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DEFENCE INDUSTRY: RELIABILITY


 Marine gyroscopes require high


precision bearings to maintain their accuracy reliably; such bearings use advanced polymer cages (inset)


Precision bearings enhance gyroscope life


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. Andy Pye reviews the work of Barden Corporation in this sphere


bearings are often oil-lubricated with ceramic balls and extremely smoothmirror finish raceways to reduce noise levels and friction. The bearings are assembled in cleanrooms and the rings are 100 per cent visually inspected for defects to ensure a long operating life. A gyrocompass is amarine device


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that is able to detect the direction of true north and notmagnetic north. Variations in the Earth’s magnetic field that deflect normal compasses do not affect a gyrocompass. The gyroscope inside a gyrocompass systemis mounted in such a way so that it canmove 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 systemor other navigational aids to feed data to the gyrocompass for correcting any errors. Typically, formarine gyroscope


applications, Barden provides angular contact ball bearings 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. 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


40 /// Environmental Engineering /// October 2017


ypicalmarine gyroscope applications use angular contact ball bearings in a back-to-back configurationmounted at either end of the drive shaft. The


sealed) angular contact ball bearings with an oil sump between the bearings. This is sealed with a wickmaterial 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 amilitary 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 aremanufactured to ABEC 7 standards, often with raceway roundness held to limits that exceed the ABEC standards requirement. The bearings are assembled in class 100


cleanroomconditions. The bearing rings are 100 per cent 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 scuffmark on the rings can


trigger wear, which could lead to a reduced operating life of the bearing and premature failure. Bearing cages are usuallymade from


specialmaterials 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 systemand bench tested. Barden gyro bearings achieve low noise levels by their low friction properties and smoothmirror finished raceways. Low friction ceramic balls and complete visual inspection of the bearing rings eliminate any defects. EE


 To readmore online about bearings, scan the QR code or visit https://goo.gl/KPwppz


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