to a forward portion 58 of L.P. shaft 34. This forward portion 58 is driven from a smaller diameter rear portion 60 of shaft 34, the drive being transmitted through splines 62 on the shaft portions. The forward end of shaft portion 60 is supported within shaft portion 58 by clamping rings (not shown).

Both bearings are supplied with lubricating oil during operation of the engine by means of an oil feed system including an inner sleeve 62 of l.P. shaft 36, the shaft and the inner sleeve being splined together at 64 and having sealing lands at 66, the sealing land on the sleeve incorporating a sealing ring 68. Oil is supplied as shown by the arrows, the oil passing along the gaps between the splines 64, flowing into small chamber 70, and passing radially through the l.P. shaft 36 via a plurality of angularly spaced drillings 72. The inner race 74 of bearing 48 has a plurality of angularly spaced grooves 76 on its radially inner surface and oil from drillings 72 fills the grooves and flows along them due to centrifugal forces.The rear of inner bearing race 74 is provided with a lip 78 to prevent the oil from spilling from grooves 76 in a rearward direction, but some of the oil from grooves 76 is centrifuged down drillings 80 in race 74 in orderto lubricate the ball bearings and cage 82 of bearing 48. The remainder of the oil from grooves 76 spills into the annular chamber 84 between shaft 36 and a forwardly extending sleeve 86 of the bearing inner race 74. A flange 88 on the forward end of sleeve 86 is bolted to the innermost portion of flange 52 of shaft 36, and there is defined between flange 88 and flange 52 a plurality of small angularly spaced apart chambers 90 formed by circular rebates in the rearward-facing surface of flange 52.Drillings 92 through the thickness of flange 52 allow the oil to enter a plurality of angularly spaced-apart chambers 94 formed by "archway"-shaped rebates in the rearface of a radially extending flange 96, which forms part of a centrifugal oil weir 98 according to the invention.

The oil weir 98 is bolted to flange 52 of shaft 36 by means of flange 96 and shares the same bolts (not shown) as flange 88 of bearing inner race 74. The rest of oil weir 98 comprises a generally cylindrical portion 100 which extends axially, i.e. perpendicularly to flange 96. Oil from chambers 94 flows over the radially inner surface of the cylindrical portion 100 of the oil weir, being held thereto by centrifugal force, and is flung off the forward end of cylindrical portion 100 by centrifugal action onto a catcher ring 102 on the bearing cage 104 of bearing 50, from where the oil penetrates into the interior of the bearing. After use in the bearings, the oil collects in the bearing chambers (not shown) which surround the bearings, and is then recycled through an oil drains system (not shown).

The use of a device like the centrifugal oil weir 98 to transfer the oil to bearing 50 is necessary because shafts 34 and 36 rotate independently of each other at different rates, making static passageways between the two shafts impossible. However, it has now been discovered that use of a conventional oil weirto transfer the oil to the bearing 50 may, under certain conditions, be detrimental to safety. This is because in the unlikely event of bearing 50 beginning to fail, e.g. by gradual break-up of the inner or outer bearing tracks or the ball elements due to fatigue, increasing "play" in the bearing will allow the L.P. shaft 34 to move rearward relative to l.P.

shaft 36 under the influence of aerodynamic forces on the fan 20 and the low pressure turbine 32, causing the inner race 56 of bearing 50 to contact the forward end of cylindrical weir portion 100. In the case of a conventional oil weir, this would cut off the oil supply to bearing 50 because the oil could no longer escape from the weir over its forward edge, but would be trapped against the rear face of the inner race 56. This would accelerate deterioration of the bearing 50 and increase frictional heating, leading to fire. In contrast, a centrifugal oil weir according to the invention maintains the supply of oil to a failing bearing for a longer period by providing alternative routes for the flow of oil in the event that the forward end of the weir 98 contacts the inner race 56.

Oil weir 98 and its operation will now be further described with particular reference to Figures 3 to 6.

In orderto provide the required alternative routes for the flow of oil over the weir 98, the cylindrical portion 100 is provided with a plurality of circumferentially spaced apertures in the form of scallops 106 in its forward edge. In order to prevent loss of oil through scallops 106 during normal running of the engine, a shallow flow channel 108 is provided between each adjacent pair of scallops 106, the oil thereby being directed between the scallops 106 to the forward edge of the cylindrical portion 100.

