Patent Description:
Gas turbine engines with larger diameter fans may incorporate a gearbox connecting the fan to a core shaft of the engine core. An advantage of doing so is that both the fan and the engine core can be designed to operate efficiently as the fan size is scaled up, since the rotational speed of the fan is limited by the tangential speed of the fan tips. The gearbox allows for a reduction in rotational speed of the fan compared to that of the engine core, at the expense of additional weight of the gearbox and some efficiency losses within the gearbox. To maintain efficiency of operation of the engine, the gearbox needs to be designed to minimise weight and maximise efficiency. Bearings are a source of losses within a gearbox, and therefore need to be optimised to seek to maximise the efficiency of the gearbox.

<CIT> relates to a gas turbine engine and a method of forming a journal support pin to support intermediate gears for use in a gas turbine engine.

The journal article by <NPL> (<NUM>-<NUM>-<NUM>), discloses some design values of gearbox bearing for turbofans.

In accordance with an aspect, there is provided a method according to claim <NUM>. The ring gear may have a pitch circle diameter of around <NUM> or greater.

An inefficiency of each journal bearing, defined as a percentage power loss under maximum take-off conditions, may be less than around <NUM>%.

A diametral clearance of each journal bearing may be between around <NUM>‰ and <NUM>‰. The diametral clearance of each journal bearing may be between around <NUM>‰ and <NUM>‰.

With the aircraft gas turbine engine operating at maximum take-off conditions, a temperature of oil flowing into each journal bearing may be less than or equal to around <NUM>.

With the aircraft gas turbine engine operating at maximum take-off conditions, a pressure of oil flowing into each journal bearing at maximum take-off conditions may be within a range from around <NUM> kPa to around <NUM> kPa.

A gas turbine engine for an aircraft may comprise:.

Where the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft, the engine core may further comprise a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor, the second turbine, second compressor, and second core shaft being arranged to rotate at a higher rotational speed than the first core shaft.

The sliding speed of each journal bearing, according to the above aspect, may at maximum take-off conditions be <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>/s or within a range defined by any two of the aforementioned values.

The specific operating load of each journal bearing, according to the above aspect, may at maximum take-off conditions be <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. <NUM> or <NUM> MPa or within a range defined by any two of the aforementioned values.

The gas turbine engine may, in the above aspect, comprise a turbine, a compressor, a core shaft connecting the turbine to the compressor and a fan located upstream of the engine core, the fan comprising a plurality of fan blades. The fan may have a moment of inertia of between around <NUM> × <NUM><NUM> and <NUM> × <NUM><NUM> kg m<NUM>, or alternatively between around <NUM> × <NUM><NUM> and <NUM> × <NUM><NUM> kg m<NUM>, or alternatively between around <NUM> × <NUM><NUM> and <NUM> × <NUM><NUM> kg m<NUM>. The same features may also apply to the other aspects of the invention.

The gearbox may, in the above aspect, have a gear ratio of <NUM> to <NUM>, and optionally <NUM> to <NUM>. The gearbox is in a star configuration.

The gas turbine engine may, in the above aspect, have a specific thrust from <NUM> to <NUM> N kg-<NUM>; and/or a bypass ratio at cruise conditions of <NUM> to <NUM> or <NUM> to <NUM>.

The gearbox receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.

The gas turbine engine as described herein may have any suitable general architecture.

The gearbox may be a reduction gearbox (in that the output to the fan is a lower rotational rate than the input from the core shaft). The gearbox is a "star" gearbox, as described in more detail elsewhere herein. The gearbox may have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example greater than <NUM>, for example in the range of from <NUM> to <NUM>, or <NUM> to <NUM>, for example on the order of or at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The gear ratio may be, for example, between any two of the values in the previous sentence. Purely by way of example, the gearbox may be a "star" gearbox having a ratio in the range of from <NUM> or <NUM> to <NUM>. In some arrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described herein, a combustor may be provided axially downstream of the fan and compressor(s).

Each fan blade may be defined as having a radial span extending from a root (or hub) at a radially inner gas-washed location, or <NUM>% span position, to a tip at a <NUM>% span position. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or on the order of) any of: <NUM>, <NUM>, <NUM><NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>. These ratios may commonly be referred to as the hub-to-tip ratio. The radius at the hub and the radius at the tip may both be measured at the leading edge (or axially forwardmost) part of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e. the portion radially outside any platform.

The radius of the fan may be measured between the engine centreline and the tip of a fan blade at its leading edge. The fan diameter (which may simply be twice the radius of the fan) may be greater than (or on the order of) any of: <NUM>, <NUM>, <NUM>, <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches) cm, <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches) or <NUM> (around <NUM> inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM> or <NUM> to <NUM>.

The rotational speed of the fan may vary in use. Generally, the rotational speed is lower for fans with a higher diameter. Purely by way of non-limitative example, the rotational speed of the fan at cruise conditions may be less than <NUM> rpm, for example less than <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> (for example <NUM> to <NUM> or <NUM> to <NUM>) may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm.

