Abstract:
A method of, and apparatus for, urging blades of a gas turbine engine radially outwardly is disclosed. The method may be used to grind blade tips of blades of a rotor stage of a gas turbine engine. The method comprises locating a fluid-tight bag is in a radial gap formed between a radially inner surface of a respective blade root and a slot in a disc which the blade root cooperates. The method comprises inflating the fluid-tight bag and rotating the rotor stage relative to a grinding surface so as to grind any blade tips that contact the grinding surface during rotation. This results in more accurate positioning of the blades during the grinding process and/or during operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from British Patent Application No. 1518570.5 filed 20 Oct. 2015, the entire contents of which are incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates to a method and apparatus for positioning blades of a gas turbine engine, for example during manufacturing the blades of a rotor for a gas turbine engine. In particular, but not exclusively, this invention can relate to a method of grinding the tips of the blades. 
         [0004]    2. Description of the Related Art 
         [0005]    A gas turbine engine typically comprises multiple rotor stages and multiple stator stages. The rotor stages comprise blades that rotate within a casing. The efficiency at which the engine operates can be affected by flow leaking between the rotor blades and the casing during use, which may be referred to as over-tip leakage. This may apply both to compressor rotor stages and to turbine rotor stages, but may be particularly important in compressor stages. 
         [0006]    In order to minimize such leakage, an abradable liner may be used to circumferentially surround the blades. This abradable liner may be abraded by the tips of the blades in use in order to try to provide a good seal during engine running. 
         [0007]    However, in order for the abradable liner to be effective, all of the tips of all of the compressor blades must be at the same radial position, or at least as close to being at the same radial position as possible. Any difference in radial tip position means that the abradable liner is abraded away by the blade tip at the further radial position, which in turn results in a radial gap between the other blade tips and the abradable liner. This radial gap may allow increased over-tip leakage, and thus decreased engine efficiency. 
         [0008]    It is therefore important to ensure that the radial positions of the tips of all of the rotor blades in a rotor stage of a gas turbine engine are as close to being the same as possible. To this end, a grinding process may be used on the blade tips during manufacture. Such a grinding process may involve rotating a rotor stage relative to a grinding surface. The grinding surface may thus be used to try to grind the tips of the blades to the same position. 
         [0009]    However, a problem occurs during this grinding process which may be said to be because of the way in which the blades are secured into the rotor disc. In particular, the blades are conventionally secured into one or more slots in the rotor disc. For example, each blade may have a root that is secured into a respective slot in the rotor disc that extends in a generally axial direction. Alternatively, each blade may have a root, and the roots of all of the blades may be secured into a single circumferentially extending slot in a rotor disc. 
         [0010]    Regardless of whether the roots of the blades are secured in an axially extending slot or a circumferentially extending slot, there is necessarily a degree of play or movement allowed between the blade root and disc slot in order for the blades to be slotted into place during assembly. However, this play or movement means that the blade can adopt a range of different positions when the rotor stage is rotated depending on, for example, exactly where the blade root and disc slot engage as the rotor speed is increased and the blade is centrifuged radially outwardly. Accordingly, during the tip grinding process, the engagement of one blade root in a slot may be at a different position to another blade root in its slot. This may result in the shape and/or absolute radial extent of the blades being different after the grinding process because, although all of their tips may engage the same grinding surface, their root may be in different positions and/or the blades may be at slightly different angles (i.e. they may not all be exactly radial and/or may have different axial lean components, again depending on the engagement position of the blade root with the slot). When the resulting different shaped and/or sized blades are rotated during running of a gas turbine engine, the differences in their geometry may lead to increased unwanted abrasion of the abradable liner. In turn, this may result in increased over-tip leakage, and thus decreased stage and engine efficiency. 
       OBJECTS AND SUMMARY 
       [0011]    Accordingly, it is desirable to ensure that the tip grinding process results in blade tips with reduced over-tip leakage and/or improved engine efficiency. It is also desirable more generally to ensure that the tips of the blades are in a known position during use as well as during grinding. 
