Abstract:
A balancing kit and balancing method is provided for balancing a rotating component, The balancing kit comprises a receptacle and a balancing mass. The balancing mass comprises a pin that may be passed through a corresponding slot in a plate of the receptacle during installation. To complete installation, the balancing mass is rotated so that the pin can no longer pass through the slot, and is instead retained in an installed position in which the pin rests on a lower surface of the plate. The balancing mass can be removed by the reverse process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from British Patent Application Number 1409245.6 filed 23 May 2014, the entire contents of which are incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates to balancing a rotating part, such as a rotating part of a gas turbine engine. Aspects of the present disclosure relate to a balancing kit, a balancing mass, a balancing receptacle, and a balancing method. 
         [0004]    2. Description of the Related Art 
         [0005]    The rotor discs of compressors and turbines of gas turbine engines rotate at very high speed in use. It is important for the rotating components, including the discs, to be properly balanced, because any imbalance may result in unwanted movement of the rotating components, for example due to bending of a shaft about which the rotating components rotate. This may result in unwanted contact between rotating and stationary parts and/or an increase in the required tolerance gap between rotating and stationary parts, such as between rotating blades and surrounding cases. In turn, this may result in efficiency losses and/or vibrations, which may lead to damage, for example to bearings. 
         [0006]    Accordingly, rotating discs of gas turbine engines need to be precisely balanced during manufacture and assembly. The discs may be balanced firstly during manufacture of the discs themselves, for example by removing material in a conventional manner. The discs may then be precision balanced again after assembly with other rotating components, such as blades that are attached to the disc. It may also be necessary to re-balance rotating components, such as discs, during service of the engine. 
         [0007]    One method for precision balancing rotors involves providing grub screws to a rotor disc at various positions around its circumference. The grub screws may be screwed into corresponding threads in the disc. In this way, the rotor can be balanced by selecting the appropriate position and/or number of grub screws around the disc. 
         [0008]    As mentioned above, a disc may need to be re-balanced through its service life, for example due to wear. However, where grub screws are used to balance a disc, the screws may seize in their threads during operation. This may be due to the extreme operating environment, including very high temperatures. This means that rebalancing of a disc may be extremely difficult, particularly rebalancing of a disc in-situ. The problem is exacerbated because the grub screws need to be particularly tight during operation of the engine to ensure that they do not become loose, because if a screw were to come free during operation it could cause significant damage to the engine. Thus, the grub screws need to be tight, but this increases the possibility of the seizing. 
       OBJECTS AND SUMMARY 
       [0009]    Accordingly, it is desirable to be able to balance and optionally re-balance a rotor in a reliable and straightforward manner. 
         [0010]    According to an aspect, there is provided a balancing kit for balancing a rotating part of a gas turbine engine. The balancing kit comprises a balancing mass. The balancing mass comprises an interface portion defining tooling interface. The balancing mass comprises a retaining member. The balancing mass comprises a resilient member located between the interface portion and the retaining member. The balancing kit comprises a receptacle for receiving the balancing mass. The receptacle comprises a receiving opening for receiving the balancing mass. The receptacle comprises a retaining plate having an upper surface and a lower surface, and a hole extending therethrough. The hole is non-axisymmetric about a longitudinal (or central) axis of the receptacle. The retaining member of the balancing mass is arranged to be able to pass through the hole for installation in the receptacle. The balancing mass is rotatable via its tooling interface. The retaining member can thereby be rotated so as to rest on (and/or be biased towards) the lower surface of the retaining plate to thereby removably secure the balancing mass to the receptacle in an installed position under the action of the resilient member. 
         [0011]    According to an aspect, there is provided a balancing mass as described and claimed herein in relation to the balancing kit. According to an aspect, there is provided a balancing mass for balancing a rotor. The balancing mass comprises an interface portion defining tooling interface. The balancing mass comprises a retaining member. The balancing mass comprises a resilient member located between the interface portion and the retaining member. The retaining member is arranged (for example shaped) to be able to pass through a hole in a retaining plate provided to the rotor for installation into the rotor. The tooling interface is arranged to transmit a torque to the balancing mass (for example from a tool) to rotate it to an installed position. In the installed position, the retaining member rests on a lower surface of the retaining plate to retain the balancing mass in the installed position under the action of the resilient member. The balancing mass may be described as non-threaded. 
