3 minute read
Bearing protection
Next maintenance in 500,000 Kilometres
Tom Nusch, Product Manager at Schunk, explains how shaft grounding systems made of carbon fibres protect against bearing damage and extend the service life of high speed and light rails trains.
It's an all too common story; suddenly, metros or trains are at a standstill due to damaged engine or transmission bearings. Such breakdowns are not only annoying but, above all, expensive. The cause is often vagrant shaft voltages that discharge in the bearings and cause lasting damage. Shaft grounding systems made of infiltrated carbon fibres can provide a remedy, reliably dissipating voltages and protecting against bearing damage.
From subways to high-speed trains, we know from experience that there are many vehicles with inverter-controlled electric motors that have a problem with shaft voltages in the rail market. And the impact of this voltage is great. Our customers are not only struggling with damaged motor and gearbox bearings. In some cases, we even found that the viscosity of the lubricants had changed to such an extent that there was no lubricating effect at all. But all problems have one thing in common; they cause costly unscheduled train downtimes and high maintenance costs. But what can be done to safely dissipate vagrant shaft voltages and extend the service life of the vehicles? This is a question that Schunk has been dealing with since 2013.
TWO SOLUTIONS FOR ALL REQUIREMENTS
It is almost impossible to develop one standard solution. Because no two trains are the same, and the number of variants of drive units and combinations of motor and gearbox is too large. To offer our customers the best solution for their requirements, it is also necessary to take into account the demanding operating conditions under which a grounding solution should function reliably. These include vibration shocks of up to 100g, the shaft's alternating rotation, and influences from different media, such as water or oil.
Ultimately, we developed two market-ready shaft grounding systems with carbon fibres (CFGs) suitable for railroad applications and tested following current railroad standards. The difference between the two systems lies in the type of contacting.
GEAR/GEARBOX-MOUNTED CFG SYSTEM
This grounding system is mounted near the gearbox, and it makes radial contact with the motor shaft with two grounded carbon fibres and dissipates the shaft voltage in this way. The shaft grounding can be tailored to any gearbox or motor and is mounted on the existing interface with little effort.
MOTOR-MOUNTED CFG SYSTEM
This motor-mounted unit makes contact at the centre of the axial end of the drive shaft. This contact point is already sufficient to reduce the shaft tension. The system can be fixed on any motor shield.
In addition, we have supported our customers when it comes to customised designs of shaft grounding systems.
Regardless of which system customers choose, they receive a solution which dissipates vagrant shaft voltages, protects against expensive bearing damage, is resistant to strong vibrations and is insensitive to changes in the direction of rotation.
The system is reliable even in the event of water or oil ingress and is generally easy to retrofit.
In addition, even under the most challenging conditions, a typical maintenance interval with a CFG system is 500,000 to 600,000 vehicle kilometres. This is made possible because good electrical conductivity and favourable tribological properties make carbon a very useful material when it comes to power transmission – and thus also to shaft grounding. Because of these properties, carbon fibres and carbon brushes have become the most popular material for shaft grounding.
In short, for vehicle manufacturers and transport operators who rely on the performance of their vehicles and want to extend maintenance intervals, shaft grounding systems with carbon fibres offer a reliable method of protecting transmission and engine bearings.
