As technological innovations get more advanced year by year, drive systems such as gearboxes and traction motors for trains are also progressively improved, and ultimately must be powerful, sustainable, cost-effective and require minimal maintenance.
These requirements are steadily getting more important because of the preference for increasingly long maintenance intervals and developments in weight of railway vehicles.
Not only do the train traction motors deliver high speeds and handle shock loads, they also have to resist low load conditions, for example in coasting or cruise control mode.
Therefore, it is important that gearbox designs cater for a wide range of operating temperature ranges, a variety of shock loads, and increasing loads overall in combination with the drive to reduce friction within the gearbox and lower energy consumption.
In this regard, bearings play an important role. Drive system bearings must be designed to cope with high levels of contamination and perform in humid environments, as well as in conditions with variations in speed, temperature, load, vibration, and shocks.
But gaining more importance is the ability of the bearings to withstand electrical discharge, especially frequent with the AC motors employed in trains.
Alternating current (AC)-powered motors have come into common use in locomotives because they offer many advantages over their direct current counterparts, including higher adhesion level, good torque control, more power saving due to better regenerative braking, and overall reduced maintenance levels.
However, currents flowing through induction motor bearings have the potential of creating premature failure of these components (rings and rolling elements made of bearing steel).
Electric motors powered by fast switching voltage source inverters experience high frequency short, sharp voltage pulses that can flow through paths that would normally be considered to be insulators. Currents now can flow through the magnet wire insulation, stator slot liners, motor air gap, bearing grease and stator slot top sticks, offering new opportunities for shaft and bearing current flow that can result in premature bearing failure.
It’s therefore vital that from a bearing design perspective, sensitivity towards electrical discharge must be considered.
SKF LEADS THE WAY
As the world’s largest bearing manufacturer, it can be truly said that SKF provides reliable rotation for the real world.
The company’s expertise is built on the development, design and manufacture of bearings – as well as seals and lubrication systems – while also providing machine health assessment, engineering and remanufacturing services.
And it offers an arena for innovation – an opportunity for its customers to use the tools it provides to explore, improve or rethink the performance of their rotating equipment; a place where even the smallest adjustment can make a big difference.
As SKF railway engineer Stephen Fasolo explains, roller bearings are used widely in the rail sector, and for good reason. They support and guide rotating or oscillating machine elements – such as shafts, axles or wheels – and transfer loads between machine components.
“They provide high precision and low friction and therefore enable high rotational speeds while reducing noise, heat, energy consumption and wear,” he said.
“The rolling bearings are cost-effective and exchangeable machine elements that typically follow national or international dimension standards. And they come in a wide assortment of shapes, sizes and specifications: cylindrical, needled, tapered, or spherical, for example.”
SKF offers the full range of bearing types, many customisable to meet the requirements of applications of varying loads and speeds, and to enable easier maintenance in difficult-to-access positions.
Plus, the company constantly engineers specially-designed components to improve performance of the bearings for whatever use they are made for, from the small machines to large locomotives.
Chief among these is a bearing designed specifically to meet the electrical discharge challenges described above.
HYBRID BEARINGS
This bearing typically consists of bearing steel and rolling elements made of bearing grade silicon nitride (Si3N4), which make the bearings electrically insulating.
Used in railway drive applications such as traction motors and gearboxes, they have been developed to improve performance and reliability under extreme application conditions.
The benefits include:
- Superior electric insulation properties even if exposed to high frequency currents
- Extended maintenance intervals due to longer grease life compared to all-steel bearings
- Energy saving capabilities because of lower friction and higher precision
- Extended maintenance intervals because of lower operating temperature
“Combining rings made of bearing steel and rolling elements made of bearing grade silicon nitride, these bearings are electrically insulating even at very high frequencies,” Fasolo said.
“They also have excellent tolerance towards poor lubrication and contribute to lower operating temperatures, leading to longer grease life.
“Hybrid bearings are dimensionally interchangeable with similarly sized all-steel bearings and can substantially improve reliability and robustness when incorporated into new or existing industrial equipment.”
And thanks to the ceramic rolling elements used, hybrid bearings have features that other bearings lack.
Silicon nitride is an electrical insulator and prevents passage of electric current, with a density 60 per cent lower than steel.
Lighter rolling elements means lower inertial forces, which enable higher speeds and more rapid starts and stops.