In operation, the oil fills supply chambers 94 and is held therein by centrifugal forces whilst shaft 36 is rotating. The oil escapes from chambers 94 by flowing over the rear lips 110 of flow channels 108, which lips collectively can be termed the "inflow edge" of the radially inner weir surface 112. It then flows over the weir surface 112, confined within channels 108, until it reaches the front lips 114 of flow channels 108, which lips can collectively be termed the "outflow edge" of weir surface 112.

Thereafter, the oil is dispensed from the outflow edge and collected by the lipped catcher ring 102.

Centrifuging of oil off the weir 98 to catcher ring 102 is aided by the small lip 115 on the edge of the weir.

In the event of a reduction in the axial clearance between inner race 56 and the outflow edge 114 of weir surface 112 sufficient to cause complete or partial blockage of the outflow edge, the oil which would normally be dispensed therefrom overflows the channels 108 and is dispensed from the scallops 106 instead.

It will be noticed that the weir surface 112, (and the channels 108 therein) slopes radially outwards from its inflow edge to its outflow edge. This ensures that there is always a positive feed of oil through the channels and over the outflow edge in normal circumstances, or through the channels and over the edges of the scallops if the outflow edge is blocked.

If the inner bearing race 56 does contact the outflow edge of weir surface 112, the weir 98 will gradually be worn away, but the bearing 50 will continue to receive oil until all the scallops have been worn away by continued rearward movement of the race.

In the embodiment shown in the drawings there are eight channels 108 and eight scallops 106. On a weir surface diameter of about 170 mm, it has been found that a channel depth of 0,75 mm is sufficient to enable one channel to carry about 218 litres per hour of lubricating oil at a viscosity of about 10 centistokes. Satisfactory dimensions for the scallops are about 25 mm at the front edge of weir 98 and about 7,5 mm in extent from front to rear.

Although in the above description the apertures which provide the alternative paths for the oil flow are scallops in the edge of the weir, they could of course be notches or slots extending back from the edge. However, scallops are easier to machine than other shapes, since only a rotary cutter is required.

As a further alternative, the apertures could comprise rows of holes at various distances from the outflow edge so that as the weir wears away the oil can pass through each row of holes in turn.

In order to ensure maximum duration of the oil supply under emergency conditions, it should be noted that the oil catcher ring 102 must be of a softer material than the oil weir 98 so that it does not sever the oil weir from its attachment to l.P. shaft 36 under extremes of rearward movement by L.P. shaft 34.

Claims (7)

1. A centrifugal oil weir suitable for supplying oil from oil feed means to a rolling element bearing coaxial therewith but having an axial clearance therefrom, the weir having an axially extending cylindrical portion with a radially inner weir surface, said weir surface having an inflow edge for receiving oil from said oil feed means and an outflow edge for dispensing oil therefrom, an alternative flow path being provided for the oil, said alternative flow path comprising aperture means extending through said cylindrical portion from said weir surface, the arrangement being such that in the event of reduction in said axial clearance sufficient to cause blockage of the outflow edge of said weir surface, that oil which would normally be dispensed from the outflow edge is dispensed through the aperture means instead.

2. A centrifugal oil weir according to claim 1 in which the weir surface is provided with flow channels arranged such that during normal operation the oil does not flow through the aperture means, but if the outflow edge of the weir surface is blocked, the oil overflows the channels and is dispensed through the aperture means.

3. A centrifugal oil weir according to claim 1 or claim 2 in which the weir surface slopes radially outwards from its inflow edge to its outflow edge to assist flow of the oil.

4. A centrifugal oil weir according to any one of claims 1 to 3 in which the aperture means comprise circumferentially spaced-apart notches or scallops in the outflow edge of the weir.

5. A centrifugal oil weir according to any one of claims 1 to 3 in which the aperture means comprise circumferentially spaced-apart slots cut back from the outflow edge of the weir.

6. A centrifugal oil weir according to anyone of claims 1 to 3 in which the aperture means comprise a plurality of rows of circumferentially spaced-apart holes, said rows being at various distances from the outflow edge of said weir.

7. A centrifugal oil weir substantially as described in this specification with reference to, and as illustrated by, Figures 2 to 6 of the accompanying drawings.