Benefits of roller bearings - Technical Topics - V11LeMans.com

Rollers are heavy, noisy, bulky, expensive, weaker and less stiff then plain bearings. Plain bearing are light, quiet, compact, cheap, strong and stiff.

The myth that roller bearings have less friction comes from people spinning an engine by hand and seeing how much more easily the roller bearing engine spins. What these people are missing is that when the oil system is supplying the high pressure oil that allows the two bearing surfaces of plain bearings to "float" over each other without contact the plain bearing engines spins just as easily as a roller engine.

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Rolling element bearings are less, not more, sensitive to particulates in the oil. Rolling element bearings do need less oil pressure. The ability to live with low oil pressure and tolerance for "dirt" is why they were used for decades in motorcycles after they stopped being used in cars. The oiling system in bikes were poor and often unfiltered so the rollers gave better service. The first Japanese four-strokes and Ducati twins started with roller bearings and slowly converted to plain bearings over time. The same pattern happened on race bikes so the change was not entirely due to noise or cost reduction.

The differences between two types of bearings are well known to any mechanical engineer, nothing above is controversial or new information. Rather than get into a discussion with anyone who believes the myths about low friction from rollers I'd ask them to explain why every modern engine uses plain bearings for main, rod and cam bearings. This runs from the cheapest Toyota to the engine in every MotoGP bike and Formula One car. Roller bearings are limited to places where oil flow is poor but low friction is required.

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When you find an engine with roller bearings there is a reason. My Harley has rollers on the crankshaft and rods because weird pressed together "knife and fork" crankshaft design requires them; two strokes can't provide oil to the bearings; a few singles use pressed together crankshafts that require roller bearings, other bikes have poor lubrication (sounds like the Hyland may be one) and need rollers for survival.

Lex

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Hello

We are in the cement manufacturing industry and we have a primary air fan that operates at rpm and over the years we were using self-aligning ball bearing C3 as specified and provided by the equipment manufacturer. Until recently we have been changing the bearings at a frequency of once every two weeks. On mounting we check the fan for balancing and it is within specifications. On initial start up and after a week of running both the fan and motor bearing vibrations are within specifications but the bearing condition of the fan is out of spec. Thus after the fan runs for two weeks the fan starts vibrating and the bearings eventually seize. The lubrication that we use is grease (Shell Alvania R3) as specified by the equipment manufacturer. My question is what could be the cause of this? Is it also possible to substitute the bearing on the impeller side with a cylindrical roller bearing of the NU or N type, whilst keeping the self aligning ball bearing on the motor side of the fan because I think we are easily exceeding the load limit of the self aligning ball bearings. The bearings are mounted in SNR316 plummer block housing.

rpm*24*7*2= 1 million cycles. That is way less than any high speed life I have seen for a bearing. Is it underspecified?

Replacing an SA bearing by a roller bearing may work, unless you are seeing significant misalignment or shaft bending.

bearing life calcs are fairly easy, why not get your bearing supplier to look at the design - you must be one of their best customers!



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips. That speed is too fast for grease lubrication. NTN shows a limiting speed of RPM for grease, and RPM for oil lubrication. You may be able to rig the bearing housing to a recirculating oil system. You could also try a , although the limiting speeds are even lower, it may stand up to the vibration better, maybe even try a shaker screen bearing.

Russell Giuliano
The life of the product is about 60,000,000 cycles which would give a C/P allowed of 3.91 (3.91^3=60). Thus the equivalent load on the bearing should be around 34.5 kN ( Lb). If the load on the bearing is in line with the above, you have an application that is designed poorly (the bearing appears to be much smaller than required). If your bearing loading is not close to the load above, then you must look at other possible sources of failure.



I would look to see if anything has changed since the failures started, is the fan running faster or hotter? Is the shaft aligned properly? Are the bearings installed in the pillow blocks properly? Are the housings being overfilled with grease preventing the bearings from expelling the excess?

I would not replace with a cylindrical bearing without checking with a bearing applications engineer. The self aligning bearing might have been installed for misalignment purposes and the cylindrical bearings are less able to tolerate this. A spherical roller bearing might be a better choice, your bearing supplier is best able to help you with this. We machined a new shaft for the fan when we noticed that the shaft had bent due to vibration. The bearing will fail such that the clearances will be greater than the recommended. The grease application is to recommendations and there are no possible sources of contamination due to cement as the bearings are covered. The fan is a centrifugal type. Recently when we tested for vibrations the bearing vibrations where in spec, but the bearing conditions where out of specification, what possible solution can we have? I'd get the bearing application or fan manufacturer on site to change bearings with me. I'd write a PO to get them there if necessary. In the mean time I'd order the largest diameter standard sheaves/pulleys I could get to maintain the ratio I need, with the fewest belts allowed for the power required. I'd order a belt tension gage at the same time, and call the belt manufacturer's engineering department to provide a calculated tension specification. Until the belts arrived I'd loosen the existing belts until they just stop squealing at speed. At 9 am each morning I'd s-l-o-w-l-y apply 1 stroke of Mobil SHC 100 grease to each bearing from a dedicated standard grease gun after wiping off the grease fitting.

When the new large 1 or 2 groove sheaves and narrow section belts arrived I'd get the belt supplier to install the belts with me. If the belts did not squeal on start up I'd loosen them a little, unitl they just stop squealing at speed.