Abstract:
A nozzle with changeable press fit and a method for calibrating a nozzle is described. The nozzle may be used in a nozzle/flapper type servovalve. The method comprises the steps of providing a nozzle within a cylindrical body defining a cylindrical bore, the nozzle having a tubular shape with an outer cylindrical surface and an inner radial surface., the method further comprises the steps of positioning a first, tubular locking member within the nozzle, and axially moving the first, tubular locking member within the nozzle. The first tubular locking member is configured to cause the nozzle to become positionally fixed at a selected position within the bore in response to the first, tubular locking member being axially moved relative to the nozzle. A nozzle positioning system is also described herein.

Description:
FOREIGN PRIORITY 
       [0001]    This application claims priority to European Patent Application No. 16155188.2 filed Feb. 11, 2016, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The examples described herein relate to a method for positioning and locking a nozzle in place within a valve body. The examples described herein also relate to a nozzle for positioning and locking in place within a servovalve body. The nozzles and method may be used, amongst other applications, in conjunction with a flapper/type servovalve. 
       BACKGROUND 
       [0003]    A hydraulic servovalve is a servo with a device (either flapper nozzle or jet pipe) used to position the servo. When servovalves are controlled through an electrical signal they are called electrohydraulic servovalves. Servovalves are normally used when accurate position control is required and this position control may be achieved through a closed loop control system, consisting of command sensor, feedback sensor, digital or analogue controller, and the servovalve. 
         [0004]    Flapper nozzle systems for use in servovalves are well known. Flapper position is controlled by the electromagnetic torque motor and the torque developed by the torque motor is proportional to the applied current, with currents generally being in the milliamp range. A torque motor consists of two permanent magnets with a coil winding attached to a magnetically permeable armature. The armature is part of the flapper piece. When a current is applied to the coils, magnetic flux acting on the ends of the armature is developed. The direction of the magnetic flux (force) depends on the direction of the current. The magnetic flux will cause the armature tips to be attracted to the ends of the permanent magnets (current direction determines which magnetic pole is attracting and which one is repelling). This magnetic force creates an applied torque on the flapper assembly, which is proportional to the applied current. In the absence of any other forces, the magnetic force would cause the armature to contact the permanent magnet and effectively lock in this position. However, other forces are acting on the nozzle, such that flapper position is determined through a torque balance consisting of magnetic flux (force), hydraulic flow forces through each nozzle, friction on the flapper hinge point, and any spring (wire) connecting the flapper to the spool (which is almost always installed used in servovalves to improve performance and stability). 
         [0005]    As the applied current is increased, the armature and flapper will rotate. As the flapper moves closer to one nozzle, the flow area through this nozzle is decreased while the flow area through the other nozzle increases. 
         [0006]    Servovalves can be used to control hydraulic actuators or hydraulic motors. When a servoactuator is used to control an actuator, the servovalve and actuator combination are often referred to as a servoactuator. The main advantage of a servovalve is that a low power electrical signal can be used to accurately position an actuator or motor. The disadvantage is their complexity and the resulting costs of components consisting of many detail parts manufactured to very tight tolerances. Therefore, servovalves are generally only used when accurate position (or rate) control is required. 
       SUMMARY 
       [0007]    An example of a method of positioning a nozzle within a cylindrical body is described herein. The method comprises the steps of providing a nozzle within a cylindrical body defining a cylindrical bore; said nozzle having a tubular shape with an outer cylindrical surface and an inner radial surface. The method further comprises the step of moving the nozzle within the cylindrical bore and positioning a first, tubular locking member within said nozzle, and axially moving said first, tubular locking member within the nozzle, said first tubular locking member being configured to cause the nozzle to become positionally fixed at a selected position within the bore in response to the first, tubular locking member being axially moved relative to the nozzle. 