         [0012]    According to an aspect, there is provided a receptacle for receiving a balancing mass as described and claimed herein. According to an aspect, there is provided a receptacle comprising a receiving opening for receiving the balancing mass. The receptacle comprises a retaining plate having an upper surface and a lower surface, and a hole extending therethrough. The hole is non-axisymmetric about a longitudinal axis of the receptacle. The hole is shaped to allow a retaining member of the balancing mass to pass therethrough for installation of the balancing mass in the receptacle. The retaining plate is arranged to allow the retaining member to rest on the lower surface thereof following rotation of the balancing mass after the retaining member has passed through the hole, thereby removably securing the balancing mass to the receptacle in an installed position under the action of the resilient member. 
         [0013]    According to an aspect, there is provided a method of balancing a rotor of a gas turbine engine. The method comprises providing the rotor with at least one receptacle for receiving a respective balancing mass. The receptacle comprises a receiving opening for receiving the balancing mass. The receptacle comprises a retaining plate having an upper surface and a lower surface, and a hole extending therethrough. The hole is non-axisymmetric about a longitudinal axis of the receptacle. The method further comprises installing a balancing mass to one or more selected receptacles in order to balance the rotor. The balancing mass comprises an interface portion defining tooling interface; a retaining member; and a resilient member located between the interface portion and the retaining member. The step of installing the balancing mass to its respective receptacle comprises: passing the retaining member through the hole; and rotating the balancing mass into an installed position. The balancing mass may be rotated by providing torque to its tooling interface. In the installed position, the retaining member rests on the lower surface of the retaining plate to thereby removably secure the balancing mass to the receptacle under the action of the resilient member. 
         [0014]    The method may comprise a determining step in which the selection of receptacles to which balancing masses should be provided and/or the mass of the balances is determined. Any suitable technique may be used in such a determining step, for example previously known techniques for deciding on the position/mass of one or more balancing masses to balance a rotating part. 
         [0015]    In any arrangement and/or method described and/or claimed herein, the non-axisymmetric hole may comprise at least one slot. The slot may extend from an axisymmetric (or circular) hole, so as to form the non-axisymmetric hole. In such an arrangement, the retaining member may be arranged to pass through the slot(s). 
         [0016]    The hole of the receptacle and the opening of the receptacle may be said to be in communication with each other, that is they may be said not to have any walls separating them. 
         [0017]    The longitudinal axis of the receptacle may be aligned with (for example collinear with) a longitudinal axis of the balancing mass during installation and/or when the balancing mass is in the installed position. The longitudinal axis may be the axis along which the balancing mass is translated and/or about which the balancing mass is rotated for installation. 
         [0018]    During the step of passing the retaining member through the hole, the resilient member may be supported on the upper surface of the plate so as to be compressed between the plate and the interface portion. 
         [0019]    In straightforward terms, the method may be said to involve installation and/or removal of a balancing mass. Installation may involve pressing the balancing mass along its longitudinal axis, rotation (or turning) of the balancing mass about its longitudinal axis into an installed position, and then release of the balancing mass in the installed position. Removal may involve the same steps, but with the rotation being in the opposite direction about the longitudinal axis. 
         [0020]    The methods, balancing kits, receptacles and balancing masses described and claimed herein allow straightforward and reliable balancing of a rotating part. The balancing masses are not susceptible to seizure, may be easily removed and re-fitted after engine running, and/or allow straightforward re-balancing of a rotor after a period of use. 
         [0021]    It will be appreciated that any features described and/or claimed herein may apply to a balancing mass, receptacle, balancing kit and/or method as applicable. 
