Abstract:
A method for producing a manifold is disclosed, wherein an additive manufacturing process is used to produce the manifold.

Description:
TECHNICAL FIELD 
       [0001]    The present invention concerns improvements in and relating to the manufacture of servo valve manifolds. 
       BACKGROUND OF THE INVENTION 
       [0002]    Servo valves are used in a wide variety of industries to control the movement of hydraulic or pneumatic actuators in response to an input signal and are employed in industries where precise control of an actuator is required, for example in the aerospace industry. Servo valves alter the flow of a fluid through the valve in order to control the position, velocity, acceleration or force generated by an actuator, for example a hydraulic or pneumatic cylinder or motor. 
         [0003]    A servo valve typically comprises a moving element (spool) and a fixed element. The relative movement of these two elements controls the flow of fluid through the valve. 
         [0004]    A fluid manifold may be defined as any element which provides at least one flow path for fluids between one or more fluid inputs and one or more fluid outputs. As such, manifolds are found in many different applications. A typical servo valve manifold differs from manifolds in general in that it must accommodate a moveable spool within a cavity formed inside the manifold. As it is the position of the spool within the cavity which controls the fluid flow through the servo valve the internal dimensions of the manifold, and in particular of the cavity and the flow ports which open onto the cavity, must be produced to tight tolerances in order to achieve the necessary degree of precision in the control of the flow. 
         [0005]    In contrast to the manifolds that may be found in other applications, size is commonly a key driver in servo valve design. For example, in aerospace applications the space and weight limitations within an aircraft may place severe constraints on the size of servo valve that can be accommodated. 
         [0006]    A servo valve manifold may also include further cavities into which one or more flow management devices may be inserted. Flow management devices, for example check valves, flow restrictors and fluid filters act to alter the flow and ensure the smooth functioning of the fluid system of which the manifold forms a part. 
         [0007]    Typically, servo valve manifolds are created by subtractive manufacturing processes. For example, a servo valve manifold may be created by removing material from a billet by machining. Alternatively, a servo valve manifold may be created by a combination of casting and subtractive manufacturing processes. For example the basic form of the manifold may be cast and the flow paths then created by removing material by machining (e.g. boring). Where greater precision is required a sleeve may be inserted within the manifold to isolate precision features from pressure and thermal distortions within the manifold and to enable the use of different materials, for example materials which have improved mechanical properties or are available at lower cost. Such manufacturing processes are relatively complex and involve a number of steps as well as a variety of different production techniques. These manufacturing processes also place limitations upon the type of manifold that can be produced. Consequently, it would be advantageous to provide an improved manufacturing process which is less complex and allows more flexibility in the design of servo valve manifolds. It would also be advantageous to provide an improved manifold which exploits the increased flexibility offered by such an improved manufacturing process. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect of the invention there is provided a method for producing a servo valve manifold, wherein the manifold is formed using an additive manufacturing process. 
         [0009]    Additive manufacture, also known as 3D printing, is a term applied to processes whereby three-dimensional articles are manufactured by building up successive layers of material in different shapes. This is in contrast to traditional manufacturing techniques (known as subtractive manufacturing) such as milling or boring in which material is removed in order to create the final form of an article. Using an additive manufacturing technique allows the manifold to have a single piece construction, whilst still incorporating the features included in a manifold produced using traditional manufacturing techniques. Moreover, the design flexibility offered by the additive manufacturing techniques means that the design of the manifold itself may be improved. Thus, the use of additive manufacturing techniques may result in an improved design of manifold which is easier to produce. 
         [0010]    At least one curvilinear flow gallery may be formed within the manifold during its production. The method may further include the step of: forming at least one curvilinear flow gallery within a single piece manifold during the additive manufacturing process. Thus, the design of the valve may be improved by using an additive manufacturing process to create a curvilinear flow gallery within the manifold. Varying the curvature of the galleries allows the arrangement of the galleries within the manifold to be optimised. Curvilinear flow galleries may be packed more tightly within the body of the manifold than rectilinear galleries thereby reducing the space occupied by the manifold. Gradually varying the curvature of the galleries when connecting flow paths also allows fluid momentum to be maintained (as compared with the loss of momentum experienced when a fluid must flow around a right-angled bend for example). 