         [0008]    Another example of a method described herein comprises a method for positioning a nozzle in the body of a valve, said method comprising providing said nozzle, said nozzle having an outer cylindrical surface and an inner cylindrical surface; providing a first, male, locking member within said nozzle, said first, male, locking member comprising a hollow tube having an outer cylindrical surface and an inner cylindrical surface, and wherein the outer surface of the first locking member comprises a conical shape that is tapered at a first angle; providing a second, female locking member within said nozzle, said second, female, locking member comprising a hollow tube having an outer cylindrical surface and an inner cylindrical surface; said outer cylindrical surface of said female locking member being in contact with said inner cylindrical surface of said nozzle, and wherein the inner surface of the second locking member has a conical shape that is tapered at a second angle; and wherein said first angle and said second angle are equal; said method further comprising positioning said nozzle in said body of said servovalve; and moving said first, male, locking member inside said second, female, locking member, said first and second locking members being sized relative to each other so that said first locking member fits within said second locking member with said tapered surfaces contacting each other. 
         [0009]    In some examples described herein, the male locking member may be moved using a first push rod and the nozzle may be positioned using a second push rod. 
         [0010]    In some examples described herein, the female locking member may be moved using said second push rod. 
         [0011]    In some examples described herein, the method may further comprise the step of providing a fluid through said first push rod, through said nozzle and said locking members and out of said nozzle. 
         [0012]    In some examples described herein, the method may further comprise the step of removing said first and second push rods following the step of moving said male locking member. 
         [0013]    In some examples described herein, the female locking member extends between a first end and a second end and the male locking member extends between a first end and a second end, and the outer tapered surface of said male locking member tapers outwards at said first angle from said first end to said second end, and the inner tapered surface of said second female locking member tapers outwards from said first end to said second end and wherein said step of moving said male locking member comprises moving said first end of said male locking member in the direction of the first end of said female locking member. 
         [0014]    In some examples described herein, the female locking member extends between a first end and a second end and the male locking member extends between a first end and a second end, and wherein said male locking member has a wedge-shaped cross section between its first end and its second end, said wedge shaped cross-section being narrower at said first end than said second end of the male locking member and wherein said female locking member has a wedge-shaped cross section between its first end and its second end said wedge shaped cross-section being narrower at said second end than said first end of the female locking member and wherein said step of moving the male locking member comprises moving the narrower end of the wedge of the male locking member in the direction of the wider end of the wedge of the female locking member. 
         [0015]    A nozzle positioning system is also described herein comprising: a body defining a cylindrical bore; a nozzle having a tubular shape with an outer cylindrical surface and an inner radial surface being initially movable within the cylindrical bore; and a tubular member positionable within the nozzle configured to cause the nozzle to become positionally fixed at a selected position within the bore in response to the tubular member being axially moved relative to the nozzle after the nozzle has been positioned at the selected position. 
         [0016]    A nozzle positioning system for positioning a nozzle within a body of a valve is also described herein. In some examples, the nozzle may have an outer cylindrical surface and an inner radial surface. The assembly may further comprise a first, male, locking member provided within said nozzle, said first, male, locking member comprising a hollow tube having an outer cylindrical surface and an inner radial surface, and wherein the outer surface of the male locking means comprises a conical shape that is tapered at a first angle. The assembly may further comprise a second, female locking member provided within said nozzle, said second, female, locking member comprising a hollow tube having an outer cylindrical surface and an inner radial surface; said outer cylindrical surface of said female locking member being in contact with said inner radial surface of said nozzle. The inner surface of the female locking member has a conical shape that is tapered at a second angle and wherein said first angle and said second angle are equal. The first and second locking members may be sized relative to each other so that said first locking member fits within said second locking member with said tapered surfaces contacting each other after the nozzle has been correctly positioned within the valve. 
         [0017]    In some examples described herein, said female locking member extends between a first end and a second end and said male locking member extends between a first end and a second end. The outer tapered surface of said male locking member may taper outwards at said first angle from said first end to said second end, and said inner tapered surface of said female locking member may taper outwards from said first end to said second end. The first end of said male locking member may contact said first end of said female locking member and said second end of said male locking member may contact said second end of said female locking member. 
