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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to brushes used for cleaning wellbores and subterranean tubular members. 
     2. Description of the Related Art 
     Brushes are used to clean and remove debris from wellbore casings, liners and other tubular members in a wellbore. A common occurrence requires these tools to pass through restrictions in the diameter of the tubular member being cleaned. Many conventional tools rely upon the flexibility of brush bristles to allow the brush to pass through such restrictions. In practice, unfortunately, damage often occurs easily to the bristles of downhole brush tools as they are passed through tubular restrictions. 
     SUMMARY OF THE INVENTION 
     The invention provides devices and methods for protecting the bristles of downhole brushes during operation. Downhole expandable brushes are described which include a housing that can be affixed to a running string and a brush assembly with at least a single set of brush bristles that extend outwardly through the housing and can be radially extended with respect to the housing to radially expand the brush. Additionally, the bristles of the brush may be radially retracted with respect to the housing so that the brush may be passed through restrictions in the surrounding tubular member without damaging the bristles. According to described embodiments, axial movement of a piston member with respect to the housing in a first axial direction causes the brush assembly to move radially outwardly while axial movement of the piston member with respect to the housing permits the brush assembly to retract radially with respect to the housing. 
     According to a first exemplary embodiment, the housing of the brush defines an interior axial flowbore through which fluid can be flowed. A piston member is retained within the flowbore and is axially moveable therein. Axial movement of the piston member with respect to the housing rotates one or more cams having eccentric profiles. The cam(s) will urge a brush assembly, having a shoe and bristles, radially outwardly. Reverse rotation of the cam(s) will permit the brush member to retract into the housing. 
     In a second described embodiment, a piston member is retained within the flowbore and is operably interconnected with a flexible linkage. Axial movement of the piston member with respect to the housing causes the linkage to flex and move a brush member radially outwardly. Reverse axial movement of the piston member will cause the linkage to unflex and move the brush member radially inwardly with respect to the housing. 
     According to a third embodiment, a brush member is mounted upon a shoe, which has an angled inwardly-directed ramp surface. A piston member is moveably disposed within a flowbore of the housing and presents an angled or conical surface which contacts the ramp surface of the shoe. Axial movement of the piston member moves the shoe and affixed bristles radially outwardly with respect to the brush housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
         FIG. 1  is a side, cross-sectional view of an exemplary wellbore containing a running string with an affixed downhole brush tool in accordance with the present invention. 
         FIG. 2  is a side, cross-sectional view of an exemplary expandable brush in accordance with the present invention in a reduced diameter condition. 
         FIG. 3  is a side, cross-sectional view of the brush shown in  FIG. 2 , now in an expanded diameter condition. 
         FIG. 4  is a detail view of a cam member used within the brush shown in  FIGS. 1-2 . 
         FIG. 5  is a side, cross-sectional view of an exemplary alternative embodiment for an expandable brush in accordance with the present invention in a reduced diameter configuration. 
         FIG. 6  is a side, cross-sectional view of the brush shown in  FIG. 5 , now in a radially expanded condition. 
         FIG. 7  is a side, cross-sectional view of another exemplary alternative embodiment for an expandable brush in accordance with the present invention in a reduced diameter configuration. 
         FIG. 8  is a side, cross-sectional view of the brush shown in  FIG. 7 , now in a radially expanded condition. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an exemplary wellbore  10  that has been drilled through the earth  12  from the surface  14 . The wellbore  10  is lined with metallic casing  16  of a type known in the art. The casing  16  presents a diametrical restriction  18 . It is desired to clean or polish casing  16 , particularly below the restriction  18 . 
     A running string  20  is shown disposed into the wellbore  10  from the surface  14 . The running string  20  may be coiled tubing or be made up of conventional drill string tubulars or have other constructions known in the art. An expandable brush  22  is shown affixed to the running string  20 . 
     A first exemplary expandable brush  22  is illustrated in  FIGS. 2 and 3 . The brush  22  includes a generally cylindrical outer housing  24  which is affixed to the running string  20  via threaded connection  25 . The housing  24  is shaped and sized to fit within a surrounding tubular member which it is desired to clean, such as casing  16 . The housing  24  has an axial fluid flowbore  26  defined along the length of the housing  24 . An axially-facing stop shoulder  28  is formed within the flowbore  26 . Lateral windows  30  are formed within the housing  24 . In the depicted embodiment, there are two windows  30  shown. It should be understood, however, that this is for illustrative purposes only, and that there may be more or fewer than two windows  30  in practice. A piston member  32  is disposed within the flowbore  26 . The piston member  32  includes an inner axial fluid passage  34  along its length. A ball or plug seat  36  is formed within the fluid passage  34 . The piston member  32  presents an enlarged diameter section  38  and a reduced diameter section  40  which extends axially downwardly from the enlarged diameter section  38 . A downward-facing shoulder  42  is presented on the outer radial surface of the piston member  32  between the enlarged diameter and reduced diameter sections  38 ,  40 . The reduced diameter section  38  presents a radially outer toothed or notched profile  44 . 