         [0022]    In the installed position the resilient member may be compressed between the interface portion of the balancing mass and the upper surface of the retaining plate of the receptacle. 
         [0023]    The balancing mass may be said to have a longitudinal axis. The interface portion may be a proximal end region of the longitudinal axis. The retaining member may be at an opposing distal end region of the longitudinal axis. In the balancing method, the step of passing the retaining member through the slot may involve moving the balancing mass (and thus the retaining member) relative to the receptacle (and thus the retaining plate) along the longitudinal axis. The step of rotating the balancing mass into an installed position may involve rotating the balancing mass about the longitudinal axis. The retaining plate of the receptacle may be offset from the receiving opening along a direction that corresponds to a longitudinal axis. When installed, the balancing mass and the receptacle may have respective longitudinal axes that are collinear. 
         [0024]    The interface portion may have a cross-section that has an outer perimeter that is axisymmetric about the longitudinal axis. The receiving opening may have a cross-section that has an inner perimeter that is axisymmetric about the longitudinal axis. The tooling interface may be arranged so as to be able to rotate the balancing mass about the longitudinal axis into the installed position. 
         [0025]    The cross-sectional shape of the interface portion may correspond to the cross-sectional shape of the receiving opening. The cross-sectional shapes of the (outer perimeter of) the interface portion and the receiving opening may be circular. During installation or removal of a balancing mass from a respective receptacle, the tooling interface may rotate within the receiving opening. 
         [0026]    The balancing mass may comprise a main body extending from the interface portion along the longitudinal axis. The retaining member may extend from the balancing mass, for example from the main body of the balancing mass. The retaining member may not be axisymmetric about the longitudinal axis. 
         [0027]    The shape of the hole in the retaining plate, including any slot, is not axisymmetric, for example about an axis normal to the hole. The shape of the hole in the retaining plate may, however, be axisymmetric (for example circular) without a slot. Thus, one or more slots may extend from an axisymmetric (or circular) portion of the hole. The shape of the slot may correspond to the shape of the retaining member. 
         [0028]    Where the balancing mass is said to have a main body, the main body may be axisymmetric about the longitudinal axis. 
         [0029]    The retaining member may take any suitable form. For example the retaining member may comprise at least one elongate pin extending in a plane perpendicular to the longitudinal axis. Such an elongate pin may be said to extend in a plane that is perpendicular to the direction in which the balancing mass moves in order to pass the retaining member through the slot. Such an elongate pin may be said to extend in a plane that is perpendicular to the axis about which the balancing mass is rotated after the retaining member has been passed through the hole (for example through a slot), to the installed position. 
         [0030]    The retaining member may comprise more than one pin, for example two, three, four or more than four pins. Where the retaining member comprises more than one pin, the pins may be equally spaced around the balancing mass, for example around the longitudinal axis of the balancing mass. For example, where the retaining member has two pins, these may be diametrically opposed. 
         [0031]    In addition to the retaining member, at least a part of the balancing mass from which the retaining member extends, for example at least a part of a main body, may also pass through the hole in the retaining plate when in the installed position. The hole in the retaining plate may be said to have a first portion (which may be axisymmetric, such as circular) through which a main body of the balancing mass may pass, and a second portion (which may not be axisymmetric) through which the retaining member may pass. The shape of the first portion may be said to correspond to the shape of the cross-section of the main body, and the shape of the second portion may be said to correspond to the shape of the retaining member. 
         [0032]    In the installed position, the retaining member may rest on any part of the lower surface of the retaining plate. Optionally, the lower surface of the retaining plate may comprise a retaining recess having a shape that corresponds with the retaining member, such that in the installed position, the retaining member engages the retaining recess. This may help to retain the retaining member, and thus the balancing mass, in a fixed position during use. 
         [0033]    According to an aspect, there is provided a rotor for a gas turbine engine comprising at least one balancing kit as described and/or claimed herein. In such an arrangement, the receptacle of the or each balancing kit may be provided to the rotor (for example around the circumference of the rotor), and at least one of the or each receptacle may be provided with its respective balancing mass. Any number of receptacles may be provided to the rotor, for example between 1 and 50 or more, for example 2 and 40, for example 5 and 30, for example 10 and 20. Such a rotor may be particularly straightforward to balance and/or re-balance, for example in situ in an engine. 