         [0012]    According to an aspect, there is provided a method of grinding blade tips of blades of a rotor stage of a gas turbine engine. The rotor stage comprises a rotor disc and a plurality of radially extending blades, each blade comprising a root that cooperates with a slot in the rotor disc to secure the blade to the rotor, with a radial gap being formed between a radially inner surface of a respective root and the slot. The method comprises: locating a fluid-tight bag such that it is positioned in the radial gap; inflating the fluid-tight bag; and rotating the rotor stage relative to a grinding surface so as to grind any blade tips that contact the grinding surface during rotation. The rotation of the rotor stage relative to the grinding surface may be about an axial direction. 
         [0013]    The tips of the blades (and/or the blades themselves) may be pushed/urged radially outwardly by inflation of the fluid-tight bag (which may be referred to as a fluid-impermeable bag). The tip of each blade may be pushed/urged to its maximum radially outward extent by inflation of the fluid-tight bag. The inflated fluid-tight bag may ensure that the blade extends (for example the longitudinal or spanwise direction of the blade extends) in a radial direction, or at least that the blade extends in a known direction, such as a design direction. The blade may be said to be fully extended by the inflated bag. 
         [0014]    An apparatus for use in grinding the tips of rotor blades that extend radially from a root that cooperates with a slot in a rotor disc to secure the blade to the rotor of a gas turbine engine to a tip is provided. The apparatus comprises a fluid-tight bag for locating in a radial gap between a radially inner surface of a respective root and the slot. The apparatus comprises a fluid supply source for connecting to the fluid-tight bag in order to provide positive pressure to the fluid-tight bag in order to urge the rotor blades in a radially outward direction. 
         [0015]    Each and every blade may have a fluid-tight bag located in its respective radial gap. Each and every blade may be urged radially outwardly by inflation of a respective fluid-tight bag. 
         [0016]    The methods and apparatus of the present disclosure may help to ensure that each blade root is in the same position within its slot (and/or relative to its slot) as the other blade roots are within (and/or relative to) their blade slots. This may be beneficial during a grinding operation of the blade tips and/or during normal engine operation. For example, the methods and apparatus may help to ensure that the grinding operation produces blades having uniformly positioned blade tips. The methods and apparatus of the present disclosure may be said to result in more accurate positioning of the blades during the grinding process and/or during use, for example during engine running. 
         [0017]    The root of each rotor blade may extend substantially in an axial direction (for example in a substantially axial direction when assembled to the rotor disc). The slot with which each root cooperates may be a respective axially extending slot in the rotor disc. The radial gap formed by each root may be a respective axially-extending radial gap. The method may comprise locating and inflating a separate fluid-tight bag into each individual axially-extending radial gap before rotating the rotor stage to grind the tips. Where a gap, root, or slot is said to be axially-extending (or substantially axially-extending), it will be appreciated that this includes directions that are not precisely axial but have a substantial axial component, for example a greater axial component than radial component. 
         [0018]    The method may comprise connecting more than one fluid-tight bag (for example all fluid-tight bags) to a single fluid supply. Such a single fluid supply may be used to inflate all of the fluid-tight bags to which it is connected. This may be referred to as synchronous filling of multiple bags. 
         [0019]    Alternatively, each fluid-tight bag may be connected to a separate respective fluid supply. 
         [0020]    The root of each rotor blade may extend substantially in a circumferential direction (for example in a substantially circumferential direction when assembled to the rotor disc). The slot with which each root cooperates may be a single slot that extends circumferentially (including substantially circumferentially) around the rotor disc. Each root may cooperate with the same circumferentially-extending slot. The radial gap may be part of a continuous circumferentially-extending radial gap that extends around the entire circumference of the rotor disc. Such a continuous circumferentially-extending radial gap may be formed by a plurality of individual radial gaps, each formed by the roots of a rotor blade and each extending around a segment of the circumference. The method may comprise locating and inflating a single circumferentially extending fluid-tight bag in the continuous circumferentially-extending radial gap before rotating the rotor stage. 