         [0018]    In some examples described herein, said female locking member may extend between a first end and a second end and said male locking member may extend between a first end and a second end, and said male locking member may have a wedge-shaped cross section between its first end and its second end, said wedge shaped cross-section being narrower at said first end than said second end of the male locking member. The female locking member may also have a wedge-shaped cross section between its first end and its second end, said wedge shaped cross-section being narrower at said second end than said first end of the female locking member and said narrower end of the wedge of the male locking member may contact the wider end of the wedge of the female locking member when the nozzle is in position within the valve. 
         [0019]    In some examples described herein, said female locking member may extend between a first end and a second end, and said male locking member may extend between a first end and a second end, and the assembly may further comprise means for moving said male locking member inside said female locking member and for moving said first end of the male locking member in the direction of the first end of the female locking member. 
         [0020]    In some examples described herein, said female locking member may extend between a first end and a second end, and said male locking member may extend between a first end and a second end, and said male and female locking members may have a wedge-shaped cross section between their first and second ends and the narrow end of the wedge of the male locking member may be in contact with the wide end of the wedge of the female locking member when the nozzle is in position in the valve. 
         [0021]    Any of the methods or assemblies may be used in conjunction with a servovalve. Any of the nozzles described herein may be positioned within a tubular body of the servovalve and the outer surface of the nozzle may be in contact with an inner surface of the tubular body. 
         [0022]    In any of the examples described herein, the first and second locking members may be made of a different material or materials than the nozzle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram showing a section of a known nozzle/flapper type servovalve. 
           [0024]      FIG. 2  is a schematic diagram showing a cross section of an example of a new nozzle positioning assembly during calibration. 
           [0025]      FIG. 3  is a schematic diagram showing a cross section of an example of a new nozzle positioning assembly after calibration. 
           [0026]      FIG. 4  is a schematic diagram showing the flow of fluid or air during calibration. 
           [0027]      FIG. 5  shows a schematic diagram of another example of a nozzle positioning assembly 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    An example of a known type of double flapper nozzle  100  that may be used with a servovalve is depicted, for example, in  FIG. 1 .  FIG. 1  does not depict the entire servovalve, but only the main features of the nozzles and flapper. The device has a torque motor having an armature  17  with one or more coils  16 , flapper  14  and nozzles  15 . In use, fluid pressure is supplied to the points  10 A and  10 B. Orifices  13 A and  13 B are formed on each side between the flapper  14  and the opposing nozzles  15 . As long as the flapper is centered, the orifices  13 A and  13 B are the same on both sides and the pressure drop to the return is the same. Pressure at  10 A equals the pressure at  10 B, and the spool connected to the servo (not shown) is in force balance. If the torque motor  17  rotates the flapper  14  clockwise then the orifice  13 A on the left is smaller than the orifice  13 B on the right, and the pressure at  10 A will be greater than the pressure at  10 B. This pressure difference shifts the spool of the servovalve to the right. As the spool shifts, it deflects a feedback spring connected to the servo (not shown). The spool continues to move until the spring force produces a torque that equals the electromagnetic torque produced by the current flowing through the coil  16  around the armature  17 . At this point, the armature  17  is moved back to the center position, the flapper  14  is centered, the pressure becomes equal at  10 A and  10 B, and the spool stops. The spool of the servovalve stays in this position until the current through the coil  16  changes. Because of the feedback spring, the spool has a unique position corresponding to each current through the coil  17  ranging from 0 to rated current. At rated current, the spool is shifted to its full open position. 
         [0029]    In order to control flow in a linear manner, the circumferential area created by the flapper distance to the nozzle must be smaller than the nozzle diameter, such that the circumferential area controls flow and not the nozzle diameter. In this way, the flow area varies linearly with flapper position. Also, the torque motor materials, windings and overall design features lead to accurate control of torque such that small movements of the flapper are possible. This leads to accurate control of the pilot spool, which in turns provides accurate control of the actuator. 
         [0030]    The goal of the flapper and nozzles is to control the pressure acting on both sides of the pilot spool. When the flapper is in the neutral position, the nozzle flow areas are equal and the pressures on both side of the spool are equal. 
