Power is transferred from turbines to compressors through shaft and spline systems, with bearings providing axial and radial positioning of the rotating components. The basic structure of the engine support the engine rotors while allowing the engine airflow to pass through. These support structures, or bearing housings, are large, strong, weight-efficient castings or fabrications that support the engine rotors. For example, on a three shaft Trent engine, the intercase houses the location bearings for all 3 shafts, supporting them at mid-span. Structures are also hosts of many secondary systems: anti-icing, bearing chambers, internal air system... Hence they are both large and complex. They are amongst the most challenging components to manufacture.
In order to extract power from the engine to drive engine accessories and aircraft systems, drives transmits power from the gears mounted on the engine mainline shafts to the external gearbox. This in turn runs the engine accessories (fuel pump, oil pump, …) and provides hydraulic and electrical power to the aircraft (for air control surfaces actuation, cabin entertainment, …) During starting, the drives are used “in reverse”, transmitting power from the starter motor to the IP or HP shaft.
With all the fluids involved in a gas turbines, effective sealing of revolving is critical. Air-air seals control the secondary air system flows - they minimise the air quantity required for component cooling, prevention of turbine disc rim air ingestion, or bearing load management and have a direct influence on engine fuel burn. Air-oil seals prevents oil from leaking out of the bearing chambers.
Bearing housing casings, required to locate accurately up to three rotating shafts, are large and complex and need to be manufactured with great accuracy.
They are made currently either as fabrications – an assembly of many parts, requiring excellent process control – or in a single piece as a casting. This calls for excellent casting control and latest machining techniques for such large parts. Advanced manufacturing techniques including automated fabrication, metallic deposition and composites are under development to increase material yield while reducing lead-time, manufacturing time, cost and weight.
3D dynamic design methods are used to understand component and gear system behaviours and to produce optimum weight and reliability.
3D modelling of complex features, such as helicoidal splines, allows a full dynamic load assessment of contact geometry, peak stress, low and high-cycle fatigue and damage tolerance.
High-precision bearings and bearing load management systems are essential for the lightweight and high-reliability demands of the aero engine.
Design challenges for bearings are considerable as they have to avoid debris contamination, ball skidding under low load and under load crossover, outer race rolling fatigue under excessive radial load, and inner race lip-edge operation under excessive axial load. The bearing load management system balances the axial load that results from compressors’ and turbines’ axial aerodynamic load by using compressed air acting on air seals at suitable radii to provide balance “pistons” and offset the load on location bearings.
Whole engine modelling facilitates the design of complete structures for optimum weight and rotor dynamics ahead of physical testing.
Large engine structures are optimised for rotor dynamics, low weight, blade tip clearance – enhancing engine performance and operability – and engine flight load distribution. Historically, validation was reliant on test bed runs or flight tests using real hardware.
Whole engine modelling provides an environment for analysing advanced rotor dynamics, representative engine load distribution and integrating individual structural models. This ‘pre-work, not rework’ approach supersedes much costly and time-consuming hardware testing.
Highly accurate analysis tools are replacing physical testing to validate shaft designs.
The point at which shafts buckle has traditionally been demonstrated by full-scale hardware tests, but today a validated method for predicting shaft buckling – developed in conjunction with the Rolls-Royce University Technology Centre network – has been accepted by Europe’s certification authorities to replace physical testing. The method involves combining axial and torque loadings, temperature variations and material properties to predict peak torque and the location and form of failure. Fan retention analysis methods have been developed that now take account of the energy absorption capability of the material. This non-linear method has also been validated by component testing for use in the analysis of fan retention shafts.
As shafts are very long and have hollow parts, they require a high degree of accuracy during manufacture.
Traditional shaft manufacturing methods present considerable machining and inspection challenges. A flow forming method, already in use on some drive shafts, is being developed for mainline shafts. This fast and economical cold extrusion process, yielding a preform to a desired shape, minimises material wastage. Several preforms combine to give a seamless component of improved strength and hardness.
Advanced seal designs are important in helping improved engine efficiency as they permit more air to be used in the engine cycle.
While conventional air seals use controlled clearance labyrinth technology, relying on abradable materials and seal clearance matching, advanced seals accommodate the variation of radial clearance over the full operational range. Such designs include circumferential carbon seals and air-riding carbon seals.
The engine manages the supply of electrical, pneumatic, hydraulic or mechanical power to the aircraft.
Classic ‘power extraction’ systems include mechanical drives that transmit power from the gears mounted on the engine mainline shafts to the external gearbox driving accessories such as pumps and generators. Mechanical drives can be replaced by ‘more electric’ systems comprising embedded electrical generators mounted directly on the engine mainline shafts. Power electronics convert the variable frequency electrical power the engine delivers into AC or DC current for the aircraft. Alternatively, continuously variable mechanical systems can make generators run at constant speeds. Astute management of the aircraft load demand offers engine handling improvement opportunities.