         [0034]    Any selected combination of balancing masses may be used in order to balance the rotor. For example, where a rotor is provided with at least two balancing kits, at least one of the receptacles may not be provided with a respective balancing mass. Additionally or alternatively, at least two receptacles may be provided with balancing masses that have different masses to each other. 
         [0035]    The balancing kit and/or method described and claimed herein may be used to balance any type of rotor or rotatable part of a gas turbine engine. For example, the rotatable part may be or may comprise a rotatable disc of a gas turbine engine, such as a compressor or a turbine disc, to which blades may be attached. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    According to an aspect, there is provided a gas turbine engine comprising a rotor balanced using a balancing kit as described and/or claimed herein and/or using the method as described and/or claimed herein. 
           [0037]    Embodiments of the disclosure will now be described by way of non-limitative example with reference to the accompanying drawings in which: 
           [0038]      FIG. 1  shows a cross-section through a gas turbine engine according to the present disclosure; 
           [0039]      FIG. 2  shows a balancing kit according to the present disclosure being used to balance a rotor disc; 
           [0040]      FIG. 3  shows a detailed cross-section through a balancing kit according to the present disclosure; 
           [0041]      FIG. 4   a  shows a detailed cross-section through a receptacle according to the present disclosure; 
           [0042]      FIG. 4   b  shows a cross-section through the line A-A of the receptacle of  FIG. 4   a;    
           [0043]      FIG. 5   a  shows a detailed cross-section through a balancing mass according to the present disclosure; 
           [0044]      FIG. 5   b  shows a cross-section through the line B-B of the balancing mass of  FIG. 5   a ; and 
           [0045]      FIG. 6  shows a detailed schematic of a balancing mass being rotated within a receptacle for installation to or removal from a receptacle. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0046]    With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis X-X. The engine  10  comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , and intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . The engine also has a bypass duct  22  and a bypass exhaust nozzle  23 . 
         [0047]    The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate pressure compressor  13  and a second air flow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
         [0048]    The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  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  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  16 ,  17 ,  18  respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
         [0049]    Each of the high, intermediate and low-pressure turbines  16 ,  17 ,  18  and the intermediate and high-pressure compressors  13 ,  14  comprises at least one stage comprising a set of rotor blades and a set of stator vanes. In use, the rotor blades rotate around the engine axis X-X, whilst the stator vanes are stationary within the engine. Purely by way of example, in  FIG. 1 , the rotor blades of the intermediate pressure compressor  13  are given the reference numeral  130 , and the stator vanes are given the reference numeral  140 . 
         [0050]    A rotor disc  132  may have one or more sets of rotor blades  130  attached thereto, from one or more stages. The rotor blades  130  may be attached to the rotor disc in any suitable manner, for example by manufacturing slots in the disc  132  into which the blades  130  are inserted, or by permanently joining the blades  130  to the disc  132  (for example by welding, for example linear friction welding or inertia welding), or by forming the disc  132  and the blades  130  from a single metal sheet. 
         [0051]    The rotor blades  130  and the rotor disc  132  of any stage of the compressor or turbine rotate together at high rotational speed in use of the gas turbine engine  10 . Accordingly, it is important to be able to precision balance the rotating parts of the gas turbine engine  10 , such as the combined rotor disc  132  and rotor blades  130 , for at least the reasons described herein. It may be advantageous to be able to precision balance the rotating parts when they are in-situ in the engine, and to be able to adjust the balance of the rotating parts after a period of service in a convenient manner. 
         [0052]    In  FIG. 1 , the rotor blades are shown as being radially inside an engine core casing  150 , The engine core casing  150  is shown schematically in  FIG. 1 . In  FIG. 2 , the casing  150  is shown by way of example as comprising an inner casing  152  and an outer casing  154 , the terms inner and outer referring to their relative radial positions. The stator vanes  140  may extend from the inner core casing  152 , as in the  FIG. 2  example. 