         [0031]    In known devices and methods, the servovalve is calibrated by movement of the nozzles  15  into the correct position within the body of the servovalve. Once the nozzle is in the correct position and calibration has been completed, it is no longer possible to then move the nozzle at a later date. The nozzles  15  in known devices are only kept in place by close fit between the nozzle  15  outer surface and the inner surface of the body  18  within which it is placed. The tolerances and dimensions of these are therefore very tight, making the whole calibration process very difficult and also expensive. In addition to this, due to the high forces in the body of the servo, the nozzle and/or servo can often become damaged during calibration. 
         [0032]    The examples described herein with reference to  FIGS. 2 to 5  overcome such disadvantages as they provide a new and improved means of positioning and locking the nozzle in place during calibration that not only provides a sufficient press fit to hold the nozzle in the correct place, but also greatly improves the ease with which the calibration can be obtained in the first place. 
         [0033]    A nozzle with changeable press fit and a new method and means for positioning and locking in place a nozzle within the body of a servovalve will now be described with reference to  FIGS. 2 to 5 . The same reference numerals are used to represent the corresponding features in each of the  FIGS. 2 to 5 . 
         [0034]      FIG. 2  depicts a cross section of an example of a nozzle  200  positioned within the body  210  of a servovalve during calibration. As can be seen in this figure, the body of the servovalve in which the nozzle is positioned is a hollow tube. During calibration, fluid flow (fuel or air)  291  is provided through the supply port(s)  290  and directed towards and out of the end  260  of the nozzle  200 , as shown in  FIG. 4 , the arrows  291  in this figure depicting the flow of fluid (or air). 
         [0035]    The nozzle  200  is also hollow and comprises an elongated cylindrical wall extending between a first end  250  and a second end  260 . In the example shown in  FIGS. 2, 3 and 4 , in use, the second end  260  of the nozzle is the end which would be closest to the flapper, as described above, with reference to  FIG. 1 . The first end  250  of the example shown in  FIGS. 2 to 4  has a first opening which is wider than an opening at the second end  260 . 
         [0036]    In the example shown in  FIG. 2 , the means for locking the nozzle in position comprises a first, male, tubular, locking member  280  and a second, female, tubular, locking member  270 . The female locking member  270  is provided within the nozzle and, in this example, results in an area within the valve wherein the inner circumference is reduced. The male locking member  280  is then shaped and sized so that it can be moved axially and relative to and within the female locking member  270  until at least a part of the outer surface of the male locking member  280  abuts at least a part of the inner surface of the female locking member  270 . In the example shown in  FIG. 2 , the female locking member  270  is also movable axially relative to the nozzle. 
         [0037]    As can be seen in  FIGS. 2 to 4 , the female locking member  270  may have an outer surface  273  and an outer circumference that is sized so as to be in contact with the inner surface and inner circumference of the nozzle  200 . In the example shown in  FIG. 2 , the female locking member  270  comprises a hollow tube with a cylindrical wall extending between first  271  and second  272  ends. 
         [0038]    In some examples, the female locking member  270  may be integrally formed with the inner surface of the nozzle, and may even be the inner surface of the nozzle itself, as shown in  FIG. 5 . In this example, the female locking member simply comprises the inner surface of the nozzle  200 , the inner surface being tapered. 
         [0039]    At least a part of the inner circumference of the female locking member  270  (e.g. at its second end  272 ) is greater than at least a part of the outer circumference of the male locking member  280  (e.g. at its first end  281 ) so that the male locking member  280  may be moved inside the female locking member  270  as shown in  FIG. 2 . 
         [0040]    In  FIGS. 2 to 4 , it can be seen that the thickness of the first and second locking members&#39; cylindrical walls differ along their length to each form a wedge-shaped cross section. In detail, the outer circumference of the female locking member (i.e. that surface  273  which is in contact with the inner radial surface of the nozzle) remains the same between the first end  271  and second end  272  of the female locking member  270  (although this is not crucial, this is useful in that it results in an even distribution of force as against the nozzle), whilst the thickness of the wall of the female locking member  270  decreases in the direction of the second end  272  of the female locking member. 
