21-07-2015 | BARDEN CORPORATION | PLYMOUTH, UK
Custom super precision bearings manufactured by The Barden Corporation, part of the multinational Schaeffler Group, are playing vital roles in the thrust reversing actuator system (TRAS) on the Airbus A350 XWB and electro-hydraulic actuator (EHA) systems on the Airbus A380.
As part of the aerospace division of leading automotive and industrial supplier The Schaeffler Group, Barden specialises in the design and manufacture of super precision rolling bearings, integrated bearing assemblies and high precision components for the aerospace industry. Bearings for this sector have to withstand harsh conditions: extreme temperatures, demanding load profiles and high speeds and can therefore be custom engineered for a specific application.
As Robin Kyte, Sales Manager at Barden explains: "Barden bearings are capable of high speed, reliable operation and running quietly with minimum power losses, making them well suited to applications in the aerospace sector. We have supplied bearings for a variety of auxiliary aircraft positions, including navigational gyroscopes, air cycle machines, actuators, primary and secondary flight controls, starter generators, hydraulic pumps and cabin fan bearings. We are committed to joint development partnerships with aerospace technology leaders such as UTC Aerospace Systems, a customer we have been working closely with for more than 40 years."
Photo courtesy of Airbus SAS
Close working partnership
Based in Hemel Hempstead, Herts, UTC Aerospace Systems (UTAS) is a market leader in the development of electro-hydraulic (EHA) technology for military and civil aircraft applications. UTAS supplies thrust reversing actuator systems (TRAS) for the Airbus A350 XWB and primary flight control actuation systems for the Airbus A380. Both of these actuation systems utilise Barden super precision bearings.
More electric aircraft
Driven by the demand to optimise aircraft performance, decrease operating and maintenance costs and reduce gas emissions, the aircraft industry is pushing towards the concept of more electric aircraft (MEA) and, ultimately, an all-electric aircraft.
The MEA concept provides for the utilisation of electric power for all non-propulsive systems. Traditionally, these non-propulsive systems are driven by a combination of different secondary power sources such as hydraulic, pneumatic, mechanical and electrical. However, recent technological advances in the field of power electronics, fault-tolerant architecture, electro-hydrostatic actuators, flight control systems, high-density electric motors, power generation and conversion systems, have ushered in the era of MEA. This trend is accelerating, as aircraft OEMs collaborate with their suppliers to design new systems and implement new electrical-intensive architectures.
A380 primary flight controls
The primary flight control actuators on the Airbus A380 are a fine example of the push towards more electric aircraft (MEA). Custom super precision bearings from Barden play a key role in these actuation systems. Not only do the bearings incorporate sealing technology, which helps save space, they are also designed to withstand very high axial loads generated by the system's hydraulic pumps.
EHA reduces the need for central hydraulic power generation and distribution systems on the aircraft. The system uses aircraft electrical power to produce the required hydraulic pressure and flow within the actuator at the point of use. EHAs generate hydraulic pressure which flows locally within the actuator, using the aircraft electrical bus as the power source.
Bearings key to aircraft weight reduction
The Airbus A380 is the first civil aircraft to use 5,000psi Fly-by-Wire actuators and modern Power-by-Wire EHAs. Typically, a civil aircraft uses a 3,000psi system or 4,000psi, but most modern military aircraft use 5,000psi systems. Increasing the system pressure to 5,000psi for the rudder, elevator and aileron controls enables a reduction in the diameter of the hydraulic pipes and couplings while delivering the same force. This provides additional benefits such as aircraft weight reduction.
For the EHAs to function correctly and withstand such high system pressures, bearings and seals are vital. Barden supplies bearings to UTAS for the EHAs on the rudder, elevator and aileron flight control systems. The bearings are local to the point of actuation, on the wing for example, and so high vibration levels can be expected.
As Robin Kyte explains: "The design of the bearings was quite a challenge as we had such a small design envelope to work in. UTAS also required wide ranging duty cycles to be considered. Because of the restricted design envelope we sat down with their engineers at an early stage in the design process to discuss alternative design solutions."
Ultimately, sealing technology was incorporated into the bearings to save design space and increase the capacity of the bearings. The internal geometry, curvature, ball complement and shoulder height of the bearings were optimised. Super precision geometric tolerances were also applied to track roundness and weave. The raceways are super finished to improve lubrication film generation.
Bearings for the A350 XWB
Barden also supplied bearings to UTAS for the electric motor on the Airbus A350's thrust reversing actuator system (TRAS). During landing of the aircraft, this high-speed electric motor provides reverse thrust during landing.
The motor and therefore the bearings are non-operational for 99.99% of their life. The bearings are therefore provided with ceramic balls to minimise friction and adhesive wear between the balls and raceways due to false brinelling, i.e. wear caused by the slight axial sliding movement (due to vibrations) of the rolling elements while the bearing is stationary.
Loads on the motor bearings are relatively small but vibrations are high and so the bearings are spring loaded to minimise tolerance stack-up, which again helps to prevent false brinelling.
The bearings are lubricated-for-life with a special grease that provides high mechanical stability. The viscosity of the grease also ensures that friction is minimised in the cold aircraft operating temperatures.
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