         [0021]    Such a circumferentially extending fluid-tight bag may be circumferentially split so as to have a first end and a second end. The first end and the second end may be slideable relative to each other, for example slidable in a circumferential direction. Sliding the first end relative to the second end may allow the circumference of the bag to be adjusted. 
         [0022]    In an arrangement having a circumferentially split fluid-tight bag having a first end and a second end, the method of urging blades and/or grinding blade tips may comprise sliding the first end over the second end and adjusting the position of the first end relative to the second end so as to match the circumference of the fluid-tight bag to the circumference of the circumferentially extending slot and/or circumferentially extending radial gap. This may allow the same fluid-tight bag to be used during the grinding of the blades of a range of rotor stages that may be of different sizes and/or may have different circumferences (for example different rotor disc circumferences). 
         [0023]    A circumferentially extending fluid-tight bag may comprise a circumferential locking feature. The circumferential locking feature may prevent circumferential rotation of the blades relative to the slot. The circumferential locking feature may extend radially between two blade platforms. The two platforms between which the circumferential locking feature extends may be provided with cutouts to allow the circumferential locking feature to pass therethrough. 
         [0024]    The circumferential locking feature may comprise a one-way valve having a threaded outer portion. A bolt may be provided to the threaded portion. Such a bolt may be adapted to receive a tightening tool, for example one or more recesses or notches on its radially outer surface. This may be useful for arrangements in which it is not possible to access the flats of the bolt with a tightening tool (such as a spanner) due to the proximity of the neighbouring blade platforms. 
         [0025]    The or each fluid-tight bag may be provided into the position in which it is inflated at any suitable stage of the method. For example, the or each fluid-tight bag may be located into position before blades are assembled to the rotor disc to form the or each radial slot. Alternatively, if space permits, the or each fluid-tight bag may be provided into position after at least one (for example one, more than one or all) of the rotor blades have been assembled to the disc. 
         [0026]    According to any arrangement, the or each fluid-tight bag may comprise a one-way valve. Such a one-way valve may allow fluid to pass into the fluid-tight bag, but not back out of the fluid-tight bag. The method may comprise connecting the or each fluid-tight bag to a fluid supply via its one-way valve prior to the inflation step. 
         [0027]    The method may comprise disconnecting the or each fluid-tight bag from its fluid supply after the inflation step. Where the or each fluid-tight bag is used during a tip grinding process, the method may comprise disconnecting the or each fluid-tight bag from its fluid supply prior to the rotation step. 
         [0028]    Any suitable fluid (gas or liquid) may be used to inflate the or each fluid-tight bag. For example, a gas such as air (for example compressed air) may be used. The fluid supply may comprise an air supply, which may provide compressed air (that is, air that has a high pressure than atmospheric and/or air that has been compressed using a compressor). Purely by way of non-limitative example, the fluid may have a pressure in the range of from 1 and 10 bar, for example 2 and 5 bar, for example 3 and 4 bar. 
         [0029]    Where the or each fluid-tight bag is used during a tip grinding process the method may comprise removing the or each bag after the step of rotating the rotor stage to grind the blade tips. The or each bag may be deflated before removal. Thus, the rotor stage may not comprise the fluid-tight bag after final assembly and/or during normal engine running, such as in-service engine running. Alternatively, as described elsewhere herein, and inflated fluid-tight bag such as those described and/or claimed herein may be in position during normal engine running, such as in-service engine running. 
         [0030]    Any suitable material may be used for the fluid-tight bag. Purely by way of non-limitative example, the fluid-tight bag may be an elastic material and/or may be expandable under pressure. The fluid-tight bag may be re-usable (for example, each bag may be used in at least one, for example more than one, tip-grinding operation). 