In use of the gas turbine engine, the fan (with associated fan blades) rotates about a rotational axis. This rotation results in the tip of the fan blade moving with a velocity Utip. The work done by the fan blades <NUM> on the flow results in an enthalpy rise dH of the flow. A fan tip loading may be defined as dH/Utip<NUM>, where dH is the enthalpy rise (for example the <NUM>-D average enthalpy rise) across the fan and Utip is the (translational) velocity of the fan tip, for example at the leading edge of the tip (which may be defined as fan tip radius at leading edge multiplied by angular speed). The fan tip loading at cruise conditions may be greater than (or on the order of) any of: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> (all units in this paragraph being J kg-<NUM>K-<NUM>/(ms-<NUM>)<NUM>). The fan tip loading may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>, or <NUM> to <NUM>.

Gas turbine engines in accordance with the present disclosure may have any desired bypass ratio, where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core at cruise conditions. In some arrangements the bypass ratio may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The bypass ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of form <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The bypass duct may be substantially annular. The bypass duct may be radially outside the core engine. The radially outer surface of the bypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described herein may be defined as the ratio of the stagnation pressure upstream of the fan to the stagnation pressure at the exit of the highest pressure compressor (before entry into the combustor). By way of non-limitative example, the overall pressure ratio of a gas turbine engine as described herein at cruise may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The overall pressure ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>.

Specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow through the engine. At cruise conditions, the specific thrust of an engine described herein may be less than (or on the order of) any of the following: <NUM> N kg-<NUM>s, <NUM> N kg-<NUM>s, <NUM> N kg-<NUM>s, <NUM> N kg-<NUM>s, <NUM> N kg-<NUM>s, <NUM> N kg-<NUM>s or <NUM> N kg-<NUM>s. The specific thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> Nkg-<NUM>s to <NUM> N kg-<NUM>s, or <NUM> N kg-<NUM>s to <NUM> N kg-<NUM>s. Such engines may be particularly efficient in comparison with conventional gas turbine engines.

A gas turbine engine as described herein may have any desired maximum thrust. Purely by way of non-limitative example, a gas turbine as described herein may be capable of producing a maximum thrust of at least (or on the order of) any of the following: 160kN, 170kN, 180kN, 190kN, 200kN, 250kN, 300kN, 350kN, 400kN, 450kN, 500kN, or 550kN. The maximum thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). Purely by way of example, a gas turbine as described herein may be capable of producing a maximum thrust in the range of from 330kN to <NUM> kN, for example 350kN to 400kN. The thrust referred to above may be the maximum net thrust at standard atmospheric conditions at sea level plus <NUM> degrees C (ambient pressure <NUM>. 3kPa, temperature <NUM> degrees C), with the engine static.

In use, the temperature of the flow at the entry to the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the exit to the combustor, for example immediately upstream of the first turbine vane, which itself may be referred to as a nozzle guide vane. At cruise, the TET may be at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The TET at cruise may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The maximum TET in use of the engine may be, for example, at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The maximum TET may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>. The maximum TET may occur, for example, at a high thrust condition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described herein may be manufactured from any suitable material or combination of materials. For example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a composite, for example a metal matrix composite and/or an organic matrix composite, such as carbon fibre. By way of further example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a metal, such as a titanium based metal or an aluminium based material (such as an aluminium-lithium alloy) or a steel based material. The fan blade may comprise at least two regions manufactured using different materials. For example, the fan blade may have a protective leading edge, which may be manufactured using a material that is better able to resist impact (for example from birds, ice or other material) than the rest of the blade. Such a leading edge may, for example, be manufactured using titanium or a titanium-based alloy. Thus, purely by way of example, the fan blade may have a carbon-fibre or aluminium based body (such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described herein may comprise a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may comprise a fixture which may engage a corresponding slot in the hub (or disc). Purely by way of example, such a fixture may be in the form of a dovetail that may slot into and/or engage a corresponding slot in the hub/disc in order to fix the fan blade to the hub/disc. By way of further example, the fan blades maybe formed integrally with a central portion. Such an arrangement may be referred to as a bladed disc or a bladed ring. Any suitable method may be used to manufacture such a bladed disc or bladed ring. For example, at least a part of the fan blades may be machined from a block and/or at least part of the fan blades may be attached to the hub/disc by welding, such as linear friction welding.

The gas turbine engines described herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the exit area of the bypass duct to be varied in use. The general principles of the present disclosure may apply to engines with or without a VAN.

The fan of a gas turbine as described herein may have any desired number of fan blades, for example <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> fan blades.

As used herein, cruise conditions may mean cruise conditions of an aircraft to which the gas turbine engine is attached. Such cruise conditions may be conventionally defined as the conditions at mid-cruise, for example the conditions experienced by the aircraft and/or engine at the midpoint (in terms of time and/or distance) between top of climb and start of decent.

