Seal for a joint or juncture – Seal between relatively movable parts – Brush seal
Reexamination Certificate
1999-01-11
2001-07-31
Brittain, James R. (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Brush seal
Reexamination Certificate
active
06267381
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention relates generally to a seal arrangement and in particular to improvements to leaf seals.
BACKGROUND OF THE INVENTION
It is frequently necessary to effect a seal in a clearance gap between two components that are capable of relative movement. In particular seals are often required to seal between a rotatable shaft and a housing surrounding the shaft, the housing having a bore through which the shaft extends.
Within a gas turbine engine there are shafts that rotate at relatively high speeds and are exposed to pressurised hot gases. Seals are required for these shafts and the seal performance can have a significant effect on the overall efficiency of the gas turbine engine. There are a number of seal designs that have been proposed for such purposes and that are designed to operate within the harsh environment found within a gas turbine engine.
One type of seal that has been developed for shafts that rotate at relatively high speeds, as found in gas turbine engines, is a brush seal. In such a brush seal, a plurality of fine bristles (such as bronze or stainless steel wire) are held in a carrier mounted on a housing. The tips of the bristles wipe against the shaft so as to effect a seal thereagainst.
A problem with such brush seals is that in use the bristles tend to flex in an axial, sealing direction, away from the high pressure side of the seal towards the low pressure side. Bending of the bristles in this way reduces the effective radial length of the bristles and so reduces the contact between the bristle tips and the shaft. In turn this reduces the sealing efficiency. Additionally flexing of the bristles can clamp the bristles against each other and against a backing member of the seal. Friction between the bristles and the backing member stiffens the bristles and prevents free movement of the bristles so that they no longer follow the shaft. Excursions of the rotating shaft are therefore not accommodated by the bristles. This leads to clearance gaps opening up between the bristle tips and shaft, and at the other extreme increased wear of the bristles, both of which reduce the seal performance.
An alternative type of seal that has been proposed for gas turbine engines, and also other applications, is a leaf seal. Leaf seals, which are sometimes called foil seals, comprise a number of thin resilient yet flexible strips which are densely packed together with the strips substantially aligned in the sealing direction. One end of the strips is held within a housing whilst the other, distil end of the strip extends across the seal gap and rests on the shaft. Such a seal design is described in European Patent Application EP 0,391,676. As described the seal elements, comprising the resilient strips in this type of seal are axially stiff and do not tend to bend in the axial, sealing, direction. This type of seal thereby alleviates some of the problems described above in relation to brush seals.
In EP 0,391,676 the resilient strips are described as being packed essentially solidly. This will prevent or restrict the radial movement of the strips. Consequently the seal described is also stiff in a radial direction. Problems can therefore occur during radial excursions of the shaft into the strips. The radial stiffness of the seal reacts against the radial movement of the shaft, causing high tip loads and increased wear of the strips.
It is therefore desirable to provide an improved seal design that alleviates the above problems and/or offers improvements generally.
SUMMARY OF THE INVENTION
According to the present invention there is provided a seal arrangement for effecting a seal between two components to inhibit a flow of fluid, between the two components, in a sealing direction from an upstream side of the seal to a downstream side of the seal, the downstream side of the seal arranged in use to be at a lower pressure than the upstream side of the seal, the seal comprising a carrier adapted for mounting from one of the components, and a plurality of densely packed resilient strips mounted on the carrier to extend therefrom, each strip having a tip which is adapted to wipe on the other component to effect a seal thereagainst in the sealing direction, the strips arranged substantially parallel to the sealing direction,
characterised in that there is provided a gap between adjacent strips, at least at the tip portions of the strips, to allow the tip portions of the strips to move relative to each other and so maintain in operation a flexible seal.
By providing a small discrete gap between the individual strips of the seal at the point where the strips touch the shaft the strips can move relative to each other. A degree of radial movement of the shaft into the seal can thereby be accommodated by movement of the strips without the seal becoming effectively solid.
The seal is also straightforward to manufacture and is relatively cheap to produce.
Preferably one component is a rotatable shaft and the other component is a stationary housing.
Preferably the strips are tapered such that the thickness at the tip of each strip which is arranged to wipe against the other component is less than the thickness at the end portion of the strips that is mounted on the carrier.
Furthermore along the length of each strip, toward its tip, there may be a step change reduction in the thickness of the strip. The thickness of each strip may preferably be reduced in a series of steps along the length of the strip towards its tip.
A spacer strip may be interposed between adjacent strips there may be provided a spacer strip, the spacer strip extends from the carrier and is of a shorter length than the strips thereby providing a gap between the tips of the strips.
Preferably when viewed in the sealing direction the strips extend from the carrier at an angle to a direction perpendicular to the carrier.
The strip may be bent part way along its length such that the strip is divided into two portions, a first portion that is mounted on and extends from the carrier, and a second portion which extends from the first portion and is arranged to wipe against the other component, the second portion extending at an angle to the first portion. Furthermore the strips may be bent at a plurality of positions along their length such that the strips are divided into a plurality of portions, each portion extending at a different angle to the other portion of the strip. The strips may be curved in the direction of the length of the strips extending from the carrier to the tips of the strips that are arranged to wipe against the other component.
Bending, or curving the individual strips of the seal provides an improved method of ensuring that there is a gap between individual strips whilst allowing the strips to abut each other at the other end. By abutting the seal at one end, where a gap between strips is not needed, the overall leakage through the seal is reduced. Bending of the strips to control the gap between strips is also simpler to manufacture than radially tapering the thickness of the strips. The individual strips, since they abut at one end, can also be simply and easily assembled and fixed in position by welding or other means.
Preferably the strips are mounted with the width of the strips at an angle to the sealing direction of the seal.
The seal may be provided with a damper which acts on the strips to damp relative movement of the tips of the strips which are arranged to wipe against the other component. Preferably the damper comprises a front skirt and back skirt which extend from the carrier on opposite sides of the strips, the front and back skirts are separated from the edges of the strips by a gap and enclose a substantial portion of the strips therebetween such that, in use, viscous damping of the strips is produced. The damper may comprise at least one finger member that extends from the carrier, the tip of the finger member touches and is urged against one of the edges of a plurality of the strips part way along the length of the strips. The damper may comprise
Beres John L.
Brittain James R.
Manelli Denison & Selter PLLC
Rolls-Royce plc
Taltavull W. Warren
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