         [0011]    At least one flow control feature may be formed by the manifold itself. The at least one flow control feature may be formed within the manifold during its production. The at least one flow control feature may be formed as a single piece with the manifold body. The method may also include the step of: forming at least one flow control feature as part of a single piece manifold during the additive manufacturing process. 
         [0012]    The flow control feature formed as part of the single piece manifold may be a fluid filter. Thus, the method may include the step of: forming a fluid filter within a single piece manifold during the additive manufacturing process. 
         [0013]    The flow control feature formed as part of the single piece manifold may be a flow restrictor. Thus, the method may include the step of: forming a flow restrictor within a single piece manifold during the additive manufacturing process. 
         [0014]    The flow control feature formed as part of the single piece manifold may be a sleeve. Thus, the method may include the step of: forming a sleeve within a single piece manifold during the additive manufacturing process. 
         [0015]    Thus, the process of manufacturing the manifold may be further improved (simplified) by using the additive manufacturing process to form flow features as part of the single piece manifold. Additionally, the performance of a single piece manifold may be improved by including control features therein. 
         [0016]    A check valve may be formed at least in part by the single piece manifold. Thus, the method may include the step of: forming at least part of a check valve, for example, a moveable valve member, within a single piece manifold during the additive manufacturing process. 
         [0017]    The method of forming the check valve may include one or more of the following steps:
       providing a single piece manifold using the additive manufacturing process including a cavity, defined by an inner surface of the manifold, having a valve locating portion and a valve opening;   providing a valve member including at least one protrusion within the cavity using the additive manufacturing process; and/or   exerting a force on the valve member such that the at least one protrusion of the valve member engages the valve locating portion thereby locating the valve member within the cavity;   exerting a force on the valve member such that the valve member is brought into contact with the inner surface of the manifold that defines the valve opening.       
 
         [0022]    The valve member may be displaced axially within the cavity such that the at least one protrusion engages the valve locating portion. The valve member may be twisted within the cavity such that the at least one protrusion engages the valve locating portion. 
         [0023]    The valve locating portion may be an area of the cavity having a reduced diameter. The method may involve pushing or pulling the valve member such that the at least one protrusion is forced past the area of reduced diameter. The method may involve the step of twisting the valve member such that the at least one protrusion passes the area of reduced diameter. Thus, the method may involve exerting a force on the valve member, in a first direction, such that the at least one protrusion of the valve member passes the area of reduced diameter. Thus, the valve member may be located within the cavity by the contact between the protrusion and the area of reduced diameter. 
         [0024]    The steps of locating the valve member within the cavity and bringing the valve member into contact with the inner surface of the manifold which defines the valve opening may be carried out simultaneously. Thus, the method may include the step of exerting a force on the valve member, in a first direction, such that the at least one protrusion of the valve member passes the area of reduced diameter and the valve member is brought into contact with the inner surface of the manifold that defines the valve opening. 
         [0025]    The valve locating portion may be a recess formed by the inner surface of the cavity. The valve locating portion may be a slot in the inner surface of the cavity. For example, the valve locating portion may be an L-shaped slot such that the valve locating portion and protrusion form a bayonet fitting. 
         [0026]    The valve member may be elastically deformable. The valve member may act as a compression spring. The method may include the step of axially compressing the valve member. The valve member may act as a torsion spring. The method may include the step of twisting one end of the valve member relative to the other end of the valve member. 
         [0027]    The valve member may elastically deform as it passes the area of reduced diameter. The valve member may deform radially as it passes the area of reduced diameter. Thus, the sides of the valve member may be pushed inwards as they pass the area of reduced diameter before returning to their original shape once they are past. Once the at least one protrusion of the valve member is past the area of reduced diameter the check valve may be maintained in a first, axially compressed, state by contact with the inner surface of the manifold. The valve member may be maintained in an axially compressed state as a result of the engagement between the valve locating portion and the at least one protrusion. In the first state the contact of the at least one protrusion at a first end of the valve member against the internal surface of the manifold at the area of reduced diameter and the contact of a second, opposite, end of the valve member against the inner surface of the manifold defining the valve opening may maintain the valve member in a compressed state. Thus, the area of reduced diameter may prevent the valve member expanding. 
