Custom-engineered bearings ensure reliability of satellite reaction wheels

Increasing numbers of smaller, more cost-effective telecommunications satellites are being launched into the Earth’s orbit, which has led to a growing need for high precision, high reliability bearings to support onboard subsystems such as reaction wheels and other critical mechanisms, says Gary Hughes, product engineering manager at Schaeffler subsidiary, The Barden Corporation (UK) Ltd.

In the past, building and launching a civil telecommunications satellite into space required a multimillion-pound investment, but more recently, the size and cost of satellites have reduced. An increasing number of satellites are now being launched into space from Europe, the US, China, Turkey, India and the Far East.

For bearing suppliers such as Barden UK, high requirements are now being placed on bearings that support satellite subsystems such as reaction wheels and other onboard precision mechanisms.

A reaction wheel is a type of flywheel used primarily by spacecraft and satellites for attitude control without using fuel for rockets or other reaction devices. A satellite typically comprises three reaction wheels (for X-, Y- and Z-axes), which are particularly useful when the spacecraft must be rotated by very small degrees. They may also reduce the mass fraction needed for fuel. This is accomplished by equipping the satellite with an electric motor attached to a flywheel which, when its rotation speed is changed, causes the satellite to begin to counter-rotate proportionately through conservation of angular momentum.

The failure of one or more reaction wheels can cause a satellite to lose its ability to maintain position and thus potentially cause a mission failure. Therefore, the reliability of reaction wheel bearings is absolutely critical. The bearings must satisfy demanding requirements, by continuing to reliably spin for long periods of 10-15 years while orbiting in space, using only a small amount of lubricating oils on the bearing surfaces in a vacuum environment. At the same time, the bearings must minimise any vibration, so as not to negatively affect the observational performance of the satellite. The bearings must also withstand the vibration and loads at launch, as well as endure the temperature variations while in orbit.

At Barden UK, reaction wheel bearings are typically custom-engineered angular contact ball bearings that are manufactured with high surface-finish requirements and visual appearance of the raceways. The bearings typically comprise 440C stainless steel rings and balls with phenolic cages. Two bearings are required per reaction wheel.

The bearings are assembled in clean-room conditions (exceeding flight-critical aerospace applications) using special handling techniques. Every part of the rolling contact surface of the bearing is inspected at Barden UK. Any slight defect is captured, commented on and documented. The bearings are typically shipped as a kit ready for further examination and screening by the customer prior to assembly/fitting.

Full traceability

Barden’s Quality Management Systems are accredited to Aerospace Standard AS9100. The company is also able to satisfy specific customer requirements such as The National Aerospace and Defense Contractors Accreditation Program (NADCAP) for its heat treatment and non-destructive testing processes. These controls are coupled with a planned flexibility that enables Barden to comply with specific requirements of individual customers through a system of bespoke quality levels and formal certification of its products.

Bearings are manufactured and controlled under strict aerospace procedures, providing full traceability, controlled lubrication and complete retention of records. Full traceability is provided for satellite bearings. Torque test traces and functional tests can be performed on the bearings, as well as 100% inspection of bearing raceway surfaces. Documentation and measurement reports can also be provided on every bearing in every batch.