Purely by way of example, the forward speed at the cruise condition may be any point in the range of from Mach <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example on the order of Mach <NUM>, on the order of Mach <NUM> or in the range of from <NUM> to <NUM>. Any single speed within these ranges may be the cruise condition. For some aircraft, the cruise conditions may be outside these ranges, for example below Mach <NUM> or above Mach <NUM>.

Purely by way of example, the cruise conditions may correspond to standard atmospheric conditions at an altitude that is in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM> (around <NUM> ft), for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> (around <NUM> ft) to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example on the order of <NUM>. The cruise conditions may correspond to standard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to: a forward Mach number of <NUM>; a pressure of <NUM> Pa; and a temperature of -<NUM> degrees C. Purely by way of further example, the cruise conditions may correspond to: a forward Mach number of <NUM>; a pressure of <NUM> Pa; and a temperature of -<NUM> degrees C (which may be standard atmospheric conditions at <NUM> ft).

As used anywhere herein, "cruise" or "cruise conditions" may mean the aerodynamic design point. Such an aerodynamic design point (or ADP) may correspond to the conditions (comprising, for example, one or more of the Mach Number, environmental conditions and thrust requirement) for which the fan is designed to operate. This may mean, for example, the conditions at which the fan (or gas turbine engine) is designed to have optimum efficiency.

In use, a gas turbine engine described herein may operate at the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (for example the mid-cruise conditions) of an aircraft to which at least one (for example <NUM> or <NUM>) gas turbine engine may be mounted in order to provide propulsive thrust.

As used herein, a maximum take-off (MTO) condition has the conventional meaning. Maximum take-off conditions may be defined as operating the engine at International Standard Atmosphere (ISA) sea level pressure and temperature conditions +<NUM> at maximum take-off thrust at end of runway, which is typically defined at an aircraft speed of around <NUM>. 25Mn, or between around <NUM> and <NUM> Mn. Maximum take-off conditions for the engine may therefore be defined as operating the engine at a maximum take-off thrust (for example maximum throttle) for the engine at ISA sea level pressure and temperature +<NUM> with a fan inlet velocity of <NUM> Mn.

However, according to claim <NUM>, the epicyclic gearbox <NUM> is a star arrangement, in which the planet carrier <NUM> is held fixed, with the ring (or annulus) gear <NUM> allowed to rotate.

Accordingly, the present disclosure extends to a gas turbine engine having any arrangement support structures, input and output shaft arrangement, and bearing locations.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in <FIG> has a split flow nozzle <NUM>, <NUM> meaning that the flow through the bypass duct <NUM> has its own nozzle <NUM> that is separate to and radially outside the core engine nozzle <NUM>. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct <NUM> and the flow through the core <NUM> are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area.

The axial, radial and circumferential directions are mutually orthogonal.

<FIG> illustrates schematically an example planet gear <NUM> of the type shown in the example epicyclic gearbox <NUM> of <FIG>. The planet gear <NUM> is mounted around a pin <NUM>, the inner surface of the planet gear <NUM> and the outer surface of the pin <NUM> forming sliding surfaces of a journal bearing <NUM>, allowing the planet gear <NUM> to rotate relative to the pin <NUM>. In use, the sliding surfaces are lubricated with oil to allow the planet gear <NUM> and pin <NUM> to rotate smoothly relative to each other. The gap between the sliding surfaces is shown exaggerated in <FIG> for clarity.

The sliding surfaces of the journal bearing <NUM> in the example of <FIG> are shown as the inner surface of the planet gear <NUM> and outer surface of the pin <NUM>. In alternative examples, a sleeve may be provided around the pin <NUM>, an outer surface of which provides the inner sliding surface of the journal bearing <NUM>, the sleeve being fixed to the outer surface of the pin <NUM>, for example by an interference fit. A bush (or bushing) may alternatively or additionally be provided, the inner surface of which provides the outer sliding surface of the journal bearing <NUM>, the bush being fixed to the inner surface of the planet gear <NUM>, for example by an interference fit between the bush and the gear <NUM>. An advantage of forming the sliding surfaces of the journal bearing <NUM> from the pin <NUM> and planet gear <NUM> themselves is that tolerances of the journal bearing <NUM> can be more tightly controlled, while an advantage of using one or both of a sleeve and a bush is that the journal bearing may be more readily repaired by replacing one or both components when worn.

The planet gear <NUM> is defined by an inner surface diameter <NUM>, which may also be defined as the diameter of the journal bearing <NUM>, and an outer pitch circle diameter <NUM>. The planet gear <NUM> comprises a plurality of teeth <NUM> extending around the outer circumference of the gear <NUM>. The total number of teeth <NUM> may differ from that shown in <FIG> depending on the specifics of the application. The teeth <NUM> may be arranged in a spur gear or helical gear form, i.e. either parallel or at an angle to the rotational axis of the gear <NUM>. A helical gear form is a more common arrangement because this allows for a smoother transition between the gear teeth <NUM> of the planet gear <NUM> and corresponding teeth on the ring gear <NUM> and sun gear <NUM> (<FIG>) as the gears rotate relative to each other.

