Brush seal assembly

A brush seal assembly for effecting a seal between a first component (12) and a surface of a second component (11) movable relative to the first component. The assembly has a first row of bristles (17) arranged to be supported by the first component (12) and to extend towards but not touch the surface of the second component (11), and a second row of bristles (19) arranged to wipe over the surface of the second component which second row of bristles (19) lies against one side of the first row of bristles (17). The bristles (17) of the first row are shorter than those of the second row, and have a greater stiffness. The seal assembly includes a backing plate (15) intended to be disposed on the low pressure side of the seal and which supports the adjacent row of bristles. Depending upon the intended use, either the first row or the second row of bristles may be supported by the backing plate (15).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a brush seal assembly adapted to effect a seal 
between a first component and a surface of a second component which is 
relatively movable with respect to the first component. 
2. Description of the Related Art 
It is frequently necessary to effect a fluid-tight seal between two 
relatively movable components, such as between a rotatable shaft and a 
housing having a bore through which the shaft extends. Innumerable seal 
designs have been proposed for this purpose and a machine designer has to 
select the most appropriate type of seal, having regard to the likely 
operating conditions to be encountered in service. 
For the case of a shaft extending through a bore in a housing and which 
shaft rotates at a relatively high speed in the presence of pressurised 
hot gases, such as may be encountered in a gas turbine or a jet engine, 
various designs of brush seal have been developed in an attempt to 
overcome the unsatisfactory performance of simpler seals having for 
example a synthetic elastomeric annular sealing member mounted in the bore 
of the housing and rubbing on the surface of the shaft. In a brush seal, a 
plurality of fine bristles (such as of bronze or stainless steel wire) are 
held in a carrier mounted on the housing, with the tips of the bristles 
wiping the surface of the shaft so as to effect a seal thereagainst. 
In a typical brush seal intended for use with a pressure difference 
thereacross, the bristles are supported against deflection in a direction 
parallel to the shaft axis for the greater part of the length of the 
bristles by means of an annular backing plate on the low-pressure side of 
the seal, with the tips of the bristles projecting radially beyond the 
backing plate to wipe against the shaft surface to be sealed. If it can be 
expected that the shaft is likely to perform relatively large radial 
excursions--for instance, as may occur when an engine rotor is being run 
up to operating speed--the clearance between the backing plate and the 
shaft must be increased, so leading to a greater unsupported length of 
bristle, projecting beyond the backing plate. If then the seal is 
subjected to a relatively large pressure difference, the unsupported 
bristle tips projecting beyond the backing plate will tend to deflect 
towards the low pressure side and so lift clear of the shaft, and the 
sealing effect will be lost. If the bristles are made stiffer (for example 
by increasing the diameter of each bristle) the bristles may be too stiff 
to give an adequate sealing function able to accommodate minor radial 
excursions between the shaft and the housing; also rapid wear of the shaft 
may be expected as a consequence of the relatively stiff bristles wiping 
the shaft. 
In addition to the above problem, it is possible for a brush seal to be 
subjected to an abnormal pressure surge where the side of the seal 
normally subjected to high pressure is suddenly subjected to a lower 
pressure than is present on the other side of the seal. If this occurs, 
the seal can be destroyed. It will be appreciated that it is in general 
not possible to employ a second backing plate on the high pressure side of 
the seal, since the pressure of the two backing plates is likely to 
interfere with the free movement of the bristles. 
It is a principal aim of the present invention to provide a brush seal 
assembly adapted to effect a seal between two relatively movable 
components and which is able to accommodate relatively large excursions of 
one component with respect to the other and out of the normal line of 
movement between the two components, and yet which seal assembly is still 
able to achieve a good sealing effect notwithstanding a significant 
pressure difference thereacross. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a brush seal assembly 
for effecting a seal between a first component and a surface of a second 
component relatively movable with respect to the first component, which 
seal assembly comprises a backing plate adapted for mounting on the first 
component to extend towards said surface of the other component, a first 
row of bristles projecting beyond the backing plate towards said surface 
of the other component, and a second row of bristles lying against the 
bristles of the first row, the bristles of the second row projecting 
beyond the tips of the bristles of the first row and adapted to contact 
and effect a seal against said surface of the other component, and a 
majority of the bristles of the second row each having a lesser stiffness 
than the stiffness of each of a majority of the bristles of the first row. 
In the following further description of the present invention, reference 
will primarily be made to the sealing of a housing (the first component) 
to a rotatable shaft (the second component) extending through a bore in 
the housing. It will however be understood that the brush seal assembly 
may be used to effect a seal between other relatively movable components 
besides a housing and a shaft, and references herein to a housing and to a 
shaft should be construed accordingly. 