         [0041]    This results in the female locking member  270  having an inner radial surface  274  that has a truncated conical shape, with the wider part of the cone being at its second end  272 . In other words, the female locking member  270  has walls that taper so that the size of the circumference of the inner surface of the female locking member  270  at its first end  271  is smaller than the circumference of the inner surface at its second end  272 . 
         [0042]    The male locking member  280  of the example shown in  FIG. 2  also comprises a hollow tube with a cylindrical wall extending between a first  281  end and a second  282 . The outer circumference of the male locking member  280  is smaller than the inner circumference of the nozzle  200  and so, in this example, is not in contact with the inner surface of the nozzle during calibration. 
         [0043]    In contrast to the female locking member, the inner circumference of the surface of the male locking member  280  remains the same between its first and second ends, (this is not crucial but is useful as this is the surface that is in contact with the push-rod, as described later), whereas the walls taper such that the size of the circumference of the outer surface  283  of the male locking member  280  is smaller at its first end  281  than the circumference of the outer surface  283  at its second end  282 . As can be seen in  FIG. 2 , the thickness of the wall of the male locking member  280  increases in the direction of the second end  282  of the male locking member  280 . These features of the male locking member result in a hollow tube that has an outer surface  283  having a truncated conical shape. 
         [0044]    The truncated conical shape of the inner surface  274  of the female locking member and the truncated conical shape of the outer surface  283  of the male locking member have a matching taper of equal angle (depicted by reference numeral  300  in  FIG. 5 ). The locking members are also sized relative to each other so that the male member having a conical external shape fits inside the female member having a corresponding conical internal shape so that they contact each other and self-lock. The coefficient of friction (μ) of the materials from which the locking members are made should be equal to or bigger than the tangent of an angle of the taper (α): μ≧tan α and for small angle (tan α≈α): μ≧α. For example, if the locking members are made of aluminium, α will be lower than 45 degrees (μ=1 for aluminium/aluminium). In some examples, the angle of the taper may be relatively small (from 1 to 2 degrees). 
         [0045]    Due to the conical internal and external shapes of the female and male locking members, and as can be seen in  FIGS. 2 to 5 , the male locking member  280  may be described as having a wedge-shaped cross section between its first end  281  and its second end  282 , the wedge shaped cross-section being narrower at the first end  281  than the second end  282 . The female locking member  270  may also be described as having a wedge-shaped cross section between its first end  271  and its second end  272 , the wedge shaped cross-section being narrower at the second end  272  than at the first end  271  of the female locking member  270 . This means that when the male locking member  280  is moved within the female locking member, the narrower end  281  of the wedge of the male locking member  280  moves in the direction of the wider end  282  of the wedge of the female locking member. In other words, the first end  281  of the male locking member  280  is moved within the female locking member  270  in the direction of the female locking member&#39;s first end  271 . The tapered outer surface  283  of the male locking member  280  therefore eventually contacts the tapered inner surface  274  of the female locking member  270  and the correspondingly tapered surfaces  283 ,  274  fit and slide against each other with increased pressure the further the male locking member is moved relative to and within the female member. Since the female locking member is in contact with the body  210 , this, in turn, increases the pressure with which the nozzle  200  is pressed against the inner surface of the body and therefore locks the nozzle in place within the body via a self-locking press fit. 
         [0046]    The movement of the nozzle and locking members may be achieved via the use of push rods. 
         [0047]    As shown in  FIG. 2 , this may be achieved using first push rod  220  and second push rod  230 . In this example, the second push rod  230  comprises a hollow cylindrical tube and the first push rod  220  is inserted within the hollow centre of the second push rod  230 . An O-ring  231  may be provided to seal the space between the push rods  220  and  230 . The first push rod  220  is also hollow. This is necessary, as during calibration, fluid flow (fuel or air)  291  must be provided through the servovalve body as shown in  FIG. 4 . 