         [0053]    The rotating parts may be balanced using a balancing kit  100 . The balancing kit comprises a receptacle  110  and a balancing mass  120 , both of which are described in greater detail below and elsewhere herein. The balancing kit  100  is shown in  FIG. 2  in a slot (which may be a circumferentially extending slot, such as a dovetail slot)  134  formed in the rotor disc  132 . During assembly of the gas turbine engine  10 , the rotor blades (more precisely a root portion of each rotor blade  130 ) may be slotted into the slot  134  in the rotor disc  132  in order to secure the rotor blades  130  to the disc  132 . The balancing kit  100 , for example the receptacle  110  thereof, may be secured in the slot  134  during assembly, for example between rotor blades  130 . Accordingly, the receptacle  110  may be fixed in (or integral with) the disc  132 . 
         [0054]    A rotating part, such as the disc  132 , may be provided with more than one balancing kit  100 , or at least more than one receptacle  110  of a balancing kit  100 , spaced around its circumference. The rotating part  132  may be balanced by inserting suitable balancing masses  120  into respective selected receptacles. The choice of mass of the balancing mass  120  and/or receptacle  110  into which to insert a balancing mass  120  in order to balance the rotating part  132  may be determined by any suitable method. 
         [0055]    A balancing mass  120  may be inserted or removed into a respective receptacle prior to final assembly of the gas turbine engine  10  (for example during assembly of the rotor stage) and/or in situ (for example when the rotor stage is within the casing  150 , as shown in  FIG. 2 ). Accordingly, the rotating part  132  may be balanced after a period of service of the gas turbine engine  10  without stripping the engine. 
         [0056]    As shown in  FIG. 2 , the balancing receptacle  110  may be accessible from outside the casing  150  (for example from outside an inner casing  152  and an outer casing  154 ) through one or more inspection holes  153 ,  155  formed in the inner and outer casings  152 ,  154 . The inspection holes  153 ,  155  may be used for other purposes, for example for inserting a baroscope or other inspection into the engine core to inspect the core of the engine  10 , for example during service or maintenance. 
         [0057]    In some arrangements the receptacle  110  may be a part of, for example formed with and/or integral with, the rotating part  132 . 
         [0058]    In order to insert or remove a balancing mass  120  into a respective receptacle  110 , a tool  200  may be used. Such a tool  200  is shown by way of example in  FIG. 2 . The tool  200  may be of any suitable form. For example, the tool may comprise an outer body  250  and a tightening body (which may be an elongate tightening body)  210 . The tightening body  210  may be placed within a through hole in the outer body  250 . In such an arrangement, the tightening body  210  is rotatable relative to the outer body  250  about their common longitudinal axes Y-Y. 
         [0059]    As described elsewhere herein, in order to install a balancing mass  120  into a respective receptacle  110 , it may be pressed (and thus translated) in the direction along its axis Y-Y (which may correspond to the radial direction of the rotating part  132 ), and then rotated about the same axis. This may be performed using a tool, such as the tool  200  shown in  FIG. 2 . In order to remove a balancing mass  120  from its receptacle  110 , it may be rotated (for example by tool  200 ) to a release position, at which point it may be released and/or separated from the receptacle  110 . When the balancing mass  120  is not held in its receptacle  110 , it may, for example, be removably secured to the tool  200 , for example in any suitable manner, such as using a magnet and/or a recess (not shown) in the tool  200  into which the balancing mass may be drawn. 
