Nick Dowding discusses how the design of bearings can be optimised in order to ‘add value over life’ in aircraft actuation systems, auxiliary equipment and instrumentation applications
Bearings are critical components in a variety of auxiliary aircraft applications, including navigational gyroscopes, air cycle machines, actuators, gearboxes, primary and secondary flight controls, starter generators, hydraulic pumps and cabin fan bearings.
However, the decision in favour of a specific bearing solution should always be taken after analysing the whole life costs of the bearing and not merely on the basis of purchase price alone. Today’s advanced high technology bearings offer many improved features that enable whole life cost reductions to be achieved – despite an overall higher bearing price – providing added value over the complete life of aircraft systems and equipment.
For a bearing designed/selected for a given aerospace application, the whole life cost is equivalent to the sum of the following:
Initial cost/purchase price
Maintenance cost (routine and planned)
While the initial purchase price of an advanced bearing solution will be higher than a standard bearing, the potential savings that can be achieved in the form of reduced assembly times, improved energy efficiency and reduced maintenance costs, often more than outweigh the initial higher purchase price of the advanced bearing solution.
Adding value over life
The influence of an improved design in reducing whole life costs and adding value over life can be significant, as designed-in savings are often sustainable and permanent. For example, a 5% price reduction from a bearing supplier held at that reduced price over five years is not likely to last beyond that point. However, a 5% reduction in assembly time/cost, or a 5% reduction in maintenance costs over the same 5-year period is much more desirable for the customer. Sustained reductions over the life of the system or equipment are worth far more to the customer in terms of savings than a reduction in the initial purchase price of the bearings.
To aerospace OEMs, the design of bearings can add value to their own products in many ways. By engaging with aircraft OEMs early in the development stages, bearing suppliers can customise fully optimised, integrated bearings and assemblies, which meet the specific requirements of an application.
For aircraft actuation systems in which the bearings are local to the point of actuation (ie, high vibration levels are expected), super precision bearings can be designed with dissimilar ball and race materials (e.g. ceramic balls) in order to reduce adhesive wear during vibrational or non-operational duty cycles.
Customised internal bearing designs can be created that maximise load carrying capacity and stiffness. In applications where design envelopes are small, the bearing design can be optimised for ease of assembly and to reduce assembly times. For example, screw threads on assembly mating surfaces can be incorporated into the bearing design. It may also be possible to incorporate components from the surrounding shaft and housing into the bearing design. Features such as these add real value to the OEM customer’s system and can potentially lead to cost savings over the whole life of the aircraft.
Other features can be added to the bearings that add further value over the life of the aircraft system. These include special sealing technology within the bearing to help save space; super finished raceways to improve bearing lubrication film generation; anti-rotation features to prevent slippage under the effects of rapid changes in speed and direction of rotation; and optimising bearing operation under boundary lubrication conditions.
For aircraft auxiliary equipment such as pneumatic and electric starters, generators and gearboxes, bearing configurations range from standard deep groove bearings to intricate split inner ring designs. Bearing designs can be customised to meet high speed, reliable operation and quiet running with minimum power losses.
Due to their unique design, split inner ring configurations can accept reversing thrust and combination loads. The bearings are assembled with one-piece high strength cages that are often silver-plated for improved operation under marginal lubrication conditions. Bearing designs can also include puller grooves and flanges. Ceramic balls can be included to enable higher speed operation.
Other typical configurations for aircraft auxiliary equipment include deep groove bearings that are greased for life at the factory in clean assembly room conditions. ‘T’ cages are often recommended for these bearings as they are lightweight and strong, but also enable high speed operation.
Case study: cabin fan bearings
Cabin fan bearings on a commercial passenger aircraft are essential to the safety and comfort of passengers and crew. Located within the fuselage, the function of these fans is to re-circulate air within the cabin. Fan bearings are therefore critical components.
In terms of the engineering challenges faced by the bearing supplier, the bearings need to be energy efficient with low noise, as well as having the ability to operate reliably at high rates of rotational acceleration, high loads (preload, rotor weight, rotor dynamic imbalance, etc.) and at speeds of 300,000 to 500,000 N.dm. In addition, there is increasing pressure on aircraft designers to reduce the total energy requirements of all electrical/power systems, including engine and auxiliary systems.
Historically, cabin fan bearing solutions comprise a double sealed deep groove ball bearing, very lightly spring loaded, with a ribbon cage and steel (SAE 52100) rings and balls. The bearings would be grease lubricated.
Barden worked closely with a cabin fan manufacturer to develop a much improved, energy efficient bearing solution. This is a deep groove ball bearing with the same ring material (ie, SAE 52100) but with low friction ceramic (silicon nitride) balls, low friction two-piece phenolic riveted cage, double sealed with Barden Flexeals (ie, high speed, low friction seals). The bearings have an increased width and are lubricated with a special grease that is suitable for high speed operation and longer operating life. The result is a cabin fan that requires less power to drive the bearings (ie, more energy efficient) and that is more reliable, resulting in a longer operating life (ie, reduced servicing and maintenance costs).
Custom designed and manufactured aerospace bearings are the cornerstone of the Barden product range. 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 industry. The company has 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. Aerospace bearings can be manufactured to ABEC 7/9 (ISO P4/P2) and fully certified.
Nick Dowding is Business Development Manager, The Barden Corporation (UK) Ltd, Estover, Plymouth, Devon, UK.