         [0048]    The first push rod  220  may be connected to the inner surface  284  of the male locking member  280  by a screw thread  242 . The second push rod  230  may be connected to/with the first end  250  of the hollow nozzle  200  by a screw thread  240 . During calibration, the first, male locking member  280  is first placed within the nozzle  200  using the first push rod  220  and then the second, female, locking member  270  is placed in the nozzle using the second push rod  230 . 
         [0049]    The second push rod  230  is used to control the position of the nozzle  200  within the servo body  210 . The second push rod  230  also positions the female locking member  270  in the correct place within the nozzle. Calibration is then performed by inserting a fluid or gas  291  through the port  290  as shown in  FIG. 4 . 
         [0050]    Once calibration of the nozzle is complete and the nozzle  200  is in the correct position, the first push rod  220  may then be used to axially move the male locking member  280  by pulling the male locking member  280  at least partially and/or fully inside of the female locking member  270 . As described above, due to the fact that the female locking member  270  has a tapered inner surface  274  having a truncated conical shape and the male locking member  280  has a correspondingly tapered outer surface  283  also having a truncated conical shape, the angles of both tapers being equal, as the male locking member  280  is pulled at least partially inside of the hollow female locking member  270 , a press fit is achieved. As the male locking member is pulled further and further inside of the female locking member, the press fit increases and the locking members self-lock. The nozzle is then held in place within the valve body due to these locking members. 
         [0051]      FIG. 5  shows an alternative arrangement to  FIGS. 2 to 4 . The reference numerals in this figure represent the corresponding features found in  FIGS. 2 to 4 . As described above, in this example, the female locking member  270  is integrally formed with the inner surface of the nozzle, and in this case comprises the inner surface of the nozzle itself. For geometric reasons, in order for the male locking member  280  to be able to be inserted into the nozzle and female member, the slope of the tapered inner and outer surfaces of the female and male locking members must lie in the opposite direction to the examples shown in  FIGS. 2 to 4 . As can be seen in this figure, the first end  271  of the female locking member  271  (i.e. the nozzle  200 ) is therefore closest to the nozzle end  260 . This is in contrast to the examples shown in  FIGS. 2 to 4  where the first end  271  of the female locking member  270  is further away from the nozzle end  260 . In this example, the first end  281  of the male locking member  280  is also closest to the nozzle end  260  in this example, with the second end  282  of the locking member  280  being further away from the nozzle end  260 . 
         [0052]    Contrary to known techniques, with the systems and methods described herein with reference to  FIGS. 2 to 5 , there is no risk that the female locking member  270  will be moved during this calibration process because it is being held in position by the first push rod  220  at one end and by the male locking member  280  at the other end. 
         [0053]    Once calibration is complete, and the nozzle  200  is locked into the correct position within the body of the valve, the first and second push rods  220   230  may be unscrewed and removed, as shown in  FIG. 3 . 
         [0054]    The examples described herein therefore provide significant advantages over known techniques in that they enable a much easier process of calibration than was previously possible. 
         [0055]    The examples described herein also require much lower requirements for dimensions of nozzles and body. This is in contrast to known devices where the requirements are very high. 
         [0056]    In addition to the above, the examples described herein have further advantages over known devices in that the press fit of the nozzle would not be affected by changes in temperature. In known devices, the nozzle is made from steel, whereas the body within which it is positioned is made from aluminium. Since aluminium has a higher thermal coefficient of expansion than steel, when there is an increase in temperature, the body of the valve expands at a greater rate and so the press fit between the outer surface of the steel nozzle and the inner surface of the body is reduced. The nozzle can therefore slip and the valve would then be damaged. In contrast to this, in the examples described herein, since it is the first and male locking members and not the nozzle surface itself that is holding the nozzle in position, in some examples, the female and male locking members may be made from the same material as the body so that with an increase of temperature, the press fit is not adversely affected. 
         [0057]    In addition to the above, in current devices, it is not possible to change the position of the nozzle once it has been calibrated, as it would crack under the pressure from the movement. With the examples described herein, however, the nozzle may be more easily removed by simply removing the male locking member  280  from inside the first locking member  270  and thereby removing the press fit and allowing the nozzle to be moved or removed completely.