         [0060]      FIG. 3  shows a cross-section through an example of a balancing kit  100  including a receptacle  110  and a balancing mass  120  in greater detail. In  FIG. 3 , the balancing mass  120  is shown in an installed position in the receptacle  110 .  FIG. 4   a  shows a cross-section through the centreline of an example of a receptacle  110  that may be used as part of a balancing kit  100  such as that described herein, and  FIG. 4   b  shows a cross-section through the line A-A in  FIG. 4   a .  FIG. 5   a  shows a cross-section through the centreline of an example of a balancing mass  120  that may be used as part of a balancing kit  100  such as that described herein, and  FIG. 5   b  shows a cross-section through the line B-B of  FIG. 5   a . Like features in the Figures are given like reference numerals, and reference may be made to all of the Figures in the description provided herein. 
         [0061]    The balancing mass  120  has an interface portion  121 . The interface portion  121  is arranged to receive a force and/or torque for installing and/or removing the balancing mass  120  from an installed position (for example in a receptacle  110 ) to a removed, or detached position. The interface portion  121  may define a tooling interface  122  for this purpose. The tooling interface  122  may be arranged to receive any suitable tool for providing the torque and/or force, such as the tool  200  shown purely by way of example in  FIG. 2 . For example, the tooling interface  122  may be a key shape, such as a hex-key shape (or Allen key shape), a slot, a cross, a torque drive shape or a star drive shape. 
         [0062]    The balancing mass  120  also comprises a retaining member  123 . In the example shown in  FIGS. 3 and 5 , the retaining member  123  comprises two pins  124 . The two pins extend from a main body  125  of the balancing mass  120 . The main body  125  itself may be axisymmetric about a central axis Y-Y, and may have a circular cross-section. However, a cross-section through the balancing mass  120  that is perpendicular to the central axis Y-Y and passes through the main body  125  and the retaining member  123  may not be axisymmetric, for example as shown in the  FIG. 5   b  example. The balancing mass  123  may comprise any suitable retaining member  123 . For example, as alternatives to the arrangement shown in  FIG. 5   b , a retaining member  123  may comprise just one pin  124 , or more than two pins  124 , for example 3, 4, 5 or more than 5 pins  124 . 
         [0063]    In the installed position shown in  FIG. 3 , the retaining member  123  rests on a retaining surface  113  of the receptacle  110 , which is shown clearly in  FIG. 4   b . Optionally, the retaining member may rest or sit in a retaining recess  114 , shown by the dashed line in  FIG. 3 , which may extend into the retaining surface  113  (which may be referred to elsewhere herein as the lower surface  113 ). The retaining surface  113  of the receptacle  110  may be defined by another feature of the receptacle  110 , such as by a retaining plate  112  (which may be any sort of body). 
         [0064]    The retaining surface  113  (and/or the retaining plate  112  by which it is defined) has a hole  115  therein. The hole  115  in the  FIGS. 3 and 4  example extends entirely through the retaining plate  112 . In the example of  FIGS. 3 and 4 , the hole  115  has two slots, or slot portions,  116 . The slots  116  extend from a central hole portion  117 . The central hole portion  117  may be axisymmetric, as shown in the  FIG. 4   b  example. 
         [0065]    Other arrangements of retaining plate  112  may have different non-axisymmetric shapes, for example different numbers of slots  116 , for example 1, 3, 4, 5 or more than 5 slots  116 . In general, the number of slots  116  may be equal to the number of retaining members  123  of the balancing mass  120 . In general, the shape of the or each slot  116  may correspond to the shape (for example the plan form shape) of the or each retaining member  123 . In general, the shape of a central hole portion  117  from which the slots  116  extend may correspond to the shape of the main body  125  from which the pins  124  of the balancing mass extend (for example at least to the shape of a cross-section through the main body at the position of the pins  123 ). In general the shape of the hole  115  may correspond to the shape of the balancing mass  120  at a cross-section through the balancing mass  120  perpendicular to the central axis Y-Y. The term correspond to may mean the same as or slightly offset from. For example, the hole  115  may be slightly larger (or offset from) the cross-section through the balancing mass  120  to allow the balancing mass  120  to pass therethrough. 
         [0066]    The slot or slots  116  may actually be any suitable shape, for example any shape that results in the hole  115  not being non-axisymmetric and/or any shape that corresponds to the shape of the retaining member  123 . 