<FIG> illustrates schematically an axial section through the planet gear <NUM> and pin <NUM> of <FIG>. The transverse section shown in <FIG> is taken along the line A-A' indicated in <FIG>. The pin <NUM> is mounted to the planet carrier <NUM>, in this example by extending through the thickness of the planet carrier <NUM>. The pin <NUM> may be fixed to the carrier <NUM> by welding, bolting or by otherwise securing the pin <NUM> and carrier <NUM> to prevent relative movement between the pin <NUM> and carrier <NUM> when in use. In operation, forces are transmitted between the pin <NUM> and carrier <NUM> primarily through shear forces on the pin <NUM> transverse to the axis <NUM> of the pin <NUM>, which also result in bending moments applied to the pin <NUM> along the axis <NUM>. In a star gearbox arrangement, in which the planet carrier <NUM> is fixed relative to the engine frame, the net forces on the planet gears <NUM> act in a direction tangential to a diameter of the planet gear <NUM> centres. In a planetary gearbox arrangement, in which the outer ring gear <NUM> (<FIG>) is fixed, the net forces on the planet gears <NUM> are tilted towards the centre of the sun gear due to the additional centripetal force component required to maintain the planet gears <NUM> rotating about the sun gear <NUM>, the centripetal force being a function of the rotational speed of the planet carrier <NUM>. An advantage of the gearbox being configured in a star arrangement is that loading on the pins is reduced when the gearbox is operating at high speeds.

The planet gear <NUM> is shown in <FIG> with a journal bearing portion <NUM> having a length L smaller than a total width <NUM> of the planet gear <NUM>. The length L of the journal bearing <NUM> may be selected according to the loads experienced during operation of the gearbox and to optimise a ratio between the journal bearing length L and the diameter D of the journal bearing <NUM>. The diameter D may be defined by either the outer sliding surface, corresponding to the inner surface of the planet gear <NUM> in the example shown in <FIG>, by the inner sliding surface, corresponding to the outer surface of the pin <NUM>, or by a mean diameter between the two. In practice, the difference between the two diameters, termed the diametral clearance c (the distance c/<NUM> being shown in <FIG>), is small, typically within less than <NUM>% of either diameter. For an example range of diameters of between <NUM> and <NUM>, the difference may be between around <NUM> % and <NUM> %, i.e. between around <NUM> and around <NUM>, with a typical diametral clearance of around <NUM>.

The length <NUM> of the journal bearing <NUM> may in some examples be the same as, or greater than, the total width of the planet gear <NUM>.

In particular examples, a ratio LID of the length L of the journal bearing <NUM> to the diameter D of the journal bearing <NUM> may be in a range from around <NUM> to <NUM>, optionally between around <NUM> and <NUM>. A lower L/D ratio reduces misalignment of the gears <NUM> relating to the pins <NUM>, in part by reducing the bending moment applied to the pins, thereby keeping the pins <NUM> more parallel with the gears <NUM>. The LID ratio should, however, be kept above around <NUM>, or optionally around <NUM>, to avoid the specific loading on the journal bearing from becoming too high and adversely affecting the lifetime of the bearing.

<FIG> illustrates schematically an example structure of a surface coating <NUM> that may be applied to either sliding surface of the journal bearing <NUM>. The underlying material <NUM> may be either the pin <NUM> or the ring gear <NUM>, or in alternative examples may be a sleeve or bush of the type described above. The overall thickness of the surface coating <NUM> may be in the region of between <NUM> and <NUM> micrometres thick, with a specific example thickness in the region of around <NUM> micrometres.

Although the surface coating <NUM> may be applied to either surface of the journal bearing <NUM>, applying the coating <NUM> to the outer surface of the pin <NUM> may in practice be preferable due to practical limitations of deposition methods for internal surfaces.

Common deposition methods such as physical vapour deposition (PVD) may be more suitable for application of coatings to an external rather than internal surface. Other techniques such as casting may be more applicable for application of a coating to an internal surface, although casting is generally less suitable for creating a coating of the thickness range defined above, and with the tolerances required for journal bearings.

An example surface coating <NUM> may comprise three layers 61a-c. A first layer 61a is deposited that has a thermal expansion coefficient between that of the underlying material <NUM> and the second layer 61b. With steel as the underlying material, the first layer 61a may for example be a copper-based alloy. The second layer 61b, which typically forms the largest thickness layer in the surface coating <NUM>, i.e. having a thickness of between around <NUM>% and <NUM>% of the total thickness of the surface coating <NUM>, may be composed of a copper- or aluminium-based alloy, i.e. a metallic alloy having either copper or aluminium as a primary constituent, an example being a leaded bronze, i.e. an alloy of copper, lead and tin. Such an alloy is selected to have a lower hardness compared with that of the material forming the other surface of the journal bearing, so that any particles that are not filtered out from the oil may instead become embedded in the second layer 61b, reducing their ability to wear the surfaces of the journal bearing.