         [0028]    A stylus may be inserted into the cavity in order to exert force on the valve member in the first direction. Alternatively, the force used to urge the valve member in the first direction may be generated by hydraulic pressure. For example, a sphere may be inserted into the cavity above the valve member and under the action of a pressurised fluid the sphere may push on the valve member thereby activating the locking barb. 
         [0029]    In order for a fluid to flow through the check valve, the pressure of the fluid must generate sufficient force on the valve member to compress the valve member further and thereby create a gap between the inner surface of the cavity which defines the valve opening and the valve member allowing fluid to flow through the valve. 
         [0030]    According to a second aspect of the invention there is provided a manifold including at least one curvilinear flow gallery, characterised in that the manifold is of a single piece construction. Thus, the design of the manifold may be improved without increasing the complexity of the manufacturing process. 
         [0031]    The manifold may include at least one flow gallery. Each gallery may form a fluid flow path. The flow gallery may form a flow path between a fluid inlet and a fluid outlet. Thus, fluid may flow along the fluid gallery. The manifold may include a plurality of fluid galleries. It may be that the at least one flow gallery is not straight, but follows a nonlinear path in three-dimensional space. Thus, the gallery may curve within the manifold. The manifold may include at least one flow gallery in which the curvature of the gallery varies along the majority of the length of the gallery. The manifold may include at least one flow gallery with a continuously varying curvature. The curvilinear gallery may be incorporated within the form of the manifold produced by the additive manufacturing process. 
         [0032]    As the additive manufacturing process involves building up layers of material in order to form a finished shape, there may be limitations as to the degree of overhang (the size of a horizontal projection without a supporting layer below) that can be achieved in an article produced using this method. Thus, it may be that the gradient of the sides of one half of the flow gallery (the upper half during production) are steeper than the gradient of the sides in the other half (the lower half during production) of the gallery. It may be that the cross-section of the at least one gallery is not circular. It may be that the at least one gallery has a tear-drop shaped cross-section. 
         [0033]    The cross-section of the at least one gallery may vary along at least a portion of the length of the gallery. Thus, the at least one flow gallery may have a variable cross-section. In this manner the packing of the galleries within the servo valve manifold may be optimised while maintaining fluid momentum. The cross-section of the gallery may vary along the majority of the length of the gallery. The variation in the cross-section of the gallery may be incorporated within the form of the manifold produced by the additive manufacturing process. 
         [0034]    The at least one flow gallery may branch into two sub-galleries. A sub-gallery may have a reduced cross-sectional area as compared to the gallery that it branches off from. The at least one flow gallery may branch into more than two sub-galleries. For example, the at least one flow gallery may branch into three sub galleries. Two or more sub-galleries may merge to form a single gallery. Thus, the manifold may contain a plurality of galleries and sub-galleries which branch into, and are produced by, the merging of each other. 
         [0035]    The manifold may incorporate a flow control feature. The flow control feature may be incorporated into the form of the manifold produced by the additive manufacturing process. Thus, the design of the manifold can be improved without adding complexity to the manufacturing process. 
         [0036]    The flow control feature may be a fluid filter. Thus, a manifold may filter the fluid flowing through it without requiring any additional components. The fluid filter may be incorporated into the form of the manifold produced by the additive manufacturing process. The fluid filter may comprise a plurality of filter flow paths formed within the manifold. 
         [0037]    The filter may be in the form of a grate incorporated into a single piece manifold. The grate may comprise a plurality of intersecting filter walls defining holes therebetween. The holes of the grate may be round. Thus, the cross-sections of the filter flow paths may be round. The holes of the grate may be square. Thus, the cross-sections of the filter flow paths may be square. The grate may be located in a flow gallery transverse to the direction of flow. The grate may extend axially in the direction of flow. The filter walls may extend axially in the direction of flow. The axial length of a filter wall may be very much greater than its thickness. Thus, the filter may be relatively strong in the direction of flow. The length of a filter wall may be more than five times its thickness. For example, the length of a filter wall may be 10 times its thickness. 
