Andy Pye discovers how precision ball bearings manufactured in cleanroom conditions are used in premium applications.
A gyrocompass is a marine device that is able to detect the direction of true north and not magnetic north. Variations in the Earth’s magnetic field that deflect normal compasses do not affect a gyrocompass. The gyroscope inside a gyrocompass system is mounted in such a way so that it can move freely to enable its axis of spin to settle parallel with true north when influenced by the Earth’s rotation. A gyrocompass is used to navigate the ship, find positions and record routes. Modern ships now use a GPS system or other navigational aids to feed data to the gyrocompass for correcting any errors.
As a complex electromechanical system, a gyroscope requires bearings that are low friction, quiet and reliable. For more than 25 years, Barden has been supplying low-friction angular contact ball bearings to manufacturers of marine gyroscopes. The company specialises in intricate configurations designed to meet unusual performance or application problems.
Typically, for marine gyroscope applications, angular contact ball bearings are used with an extremely robust, single piece polymer cage, with either steel or ceramic (silicon nitride) balls. An increased ball complement is usually provided to ensure lower noise levels and an increased operating life. As gyroscopes run continuously at all times, the operating life of the bearings is critical.
“As critical navigational aids that run continuously around the clock, marine gyroscopes require super precision ball bearings that are reliable, low friction and quiet, ensuring a long operating life,” says Barney Eley, Applications Engineer at Barden Corporation (UK), a subsidiary of Schaeffler.
Most gyro bearings are oil-lubricated rather than greased, although some gyroscopes do use greased bearings. A typical gyroscope application will incorporate two oil-lubricated open (not sealed) angular contact ball bearings with an oil sump between the bearings. This is sealed with a wick material that soaks up the oil and transfers it, drop by drop, to the bearings. This oil is returned back to the sump and the lubrication process is repeated. The oil itself is normally a military specification synthetic oil.
Ball bearings for gyroscopes rotate at speeds of around 10,000 to 12,000 rpm, so in terms of Barden’s super precision bearings, this is relatively low speed. Barden bearings are manufactured to ABEC 7 standards often with raceway roundness held to limits that exceed the ABEC standards requirement. The bearings are assembled in class 100 cleanroom conditions. The bearing rings are also 100% visually inspected in order to check for any defects on the raceways. This process is critical to guarantee a long operating life of the bearings. Any slight scratch or scuff mark on the rings can trigger wear, which could lead to a reduced operating life of the bearing and premature failure.
Bearing cages are normally made from special materials such as polyamide-imide, a high temperature plastic with good wear resistance. These robust cages can operate in temperatures in excess of 200°C. Typically, a bearing for a gyroscope has a bore diameter of around 5mm. Two bearings are normally required – one on each end of the rotor shaft in a back-to-back configuration.
Another key requirement is that the gyroscope operates with low noise levels. The bearings are therefore fitted to the system and bench tested. Barden gyro bearings achieve low noise levels by their low friction properties and extremely smooth ‘mirror finished’ raceways. This is further enhanced by the use of low friction ceramic balls and 100% visual inspection of the bearing rings to eliminate any defects.
Airbus actuation systems
Barden’s custom super-precision bearings are also 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. These are manufactured by UTC Aerospace Systems (UTAS).
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.
“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,” says Sales Manager Robin Kyte. “We supply bearings for a variety of auxiliary aircraft applications, including navigational gyroscopes, air cycle machines, actuators, primary and secondary flight controls, starter generators, hydraulic pumps and cabin fan bearings.”
Traditionally, aircraft 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 so-called More Electric Aircraft (MEA). The primary flight control actuators on the Airbus A380 are a fine example.
Here, 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.
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.
“The design of the bearings was quite a challenge as we had such a small design envelope to work in,” adds Kyte. “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 designs.”
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.
Barden also supplies 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 – 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 preloaded 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.