Coatings optimise bearing performance
The constant requirement to improve the performance and life expectancy of bearings means that this is an area in which innovation is frequent.
However, the fundamental shape and dimensions of bearings do not alter often – not least because of the need to ensure that they are standardised. For this reason, the tendency is for innovation to take place either in the materials from which the bearings are made or – more commonly – in their coatings.
Quite how significant the difference that can be made to bearing performance by coatings can be is amply exemplified by the way in which NSK's L-PPS (linear polyphenylene sulphide) material recently helped to solve the problem of heat-induced bearing cage failures in a screw compressor.
The compressors were receiving frequent complaints from customers about failures resulting from damage to the bearings. On closer inspection and analysis, the bearings showed the failures were caused during continuous 'hot' operation above 70°C.
NSK found the bearing cages were manufactured either from brass or polyamide and it was these materials that were failing prematurely. The brass cages exhibited stress corrosion cracking, while the operation of the polyamide cages were impaired by lubricant at high operating temperatures.
After examining the failed bearings, NSK recommended the use of bearings with cages made from L-PPS material. L-PPS was developed by NSK using a special manufacturing process and it has enabled bearings equipped with L-PPS cages not only to offer greater performance and life, but to provide these benefits at a competitive price to both brass and PEEK.
The problem experienced is not uncommon for screw compressors, which are facing an ever increasing demand from customers for trouble-free, longer life with fewer overhauls and maintenance. In addition to the greater load-carrying capacity and improved lubrication capability demanded from bearings, other key requirements are thermal and chemical stability.
L-PPS is a glass fibre reinforced plastic designed for high temperature use, at up to 190°C. The material also has a high level of chemical resistance and thermal stability. Bearings equipped with L-PPS cages also offer greater load carrying capacity and have much improved lubrication capability.
Although initially developed for compressor bearing applications, the advantages of bearings employing L-PPS plastic cages are applicable in other applications where high temperature and/or special conditions exist.
An instance of the efficacy of bearing coatings in corrosive environments can be seen in NKE's new SQ171E coating. This is even thinner and provides longer-lasting protection against corrosion than the previous version. The coating protects bearings and components in applications including material handling, agricultural machinery, chemical and pharmaceutical industries as well as compressors and pumps.
The SQ171E coating can be used for standard or special bearings as well as for all metal parts that are exposed to wet or corrosive environments. Indeed, it can even be used machined surfaces. The coating provides protection against water, condensation and slightly alkaline or acidic cleaning agents. Compared to uncoated components, parts coated with SQ171E have a significantly longer service life.
As an additional option for even more effective protection, the coating is also available with a silicate-based sealing layer. Due to the reduced thickness of the coating (from 2 to 4µm) coated and uncoated parts are completely interchangeable. In accordance with DIN 58979:2008, the coating, at a thickness of 4µm, reaches service life of at least 96 hours without coating corrosion (white rust) or base material corrosion (red rust).
The SQ171E coating has been tested with the neutral salt spray test method in accordance with DIN EN ISO 9227 NSS. The passivation does not contain hexavalent chrome Cr (VI) and is therefore compliant with the RoHS directive. The company also claims that, compared to stainless steel, the SQ171E coating is more cost-effective, yet offers better anti-corrosion protection.
In automotive applications, coatings can also have a massive impact. This is particularly the case as the drive for increased engine efficiency places demands on crankshaft bearings that require new designs and materials applications. The lubrication conditions at start up are very different from those that exist during high-speed, high-load operation. While solid lubricants or dry bearing materials are effective at preventing metal-to-metal contact at low running speeds, these conventional solutions are not suited to higher speeds, which require journal bearings with a generous lubricant supply.
To this end, Federal-Mogul has expanded the performance capabilities of engine bearings by developing an innovative polymer-coated bearing shell that can reduce fuel consumption and CO2 emissions by withstanding mechanical loads produced by heavily boosted engines. Called IROX, the new technology addresses the lubrication challenges associated with frequent engine restarts found in hybrid and other future stop-start engines by protecting both the crankshaft and the bearing shells from damage where metal-to-metal contact would otherwise occur. It is estimated that the IROX bearing overlay can help increase the life of crankshafts and bearing shells by more than five times in more extreme applications, such as direct-injected engines and engines with stop-start systems.
Federal-Mogul's new system combines the best features of both these established technologies by introducing a polymer coating for traditional metallic bearing shells, integrated with solid lubricants and wear inhibitors to produce a cost-effective, robust and production-ready solution. Extensive development has led to the identification and optimisation of a number of key parameters, including layer thicknesses, substrate material specification, resin binder properties, curing conditions and functional additive specifications, and a number of patents on the technology.
The IROX bearings have an overlay that is a PolyAmideImide polymer resin binder containing a number of additives dispersed throughout the matrix. These additives provide a variety of properties to the finished coating, such as wear resistance, mechanical strength, thermal conductivity and in their ability to safely envelop loose abrasive particles).
Test results have shown a dramatic improvement in life compared to both conventional shell materials and state-of-the-art competitors. Typical bearings with aluminium overlays show significant wear after 100,000 stop-start cycles. However, the new generation of engine systems requires 250,000 to 300,000 cycles. However, claims Federal Mogul, in comparison tests where conventional aluminium overlays showed 100 microns of wear and lead-free bronze showed up to 50 microns, those with the IROX overlay still looked like new with a measurable wear of just a few microns.
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