Abstract:
A hydraulic restrictor comprising: a passage along which hydraulic fluid flows; a member which extends along the passage, the member restricting the flow along the passage; and structure for selectively securing the member at a plurality of different positions along the passage thereby to enable variation in the restriction on flow along the passage, the securing structure securing the member at each position by securing the member against the side wall of the passage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the US National Stage of International Application No. PCT/EP2005/052670, filed Jun. 9, 2005 and claims the benefit thereof. The International Application claims the benefits of English Patent application No. 0413156.1 filed Jun. 12, 2004. All of the applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   This invention relates to a hydraulic restrictor. 
   BACKGROUND OF THE INVENTION 
   In UK patent application no. (ref. 2004P01248 GB) filed on the same day as the present application, hydraulic restrictors are used in an apparatus for damping the torsional excitation of a drive shaft. The damping is linear and viscous. Accordingly, it is required that the design of restrictor used promote laminar flow, and provide a constant and repeatable restriction (pressure-drop/flow characteristic) under all operating conditions. 
   In the case of a fixed value restrictor, laminar flow and constant and repeatable restriction may be achieved by careful design of the geometry of the restrictor (this is of course up to a certain flow rate threshold). In the case of a variable restrictor, the design is much more complex, and consequently stable/repeatable operation is difficult to achieve. 
   SUMMARY OF INVENTION 
   A possible variable restrictor comprises a passage, and a series of switching devices positioned at various distances along the passage. Variable restriction is achieved by switching in circuit different lengths of the passage. Such a design would provide constant and repeatable variable restriction (pressure-drop/flow characteristics), but is a complex and cumbersome design. 
   Another possible more compact variable restrictor comprises a bore, and a spool or piston which extends along the bore. The hydraulic fluid would pass along the bore in the annular space formed between the spool and the bore. Variable restriction would be achieved by varying the length of engagement of the spool with the bore, and hence the length of the spool along which the fluid must pass. This design is especially vulnerable to variation in eccentricity between the spool and bore. For a given pressure drop across the restrictor, laminar flow may vary by up to 250 percent depending on the eccentricity between the spool and bore. This vulnerability can lead to lack of repeatability and instability. 
   According to the present invention there is provided a hydraulic restrictor comprising: a passage along which hydraulic fluid flows; a member which extends along said passage, said member restricting the flow along the passage; and means for selectively securing said member at a plurality of different positions along the passage thereby to enable variation in the restriction on flow along the passage, said securing means securing said member at each said position by securing the member against the side wall of the passage. 
   Preferably, the securing means secures the member against the side wall of the passage by acting on the opposite side of the member to the side wall. 
   The securing means suitably acts on the flat base of a groove formed in the said opposite side of the member. 
   The securing means may include a plurality of setting shims which interact with one end of the member to assist in the selective securing of the member at the plurality of different positions. 
   Preferably, the one end of the member comprises a length of the member of reduced width and the shims are secured against a shoulder formed between the length of reduced width and the remainder of the member. 
   Preferably: a section of the passage is of expanded width; an end of the member is disposed between the section of expanded width and one end of the passage; and fluid flow through the restrictor is between the section of expanded width and said one end of the passage. 
   The plurality of different positions of the member may be such that the position of said disposed end of the member may be varied between a position immediately beside the section of expanded width and a position a predetermined maximum distance away from the section of expanded width. 
   The restrictor suitably comprises a plurality of cross-holes which extend radially outward from the section of expanded width and supply fluid to/receive fluid from the section of expanded width. 
   Preferably, said passage and said member are of cylindrical form. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a longitudinal cross-section through a restrictor in accordance with the present invention; 
       FIG. 2  is a cross-section on the line B-B in  FIG. 1 ; and 
       FIG. 3  is a cross-section on the line A-A in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF INVENTION 
   Referring to  FIG. 1 , the restrictor comprises a bore  1  formed in a cartridge body  3 , and a spool  5  which extends along bore  1 . Bore  1  and spool  5  are cylindrical in form. 
   The restrictor further comprises setting shims  7 , restraining nut  9 , and locking screw  11  together with associated locknut  13 . Spool  5  includes a spool extension  15  of smaller diameter than the main body of spool  5 . 
