By working closely with OEMs, The Barden Corporation is helping to optimise the performance, reliability and durability of super precision bearings for aircraft starter-power generators.

• Bearings for combined starter-power generators for aircraft must withstand high speeds and high loads, with typical rotor speeds ranging from 10,000-30,000rpm, with high centrifugal forces as a result.
• Barden works with equipment manufacturers to improve the reliability and durability of starter generators by adding design features such as silver-plated steel cages, ceramic balls and rings produced from high fatigue strength materials.
• Performance of the bearing may be optimised by using a variety of tools including MESYS dynamic analysis software and  finite element analysis.

Custom super precision bearings from Barden play a critical role in combination starter and power generators for aircraft. These systems serve two important functions: to provide the power that drives the engine during the start-up phase; and second, to generate electrical power whilst the engine is operating. High reliability and durability are therefore key factors when designing bearings for these applications. The bearings must be capable of withstanding high speeds and loads, with typical rotor speeds ranging from 10,000-20,000rpm, with high centrifugal forces present as a result.

Pawel Bytnar, Applications Engineer at Barden UK comments: “Over the years, Barden has custom engineered a variety of different bearings to suit specific starter generator designs for aircraft. The bearings are manufactured from high specification materials, including ceramic rather than steel balls and metallic cages which are often silver-plated.”

A typical bearing arrangement for a starter generator would incorporate two bearings together with a permanent axial spring preload in order to ensure that the balls (either ceramic or steel) are dynamically controlled at all times, providing an optimum contact angle with the raceways, whilst minimising the risk of dynamic ball skid and cage ball pocket wear.

The bearings are assembled with one-piece high strength cages. In most cases, these cages are steel or bronze which, depending on the application, may be supplied silver-plated in order to facilitate improved lubricity, especially during emergency conditions. Other 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 recirculating oil.

Gothic Arch [three-point contact] bearings featuring a split 2-piece inner race are commonly recommended at the fixed end of the generator. Due to their unique design, such bearings can accept reversing thrust and combination loads whilst benefiting from a maximum ball complement (load carrying capacity). Bearings are typically manufactured from high temperature, high strength bearing steels such as AMS5898 (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 surface fretting corrosion.

Optimised by design

Barden was recently involved in a project to help improve the reliability of the bearings used in an existing variable frequency combined starter and power generator.

A number of reliability improvements were required, including better oil flow to the bearings, a more robust bearing cage design and a method of detecting cage debris.

Bytnar states: “We made a number of design improvements to the bearings. By making extensive use of FE [finite element analysis] software, we optimised the dimensions of the bearing cage in order to reduce the peak stress and cage ball pocket wear associated with centrifugal effects.”

“For improved lubrication, oil lubrication holes were added through the inner ring raceway. Prior to this, oil was injected into the bearing externally through the cage bore diameter via a nozzle. This method was proven ineffective in this particular application. By feeding oil down the shaft and straight through the inner ring, oil is supplied exactly where it is needed to provide the optimum level of cooling and lubrication.”

For detecting cage debris, says Bytnar, the silver-plated steel cage also allowed the deployment of a simple magnetic shedding detection system, which was positioned at the bottom of the oil sump.

“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,” concludes Bytnar.