Barden Cuts Bearing Costs On Hippag Air Generator

Barden has helped Ultra Electronics Precision Air Systems to reduce bearing costs and assembly times on its high-pressure pure air generator by 40-50 per cent. Ultra produces a range of mini-compressors known as Hippag (High Pressure Pure Air Generators). These are compact, lightweight, electrically powered air compressors that are carried on-board various military platforms. The Hippag airborne compressor provides a continuous source of high-pressure pure air to cool the infrared seekers of an aircraft's defensive missiles, keeping them ready for action.

Hippag also provides a controllable energy source for the pneumatic ejection of aircraft munitions, while meeting the emerging requirement for launching increased quantities of smaller, smart munitions. More than 5,600 Hippags have been delivered for defence applications around the world. Hippag has been developed for the F-35 JSF aircraft and is being supplied for Boeing's Small Diameter Bomb programme, the Eurofighter Typhoon, the US Marine Corps AV-8B Harrier, AH-1W Supercobra, F/A-18 C/D Hornet and the US Navy F/A-18 E/F Super Hornet aircraft.

At the heart of the Hippag system is a bearing assembly based on a swash plate configuration. Supplied by Barden, this super-precision bearing is a customer-designed assembly that comprises a pair of bearings mounted on a cranked shaft with a 15-degree offset between axes. The bearing's main function is to transfer rotary motion into linear motion, enabling the axial motion of the Hippag compressor pistons. The bearing assembly will typically comprise a pair of solid, pre-loaded angular-contact bearings on the swash axis and a second pair of angular-contact bearings for the shaft axis, which are spring preloaded and share a double row inner ring.

The bearings are made from carbon chrome and use standard steel balls, as the Hippag application speeds are relatively low (less than 700rpm). These two bearing sub-assemblies are then mounted onto the offset shaft. Ultra subsequently receives this completed sub-assembly from Barden, which it simply drops into its Hippag housing to mate with the brushless DC motor, which sits on one side of the bearings, with the compressor on the other side. Martin Carpenter, engineering director at Ultra, said Barden's original design was selected because it utilised lower-cost manufacturing methods that used existing tooling and (at the time) a cellular-manufacturing approach that also contributed to a lower-cost bearing solution.

The new bearing assembly integrates directly with the Hippag unit and is smaller and lighter than the original. The original flange-mounted assembly has been replaced with a threaded outside diameter that enables through-grinding. 'The latest design incorporates precision-milled "pots" in the outer ring for the piston rod ends, which reduces the component count. 'Over the years, we've made lots of design iterations, including a low-duty unit in SAE52100 and a special high-duty unit that uses our "Cronidur 30" corrosion-resistant steel with its increased fatigue strength,' said Gary Hughes, application engineering manager for aerospace at Barden.

According to Hughes, there were several engineering challenges to be overcome. For Barden, Hippag is a low-speed high-load application with a short life of less than 500 hours and so the design had to reflect this. Hughes added: 'Through a 360-degree revolution of the Hippag, there's a constantly varying load cycle on the bearings and so we had to develop new methods of modelling this. 'This led to a bearing devolved from the original Ultra design to two bearing pairs and a cranked shaft. 'Investment in new machinery at Barden also enabled us to reintegrate the bearings into the Hippag shaft. 'We were able to eliminate the use of external clamping by incorporating threaded outside diameters on the bearings,' finished Hughes.