BEARINGS, SEALS & GASKETS FEATURE
CAN YOUR APPLICATION benefit from magnetic bearings?
Oil & gas OEMs have long exploited the advantages of magnetic bearings but,
thanks to technological advances, they are becoming more affordable to other sectors of industry, as Phil Burge, marketing and communications manager at SKF, explains
A
s a non-contacting bearing technology that is
completely lubrication-free, manufacturers of oilfield equipment – such as compressors and turbomachinery – often specify magnetic bearings. With these bearings, as there is no metal-to-metal contact, they are practically frictionless in operation so there is no wear. They are also capable of high speeds, are vibration-free and – thanks to their frictionless operation – are highly energy efficient. It is also easy to incorporate condition monitoring features into a magnetic bearing, which avoids the need for additional sensors and associated bearing health monitoring hardware. A magnetic bearing is a relatively
aligned with that of the electromagnetic stator. Moreover, as the system is totally reliant upon a continuous power supply, back-up bearings are required to take over in the event of a power failure. Despite this complexity,
the many benefits that magnetic bearings can bring to an application are compelling, and while the oil & gas industry has certainly taken advantage of their features there is
Among their benefits, magnetic bearings are practically frictionless in operation so there is no wear, and they are highly energy efficient
evidence to suggest that other sectors, too, are keen to take advantage. Industry-specific algorithms,
miniaturised electronics and embedded sensors have seen the price of magnetic bearings fall, attracting new adopters to the technology.
“The many benefits that magnetic bearings can bring to an application are compelling, and while the oil & gas industry has certainly taken advantage of their features, there is evidence to suggest that other sectors, too, are keen to take advantage”
complex assembly and, traditionally, this has attracted a high price tag. Active magnetic bearings, which far outnumber those equipped with permanent magnet stators, rely on electromagnets to generate forces both radially and axially to levitate the shaft, allowing it to rotate contact-free in a stable position. A control system uses the signals from rotor gap measuring sensors to actively monitor and continuously adjust the current in the electromagnets so that the shaft axis is maintained precisely
AN ATTRACTIVE OPTION Being lubricant-free, magnetic bearings are an ideal choice for machines operating in a vacuum, at high or cryogenic temperatures, and where process incompatibilities must avoid contamination by lubricants or wear particles. They can also be hermetically sealed and are therefore an attractive option for processes handling corrosive fluids that would otherwise compromise the stator windings or laminations. And, as magnetic bearings rely on a carefully controlled air gap, certain applications can benefit directly from this - for example, where a process liquid or a
delicate biological cell culture must be transferred unimpeded past the bearing.
Of additional benefit, frictionless,
non-lubricated magnetic bearings can run at surface speeds as high as 250m/s, offering advantages to specialist applications including advanced machine tool spindles or specialised laboratory equipment such as the hydrogen circulators now in use at several US National Laboratories. A magnetic bearing is a highly
controllable mechanical device – thanks to its associated control system but more particularly to the software algorithms developed to run within this system. For example, SKF has developed a specific function called ‘Adaptive Vibration Control’ (AVC), which controls the bearing system’s response to shaft imbalance. This can be used in two ways: to allow
the shaft to rotate around its geometric centre and tightly control the shaft to eliminate the run-out caused by imbalance (machine tools benefit considerably from this feature); and to rotate the shaft around its mass centre to limit case vibration (of particular benefit to turbomolecular pumps and other front-end semiconductor manufacturing equipment). In magnetic bearing systems the bearing
force can be modulated for deliberate excitation of vibration, the excitation force being applied to the rotor without contact. It can also be precisely measured. In this way, magnetic bearings can provide valuable tools for equipment design, development and testing, as well as rotor dynamic research. The method has been used to test and verify new seal designs, as well as the performance of machine tool spindles.
MAINSTREAM USE While magnetic bearing technology will continue to be exploited by high-end users with very specific needs, undoubtedly the recent advances in areas like control systems, software and sensors are poised to propel it well beyond the offshore industry and into some of these more mainstream uses.
SKF
www.skf.co.uk DESIGN SOLUTIONS | MAY 2018 27
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