A step-change in bearing seal design
2017 January 31, 09:00 GMT
Though poor or inadequate lubrication is the prime cause of bearing failure, ingress of contaminants and their detrimental effects on the bearing surfaces comes a pretty close second. In this article, Paul Dysiewicz, Engineering Manager at SKF, describes how an innovative new bearing seal development is poised to tackle this age-old problem.
In 1919, some two years after joining SKF, the talented Swedish engineer, Arvid Palmgren proposed a design for the first self-aligning spherical roller bearing. But it wasn’t until the 1990s that SKF first introduced a sealed version of this novel and versatile bearing design. High load carrying capabilities, tolerance of misalignment and the ability to function in adverse environmental conditions have made the sealed spherical roller bearing a popular choice, and there are now more than 100 standard products in SKF’s portfolio, serving applications ranging from elevators and cranes to conveyors and materials processing equipment.
While, over the years, there has been a steady programme of improvement, as far as the bearing seals are concerned, this has introduced incremental rather than ‘step changes’ in seal performance – until now, that is. The culmination of an intensive engineering programme by SKF is now seeing the roll-out of an innovative new seal design that will eventually become a single standard throughout most of the company’s small spherical roller bearing size ranges.
The performance statistics are impressive; the new design achieves nearly 50 per cent reduction in seal friction and as much as 20 per cent lower total bearing friction – all of which helps to reduce the bearing operating temperature by as much as 20 °C. Bearings incorporating the new seals achieve better lubricant retention (such that they can be considered re-lubrication free); indeed, more applications can be considered as re-lubrication free thanks to the reduced operating temperature. Moreover, they can operate at approximately twice the limiting speed rating of previous sealed SKF bearings of the same size. So, how has all this been achieved?
On the materials front, the seals now come in a choice of two different rubber compounds: standard nitrile butadiene rubber (NBR) or hydrogenated nitrile butadiene rubber (HNBR). NBR - a copolymer manufactured from acrylonitrile and butadiene - has very good engineering properties and, as a general-purpose sealing lip material, it provides good resistance to most mineral oils and greases with a mineral oil base, fuels such as petrol, diesel and light heating oils, animal and vegetable oils and fats, and hot water. HNBR - a saturated copolymer of acrylonitrile and butadiene - is suitable for applications involving contact with aliphatic hydrocarbons and mineral oils. While NBR is the standard material of choice, if higher operating temperatures are expected, then HNBR should be considered.
Beyond these material choices, the major contributing factors to better seal performance include three novel design improvements: a new seal groove configuration in the outer ring, which improves seal anchorage; a new rubberised seal face that is more resistant to corrosion and scratches, and a new seal lip with an optimised contact pressure that helps reduce the seal frictional moment. SKF engineers used powerful finite element analysis software, SKF Seal Designer, to assist their design efforts.
The reduction in frictional moment enables higher speeds without damaging the seal lip. For most of the bearings fitted with the new seal, this means approximately twice the previous speed limit. However, when selecting sealed spherical roller bearings operating at high speeds, it is important to ensure that the operating temperature does not exceed the maximum recommended temperature for the seal material (90 °C for NBR and 150 °C for HNBR).
The reduction in operating temperature of up to 20 °C arising from the bearing seal’s lower frictional moment has a positive impact on the bearing lubricant, prolonging its life, and thus supporting longer re-lubrication intervals. In fact, a reduction of the operating temperature by just 15 °C doubles the re-lubrication interval.
SKF conducted a rigorous testing regime to confirm the performance of its new seal design. This included measuring the seal frictional moment at different speeds for bearings of three different sizes, which revealed an average seal friction reduction of approximately 50 per cent. Meanwhile, fine Arizona sand was used to test the seal’s ability to block ingress of contaminants. Performed in a dust chamber, this test measured the amount of dust that entered the bearing during the trial, and revealed the new seal lip to be at least as efficient as the current design, or even better at sealing off dry contaminants. The three tests conducted to verify correct anchorage of the seal in the outer ring indicated an improvement over existing seal designs.
Sealed spherical roller bearings are supplied lubricated and ready to mount. As rubber seals require lubrication, this is provided by the grease contained within the bearing and thus a small leakage of lubricant is to be expected. Factors that influence the grease leakage rate are speed, temperature, misalignment and mounting direction, so quantifying it in advance is difficult. The new seal was tested under conditions typically found in elevator installations, and the grease retention capability was found to be the same as that of the current design.
SKF’s new seal design now opens up the possibility of more applications benefiting from sealed bearings, thanks to the higher speed limit and the longer grease life. This means that remotely located or difficult-to-access installations become excellent candidates for the new sealed spherical roller bearings as operator maintenance costs are substantially reduced.
The above technical article was originally published in the January 2017 edition of SHD Logistics magazine.