Special bearing designs provide benefits in harsh, demanding applications

Paul Boughton

In arduous, safety-critical applications such as aerospace systems, standalone and integrated special bearing designs can significantly improve system cost, performance and operating life, says Gary Hughes.

Pressure to reduce costs in all areas of manufacturing means that the integration of bearing systems into mating components is becoming more common. The resulting assemblies are neater, more compact, faster to put together and offer the additional benefits of reducing space and mass, whilst resolving the issues of tolerance stack-up.

The designs of bearings used in these integrated assemblies are in many cases special, calling upon manufacturers to provide the highest levels of innovation and manufacturing excellence in supplying custom engineered solutions to what are often some of the most arduous operating conditions encountered by any type of machine or equipment.

Barden is at the forefront of this design transformation, producing both standalone and integrated special bearing designs, which improve performance and life in applications such as aerospace and defence, F1 motorsport, medical and surgical equipment, dentistry, machine tools, flight navigation systems and turbomolecular pumps. In these areas, the focus of Barden’s design process for bearings is the close integration of new materials, surface engineering, lubrication and sealing technologies.

In materials technology, Barden’s engineers select the most appropriate bearing materials to maximise bearing performance. Special ring materials are used successfully in aerospace and non-aerospace applications, combining superior corrosion and wear resistance with the ability to withstand higher dynamic loads than conventional bearing steels. When used in conjunction with ceramic balls, significant gains in bearing life and performance can be achieved.

Barden has also taken the lead in surface engineering technology. The role of surface engineering in rolling bearing technology is becoming increasingly important as bearings get progressively smaller, but are still required to run faster, carry higher loads and must operate reliably for longer periods. Barden collaborates with recognised industry leaders to provide advanced coatings and surface treatments that combat friction, prevent corrosion and reduce wear, even under the harshest operating conditions. The resulting benefits are improved performance, lower running costs and longer service intervals.

Lubrication also plays a major role in any special bearing design. Recent developments by Barden are focused on extending the speed and temperature capabilities of greases to enable the replacement of oil in high speed applications. This is important as removal of oil lubricating systems reduces component count, assembly time and overall system cost. In addition, grease lubrication also meets the demands of many sectors of modern manufacturing for sealed for-life operation.

The increasing use of grease in Barden’s special bearing designs has, in turn, resulted in the development of new generations of seals, shields and closures, helping to retain the lubrication medium and to prevent the ingress of contaminants.

Aerospace and defence

Barden’s special design capabilities are very much in evidence in the aerospace and defence markets. Here, the bearing designs vary considerably, but have one thing in common: they are manufactured to super precision ABEC 7/ISO P4 tolerances (as a minimum), which means they provide high speed operation with excellent levels of reliability, quiet running and minimum power losses. A number of different design configurations are available for use in a wide variety of aerospace applications, including APU generators, actuator motors, cabin air systems, gyroscopes and a range of different auxiliary aircraft functions.

Barden engineers are often involved early in the development stages of aerospace equipment. Bearing designs range from standard deep groove ball bearings to intricate split inner ring designs that are manufactured from high temperature, high strength bearing steels and can accept reversing thrust and combination loads.

Barden engineers can incorporate special design features such as flanges, shafts and housings, which make fitting easier, faster and more accurate, which in turn, reduce assembly time and overall operating costs.

In recent projects, for example, Barden has developed special bearings for use on APU generators found on commercial and military aircraft. These generators operate at high speeds, typically 10,000-20,000rpm. Depending on the required load carrying capacities of the application, these generators require either special designs of deep groove or angular contact ball bearings. The bearings are normally large diameter units, typically between 25mm and 50mm ID for operating speeds of between 10,000-20,000rpm.

A typical bearing arrangement for an APU generator would incorporate two bearings together with a permanent axial spring preload in order to ensure that the balls (either ceramic or steel) are under control at all times, providing the optimum contact angle with the raceways and minimal risk of dynamic skid.

The bearings are assembled with one-piece high strength cages. In 90 per cent of cases, these cages are metallic, but some applications call for silver-plated cages, which provide improved operation under marginal lubrication/emergency conditions. Bearing design features can include puller grooves, flanges with threaded locking holes or direct through race lubrication holes to enable both optimum lubrication and cooling of the bearings via jet oil.

For APU generators, special split inner race designs or ‘Gothic Arch’ (three-point contact) bearings are commonly recommended at the fixed end of the generator. Due to their unique design, split inner configurations can accept reversing thrust and combination loads. Bearings are typically manufactured from high temperature, high strength bearing steels such as M50 or Cronidur 30 (high nitrogen steels). Ceramic balls are often required.

At the preloaded end (or floating end) of the unit, anti-rotation slots or flanges are sometimes incorporated to prevent or restrict vibrational movement in the housing, which could otherwise lead to fretting corrosion.

Outside of APU generators, other typical aerospace bearing configurations include sealed deep groove ball bearings. These are greased and sealed for life at Barden’s factory in clean room assembly conditions. A variety of grease lubricants are available depending on the application. Barden T cages are often recommended for these bearings. Barden Flexeals are also available when higher operating speeds are required.

Full complement bearings capitalise on the space normally occupied by the ball retainer. This allows for more balls which in turn provides an increase in load capacity, either predominantly radial, in the case of filling notch designs, or unidirectional axial and radial in the case of angular contact designs. Applications here range from high temperature valves to missile fin support.

Gary Hughes is Product Engineering Manager, The Barden Corporation (UK) Limited, Estover, Plymouth, Devon, UK. www.bardenbearings.co.uk

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