         [0038]    The fluid filter may be domed. Thus, large particles may be pushed to the edge of the filter by the current. The flow control feature may be a flow restrictor. The flow restrictor may be incorporated into the form of the manifold. Thus, the performance of the manifold may be improved without the need for any additional components. The flow restrictor may include one or more flow paths having a plurality of abrupt direction changes such that transition to turbulent flow is promoted. The flow path may include a bend of at least 90 degrees over a distance of less than 5 mm. The flow restrictor may be formed by a plurality of baffles. Each baffle may extend from the side of the flow gallery perpendicular to the direction of flow. The flow restrictor may comprise at least one pair of baffles. Each pair may comprise two baffles, one baffle extending from each side of the flow gallery to form an orifice between the two baffles. The convoluted flow path of the flow restrictor may allow a given flow resistance to be achieved with a larger orifice between the baffles. Thus, the flow restrictor allows the promotion of turbulent flow without the use of the relatively small flow-constricting orifices typically used in such devices. Thus, the flow restrictor may promote turbulent flow of the fluid without being susceptible to contamination which can block such small-diameter orifices. 
         [0039]    The servo valve may further include a spool. The spool may be located within a cylindrical cavity within the manifold. The spool may be mounted concentrically within the manifold cavity. Thus, an annular cavity may be formed between the spool and the inner surface of the manifold which defines the cavity. 
         [0040]    A fluid port may be located in the inner surface of the manifold which defines the cavity. The fluid port may be a fluid inlet or a fluid outlet. The fluid port may have a non-circular cross-section. The fluid port may be of elongate shape. That is to say that the fluid port may extend over a greater circumferential distance than axial distance. For example, the fluid port may have a substantially rectangular cross-section. The fluid port may be a substantially rectangular slit in the inner surface of the manifold which defines the cavity. Therefore, the fluid port may be much smaller in the axial direction and therefore allow the corresponding features of the spool and manifold to be more closely packed in that direction than a typical, cylindrical, fluid port. 
         [0041]    The flow control feature may be a sleeve. The sleeve may be located within the cavity of the manifold. The sleeve may be substantially cylindrical in shape. The sleeve may be hollow. The spool, sleeve and manifold cavity may be concentric. The spool may be located within the sleeve. The sleeve may divide the annular cavity surrounding the spool into at least two concentric annular zones. It may be that the sleeve does not extend along the majority of the length of the spool. The sleeve may divide the annular cavity surrounding the spool into at least two concentric annular zones over a portion of the length of the spool. 
         [0042]    The manifold cavity may include at least one fluid inlet via which pressurised fluid enters the cavity. Proximate to each fluid inlet there may be a high pressure zone. Each high pressure zone may have a sleeve. The sleeve may act to pressure-balance the interface between any incoming fluid and the spool. The inner surface of the manifold may include a plurality of fluid inlets. A manifold cavity may contain a plurality of sleeves. The sleeves may be spaced apart longitudinally along the length of the spool. The central annular zone may be between the inner surface of the sleeve and the outer surface of the spool. The outer annular zone may be between the outermost surface of the sleeve and the inner surface of the manifold which forms the cavity. Thus, a spool may be mounted concentrically within a portion of the manifold such that an annular cavity is formed between the spool and the manifold and the flow control feature is a sleeve which divides the annular cavity into at least two concentric annular zones along a portion of the length of the spool. Each sleeve may include one or more apertures to allow fluid to flow into the central annular zone from the outer annular zone. A sleeve may include a single aperture, extending around the circumference of the sleeve. Alternatively a sleeve may include a plurality of holes, spaced apart around the circumference of the sleeve, to allow fluid to flow from the outer annular zone to the central annular zone. The manifold cavity may include at least one fluid outlet via which fluid exits the cavity. The at least one fluid inlet and the at least one fluid outlet may be formed in the inner surface of the manifold which defines the cavity. The fluid inlets and fluid outlets may be connected to an external hydraulic system via the fluid flow galleries contained within the manifold. Thus, the servo valve may be used to control an external hydraulic system. The inner surface of the manifold may include a plurality of fluid inlets and a plurality of fluid outlets allowing the cavity to be connected via the flow galleries of the manifold with a plurality of external hydraulic systems. 