The third layer 61c may be one that is considerably thinner than the first and second layers 61a, 61b and composed of a material having a lower hardness than the second layer 61b, for example a lead-based alloy. The third layer acts to reduce friction between the surfaces of the journal bearing, particularly when starting from a stationary position where an oil layer between the surfaces has not been built up. The third layer 61c may for example have a thickness of between <NUM> and <NUM> micrometres.

In a particular example, the first layer 61a may be between <NUM> and <NUM> micrometres in thickness, the second layer between around <NUM> and <NUM> micrometres in thickness and the third layer between around <NUM> and <NUM> micrometres or between <NUM> and <NUM> micrometres or between <NUM> and <NUM> micrometres or between <NUM> and <NUM> micrometres in thickness.

The bearing materials have been developed to provide an optimum compromise between 'hard/strong' and 'soft/flexible' for the specific application in a gearbox for a gas turbine engine. 'Hard' properties address the requirements of contact wear resistance, fatigue, and load carrying capacity. 'Soft' properties are advantageous to provide compatibility (to the countersurface), conformability, and embeddability of the surface. It has been found that this may help to ensure continued operation in imperfect conditions. In addition, the proposed arrangement has been developed to address environmental factors such as corrosion and oxidation resistance.

In a specific example, the coating layer 61b may be an (aluminium-tin-copper alloy (for example SAE783). In another specific example, a leaded bronze alloy (such as SAE49) may be used for the coating layer 61b. Such alloys have been found to be suitable for an arduous duty cycle, for example where the maximum operating specific load multiplied by the maximum operating sliding speed is around <NUM> MPa m/s or greater. The soft properties at the running surface can be further enhanced with a thin overlay coating 61c (for example up to around <NUM>) such as SAE <NUM> lead-indium without compromising the load carrying capacity of the underlying material.

<FIG> illustrates a schematic cross-sectional view of a gear <NUM> mounted around a pin <NUM>, forming a journal bearing <NUM> between the outer surface of the pin <NUM> and the inner surface of the gear <NUM>. A difference in inner and outer diameter between the gear <NUM> and pin <NUM> respectively, i.e. the diametral clearance, is exaggerated to show a variation in oil film thickness that arises when the gearbox is in operation. The oil film thickness is lower over a portion <NUM> of the journal bearing where a load F is transferred between the pin <NUM> and the gear <NUM>, for example in the direction indicated by arrow <NUM>. A specific loading on the journal bearing <NUM> is defined by the load F on the bearing <NUM> applied over an area defined by the length L and diameter D of the bearing <NUM>, i.e. the specific loading is F/LD, typically measured in MPa or N/mm<NUM>.

An oil flow path through the journal bearing <NUM> passes through a central bore <NUM> of the pin <NUM> through an inlet passage <NUM> and into a clearance between the pin <NUM> and gear <NUM>. The oil flows around the journal bearing, dragged through the minimum clearance by the relative rotation between the pin <NUM> and gear <NUM>, and exits via the edges of the bearing <NUM>. Oil is cooled and recirculated via a scavenge and pump (not shown). The oil flowing into the journal bearing may be pressurised to between around <NUM> and <NUM> kPa (<NUM> to <NUM> bar). A minimum oil pressure is required to provide sufficient oil to the bearing so that the area over which force is applied is covered with a supply of oil. Higher pressures will tend to force greater amounts of oil through the bearing, but have diminishing effects on lubricating and cooling the bearing as greater amounts will tend to travel via the wider portion of the clearance between the pin <NUM> and gear <NUM> rather than via the minimum clearance portion <NUM>. Higher oil pressures will tend to reduce the temperature difference between the inlet and outlet oil flows, which makes extraction of heat more difficult, requiring larger heatsinks. An optimum oil flow pressure and temperature difference will therefore tend to be required to minimise on weight in relation to oil pumps and heatsinks. The pressure and temperature differences defined herein have thus been chosen to provide the required lubrication, but with a sufficiently high temperature difference to enable sufficiently low weight of heat exchangers to remove the heat. The low weight of heat exchanger may be a particularly important consideration for gearboxes to be used in a gas turbine for an aircraft, because of the importance of weight on the overall fuel consumption of the aircraft to which the engine is provided.

