Bearing design choices for electric motors
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 an 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 program 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 optimizing the lubrication technique, the maintenance period can be extended to coincide with other components such as the wheel set bearings. This offers the client better savings in maintenance costs and lost productivity.
The key to optimizing 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 Z Steel which has 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.