         [0043]    A check valve may be included within the manifold. The check valve may be formed at least in part by the manifold. The check valve may include a cavity defined by an inner surface of the manifold and a valve member. The valve member may be elastically deformable. That is, the valve member may change shape under a load, and thereby store mechanical energy. When the load is removed the valve member may return to its original shape. The valve member may be axially compressible. Thus, the length of the valve member may reduce under load. The valve member may be radially compressible. Thus, the radius of the valve member may reduce when subject to a load. The valve member may be torsionally deformable. Thus, one end of the valve member may rotate relative to the other when subject to a load. 
         [0044]    The cavity may form part of a flow path within the manifold. One end of the cavity may include a valve opening. The valve opening may be defined by the inner surface of the manifold. The cavity may also include a valve locating portion. The valve locating portion may be defined by the inner surface of the manifold. The valve locating portion may be a region of reduced diameter. The inner surface of the cavity may include at least one projection. The inner surface of the manifold may include a single projection which extends circumferentially around the cavity. Alternatively, the inner surface of the manifold may include one or more individual projections, arranged circumferentially around the cavity. Thus, the region of reduced diameter may be formed by at least one projection on the inner surface of the manifold. The at least one projection may have a triangular cross-section. The diameter of the cavity may reduce gradually to a point of minimum diameter as the distance to the spring decreases. Beyond the point of minimum diameter, the diameter may increase relatively abruptly as the distance from the spring continues to reduce. The shape of the cavity may correspond to the shape of the valve member. The cavity may be substantially symmetrical. 
         [0045]    The valve member may be located within the cavity. The valve member may be formed within the cavity during the additive manufacturing process. The surface at a first end of the valve member may include at least one protrusion. The other, second end, of the valve member may be axially displaceable relative to the first end. Alternatively, the other, second end, of the valve member may be axially rotatable relative to the first end. The outer surface of the valve member may include a single protrusion which extends circumferentially around the valve member. Alternatively, the outer surface of the valve member may include one or more individual protrusions, arranged circumferentially around the cavity. The valve member may be barbed. The valve member may be substantially symmetrical. The valve member may be more flexible than the manifold. 
         [0046]    In a first state the second end of the valve member may be in contact with the inner surface of the manifold which defines the valve opening. Thus, in the first state the second end of the valve member may be blocking the valve opening such that fluid cannot flow through the valve opening. In the first state, the valve member may be located within the manifold by the contact between the valve locating portion and the at least one protrusion. In the first state, the at least one protrusion on the valve member may be in contact with the inner surface of the manifold cavity at the area of reduced diameter. In the first state, the at least one protrusion on the valve member may be in contact with the at least one projection on the inner surface of the manifold cavity at the area of reduced diameter. Thus, in the first state, the valve member may be compressed between the inner surface of the manifold at the valve opening and the area of reduced diameter. 
         [0047]    In a second state, it may be that the second end of the valve member is not in contact with the inner surface of the manifold cavity which defines the valve opening. In the second state there may be a gap between the inner surface of the manifold cavity and the valve member. Thus, in the second position, the gap between the inner surface of the manifold cavity and the second end of the valve member may permit fluid to flow through the check valve. In the second state the at least one protrusion on the valve member may remain in contact with the inner surface of the manifold at the area of reduced diameter. The area of reduced diameter may prevent the valve member from expanding away from the valve opening. The valve member may include one or more flow galleries that permit fluid to flow through the valve member at the region of reduced diameter. The manifold may include one or more flow galleries that permit fluid to flow within the manifold at the region of reduced diameter. 
         [0048]    The valve member may change from the first state to the second state when a compressive force is exerted on it. The compressive force may act upon the valve member in a second direction, opposite to the first direction. Thus, the check valve may only open when fluid flowing in the second direction exerts sufficient force on the valve member to further compress the valve member and open a gap between the valve member and the inner surface of the manifold which defines the valve opening. 
