Improved bearings for trains
Original article date: February 2000
Insulated bearings ensure trains go further without stops. Tommy Miller reports
Research has identifed the train as being an mode of travel for journeys up to 1000km. This assumes however that we have a new generation of trains able to go faster than ever before and to travel much longer distances without maintenance. These twin demands place added stress on traction motors and the rolling bearing which are fundamental to reliable operation.
To achieve these goals, designers are turning to the use of AC motors. Since an AC motor has neither a commutator nor brushes, high speed and maintenance-free running are possible. And as the adoption of AC traction motors has gathered pace, so has the demand for maintenance-free traction motor bearings.
NSK calculates that on conventional trains of the future, maintenance-free running intervals of 2,000,000km will be required. This compares with intervals of 900,000km for current generation trains equipped with AC traction motors.
The biggest obstacle to achieving the higher performance is electric erosion of traction bearings. Electric erosion occurs when electric current is allowed to flow through motor bearings to earth. This causes damage to the bearing, the extent of which depends upon the magnitude of the current and the duration of the conditions. The damage is usually in the form of craters on the bearings and rolling element surfaces. Often, it results in premature bearing failure.
With the new performance requirements being placed on trains, the importance of preventing this problem has increased. NSK has responded by developing a range of insulated bearings for traction motors.
Plasma spraying technology is employed to coat the outside and end faces of the outer ring of the bearing with a ceramic material consisting mainly of alumina (Al2O3). A thin metal layer is applied prior to the application of the ceramic coating to improve its adhesion.
Although the thickness of the ceramic coating may vary from 0.05 to 0.5mm, the range and outside dimensions of the coated outer rings are the same as conventional bearings without the coating. Therefore, it is not necessary to adjust bearing housings to accommodate special dimensions.
Following the plasma spraying process, the bearings are coated with a special acrylic resin. The resin impregnates the pores of the ceramic coating and seals in the surface to eliminate the harmful effect of humidity.
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Accurate PVT data makes it easy to meet tight tolerances on plastic products Understanding the relationship between pressure, volume and temperature (PVT) is at the core of successful processing, particularly injection moulding. With accurate PVT data, the amount of shrinkage (and hence warpage) of a component can be predicted for a set of processing conditions. This is vital for parts with critical dimensions, and as tolerances get tighter, the ability to predict confidently the final size and shape of plastic components is becoming more important. Commercially available software packages seek to reduce the risk of producing parts with shrinkage or warpage problems by providing quantitative predictions based on reliable data. A crucial input data set to these software packages is the PVT behaviour. Until recently, PVT data has only been available at, or near, equilibrium temperature conditions. However, in injection moulding, it is important to cool as rapidly as possible to reduce cycle times and maximise equipment utilisation. So a method of measuring PVT behaviour during rapid cooling is essential. Now, NPL has demonstrated a method for cooling rates up to 250degC/min and pressures up to 250MPa. Test samples are compressed in-situ from pellets or powder with pressure applied via a piston and then heated. The volume is determined from the diameter and piston position. As the sample is cooled, the changing volume is measured at constant pressure. Linear cooling rates are achieved by passing air through liquid nitrogen. For measurements below 250degC, a novel PTFE sealing is employed. Above that, a polyimide (Vespel) system is used. Families of volume-shrinkage curves are simply obtained by re-heating and applying different pressures. It is found, not surprisingly, that for industrial injection moulding employing high cooling rates, the PVT behaviour is very different to that found at near equilibrium conditions. |
- NSK
February 2000