One side of the first row of bristles may lie against and be supported by 
the backing plate, with the second row of bristles lying against the 
bristles of the first row on the other side thereof. Such a seal assembly 
is able to accommodate relatively large movements between the first and 
second components in a direction normal to a tangent to the surface of the 
second component, since a relatively large clearance may be provided 
between the backing plate mounted on the first component and said surface 
of the second component. The bristles effecting the seal against said 
surface (the bristles of the second row) may be of a relatively low 
stiffness, such as are commonly employed in the brush seal art, and yet 
those bristles are supported against deflection under an applied high 
pressure by the first row of bristles which are relatively stiff but which 
do not ordinarily contact said surface of the second component. Should 
however the second component perform a relatively large excursion with 
respect to the first component, that may be accommodated by said surface 
initially contacting and perhaps then also deflecting the bristles of the 
first row, without contact occurring between the second component and the 
backing plate. In this way, damage will be avoided because occasional 
contact between said surface of the second component and the bristles of 
the first row will not adversely affect seal performance. 
As an alternative, the second row of bristles may lie against and be 
supported by the backing plate, with the first row of bristles lying 
against the bristles of the second row on the side thereof remote from the 
backing plate. Such a brush seal is able to withstand an abnormal reverse 
pressure surge with a much reduced likelihood of damage, since the stiffer 
first row of bristles will serve to prevent excessive deflection of the 
bristles of the second row during such a pressure surge. Despite this, 
since the bristles of the first row are flexible the sealing effect of the 
bristles of the second row is not significantly affected by the pressure 
of the first row of bristles lying against one side of the second row, 
with the backing plate against the other side of the first row. 
The bristles of the first row may be given a greater stiffness than those 
of the second row by an appropriate selection of the material from which 
the bristles are made. Preferably, though, at least a majority of the 
bristles of the second row each has a smaller cross-sectional area than 
the cross-sectional area of each of a majority of the bristles of the 
first row. 
Most preferably, each bristle of the first row has substantially the same 
cross-sectional area; equally, each bristle of said second row may have 
substantially the same cross-sectional area, which area is smaller than 
that of each bristle of the first row. Moreover, each row of bristles 
advantageously comprises a mass of bristles in a closely packed manner 
both across the width of the row and along its length--that is to say, in 
the case of a shaft seal, each row has an axial width made up from a 
plurality of closely packed bristles. Typically, in the axial direction, 
the width may be made up from 2 to 10 bristles. 
In a preferred form of brush seal assembly of this invention, the diameter 
of each bristle in the first row may be from 2 to 10 times as great as the 
diameter of each bristle in the second row. Moreover, though the bristles 
in each row may be made of substantially the same material, in certain 
circumstances it may be advantageous to manufacture the bristles of the 
first row from a relatively high stiffness material and to manufacture the 
bristles of the second row from a softer material better able to effect a 
running seal against a shaft. 
For certain applications of the seal assembly of this invention, a 
relatively thin spacer ring may be disposed between the rows of bristles 
at the root end thereof (that is, the ends of the bristles remote from the 
tips adjacent or touching the surface of the shaft). Such a spacer ring 
will have the effect of separating the rows of bristles in the region of 
their root ends, whereby the bristles of the second row are supported for 
a part only of their length which overlies the bristles of the first row. 
This construction allows a more compact bristle line at the ends of the 
bristles of the second row. 
A modified form of brush seal assembly of this invention may have a third 
and possibly even further rows of bristles, with the bristle stiffness 
reducing for each subsequent row of bristles, and only the tips of the 
last row of bristles wiping the sealing surface of the second component. 
In such a modified form of brush seal, the tips of each row of bristles 
should project beyond the bristles of the preceding row, so that the 
bristles of one row are supported other than at their tips by the bristles 
of the preceding row. In this way, all of the preceding rows lend support 
to the bristles of the ultimate row, to resist deflection of the bristles 
of that row under a high pressure difference across the seal, whilst at 
the same time allowing the accommodation of relatively large excursions of 
the shaft. 
An alternative modified brush seal assembly of this invention may have a 
third row of bristles similar to the first row, with the second row 
disposed between the first and third rows. Such a seal assembly has the 
combined advantages discussed above--that is to say, it may accommodate 
unusually large shaft excursions because of the clearance between the 
shaft and the backing plate can be increased in view of the pressure of 
the first row of bristles; and it may also withstand an abnormal reverse 
pressure surge without damage to the second row, because of the presence 
of the third row of bristles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring initially to FIGS. 1 and 2, the brush seal assembly 10 of this 
invention is intended to effect a seal between a rotatable shaft 11 and a 
housing 12 having a bore 13 through which the shaft 11 extends. The seal 
assembly 10 is mounted in a counterbore 14 coaxial with the bore 13 and 
comprises an annular backing plate 15 having an internal bore 16 of a 
diameter greater than the diameter of the shaft 11, whereby a clearance C 
is defined therebetween. Lying against the backing plate 15 is a first 
annular row of bristles 17, each bristle being of about 0.010" diameter 
(about 0.25 mm), and the free tips 18 of which bristles project beyond the 
inner periphery of the backing plate 15, but which do not contact the 
shaft 11. Lying against and supported by the first row of bristles 17 is a 
second row of bristles 19, each bristle being of about 0.0028" diameter 
(about 0.07 mm), and being of such a length that the free tips 20 of those 
bristles project beyond the tips 18 of the bristles 17, and wipe against 
the shaft 11. The seal assembly is completed by a clamp ring 21, the 
backing plate 15, the root ends 22 of the bristles 17 and 19 and the clamp 
ring 21 all being welded together or otherwise bonded by a suitable 
process, and the seal assembly 10 being held in position in the 
counter-bore 14 by means of a circlip 23, located in a groove 24 formed in 
the counter-bore. 
