Feature Seals & bearings
Magnetic bearings offer high performance and reliability
Phil Burge, communication manager for SKF, looks at magnetic bearings and explains how they can offer a number of advantages in a diverse range of applications
T
he magnetic bearing systems approach to the support of rotat- ing equipment is achieving per- formance levels not previously possible with traditional rolling ele- ment bearing technologies. Magnetic bearings are a non-contacting technol- ogy, which means they offer a range of important benefits, from almost zero friction and vibration, through reduced energy consumption and bearing wear, to higher reliability and longer operating lives than their con- ventional counterparts. In addition, previously unachievable surface speeds of up to 100,000rpm can now be realised, while the need for lubrica- tion and maintenance is also elimi- nated, meaning magnetic bearings are suitable for processes that are sensitive to contamination, such as the vacuum chambers in which many semiconduc- tor manufacturing processes take place. These advantages are achieved by a combination of three complementary technologies. Firstly, a bearing stator and rotor perform the role of the electromechanical hardware by which input signals are collected, and sup- port forces applied to the machine on which they are installed. Position sen- sors identify the position of the shaft
in five axes. Finally, a control system and control algorithms provide power and control electronics for signal con- ditioning, the calculation of correcting forces, and commands to the power amplifiers for each axis of control, as well as processing input signals after conditioning and calculating the com- mand signals to the power amplifiers. The magnetic bearing stator consists of a stack of laminations with copper coils wound around it to form a series of north and south poles. In operation, a current is supplied to each coil to produce an attractive force that levi- tates the shaft inside the bearing. The controller applies the precise level of current to the coils by monitoring sig- nals from the positioning sensors in order to keep the shaft at the desired position throughout the operating range of the machine. There is usually a 0.2 to 0.5mm air gap between the rotor and stator, while the shaft is driven by a turbine, or a motor inte- grated into the shaft design.
Use of magnetic bearings has until now been limited to specialised appli- cations. However, recent technological developments have seen a reduction in the size of control systems and an substantial increase in performance.
These advances mean magnetic bear- ings are now suitable for wider usage, in commercial applications, particu- larly those where environmental impact and energy costs are priorities. Magnetic bearings are not a perfect solution for every application however and it is important to combine them with high speed brushless DC motors or permanent magnet motors to take advantage of the benefits and allow higher speed operation.
The magnetic bearing stator consists of a stack of laminations with copper coils wound around it to form a series of north and south poles
SKF T: 01582 496433
www.skf.com Enter 329
The use of monitoring equipment means magnetic bearing systems can be actively assessed to reduce mainte- nance costs and downtime. This approach can offer real time snap- shots, giving warning of anomalies. Active monitoring systems help improve machine reliability and enable better decisions to be made based on what is known about a process. The latest magnetic bearing technol- ogy can enhance the capabilities of rotating equipment by providing robust, cost effective and easy to implement solutions. When combined with the latest enhanced electronics and software systems, which allow for the better monitoring and control of motion, these revolutionary systems can now operate at optimum perfor- mance levels with exceptionally high levels of reliability – adding measur- able value to the bottom line of any company in industry.
Bearings on vegetable washer/polisher deliver significant savings
SK’s bearing substitution programme has produced annual savings of €9,542, in respect of bearing, maintenance and lost production costs, for a food processor operating a vegetable washer/polishing machine under harsh conditions, where water, soil and grit are prevalent. The food processor was encountering high failure rates on housed bearing units used on the vegetable washer, resulting in complete bearing changes every three months. To make matters worse, the failed bearings were also difficult to access. Maintenance engineers struggled to remove them, which was often impossible without resorting to hand grinders. What with all these problems, the cost of the failures was considerable: the yearly total for bearings, mainte- nance, engineering time and lost production amounting to €19,136 per year.
N NSK investigated the application and proposed
replacing the existing bearings with its stainless steel series of Molded Oil bearings. These bearings are designed to give high performance in water and dust
Factory Equipment MARCH 2011
contaminated environments. They employ a special solid lubricant to transform the performance of machinery in environments exposed to high humidity, corrosion and contamination.
A highly effective lubricant, Molded Oil offers more than twice the operating life of grease lubrication in
water contaminated environments. Moreover, the tech- nology means that operating environments are kept free of grease – very important in food applications – and there is no need for refilling of lubricant, as Molded Oil provides a continuous supply of lubricating oil. A trial was conducted on the vegetable washer using Molded Oil bearings. This was a complete success; the bearings significantly outperforming the previous housed bearings: they are still operating after more than 12 months. In addition to the improved reliability, and consequent annual cost reduction, this extended operation has brought, the food processor is also benefiting because a costly regreasing operation has now been eliminated. This represents an additional major saving in labour and downtime, enabling the food processor to introduce a reduced and planned maintenance regime.
NSK T: 01299 405454
www.nskeurope.com Enter 331 29
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