         [0031]    For tip grinding operations, the grinding surface relative to which the rotor stage is rotated in the grinding step may be any suitable surface, for example a cylindrical or frusto-conical surface. For example the grinding surface may be, or may be the same shape as, the radially inner surface of a casing of a gas turbine engine, which may be the same type of casing (for example in terms of shape and/or geometry) as that within which the blades will rotate in use. 
         [0032]    According to an aspect, there is provided a method of manufacturing a rotor stage of a gas turbine engine comprising: 
         [0033]    providing a rotor disc with a plurality of radially extending blades, each blade extending from a root which is secured into a slot in the rotor disc to a tip at its radially outer extent; and 
         [0034]    grinding the tips of the blades using any of the methods and/or apparatus described and/or claimed herein. 
         [0035]    The step of providing a rotor disc with a plurality of radially extending blades may comprise inserting a root of each rotor blade into a slot in the rotor disc to secure the blade to the rotor. 
         [0036]    According to an aspect, there is provided a method of manufacturing a gas turbine engine comprising grinding the tips of blades of at least one rotor stage of the engine according to any method or apparatus described and/or claimed herein. 
         [0037]    The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    Embodiments will now be described by way of example only, with reference to the Figures, in which: 
           [0039]      FIG. 1  is a sectional side view of a gas turbine engine; 
           [0040]      FIG. 2  is a schematic axial view of a rotor stage during a tip grinding operation; 
           [0041]      FIG. 3  is a schematic of a root region of a blade having an axially extending retention arrangement; 
           [0042]      FIG. 4  is a schematic of a root region of a blade having a circumferentially extending retention arrangement; 
           [0043]      FIG. 5  is a schematic showing an arrangement of fluid supply system; 
           [0044]      FIG. 6  is a schematic showing an alternative arrangement of fluid supply system; 
           [0045]      FIG. 7  is a schematic showing a circumferentially extending fluid bag in a rotor stage; 
           [0046]      FIG. 8  is a schematic view in a radial-circumferential plane showing a circumferential locking mechanism; and 
           [0047]      FIG. 9  is a schematic view in a circumferential-axial plane showing the circumferential locking mechanism of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0048]    With reference to  FIG. 1 , a gas turbine engine is generally indicated at  10 , having a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , an intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
         [0049]    The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
         [0050]    The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high  17 , intermediate  18  and low  19  pressure turbines drive respectively the high pressure compressor  15 , intermediate pressure compressor  14  and fan  13 , each by suitable interconnecting shaft. 
         [0051]    Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
         [0052]    The compressor and turbine rotor stages of the gas turbine engine  10  shown in  FIG. 1 , which is an example of a gas turbine engine  10  in accordance with the present disclosure, comprise blades  100 , just one example of which is labelled in  FIG. 1  for simplicity. As outlined elsewhere herein, it is important for the gap  300  between the blade  100  and the casing  200  within which it rotates to be as small as possible during use, whilst minimizing the possibility of the tips of a blade  100  rubbing against the casing  200 . 
         [0053]    Accordingly, the blades  100  of a rotor stage are subjected to a grinding operation before the engine  10  is put into service. The grinding operation attempts to ensure that all of the blades are as close as possible to being the same length as each other. 
         [0054]    With reference to  FIG. 2 , the blades  100  of a rotor stage  250  are shown schematically within a casing  500 , which may be at least a part of, or may have the same geometry as at least a part of, the casing  200  within which the rotor stage  250  rotates during operation. The blades  100  extend from a root  120  to a tip  110 . 
         [0055]    The casing  500  has a grinding surface (or abrasive surface)  510  on its radially inner surface. In a grinding operation, the rotor blades  100  are rotated (for example using a motor) within the casing, such that the tips  110  of the blades are subjected to grinding where they contact the grinding surface  510 . The rotation of the blades  100  is indicated by arrow A in  FIG. 2 , but of course could be either clockwise or anticlockwise about the rotational (or longitudinal) axis  11 . 