         [0049]    Alternatively, the valve member may change from the first state to the second state when a torsional force is exerted on it. Thus, the check valve may only open when fluid exerts sufficient force on the valve member to further rotate one end of the valve member and thereby create a gap between the valve member and the inner surface of the manifold which defines the valve opening. 
         [0050]    The valve member may be of a single piece construction. The valve member may include at least one deformable region. The deformable region may comprise a spring formed as one part with the rest of the valve member. The spring may be formed as one part with the rest of the valve member using an additive manufacturing process. The deformable region may comprise a region of reduced thickness. The valve member may include a deformable region that accommodates axial deformation. The valve member may include a deformable region that accommodates radial deformation. The valve member may include at least one rigid region which is substantially more rigid than the at least one deformable region. For example the valve member may include two rigid regions linked together by one or more deformable regions. It will be understood that as the valve member may be produced using an additive manufacturing process the deformable regions may have a different configuration to the rigid regions but still form a single piece. A manifold may include more than one flow control feature. A manifold may include more than one type of flow control feature. For example, a single piece manifold may include any combination of a fluid filter, a flow restrictor and a sleeve. A single piece manifold may include a check valve and any one of a fluid filter, a flow restrictor and a sleeve. 
         [0051]    Manifolds in accordance with the second aspect of the invention may be produced in accordance with the method of the first aspect of the invention. 
         [0052]    Any features described with reference to one aspect of the invention are equally applicable to any other aspect of the invention, and vice versa. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0053]    Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings of which: 
           [0054]      FIG. 1  is a representation of a single piece manifold including a plurality of flow galleries; 
           [0055]      FIG. 2 a    is a schematic view of a single piece manifold including a fluid filter; 
           [0056]      FIG. 2 b    is a schematic cross-sectional view of the fluid filter of  FIG. 2   a;    
           [0057]      FIG. 3  is a schematic cross-sectional view of a single piece manifold including a flow restrictor; 
           [0058]      FIG. 4  is a schematic cross-sectional view of a single piece manifold including a sleeve; 
           [0059]      FIG. 5  is a schematic cross-sectional view of a single piece manifold including a check valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]      FIG. 1  shows a manifold  2  containing flow galleries  4 ,  6 ,  8 ,  10  connected to a central cavity  12 . The flow galleries are curved, and bend around within the body of the manifold so that they are tightly packed. The aspect ratio of the cross-section of each gallery  4 ,  6 ,  8  and  10  varies along its length as the gallery curves. Two of the galleries  4 ,  6 , split into two branches,  4   a  and  4   b  and  6   a  and  6   b  respectively. 
         [0061]    Curving the flow galleries allows them to be packed more efficiently within the body of the manifold and therefore reduces the size of the manifold overall. Varying the aspect ratio of the galleries as they curve helps to maintain fluid momentum. Branching the galleries assists in efficient packing of the galleries within the body of the manifold and also offers the opportunity to use different flow control devices in different branches if so required. 
         [0062]      FIG. 2 a    shows a schematic view of a fluid filter  30  formed as part of a single piece manifold  32 . The fluid filter  30  comprises a solid matrix  34  containing multiple holes  36 , the holes having a square cross-section. 
         [0063]      FIG. 2 b    shows a cross-sectional view of the fluid filter  30  formed within a section of a single piece manifold  32 . The fluid filter is located within a flow gallery  38 . The solid matrix  34  extends across the width of the flow gallery  38 . The upstream end of the solid matrix  34  is domed. The solid matrix  34  and the holes  36  contained therein extend axially along the flow gallery  38 . The holes  36  therefore form fluid flow paths which are rectangular when viewed in cross-section in  FIG. 2   b.    
         [0064]      FIG. 3  shows a cross-section view of a flow restrictor  40  formed as part of a single piece manifold  42  within a flow gallery  48 . The flow restrictor  40  comprises a plurality of baffles  44 , arranged in pairs, the pairs being spaced apart longitudinally along the length of the flow gallery  48 . The baffles  44  are rectangular when viewed in cross-section in  FIG. 3 . The baffles  44  of each pair extend perpendicularly from either side of the flow gallery  48  to form a valve opening  46 . The transverse location of the orifice  46  varies for each pair. Thus, the orifices  46  are not aligned along the centre line but form a convoluted flow path down the flow gallery. 