         [0067]    The balancing mass  120  also comprises a resilient member  126 . The resilient member  126  may be a spring, such as a compression spring as shown by way of example in  FIGS. 3 and 5 . The resilient member  126  may comprise one or more disc springs. Such disc springs may extend around the main body  125  and/or around the central axis Y-Y. Alternatively, the resilient member  126  may take any suitable form, such as a compressible sponge and/or rubber-type material. 
         [0068]    The resilient member  126  may extend generally from a first end region (which may correspond to an interface portion  121 ) to a second end region (which may correspond to the retaining member  123 ) of the balancing mass  120 , along the direction of the central axis Y-Y. In the uninstalled position shown in  FIG. 5   a , the resilient member  126  may extend from the interface portion  121  to the retaining member  123 . The resilient member  126  may not be fixed to the retaining member  123 , such that the two features can be separated, for example during installation and in an installed position (such as that shown in  FIG. 3 ). 
         [0069]    The receptacle  110  comprises a receiving opening  118  for receiving the balancing mass  120  during installation (and through which the balancing mass  120  may be removed as required). The shape of the receiving opening  118  may correspond to (for example be the same as or slightly offset from) the outer perimeter shape of the interface portion  121  of the balancing mass  120 . In the installed position (and during at least a part of the installation process), the interface portion  121  of the balancing mass  120  may be rotatable within the receiving opening  118 . The receiving opening  118  and/or the outer perimeter of the interface portion  121  may be axisymmetric, for example circular in cross-section. 
         [0070]    Referring now to  FIG. 6 , in order to install a balancing mass  120  into a receptacle  110  (for example to balance a rotating part  132 ), the balancing mass  120  may first be inserted into the receptacle  110  through the receiving opening (or mouth)  118 . The balancing mass  120  may then be rotated, for example using a tool such as the tool  200 , about its central (or longitudinal) axis Y-Y, indicated by the arrow Ω in  FIG. 6 , so as to align the retaining member  123  with the slot  116 . If the retaining member  123  and slot  116  are not aligned, then the retaining member  123  sits, or rests, on the retaining plate  112 , for example on an upper surface  111  thereof. 
         [0071]    When the retaining member  123  and the slot  116  are aligned, the balancing mass  120  (that is, a part of the balancing mass  120 ) can be pushed through hole  115  in the retaining plate  112 , by applying a force F (for example using a tool  200 , which may be the same as the tool  200  used to rotate the balancing mass) along the central axis Y-Y. Thus, the balancing mass  120  may be said to be moved, or translated, in the direction of the central axis Y-Y relative to the receptacle  110 . As the balancing mass  120  moves through the hole  115 , the retaining member  123  and the resilient member  126  are separated. As the retaining member  123  moves through the slot portion  116  of the hole  115 , the resilient member  126  rests on the upper surface  111  of the retaining plate  112 . As the retaining member  123  moves through the slot portion  116  of the hole  115 , the resilient member  126  becomes compressed between the retaining plate  112  (specifically the upper surface  111  thereof) of the receptacle  110  and the interface portion  121  of the balancing mass  120 . The resilient member  126  remains compressed in this manner after installation. 
         [0072]    The balancing mass  120  is moved relative to the receptacle  110  until the retaining member  123  is pushed all of the way through the retaining plate  112 , as shown by the distance q in  FIG. 6 . At this point in the installation process, the balancing mass is rotated once more about its central (or longitudinal) axis Y-Y. The rotation may be performed using a tool  200 , which may be the same tool that is used throughout the installation process. The rotation may take the balancing mass  120  to any angular position relative to the receptacle  110  at which the retaining member  123  and the hole  115  (or the slot portions  116  thereof) are not aligned. The balancing mass  120  may be said to be rotated to an installed position. In the installed position, the force F may be removed from the balancing mass  120 , for example by removing the tool  200  from the tooling interface  122 . 