The dimensional and positional accuracy of the pins <NUM> and gears <NUM> of the gearbox will affect how the oil film thickness varies, as well as the viscosity and temperature of the oil. To maintain a uniform oil temperature across each journal bearing, symmetric oil feed paths may be provided in the gearbox, and a plenum for mixing oil prior to being fed into the gearbox may be sufficiently large to allow for a uniform temperature of oil being fed into the gearbox at different feed points. As a result, a temperature variation between oil fed to each of the journal bearings may be no more than <NUM> degree Celsius, for example with the engine operating at cruise conditions. A variation in oil pressure is preferably also uniform between the journal bearings, but this will typically have less effect than a variation in temperature because an increase in pressure above a minimum required will tend to simply cause more oil to flow through the bearing, having minimal effect on operation.

The operational oil film thickness, i.e. the thickness of the oil film in each journal bearing during operation of the engine, may be defined as a proportion of the journal bearing diameter. The minimum operational oil film thickness for each journal bearing during operation, for example at MTO conditions, at which loading of the gearbox is at its highest, may be less than around <NUM> micrometres for a journal bearing diameter of between around <NUM> and <NUM>, and optionally greater than around <NUM> micrometres. The clearance of the journal bearing may typically be between around <NUM> and <NUM>‰ (<NUM>% and <NUM>%) of the journal bearing diameter, for example around <NUM>‰ (<NUM>%). The journal bearing diameter may, as described above, be defined as the diameter of the inner sliding surface of the planet gear. A variation between the minimum operational thickness of each journal bearing, also for example at MTO conditions, may be less than around <NUM>% of a mean minimum oil film thickness. For example, if the mean minimum oil film thickness is around <NUM> micrometres, the maximum difference between the minimum oil film thickness across all of the journal bearings will be around +/- around <NUM> micrometres.

The operational oil film thickness will, as illustrated schematically in <FIG>, vary around each journal bearing between a minimum thickness at a point of maximum loading to a maximum thickness at a point diametrically opposite from the point of maximum loading. The point of maximum loading will tend to follow a linear path along the length of the journal bearing parallel to its axis of rotation. A ratio between the maximum and minimum oil film thickness will be highest during maximum take-off conditions and will be around <NUM> at idle conditions, i.e. with no significant load being transferred across the gearbox.

<FIG> illustrates an example plot of operating specific load (y axis, in MPa) as a function of sliding speed (x axis, in m/s) for journal bearings in a range of example gearboxes of the type disclosed herein, each operating under maximum take-off conditions. The specific loading for a journal bearing is as defined above. The sliding speed for a journal bearing is defined as the relative tangential speed of the inner and outer surfaces of the journal bearing. The dotted lines 81a, 81b, 81c represent constant values for a multiple of operating specific loading and sliding speed, which may be termed PV (being a multiple of pressure and velocity), of <NUM>, <NUM> and <NUM> MPa m/s respectively.

At higher specific loads or sliding speeds, or higher values of PV in general, a surface coating comprising a layer of an alloy having aluminium or copper as a primary constituent, for example forming the second layer 61b as shown in <FIG>, may be used. Copper as a primary constituent may be preferable for higher diameter journal bearings, for example greater than <NUM> in diameter.

The maximum operating specific loading of each journal bearing in the gearbox may be greater than 5MPa, or may be greater than any one of 6MPa, 7MPa, 8MPa, 9MPa, 10MPa, <NUM> MPa, 12MPa, 13MPa, 14MPa, 15MPa, 16MPa or 17MPa. The maximum sliding speed of the journal bearings may be defined by the corresponding sliding speed for the curves 81a-c shown in <FIG>. In particular examples, the maximum operating specific loading may be around <NUM> MPa or around <NUM> MPa, with a sliding speed in each case of around <NUM> or <NUM>/s, indicated as data points <NUM>, <NUM> respectively on <FIG>. In a general aspect, the specific loading may be within a range from around <NUM> to <NUM> MPa and the sliding speed within a range from around <NUM> to <NUM>/s at maximum take-off conditions. These ranges may apply in particular for a planetary gearbox arrangement.

Points <NUM>, <NUM> represent specific pressure and sliding speed values at maximum take-off conditions for journal bearings in two example planetary gearboxes, with journal bearing diameters of around <NUM> and <NUM> respectively and journal bearing LID ratios of around <NUM> and <NUM> respectively, both with a diametral clearance of around <NUM> ‰. The PV values at maximum take-off conditions for points <NUM> and <NUM> are around <NUM> and <NUM> MPa m/s respectively.

Points <NUM>, <NUM> and <NUM> in <FIG> represent specific pressure and sliding speed values at maximum take-off conditions for journal bearings in three example star gearboxes of different sizes, with journal bearing diameters of around <NUM>, <NUM> and <NUM> respectively and journal bearing LID ratios of around <NUM>, <NUM> and <NUM> respectively, each with a diametral clearance of around <NUM> ‰. The PV values at maximum take-off conditions for points <NUM>, <NUM> and <NUM> are around <NUM>, <NUM> and <NUM> MPa m/s. In a general aspect, the specific loading for such examples may be within a range from around <NUM> to <NUM> MPa and the sliding speed within a range from around <NUM> to <NUM>/s at maximum take-off conditions. The PV values may be in a range having a lower limit of any one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> MPa m/s and an upper limit of any one of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> MPa m/s.