   The number of setting shims  7  determines the position along bore  1  of planar end  17  of spool  5 . The shims  7  encircle spool extension  15  and are secured between restraining nut  9  and the shoulder formed between spool extension  15  and the main body of spool  5 . Spool extension  15  is drawn to the left in  FIG. 1  to hold shims  7  firmly between restraining nut  9  and the aforesaid shoulder. Locking screw  11  and its associated locknut  13  are then used to secure spool  5  in position by securing spool  5  against the side wall of bore  1 . A groove  19  machined in spool  5  receives locking screw  11 . Locking screw  11  acts against the flat bottom of groove  19 . Groove  19  also serves to limit axial movement of spool  5  during adjustment of the position of spool  5 . 
   Referring also to  FIG. 2 , spool  5 , when secured in position within bore  1 , is fully eccentric with bore  1 , i.e. the axis of spool  5  is as far as is possible from the axis of bore  1 . An elongate space  21  having parallel sides and a cross-section as shown in  FIG. 2  is defined between spool  5  and bore  1 . 
   The diameter of bore  1 , over a short length L 1  of bore  1 , is increased. Referring also to  FIG. 3 , this creates within cartridge body  3  an internal annular groove  23  concentric with bore  1 . Planar end  17  of spool  5  is located a distance L 2  along bore  1  from adjacent side  25  of groove  23 . The distance L 2  can be varied between zero and a maximum value by varying the number of setting shims  7 . Cartridge body  3  further comprises four cross-holes  27  which extend radially outward from groove  23  and are spaced at 90 degree intervals around groove  23 . 
   The restrictor may be used in “forward” or “reverse” modes of operation. In forward mode, hydraulic fluid is supplied to the restrictor by means of cross-holes  27  and leaves the restrictor by way of open end  29  of bore  1 . In reverse mode, hydraulic fluid is supplied to the restrictor by means of open end  29  and leaves the restrictor by way of cross-holes  27 . In each mode of operation the flow of hydraulic fluid between cross-holes  27  and open end  29  is restricted by spool  5 , and the amount of restriction is determined by the length L 2  of spool  5  between side  25  of groove  23  and planar end  17  of spool  5 . This length L 2  is determined by the number of setting shims  7  used. Thus, the restriction on flow (pressure-drop/flow characteristic) may be varied by varying the number of setting shims  7 . Shims  7  enable variation in restriction over a number of discrete values of restriction. Infinite adjustment could be provided by the use of a screw. The screw must not radially restrain spool  5 . 
   In  FIG. 1  the restrictor is shown secured to a unit  30  by means of screw threads  31  formed on cartridge body  3  and unit  30 . An annular space  33  concentric with bore  1  exists between cartridge body  3  and unit  30 , see also  FIG. 3 . Cross-holes  27  open into annular space  33 . The restrictor is used in the forward mode of operation. A supply passage  35  in unit  30  supplies hydraulic fluid to annular space  33 . The fluid passes down cross-holes  27  to groove  23 , along elongate space  21  between spool  5  and bore  1 , and to open end  29  of bore  1 . A receiving passage  37  in unit  30  receives the fluid from open end  29 . Passage  37  is of expanded diameter in the region where it meets open end  29 . 
   In  FIG. 1  seals  39 ,  43 ,  45  prevent egress of hydraulic fluid to the outside of the assembly and absorption of air by the fluid, and seal  41  prevents egress of fluid between annular space  33  and passage  37 . 
   In the above described restrictor variation in eccentricity between bore  1  and spool  5  is prevented by the securing of spool  5  against the side wall of bore  1 . Bore  1  and spool  5  are of plain cylindrical construction so that movement of spool  5  radially is unrestricted except by the side wall of bore  1 . Spool  5  is able to be secured fully against the side wall of bore  1  since there are no stepped diameters, threads, or other forms of obstruction or irregularity that might result in the tipping or cocking of spool  5  within bore  1 . In all set positions of spool  5  along bore  1 , the eccentricity between spool  5  and bore  1  is the same (the axes of spool  5  and bore  1  are parallel and fully eccentric). Accordingly, variation in restriction between the set positions is dependent only on length L 2 . The construction of the restrictor should be sufficiently solid that there is no vibration between spool  5  and bore  1  as this would result in unstable operation.