         [0073]    in the installed position (shown in  FIGS. 3 and 6 , for example), the retaining member  123  rests on the retaining plate  112 , in particular on the lower surface  113  of the retaining plate  112 . The resilient member  126  remains in the compressed (or more generally deformed) position, thereby holding the balancing mass  120  in the installed position in the receptacle  110 . In the installed position, the balancing mass may be held in position (via the retaining member  123  engaging the retaining (lower) surface  113  of the receptacle) through the action of any one or more of: the resilient member  126 ; centrifugal acceleration due to rotation of the rotatable member  132  in use; and/or a retaining recess  114 , where present. 
         [0074]    In order to remove the balancing mass  120  from the receptacle  110 , substantially the opposite procedure is followed. Accordingly, the balancing mass may be rotated about its central axis Y-Y until the retaining member  123  is aligned with the slot  116 , for example using a tool  200 . The retaining member  123  may then move through the slot  116  under the action of the resilient member  126  as it returns to its original shape. The balancing mass may then be free to be removed from the receptacle  110  in any suitable manner, for example by being drawn into a removal portion of a tool  200  (not shown) and/or by being magnetically attached to the tool  200 . 
         [0075]    In  FIG. 6 , the dashed line indicated by reference numeral  136  indicates the platform of the rotor blade  130 . Accordingly, the position the top (or radially outer surface) of the receptacle  110  and/or the balancing mass  120  when in the installed position may be flush with the surface of the rotor blade platform  136 . 
         [0076]    The installation and removal of the balancing mass  120  from the receptacle  110  may be said to be of a bayonet-type arrangement. In general terms, the balancing mass  120  may be considered to be an inverted bayonet with a press and turn mechanism (for example a 90 degree press and turn mechanism, or any other suitable angle). The balancing mass  120  may be described as a cylindrical bar (or central body)  125  with a tooling interface  121  at one end, and a resilient member (such as spring washers)  126  held in place between the tooling interface  121  and a retaining member  123 , such as a retention pin. 
         [0077]    The receptacle  110  may be described as having a cylindrical hole or opening  118  extending therethrough and a narrowing  115  near the bottom. The narrowing may have a shaped or keyed portion  116  (which may comprise one or more slots) that corresponds at least in part to the shape of the retaining member  123  to allow the retaining member  123  to pass through for installation or removal. 
         [0078]    In order to install the balancing mass  120  in the receptacle  110 , the retaining member  123  may slide through the keyed portion  116 , the resilient member  126  may be compressed; the balancing mass  120  may then be rotated and then released to allow the retaining member  123  to rest on the lower surface of the plate  112 , for example in a retaining recess  114 , where the balancing mass  120  may remain during engine running. 
         [0079]    Whilst aspects of the disclosure have been described herein in relation to the exemplary arrangements shown in the Figures, it will be appreciated that various different arrangements and or modifications to the method and/or apparatus disclosed herein may be made without departing from the scope of the invention. Accordingly, the exemplary arrangements/methods set forth above are considered to be illustrative and not limiting. Purely by way of example, any receptacle  110  may (or may not) be provided with a fixing interface, such as that shown by feature  119  in  FIG. 3 . Such a fixing interface  119  may be one way of fixing, or securing, the receptacle  110  into the rotating part  132 , for example into a slot (which may be a dovetail slot)  134  of a rotating disc  132 . By way of further example, the receptacle  110  may have a portion that extends from the retaining plate  112  away from the direction of the opening  118 , as shown in the Figures. This may provide sufficient space below the retaining plate  112  but within the receptacle  110  in which the retaining member  123  and at least a part of the central body  125  can rotate once the retaining member  123  has passed through the retaining plate  112 . 
         [0080]    Where reference is made herein to a gas turbine engine, it will be appreciated that this term may include may be any type of gas turbine engine, including, but not limited to, a turbofan (bypass) gas turbine engine, turbojet, turboprop, ramjet, scramjet or open rotor gas turbine engine, and for any application, for example aircraft, industrial, and marine application. Any feature described and/or claimed herein may be combined with any other compatible feature described in relation to the same or another embodiment.