Emergency bearings for magnetic bearing systems

Nick Dowding, Applications Engineering Manager at The Barden Corp. (UK) Ltd, explains why magnetic bearings are proving a popular alternative to traditional mechanical systems and the challenges in protecting those systems.

Plymouth, UK - An active magnetic bearing (AMB) provides a practical method of suspending shafts (both axially and radially) in numerous applications, including turbomolecular vacuum pumps (used in semiconductor manufacturing), dry pump bearings, compressors, blowers, air conditioning systems, gas expanders (used as venting devices in gas pipelines) and in energy storage systems as emergency back up power.

These types of bearings are attracting increasing amounts of attention due to their unique characteristics. AMBs use actively controlled electromagnetic forces to control the motion of a rotor or other ferromagnetic body in air. Contact-free suspension leads to important advantages relative to conventional rolling element or hydrodynamic bearings: reliability, low maintenance and losses, higher speeds in extreme environments without requiring complex lubrication systems.

Most magnetic bearings require continuous power input and an active control system to hold the load stable. Because of this complexity, the magnetic bearings also typically require some kind of back-up bearing in case of power or control system failure.

Between 2002 and 2007, Barden was one of ten organisations involved in a European-funded research project called “MAGFLY”, which focused on the design of AMBs for aero-engines used in civil aircraft. The project looked at the development of load-sharing bearings, the design and demonstration of AMBs for high-temperature environments and the dynamic modelling of the complete system, consisting of a rotor, AMBs, controller, load-sharing bearings, support housing and casing. In particular, the programme sought to optimise the size and performance of AMBs by using special modelling tools and software.

The five-year project provided a great deal of information about the behaviour of emergency bearings, particularly the initial shock load characteristics associated with a failure of the system, when the back-up bearings engage with the shaft. When the back-up system is activated, high gyroscopic forces are present, where the shaft skids and whirls (forwards and backwards) before running down to a stop.

As part of the MAGFLY project, engineers built test rigs and models which specifically looked at the first phase of emergency bearings, the period of time (usually around half a second) immediately after a failure takes place when the shaft engages with the bearing(s). Models were developed to simulate this phase and the initial shock loads.

From this work, engineers are now able to predict more closely the initial shock load characteristics during the crucial first phase and therefore size the bearing more appropriately. This means an emergency bearing design is not over-engineered or under-engineered for a given application. Typically, the customer specifies the life of a back-up bearing system in numbers of touch-downs, 5,10 or 20 touch-downs for example.

Various types of back-up bearings have now been developed to cope with these shock loads. These include single- and double-bearing arrangements, with the designs typically employing ceramic balls, tightly packed around the raceway with no ball separators used, and range from units that fit 4mm diameter shafts up to 200mm diameter versions. Bearing rings are typically made from corrosion resistant steel, though zirconia balls are an option for extra corrosion resistance in particularly harsh environments.

The advantages of magnetic bearings include very low and predictable friction, their ability to run without lubrication and in a vacuum. Because of these technical benefits, magnetic bearings are increasingly used in industrial machines such as compressors, turbines, pumps, motors and generators. Magnetic bearings are also used to support ‘Maglev’ trains in order to get low noise and a smooth ride by eliminating physical contact surfaces. In all cases, a mechanical back up system is needed. The only disadvantage is the relative high cost of the system.