         [0065]      FIG. 4  shows a cross-sectional view of single-piece manifold  100  for use in a servo valve which includes two sleeves  108  formed as part of the manifold  100 . A cylindrical cavity  102  is formed within the manifold  100  and contains a cylindrical spool  104 . The sleeves  108  are spaced apart longitudinally along the axis of the spool  104 . The cavity  102  is split by each sleeve into two, concentric annular zones  102   a,    102   b.  The first zone,  102   a,  is formed between the surface of the spool  104  and the innermost surface of the sleeve  108  which is formed as part of the single piece manifold. The second zone,  102   b  is formed between the outermost surface of the sleeve  108  which is formed as part of the single piece manifold and the wall of the inner surface of the manifold  100 . Viewed in cross section each cylindrical sleeve  108  appears as four rectangular protrusions which extend, parallel to the longitudinal axis of the spool  104 . Associated with each sleeve, and extending circumferentially, a gap  106  allows fluid to pass between the two annular zones  102   a  and  102   b.  An inlet port  110  is in fluid communication with annular zone  102   b  and with annular zone  102   a  via gap  106 . The presence of the sleeve helps maintain a pressure-balance within the manifold at the interface between the spool and the inlet valve. Forming the sleeve as part of the manifold using additive manufacturing techniques allows the improvement in performance associated with including a sleeve in the manifold to be retained without adding to the complexity of the manufacturing process. 
         [0066]      FIG. 5  shows a cross-sectional view of a check valve  201  formed in part by a manifold  200 . A valve member  204  is located within a cavity  202 . A protrusion  208  is located at one end of valve member  204  and is in contact with a projection  210  of the inner surface of the manifold which defines the cavity  202 . The opposite end of the valve member is tapered and is in contact with the tapered inner surface of the manifold which defines a valve opening  206 . The protrusion  208  is on the same side of the projection  210  as the valve opening  206 . The protrusion  208  extends circumferentially around the valve member and is triangular when shown in cross-section in  FIG. 5 . The projection  210  extends circumferentially around the inner surface of the manifold  202  and is triangular when shown in cross-section in  FIG. 5 . The shape of the projection  210  is such that the diameter of the cavity gradually reduces towards the valve opening  206  before abruptly increasing over a very short distance. The sides  212  of the valve member  204  are parallel to each other. The protrusion  208  is in contact with the inner surface of the manifold  200  which forms cavity  202 . The valve member  204  includes a pair of deformable spring elements  212  connecting the tapered end of the valve member to the end of the valve member  204  containing projection  208 . In this, the first state, the deformable valve member is compressed between the valve opening  206  and the projection  210  and the spring elements  212  are therefore compressed. The contact between the tapered end of the valve member  204  and the inner surface of the manifold defining the valve opening  206  prevents the flow of fluid past the valve. 
         [0067]    During installation the flexible valve member is located in the cavity with the protrusion  208  on the opposite side of the projection  210  to the valve opening  206 . Force is exerted on the valve member  204  such that it moves towards the valve opening  206  thereby compressing the spring elements  212 . Force continues to be exerted on the valve member  204  until the protrusion  208  is on the other side of the projection  210 . It is possible for the protrusion  208  to pass the projection  210  because the thin sides  212  of the valve member are flexible, and deform such that they are no longer parallel. This deformation is aided by the shape of the projection which gradually reduces the diameter of the cavity. 
         [0068]    Fluid flowing through the check valve  201 , will be prevented from passing the valve opening  206  which is blocked by the valve member  204  until the spring elements  212  are sufficiently compressed by the hydraulic pressure generated by the fluid to create a gap between the surface defining the valve opening  206  and the end of the valve member  204 . In the presence of sufficient hydraulic pressure the valve member  206  is compressed such that the tapered end moves towards the projection  210  thereby creating a gap between the valve member  204  and the inner surface of the manifold  200  which forms the valve opening  206  such that fluid may flow through the valve. Fluid flowing in the opposite direction will not be able to pass through the check valve due to the contact between the tapered end and the inner surface of the manifold which defines the valve opening  206 . 
         [0069]    Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.