As the development of more efficient electric motors continues to keep pace with upcoming regulations, it is important to ensure that all the components are engineered to the latest specifications, including the bearings. Tony Synnott, UK Engineering Manager for NSK Europe, explains how bearing design is affecting electric motor performance and reliability.
Bearings in electric motors are designed to support the rotor and maintain a consistent air gap between the rotor and the stator as well as transferring the loads from the shaft to the motor frame. Selecting the correct bearing design ensures the design efficiency of the motor is maintained with minimal friction and power losses.
For a small electric motor, a bearing failure and any subsequent damage can put it beyond economical repair, so it is desirable to install high quality bearings that are designed to withstand the stresses of the environment. In larger applications, a bearing failure can cause considerable down-time, so matching the design to the application is crucial.
For example, if a motor designed for an in-line drive application is reassigned to a radial, belt-drive application, the bearings are likely to be compromised and not perform as might be expected. In the original, specified application, the motor would be fitted with deep-groove ball bearings but these could be overloaded by the increased radial load from the drive belt.
In terms of bearing design itself, there has been considerable innovation in terms of bearing materials technology as well as precision engineering and manufacturing processes. By improving the surface finish of the rolling elements and raceways within a bearing, the friction levels are reduced, which reduces energy consumption and noise. When these improvements are combined with the advances in lubrication, the result is a much improved operating life.
One such example is the latest AC traction motors that are being used in the current generation of high speed trains, which are now expected to go faster and travel much further between maintenance periods. The bearings employed in these motors are expected to deliver exceptional performance under severe operating conditions, including high radial loads, high impact loads and high speeds.
A major obstacle to achieving extended maintenance free intervals is electrical erosion. This occurs when ‘stray’ electric current flows through the motor bearings to earth. The result is damage to the bearing, the extent of which depends upon the magnitude of the current and the duration of the conditions. The damage, known as electrical arcing, is usually in the form of tiny craters and micro welds on the bearing raceway and rolling element surfaces, which results in premature bearing failure.
One solution to the electrical arcing failures is the use of ceramic coatings or ceramic rolling elements in the bearing design, which provide electrical insulation from the shaft. Ceramic coatings are often applied using a plasma spray. The ceramic coating is treated with an acrylic resin to seal the surface and prevent the ingress of moisture. This coating also provides additional protection against strong alkalis and high temperatures used for washing the rolling stock.
The life time of a bearing, assuming it has been correctly specified for the application, is often determined by the lubrication regime and how the bearing design ensures sufficient flow to all the rolling elements. Finite element analysis and computational fluid dynamics can be used to improve bearing strength and the flow of lubrication, both of which need to be carefully specified for each application.
The maintenance programme also plays a role in bearing life expectancy and should take into account the working environment and the practicalities of access to the equipment. By improving the sealing arrangements and optimising the lubrication technique, the maintenance period can be extended to coincide with other components such as the wheelset bearings. This offers the client considerable savings in maintenance costs and lost productivity.
The key to optimising bearing design is in understanding the application and the environment, whilst having the ability to employ industry leading materials and quality control. NSK has developed the unique, long life Z Steel which has significantly extended the service life of its bearings when compared to conventional bearing steel. This has been combined with improved cage designs lubrication and seal designs, as well as ceramic and resin coatings. The end result is high quality bearings that are produced for the electric motor in all environments and applications.
NSK UK Ltd
Northern Road, Newark, Nottinghamshire, NG24 2JF, UK
Tel: 0500 2327464 Fax:
Web: www.nskeurope.com
Email: info-uk@nsk.com
NSK Europe Ltd
- 01636 605 123
- info-uk@nsk.com
- Belmont Place, Belmont Road, Maidenhead, Berkshire, SL6 6TB GB
About us
Established over 100 years ago, NSK (Nippon Seiko Kabushiki Kaisha) is a Japanese-listed company that has evolved from a regional ball bearing supplier to a roller bearing specialist and automotive supplier with a global market presence. Today, NSK employs more than 31,400 employees in 30 countries. As per March 2019, NSK achieved sales of 991,4 billion Yen. This result has been driven by ever-increasing investment in research & development, enabling the company to continuously improve the quality of its products and services. This investment supports NSK’s objective of “No. 1 in Total Quality”. In addition to a complete rolling bearing portfolio, NSK develops and manufactures precision components and mechatronic products, as well as systems and components for the automotive industry, including wheel bearing units and electric power-steering systems. In 1963, NSK’s first European subsidiary, Düsseldorf, Germany, was opened and in 1976, the first European production facility in Peterlee, England. Today, NSK Europe supports pan-European sales with production locations in England, Poland and Germany, logistics centres in the Netherlands, Germany and England and technology centres in Germany, England and Poland. In 1990, NSK purchased the UPI Group including the renowned European bearing manufacturer RHP, with its factory in Newark (UK). Additionally, NSK has developed a comprehensive network of authorised sales distributors. NSK Europe is divided into application-based business divisions: Industry rolling bearing technologies & linear and precision technology (EIBU) as well as bearing modules and steering systems for the automotive industry (EABU & ESBU). In this organisation, NSK Europe's 4,260 employees achieved a turnover of over 1,045 million Euros as per March 2019.
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