In a further general aspect therefore, the specific loading for the above-mentioned examples may be within an overall range from around <NUM> to <NUM> MPa and the sliding speed within a range from around <NUM> or <NUM> to <NUM> or <NUM>/s at maximum take-off conditions.

The higher specific loads for the planetary gearbox journal bearings (points <NUM>, <NUM>) partly reflect the additional centripetal loading on each journal bearing due to the rotation of each planet gear about the central sun gear, while the planet gears in the star gearboxes (points <NUM>, <NUM>, <NUM>) do not rotate about the central sun gear.

The y-axis spread of specific load on each of the data points <NUM>-<NUM> represents the variation in specific load over a +/- <NUM>% variation in torque load around a nominal torque load at maximum take-off conditions.

An upper limit for PV may be around <NUM> MPa m/s, while a lower limit may be around <NUM> or <NUM> MPa m/s. Upper limits may alternatively be defined by an upper limit for one or both of the sliding speed and operating specific load, for example an upper limit of around <NUM>, <NUM>, <NUM> or <NUM>/s for the sliding speed and an upper limit of around <NUM>, <NUM> or <NUM> MPa for the operating specific loading. Lower limits may be defined by sliding speeds of around <NUM>, <NUM>, <NUM> or <NUM>/s, or by specific loads of around <NUM> or <NUM> MPa, among others specified herein.

The eccentricity ratio of a journal bearing during operation of the gas turbine engine, for example while operating at MTO conditions, is defined as <NUM>-<NUM>min/c, where Hmin is the minimum oil film thickness (shown in <FIG>) and c the diametral clearance (shown in <FIG>, with the gear <NUM> and pin <NUM> arranged concentrically). <FIG> illustrates the variation in eccentricity ratio (y axis) for a range of example gearboxes as a function of percentage of the journal bearing design load (x axis). First, second and third example star gearboxes <NUM>,<NUM>, <NUM> (corresponding to the same gearbox designs having data points <NUM>, <NUM>, <NUM> respectively in <FIG>) exhibit a variation in eccentricity ratio of between around <NUM> and <NUM> between <NUM>% and <NUM>% of design load, and have eccentricity ratios that range between around <NUM> and <NUM> over this range of design loads. Eccentricity ratios <NUM>, <NUM> for first and second example planetary gearboxes (corresponding to gearbox designs having data points <NUM> and <NUM> in <FIG>), having higher absolute design loads, are between around <NUM> and <NUM> over a similar design load range, with the eccentricity ratio varying within this range by between around <NUM> and around <NUM>.

The diametral clearance, c, may be within a range of between around <NUM> and <NUM>‰, i.e. between around <NUM> and <NUM>%. A smaller diametral clearance will tend to increase the area over which the pressure between the inner and outer surfaces of the journal bearing is distributed, but this will be in combination with a narrower path through which the oil through the bearing is forced as the bearing rotates, limiting the flow rate of oil through the bearing and ultimately causing the bearing to seize as the diametral clearance is reduced further. A higher diametral clearance will tend to reduce the area over which the pressure is distributed but will also make travel of the oil through the bearing easier. An optimum balance between the factors is therefore required which, particularly for higher eccentricity ratios of between around <NUM> and <NUM>, may be between around <NUM> and <NUM> ‰, and optionally between around <NUM> and <NUM> %o.

<FIG> is a schematic plot of inefficiency as a function of eccentricity ratio for the above-mentioned range of gearboxes. The example star gearboxes tend to have higher inefficiencies, ranging between around <NUM> and <NUM>%, while the example planetary gearboxes have inefficiencies between around <NUM> and <NUM>%. The eccentricity ratios range between values as stated above. The variation of inefficiency versus eccentricity follows the general trend <NUM> shown in <FIG>, with a higher eccentricity ratio resulting in a higher efficiency, i.e. a lower inefficiency. A range of eccentricity ratios may be as previously stated, while a range of inefficiency may be less than around <NUM>%, and may be between around <NUM> and around <NUM>%. Increasing the eccentricity ratio further will tend to increase the risk of the journal bearing seizing due to the minimum thickness of the oil film becoming too small to sustain an oil film separating the pin and gear under the required range of loading.