“Ceramic rolling elements are so much harder than most solid contaminants that they will either crush contaminants or push them into the steel raceways,” Fasolo said.
“This temporarily creates a depression, with raised edges, in the raceway. The dent is quickly over-rolled and flattened by each subsequent rolling element, which is also known as a kind of self-healing effect.”
Silicon nitride rolling elements have a lower coefficient of thermal expansion, which means they are more stable over temperature gradients within the bearing and provide more accurate preload/ clearance control.
As well, when subjected to vibration, hybrid bearings are significantly less susceptible to false brinelling (formation of shallow depressions in the raceways) between the silicon nitride and steel surfaces.
“Lower bearing operating temperatures due to less frictional heat also means that the grease can last at least two times longer, depending on the application and operating conditions. Under some specific conditions, it can be up to 10 times,” Fasolo said.
“The friction coefficient between silicon nitride and steel is lower than steel-on-steel for a dry sliding contact.
“The adhesion between silicon nitride and steel is low, micro-welding does not occur and there is no risk of smearing. Hybrid bearings generate less frictional heat even when there is a very thin lubricant film.”
SKF hybrid bearings are dimensionally interchangeable with similarly sized all-steel bearings.
With their high-end qualities, the hybrid bearings can substantially improve reliability and robustness when incorporated into new or existing industrial equipment.
Silicon nitride rolling elements can extend bearing service life by offering enhanced bearing performance, even under difficult operating conditions.
Fasolo also cleared up misconceptions that the ceramic could be more brittle than steel.
“I can tell you this never happens. The ceramics endure much higher loads than steel balls before they get damaged,” he said.
“We’ve done experiments where we cut a notch into the ceramic rollers and then measure the loads they can withstand, and they’re much higher than one would ever encounter in service.”
From a cost perspective, the hybrids are also a better alternative to bearings that are insulated with coatings.
“Some bearing products use electrically insulating coating on the surfaces. The idea is the same: to try and prevent the passage of current, but it’s not a complete solution like the way the hybrid is,” Fasolo said.
“The hybrids are more cost-effective. There are costs involved with applying the coat on the bearings.
“When we compared costs to a customer for steel bearings versus the ceramic bearings, the cost difference was not significant. But when you consider the greater extension of life, the higher reliability factor, of the hybrids, then overall you would most likely end up with a cheaper bearing.
“And that’s the real benefit of hybrid rollers: they lead to lower overall costs over the life cycle.”
SUSTAINABILITY
SKF is very mindful that climate change caused by human activity presents an existential threat to life on this planet.
Greenhouse gas emissions need to reduce rapidly to net zero, and for this to happen, the global economy needs to transform from being dependent on fossil fuel to being carbon free, circular, and clean.
According to SKF rail national manager Patrick Hofstadler, the largest contribution to this transformation lies in what the company can do with, and for, its customers; they need machines that can run cleaner, longer and more safely.
“By making products lighter, more efficient, longer lasting, and repairable, we’re doing everything we can to help our customers reach significant energy and carbon while supporting their need for growth,” he said.
“We also see it as a moral obligation, not only to our customers and suppliers, but to shareholders, employees and society at large, to help realise this transformation.”
SKF already uses low friction bearing seals to help cut down the frictional moment and wear, thus reduce energy use when hauling railway vehicles.
When compared with previous generation of seals that represent the industry standard, SKF new generation seals reduce bearing friction torque by 13 per cent, representing a significant reduction in friction, leading to lower power losses and fewer resources required to generate the same railway power.
The new seal also reduces temperatures by 5°C compared to the previous seals, enabling about a 17 per cent increase in grease life to extend maintenance intervals and reduce lubrication costs.
From an environmental perspective, the increased seal and grease life increases bearing life and reduces the need to manufacture new bearings, grease and seals. Ultimately, these combined beneits will contribute to a reduction in greenhouse gas emissions and help to conserve the Earth’s resources.
The hybrid bearings are yet another string to SKF’S net zero greenhouse gas bow.
“Although the bearings aren’t very large, there are many ceramic parts and less rolling elements being produced of steel,” Hofstadler said.
“This in effect already reduces emissions by lessening the amount of steel produced at the factories.”
“Reducing friction in the rolling of the bearing also saves energy, which also is a CO2 contributor, so that’s another way it helps.”
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