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Beware of Minimum Load Specifications in Spherical Roller Bearings

In my experiences as both a Bearing Applications Engineer and Reliability Engineer there seems to be a common problem with double row “spherical roller bearings” (SRB) operating well below their minimum load ratings.  This is especially prevalent in overhung fan and blower designs utilizing couplings to transfer power.  This issue will usually raise its ugly head in the form of elevated temperature on the coupling side pillow block bearing, in some cases due to the overhung load distribution the coupling side bearing will have a load facing upward or even none at all, the worst case being the SRB being the fixed bearing and taking a thrust load despite having little or no radial load.

This condition will cause the rollers to slide rather than roll over the lubricant increasing shear forces within the lubricant and increasing friction/drag within the cage pockets, this will elevate temperatures and decrease lubrication viscosity.  This will significantly shorten bearing service life since the bearing life is related to lubrication viscosity between the rolling surfaces: “elements & raceway” and between the sliding surfaces: “cage pockets & roller side faces”.  It is also important to note that SRB’s typically need more radial load than thrust load at fan and blower operating speeds since the set of rollers “opposite of thrust load”  can become unloaded and will drag within the cage pockets, this can result in rapid cage failure of the unloaded side roller set.

As a rule of thumb that has stood the test of time in typical operating conditions, an SRB should have approximately 21cSt of lubrication viscosity at operating temperature, this can be determined from a viscosity/temperature chart once you have obtained the data and a more advanced calculation has not been performed.  For speeds that are unusually fast or slow due to a gearbox reduction or increase, a more advanced calculation would be recommended since speed is the key variable in this calculation and can change viscosity and lubrication requirements dramatically at both sides of the spectrum.

Since the SRB is widely used in fans and blowers do to its misalignment capabilities and robust design, the minimum load condition is often overlooked since more attention is usually focused on max loading conditions.  Some primitive bearing calculator tools utilizing the basic L10h bearing life formula may give millions of hours of L10h life for light loads since this basic calculation is only based on fatigue, it doesn’t take into account lubrication viscosity, contamination factors and minimum loading factors, designers beware!  More advanced online bearing programs utilize newer L10 life formulas that will take these factors into account or better yet, an Applications Engineer at a bearing company may provide this service.  If you have any questions concerning minimum loads on a specific bearing, most major bearing manufactures have these factors in their bearing catalogs and it can also be found in some online bearing programs.

If the SRB doesn’t cut it due to minimum load, there is a solution, a “double row self aligning ball bearing” (SABB) which can be used and has the same external dimensions as an SRB up to a certain size range.  This bearing has a much lower minimum load rating and in most cases will run cool and work well, however be careful not to use this bearing as the fixed bearing in this application since these bearings don’t typically do well with thrust loads unless it is relatively light in comparison to the radial load.  They should mostly be used as the floating bearing in this fan/blower scenario and utilizing the SRB as the fixed bearing on the blower side taking both the radial and axial loads.  I do realize however there might be applications that require a different approach, but this fix could certainly help some of those blower & fan applications in question.

Reader tip provided by Charles Kropewnicki
Project/Reliability Engineer
Western Refining
Gallup, New Mexico

Thank you Charles - a Reliabilityweb.com coffee mug is on its way!

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