         [0056]    The blades  100  are held in position at their root  120  by a blade retention arrangement  400 . The blade retention arrangement  400  may comprise, for example, an axially extending slot and root arrangement, or a circumferentially extending slot and root. 
         [0057]    An axially extending blade retention arrangement  400  is shown in greater detail in  FIG. 3 . The blade root  120 , which may be a dovetail root  120 , extends in an axial direction (or at least has a significant component extending in the axial direction), which is the direction about which the stage  250  rotates, i.e. the same direction as the engine rotational axis  11  shown in  FIG. 1 . The axially extending blade root of  FIG. 3  is retained within a corresponding axially extending slot  620  formed in the rotor disc  600 . The rotor disc  600  may be the same as the rotor disc on which the blades  100  are mounted in the engine  10  in operation, or may be a specifically designed rotor disc for use in the grinding operation. In either case, the axially extending slot  620  may have the same geometry as the slot in which the root  120  is retained during operation of the engine  10 . 
         [0058]    As shown in  FIG. 3 , a gap  750 , which may be referred to as a radial gap  750 , is formed between the radially inner surface or portion  140  of the blade root  120  and the base  640  (which may be referred to as the radially inner surface) of the slot  620  in the disc  600 . The radial gap  750  may be said to extend in an axial direction. 
         [0059]    During tip grinding at least, a fluid-tight bag  700  is provided in the gap  750 . The fluid-tight bag  700  is provided with a fluid (for example compressed air) from a fluid source  720  (which may be, for example, an air compressor). The fluid may be provided through a valve  710 , as in the example shown in  FIG. 3 . Such a valve  710  may be a one-way valve that may allow the fluid to be retained in the fluid-tight bag  700  after the fluid supply  720  has been disconnected. 
         [0060]    The fluid-tight bag  700  may thus be inflated by the fluid supply  720 , thereby applying a force to the blade  100 . The blade  100  may be urged, or pushed, generally radially outwardly by the force provided by the inflated fluid-tight bag  700 . In turn, this may ensure that the blade  100  is pushed to its radially outermost position, for example for a the tip grinding operation and/or for engine operation. By ensuring that each blade  100  is extended to its radially outermost position using one or more fluid-tight bags  700 , the position of each blade tip  110 , for example relative to its root  120 , can be more consistent between the blades  100  in the stage  250 . The geometry of the blades  100  may be more consistent for each blade  100  in the stage  250  through use of the fluid-tight bag  700  to urge the blades  100  radially outwards during the grinding operation. 
         [0061]    For rotor stages  250  having generally axially extending blade retention arrangements  400  such as that shown in  FIG. 3 , a separate fluid-tight bag  700  may be provided to each blade  100  in the stage  250 . In such an arrangement, a fluid supply  720  may supply fluid to one or more (for example all) of the fluid-tight bags  700 . 
         [0062]      FIG. 4  shows an example of a rotor blade  100  that has a generally circumferentially extending blade retention arrangement  400 . Other features of the arrangement may be substantially as described above in relation to  FIG. 3 . 
         [0063]    In the  FIG. 4  arrangement, the slot  620  in the disc  600  extends in a generally circumferential direction. The radial gap  750  formed between the radially inner surface  140  of the blade root  120  and the radially inner surface  640  of the slot  620  is circumferentially extending. The radial gap  750  may be continuous around the entire circumference of the rotor stage  250 . The fluid-tight bag  700  also extends in a circumferential direction, and may optionally extend around the entire circumference of the rotor stage  250 . Optionally, a single fluid-tight bag  700  may engage all of the blade roots  120 . 
         [0064]      FIG. 4  shows clearly the engagement surface  125  of the blade root  120  engaging with the engagement surface  625  of the disc slot  620 . The inflated fluid-tight bag  700  ensures that this engagement is consistent for all blades. 
         [0065]    This is also true for arrangements having axially extending blade retention arrangements  400 , such as that shown in  FIG. 3 . 