<FIG> illustrates a series of trendlines of Hmin (in µm) as a function of oil inlet temperature (in °C) for the above-mentioned example star and planetary gearboxes, all operating at maximum take-off conditions. The star gearboxes (lines <NUM>, <NUM>, <NUM>) tend to have higher Hmin values over the range of temperatures and with trendlines having a steeper gradient, while the planetary gearboxes (lines <NUM>, <NUM>) tend to have lower Hmin values and with more shallow gradients. Each trendline tends to follow a function of the form Hmin=B-AT, where T is temperature (in °C) and A and B are constants that are characteristic of the particular gearbox design. Except for one of the star gearboxes, the minimum oil film thickness Hmin at maximum take-off conditions for each gearbox is within a region having an upper bound defined by the line <NUM>, where A is <NUM>/°C and B is <NUM>. A minimum value of Hmin may be around <NUM>, below which the oil film may be insufficient to prevent seizing of the journal bearing. Two further lines <NUM>, <NUM> define further upper and lower bounds respectively, with line <NUM> defined by A=<NUM>/°C and B=<NUM> and line <NUM> defined by A=<NUM>/°C and B=<NUM>. An overall range for the inlet oil temperature may be between <NUM> and <NUM>, with an optional range of greater than around <NUM> and less than around <NUM>. At lower temperatures the oil viscosity increases, reducing lubrication efficiency, whereas at higher temperatures the resulting lower oil viscosity may cause the minimum oil film thickness to become too small.

<FIG> illustrates a series of trendlines of eccentricity ratio, E, of journal bearings of the various example gearboxes as a function of oil inlet temperature, with the gearbox in each case operating at maximum take-off conditions. The star gearboxes (lines <NUM>, <NUM>, <NUM>) tend to have lower values of E over the entire temperature range and trendlines having steeper gradients, while the planetary gearboxes (lines <NUM>, <NUM>) tend to have higher values of E and more shallow gradients. In each case the trendline tends to follow a function of the form E=AT+B where T is the oil inlet temperature and A and B constants. A maximum value for E may be around <NUM>, above which the oil film may be too small to sustain lubrication of the journal bearing. The eccentricity ratio E may be above a trendline <NUM> defined by A=<NUM>/°C and B=<NUM>, or alternatively may be above a trendline <NUM> defined by A=<NUM>/°C and B=<NUM>, and may be below a trendline <NUM> defined by A=<NUM>/°C and B=<NUM>. As for the examples in <FIG>, an overall range for the inlet oil temperature may be between <NUM> and <NUM>, with an optional range of greater than around <NUM> and less than around <NUM>.

The Sommerfeld number, S, of a journal bearing is defined as: <MAT> where d is the outer diameter of the pin <NUM> (<FIG>), c is the diametral clearance, µ is the absolute viscosity of the lubricant, N the relative rotational speed of the journal bearing (in revolutions per second) and P is the loading applied across the projected bearing area, i.e. F/LD, where L is the journal bearing length and D the diameter.

A higher PV value, resulting in a lower inefficiency value, will tend to increase the Sommerfeld number for a journal bearing. <FIG> illustrates a general relationship, given by trendline <NUM>, between inefficiency and Sommerfeld number for the above-mentioned range of gearboxes. The star type gearboxes tend to have journal bearings with a lower Sommerfeld number, ranging between around <NUM> and <NUM> at maximum take-off conditions. The two planetary gearboxes, with higher PV values, higher eccentricity and lower inefficiencies, have journal bearings that tend to have higher Sommerfeld numbers, ranging in general between around <NUM> and <NUM> under varying oil temperature and loadings. Under more optimal conditions of oil temperature, the Sommerfeld number for these planetary gearbox journal bearings tends to be between around <NUM> and <NUM>. Claim <NUM> is directed to a method of operating a gas turbine engine comprising a gearbox in a star configuration. In a general aspect, the Sommerfeld number of each journal bearing may be greater than around <NUM>, with an inefficiency of around <NUM>% or less under maximum take-off conditions. As mentioned above, the minimum inefficiency may be around <NUM>%. The maximum Sommerfeld number may be around <NUM>.

Claim 1:
A method of operating a gas turbine (<NUM>) engine for an aircraft, the gas turbine engine comprising:
an engine core (<NUM>) comprising a turbine (<NUM>), a compressor (<NUM>), and a core shaft (<NUM>) connecting the turbine to the compressor;
a fan (<NUM>) located upstream of the engine core, the fan comprising a plurality of fan blades; and
a gearbox (<NUM>) configured to receive an input from the core shaft (<NUM>) and
provide an output drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, the gearbox being in a star
configuration and comprising:
a sun gear (<NUM>);
a plurality of planet gears (<NUM>) surrounding and engaged with the sun gear (<NUM>); and
a ring gear (<NUM>) surrounding and engaged with the plurality of planet gears (<NUM>), each of the plurality of planet gears (<NUM>) being rotatably mounted around a pin (<NUM>) of a planet gear carrier (<NUM>) with a journal bearing (<NUM>) having an internal sliding surface on the planet gear (<NUM>) and an external sliding surface on the pin (<NUM>), characterised in that:
the method comprises operating the aircraft gas turbine engine at maximum take-off conditions, wherein a Sommerfeld number of each journal bearing (<NUM>) is between around <NUM> and <NUM>.