Power Transmission
arrangement is often referred to as a five-axis system. We have already mentioned turbomachinery as one application for which AMB systems are suitable; others include centrifuges for purifying nuclear isotopes, natural gas compressors, equipment for use in manufacturing semiconductors, and high-throughput blowers (see panel). In addition, the high-speed capability of AMBs makes
and says that AMBs can be scaled to carry any load - though the company also points out that the force density is
them suitable for machine tool spindles. SKF, which manufactures AMB systems, quotes a force density of 40-60 N/cm2
mechanical bearings, so the likelihood of a failure in these components is low. Given the exceptional longevity of AMBs, the life cycle
cost can be lower than that of conventional bearings, even though the initial purchase prices is usually higher due to the increased complexity.
Remote monitoring
A recent development from Waukesha Magnetic Bearings, which is a division of Waukesha Bearings Corporation and part of the Dover group, raises the possibility of further reductions in lifetime costs. This is through integrated remote connectivity that enables the machine builder or Waukesha’s engineers to monitor vibration, bearing load and rotor stability remotely. The bearing controller is equipped with a TCP/IP connection, so access is possible from any suitable device connected to the internet. As well as aiding troubleshooting during operation, the
remote connectivity also offers benefits during commissioning and later in the bearing’s life, when retuning may be necessary. The remote diagnostics also enable the static clearances to be checked either automatically or on demand, with this information providing an indication of the condition of the auxiliary bearings. One of the drawbacks with conventional AMBs is that
Fig. 2. Synchrony’s Fusion active magnetic bearings include the control system within the bearing housing for easier system integration.
lower than for other bearing types, so AMBs are likely to be bigger than conventional bearing arrangements with a similar load-carrying capacity. Unlike other types of bearing, AMB systems have to include a back-up power source and auxiliary bearings (also called emergency bearings or touch down bearings) to prevent the equipment from suffering damage in the event of a failure in the sensing, control or power amplifier components. Remember, however, that the electrical and electronic components have a far longer design life than conventional
Blower bearings S
ynchrony, a USA company specialising in magnetic bearings, high-speed motors and generators, is supplying bearings and motors to Gardner Denver for use in the recently launched
Hoffman Revolution range of high-speed, single-stage aeration blowers for water and wastewater treatment plants. Gardner Denver chose the Synchrony bearings and motors in order to achieve increased efficiency, reliability and control. Hoffman Revolution blowers feature a high-speed permanent-magnet
motor that is controlled by a variable-speed drive. The first models to be launched, the R-200 and R-300 blowers, offer flow rates from 2500 to 8500 cubic feet per minute (4250 to 14 450m3/h) and pressures from 3 to 15 psig (0.2 to 1barg), while providing up to 45 per cent energy savings from a footprint that is 50 per cent smaller than a conventional multi- stage centrifugal blower. Gardner Denver claims that the new blowers are in a class of their own in terms of efficiency and environmental credentials, thanks in part to the active magnetic bearings. l
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the controller has to be housed separately from the bearing. However, USA-based Synchrony has recently introduced the Fusion family of active magnetic bearings that integrate the control system within the bearing housing (Fig. 2). This simplifies system integration considerably, provided the increased bulk of the bearing can be accommodated and the operating environment is not too severe, as the maximum operating temperature at the stator outside diameter is 60° C. Each Fusion unit is equipped with an Ethernet port to aid commissioning, condition monitoring and troubleshooting, and MTBF (mean time between failures) is quoted as greater than 80 000 hours. To give an indication of the performance capabilities
of the integrated bearings, the model FR 35-10 has a stator outside diameter of 178mm (7 inches), can accommodate a shaft diameter of 56mm (2.2 inches), has an overall length of 97mm (3.8 inches), a load capacity of 1.9kN (424 lb) and a maximum operating speed of 34 000 revolutions per minute. In contrast, the largest model in the range, the FR 90-20,
has a stator outside diameter of 381mm (15 inches), can accommodate a shaft diameter of 147mm (5.8 inches), has an overall length of 279mm (11.7 inches), a load capacity of 27kN (6053 lb) and a maximum operating speed of 13 000 revolutions per minute. Fully integrated thrust bearings are also available.
Reduced energy consumption
Active magnetic bearings are not a new concept, but the demand from design engineers for higher performance and reduced energy consumption has led to a resurgence in interest in these bearings. As can be seen from the above, manufacturers have
responded, with the result that new developments are creating exciting and innovative opportunities for these ultra-low- friction bearings. l
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