     A compression spring  46  is disposed radially between the outer housing  24  and the piston member  32 . The compression spring  46  is located axially between the downward-facing shoulder  42  and an upward-facing shoulder  48  that is formed within the flowbore  26 . 
     A brush assembly  50  is disposed within each of the windows  30 . Each brush assembly  50  includes a brush shoe  52  that is shaped and sized to reside within its window  30  and be radially moveable inwardly and outwardly within the window  30 . Bristles  54  are fixedly secured within each shoe  52  and extend radially outwardly therefrom. In a currently preferred embodiment, each shoe  52  has perforated end portions  56  which are slidably mounted upon rods  58  that are embedded within the outer housing  24 . This permits the shoes  52  to move radially inwardly and outwardly through the windows  30 . 
     Rotatable cams  60  are also located within each of the windows  30  which are used to cause the brush assemblies  50  to be moved radially outwardly through the windows  30  when desired or withdrawn radially within the windows  30  when desired. In the depicted embodiment, there are two cams  60  shown located within each of the windows  30 . However, it should be understood that there may be more or fewer than two such cams  60  for each window  30  or for each shoe  52 , as desired. Each of the cams  60  rotate about a central pivot  62 . As best seen in  FIG. 4 , each cam  60  features an eccentric outer profile  64  having a radially reduced profile portion  66  and an extended radius profile portion  68 . The outer profile  64  of each cam  60  also features a toothed or notched portion  70  having teeth or notches that are shaped and sized to interfit with the teeth or notches of the toothed or notched profile  44  of the piston member  32  in a complementary manner. 
     The expandable brush  22  may be moved between a radially reduced configuration and a radially expanded configuration. In the radially reduced configuration ( FIG. 2 ), the shoes  52  and bristles  54  are radially withdrawn within the outer housing  24 , thereby permitting the brush  22  to be passed through wellbore restrictions, such as diametrical restriction  18  in  FIG. 1 . In the radially expanded configuration, the shoes  52  and bristles  54  are extended radially outwardly through their respective windows  30  in the housing  24  (see  FIG. 3 ). 
     In order to cause the brush assemblies  50  of the brush  22  to be moved radially outwardly to the position shown in  FIG. 3 , a ball  72  is dropped into the running string  20  and enters the flowbore  26  of the brush  22 . The ball  72  lands on the ball seat  36  of the piston member  32  and blocks fluid flow downwardly through the fluid passage  34 . Thereafter, the running string  20  can be pressurized to cause the piston member  32  to move axially downwardly within the flowbore  26  until the downward-facing shoulder  42  of the piston member  32  is brought into abutting contact with the stop shoulder  28  of the outer housing  24 . Compression spring  46  is compressed. 
     As the piston member  32  is moved axially downwardly within the housing  24 , the toothed or notched interface between profile  44  and the toothed or notched portion  70  of the cams  60  cause the cams  60  to be rotated about their pivots  62 . The cams  60  are rotated from a position wherein the reduced profile portion  66  is adjacent the shoe  52  of the brush assembly  50  (see  FIG. 2 ) to a position wherein the extended radius profile portion  68  is adjacent the shoe  52  (see  FIG. 3 ), thereby moving the brush assembly  50  radially outwardly through its window  30 . 
     In order to return the brush  22  to its reduced diameter configuration, fluid pressurization within the running string  20  is stopped. The compression spring  46  will urge the piston member  32  axially upwardly within the flowbore  26  of the outer housing  24 . As the piston member  32  is moved axially upwardly the cams  60  are rotated in a reverse direction back to their original positions, thereby permitting the brush assemblies  50  to be moved radially inwardly to the positions illustrated in  FIG. 2 . 
       FIGS. 5 and 6  illustrate an alternative expandable brush  80 . The brush  80  includes an outer housing  82  with axial flowbore  84  defined along its length. Lateral windows  86  are formed within the housing  82 . A radially enlarged piston chamber  88  and spring chamber  90  are formed within the flowbore  84 . A piston member  92  resides within the piston chamber  88  and spring chamber  90  and is axially moveable therewithin. The piston member  92  includes a radially enlarged upper portion  94  and a radially reduced lower portion  96 . An axial fluid passage  98  is formed within the piston member  92 . Ball seat  100  is formed within the fluid passage  98 . 
     A compression spring  102  resides within the spring chamber  90  to radially surround the lower portion of the piston member  92 . The spring  102  is bounded at the upper end by the enlarged diameter upper portion  94  of the piston member  92  and at its lower end by a shoulder  104  formed in the housing  82  at the lower end of the spring chamber  90 . Thus, the compression spring  102  urges the piston member  92  upwardly with respect to the housing  82 . 