As can be seen from FIG. 2, all of the bristles, though extending generally 
radially, nevertheless lie at a constant angle to the true radial 
direction; the shaft 11 should rotate in the direction marked A, in order 
that the tips 20 of the bristles 19 may wipe smoothly over the surface of 
the shaft, and deflect along their lengths should the shaft perform an 
excursion in a radial plane. 
The bristles 17 and 19 are made of essentially the same material, but the 
bristles 17 have a greater diameter than the bristles 19. Typically, the 
bristles 17 will have a diameter of about 4 times as great as the diameter 
of the bristles 19. 
When in use, if the seal assembly is subjected to a high pressure 
difference, with the high pressure on the right hand side as illustrated 
in FIG. 1, the tips 20 of the bristles 19 will tend to deflect to the low 
pressure (left hand) side of the seal assembly but in view of the support 
provided by the bristles 17, very little deflection will take place and an 
excellent sealing effect will still be maintained. If the shaft 11 should 
undertake minor excursions in a radial plane, these will be accommodated 
by deflection of the bristles 19, but should the shaft perform relatively 
large excursions, then the shaft will contact the bristles 17 and these 
too may be deflected notwithstanding their greater stiffness. Since the 
shaft ordinarily will not be in contact with the bristles 17, no rapid 
wear of the shaft will be caused despite the greater stiffness of those 
bristles. 
FIG. 3 shows an alternative form of seal assembly, and like parts with 
those of FIGS. 1 and 2 are given like reference characters; those parts 
will not be described again here. In the seal of FIG. 3, there is provided 
between the bristles 17 and 19, at the root ends thereof, a relatively 
thin spacer ring 25, this spacer ring 25 being welded in position during 
assembly of the seal. Despite the provision of this ring 25, the bristles 
of the two rows thereof will be in close contact at least in the region of 
the tips 18 of the bristles 17, as the bristles tend to spread to some 
extent, so that the space created by the spacer ring 25 will be lost in 
the region of the bore of the backing plate 15. A more compact bristle 
line at the bore may be achieved by this construction, since the bristles 
converge with a conical profile. 
FIG. 4 shows an alternative embodiment of seal assembly of this invention, 
where the bristles 17 of the first row lie at an angle different to that 
of the bristles 19 of the second row. In all other respects, the seal 
assembly is the same as that of FIG. 2, but in this alternative 
embodiment, the likelihood of the bristles of the two rows becoming 
intermingled is much reduced. 
FIG. 5 shows a linear brush seal of this invention, which is constructed in 
much the same manner as has been described above, but in this case the 
back-plate 21 is linear, and the two rows of bristles 17 and 19 extend at 
substantially equal but opposite angles to that back-plate. This seal may 
be used to seal against a flat surface, and possesses the same advantage 
as that of the embodiment of FIG. 4. 
FIG. 6 shows a further brush seal assembly, generally similar to that of 
FIG. 3 but having the second row of bristles 19 lying against the backing 
plate 15, and the shorter, stiffer first row of bristles lying against the 
second row of bristles on the side thereof remote from the backing plate 
15. As in the assembly of FIG. 3, there is a spacer ring 25 welded between 
the two rows of bristles at the root ends thereof. Thus, the bristles of 
the first row contact the bristles of the second row for a part only of 
their respective lengths, with that contact being concentrated towards the 
tip ends of the bristles. 
As with the previous embodiments of seal, that of FIG. 6 is intended to 
have the backing plate on the low pressure side but the seal is able to 
withstand a reverse pressure surge in view of the presence of the 
relatively stiff first row of bristles on the normally high pressure side 
of the seal. In view of the flexible nature of those bristles, their 
presence does not adversely effect to any great extent the operation and 
sealing characteristics of the second row of bristles 19. 
FIG. 7 shows a further embodiment, incorporating the features of the seals 
of FIGS. 3 and 6. Here, there is provided a third row of bristles 26 
substantially the same as the first row of bristles 17 and arranged on the 
high pressure side of the seal, as in the embodiment of FIG. 6. Thus, this 
seal assembly is able to withstand an abnormal reverse pressure surge in 
view of the presence of the third row of bristles 26, and also is able to 
accommodate relatively large excursions of the shaft in view of the 
presence of the first row of bristles 17, enabling the clearance between 
the shaft and the backing plate 15 to be increased.