Hybrid bearings isolate locomotive oil pumps

Edited by the Engineeringtalk editorial team Aug 26, 2008

The transformer oil pumps in Austrian national railway company OBB's Taurus high-performance locomotives are being equipped with NKE hybrid bearings to prevent passage of electrical currents.

The Taurus high-performance locomotive is one of Austrian railway operator OBB's flagships.

Assembled by OBB Technische Services - a subsidiary of OBB - in the Taurus Centre in the Austrian city of Linz, the Taurus is currently "Europe's most powerful and fastest high-performance locomotive", according to OBB.

The 1016 and 1116 series two-system high-performance locomotives are used mainly for main-line passenger and for heavy goods transports.

Thanks to its newly developed high-speed bogeys with their performance drivetrain and brake shaft, the Taurus reaches top speeds of 230km/h.

Each of the locomotive's bogies is supplied by one current convertor.

An oil-cooled transformer arranged under floor supplies the current convertors with the required propulsion power.

Each of the transformer's cooling circuits contains two oil pumps that keep the oil circulating.

An auxiliary consumer convertor supplies the oil pumps with electrical energy.

Especially during starting, the simultaneous operation of auxiliary convertor and oil pump can cause an excessive current flow through the pump's bearings.

This problem was identified by maintenance contractors OBB Technische Services and was confirmed by the engineers from the bearing supplier NKE.

To prevent the adverse current passage, the oil pumps were to be fitted with different bearings with ceramic rolling elements that provided a complete electrical isolation.

The choice fell on hybrid bearings from Austrian-based bearing manufacturer NKE Austria.

NKE impressed right from the start, with its fast submission of an offer and model selection, and the provision of samples for technical trials.

The short delivery time and cost effectiveness were also decisive advantages.

In the course of routine maintenance work on the 332 locomotives at the Taurus Centre in Linz and the other OBB Service locations, the oil pumps are removed and their existing bearings replaced with electrically insulating angular contact ball bearings and deep groove ball bearings from NKE.

The rolling elements of the new bearings are made from insulating oxide ceramic material.

Ceramic rolling elements have a theoretically infinite electrical resistance and therefore effectively prevent any current passage.

Beside hybrid bearings - bearings with ceramic rolling elements - electrically insulated bearings are also available with an oxide ceramic insulating layer on the bearing's outer race.

The ceramic layer is applied to the outer race through thin-film plasma spraying.

The insulation has a guaranteed dielectric strength of at least 1000V direct or alternating voltage.

Electrically insulated ball bearings are also used in other applications, for example in traction motors.

NKE supplies electrically insulated bearings in standard sizes with outer diameters from 80 to 400mm as well as custom sizes to order.

The electrically insulated bearings are available in a range of designs.

As their load ratings, key dimensions and tolerances are identical with those of the normal bearings, they can be easily substituted for the standard bearings in existing installations or new designs.

Electrically insulating bearings provide excellent protection from bearing damage through flashover and are therefore much more reliable in operation than their conventional counterparts.