         [0066]    The  FIG. 4  arrangement may also comprise a fluid supply  720  and optionally one or more valves  710  such as those shown in  FIG. 3 , although they are not shown in  FIG. 4  for clarity. 
         [0067]      FIG. 5  is a schematic showing at least one fluid tight bag  700  and more than one valve  710 . Each valve  710  may be associated with an individual blade  100 . The fluid-tight bag  700  shown schematically in  FIG. 5  may be continuous, or may represent more than one fluid-tight bag  700 , with, for example, each bag  700  being associated with an individual blade  100 . Each valve  710  may be supplied by a its own dedicated fluid supply  720 , or a single fluid supply  720  may feed more than one valve  710 , for example all valves  710 . Purely by way of example, the arrangement shown in  FIG. 5  may be used with axially extending blade retention arrangements such as that shown by way of example in  FIG. 3 . 
         [0068]      FIG. 6  is a schematic showing an alternative arrangement of fluid-tight bag  700  and valves  710 . In the  FIG. 6  example, the illustrated fluid-tight bag  700  is a single, continuous, fluid-tight bag  700 , having a first end  712  and a second end  714 . Purely by way of example, the fluid-tight bag  700  may extend circumferentially around a circumferentially extending radial gap  750 , such as that shown in  FIG. 4  by way of example, during the grinding process. 
         [0069]    The first and second ends  712 ,  714  may be moved (for example circumferentially moved) relative to each other, thereby adjusting the radius of the fluid-tight bag  700 . Accordingly, a single fluid-tight bag  700  may be used with a range of different rotor stages  250 , for example having radially extending gaps  750  at different radii. Although a gap is shown between the two ends  712 ,  714  in  FIG. 6 , in use the two ends may overlap, such that the fluid-tight bag  700  is continuous around the circumference. 
         [0070]      FIG. 7  is a more detailed schematic view of a part of the fluid-tight bag  700  in position in the circumferentially extending slot  620 . Features of  FIG. 7  that are equivalent to those of previous Figures are given the same reference numerals and will not be explained again in relation to  FIG. 7 . 
         [0071]    In  FIG. 7 , the fluid-tight bag  700  is provided with a circumferential locking mechanism (which may be referred to as a circumferential blade-locking mechanism)  800 . The circumferential locking mechanism  800  comprises a one-way valve  710  and a nut  810 , although it will be appreciated that the circumferential locking mechanism  800  could take any suitable form, for example any form that extends radially through the platforms  105  of two blades  100  when the rotor stage  250  is assembled. 
         [0072]    The purpose of the circumferential locking mechanism  800  is to prevent circumferential rotation of the blades  100  within the slot  620 . In  FIG. 7 , some of the blades  100 , including those that would be either side of the circumferential locking mechanism  800 , are deliberately not shown so as to provide a more clear view of the circumferential locking mechanism  800 . 
         [0073]      FIGS. 8 and 9  show the position of the circumferential locking mechanism  800  when the rotor stage is assembled, including the blade platforms  105  either side of it. The platforms  105  are provided with cutouts  106  through which the circumferential locking mechanism  800  extends. Because the circumferential locking mechanism  800  extends radially through and between the platforms  105  of neighbouring blades  100 , the neighbouring blades  100 , and thus all of the blades  100 , are prevented from unwanted circumferential movement (shown by arrow A in  FIG. 9 ) within the slot  620 . 
         [0074]    In the example of  FIG. 9 , the nut  810  is provided with tool receiving holes  812  for receiving a tightening tool, but it will be appreciated that other arrangements for tightening the nut  810  are possible. 
         [0075]    Although not shown explicitly in  FIGS. 7 to 9 , the circumferential locking mechanism  800  may itself be locked in position relative to the rest of the rotor stage  250 , for example through being fastened to and/or integral to the slot  620 . 
         [0076]    In that case, the fluid-tight bag may attach to a valve portion  710  of the circumferential locking mechanism  800  for inflation. 
         [0077]    It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.