     Brush assemblies  106  are movably disposed within each window  86 . In the depicted embodiment, there are four windows  86  (three visible) formed within the housing  82  and a brush assembly  106  is associated with each window  86 . However, it should be understood that there may be more or fewer than four windows  86  and brush assemblies  106 . Each of the brush assemblies  106  features a brush shoe  108  with bristles  110  extending radially outwardly therefrom. Each of the brush assemblies  106  also includes a flex linkage  112  that is made up of articulated arms  114  and pivot points  116  which join lower arms  114  to the shoe  108 . In the depicted embodiment, there are two arms  114  which support each shoe  108 . Pivot points  118  join the arms  114  to the housing  82 . Pivot points  120  is affixed to the piston member  92 . Thus, downward movement of the piston member  92  will cause the arms  114  to move about their pivot points  116 ,  118  and  120  so that the shoe  108  and bristles  110  are moved radially outwardly through their respective window  86  ( FIG. 6 ). 
     In order to move the brush  80  from the radially retracted position ( FIG. 5 ) to the radially expanded position ( FIG. 6 ), a ball  122  is dropped into the running string  20  from surface  14 . The ball  122  lands on the ball seat  100 , and the running string  20  is pressured up behind the ball  122  urging the piston member  92  axially downwardly within the housing  82  and compressing spring  102 . The flex linkage  112  is articulated so that the shoe  108  and bristles  110  are moved radially outwardly. To return the brush  80  to the radially reduced configuration, the running string  20  is unpressurized, allowing the compression spring  102  to urge the piston member  96  axially upwardly and returning the brush  80  to the reduced diameter configuration shown in  FIG. 5 . 
       FIGS. 7 and 8  depict a further alternative expandable brush  130 . The brush  130  includes a housing  132  having axial flowbore  134  defined along its length. Lateral windows  136  are formed within the housing  132 . A radially enlarged piston chamber  138  and spring chamber  140  are formed within the flowbore  134 . A piston member  142  resides within the piston chamber  138  and spring chamber  140  and is axially moveable therewithin. The piston member  142  includes a radially enlarged upper portion  144  and a radially reduced lower portion  146 . An axial fluid passage  148  is formed within the piston member  142 . Ball seat  150  is formed within the fluid passage  148 . 
     A compression spring  152  resides within the spring chamber  140  to radially surround the lower portion  146  of the piston member  142 . The spring  152  is bounded at the upper end by the enlarged diameter upper portion  144  of the piston member  142  and at its lower end by a shoulder  154  formed in the housing  132  at the lower end of the spring chamber  140 . Thus, the compression spring  152  urges the piston member  142  upwardly with respect to the housing  132 . 
     Brush assemblies  156  are movably disposed within each window  136 . In the depicted embodiment, there are four windows  136  (three visible) formed within the housing  132  and a brush assembly  156  is associated with each window  136 . However, it should be understood that there may be more or fewer than four windows  136  and brush assemblies  156 . Each of the brush assemblies  156  features a brush shoe  158  with bristles  160  extending radially outwardly therefrom. Each brush shoe  158  is shaped and sized to be moveable radially inwardly and outwardly through its respective window  136 . Preferably, a retainer lip  162  is formed at the periphery of each window  136  to prevent the brush shoe  158  from being lost outside of the housing  132 . Compression spring  164  biases the brush shoe  158  radially inwardly. 
     Each brush shoe  158  presents a radially-inward facing angled ramp face  166 . The lower portion  146  of the piston member  142  presents an angled or conical surface  168  that contacts the ramp faces  166  of the brush shoes  158 . Due to the interface of the angled surface  168  and ramp faces  166 , downward axial movement of the piston member  142  within the housing  132  will move the brush shoes  158  and bristles  160  radially outwardly with respect to the housing  132 . 
     In order to move the brush  130  from the radially retracted position ( FIG. 7 ) to the radially expanded position ( FIG. 8 ), a ball  170  is dropped into the running string  20  from surface  14 . The ball  170  lands on the ball seat  150 , and the running string  20  is pressured up behind the ball  170  urging the piston member  142  axially downwardly within the housing  132  and compressing spring  152 . The brush shoes  158  and bristles  160  are moved radially outwardly. To return the brush  130  to the radially reduced configuration, the running string  20  is unpressurized, allowing the compression spring  152  to urge the piston member  142  axially upwardly and returning the brush  130  to the reduced diameter configuration shown in  FIG. 7 . 
     The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.

Summary:
Expandable brushes for cleaning a surrounding tubular member. Brushes include a housing to be disposed within the surrounding tubular member and a brush shoe that retains at least one brush bristle, the brush shoe being radially moveable with respect to the housing between a radially retracted position and a radially expanded position.