The Bear(ing) Minimum

Minimum Loads of Spherical Roller Bearings and Some Lubrication Basics

by Charles Kropewnicki

In my experiences as both a Bearing Applications Engineer and a 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 is when the SRB acts as the fixed bearing and takes 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.  It will also significantly shorten bearing service life since the bearing life is related to lubrication viscosity between the rolling surfaces (elements and raceway) and between the sliding surfaces (cage pockets & roller side faces).  It is also important to note that SRBs 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.  For speeds that are unusually fast or slow due to a gearbox reduction or increase, a more advanced calculation would be recommended.  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 because of its misalignment capabilities and robust design, the minimum load condition is often overlooked, with more attention 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.  However, it doesn’t take into account lubrication viscosity, contamination factors and minimum loading factors, so 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 manufacturers have these factors in their bearing catalogs, or the information 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) 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 with the SRB utilized as the fixed bearing on the blower side, taking both the radial and axial loads.  I do realize, however, that there might be applications requiring a different approach, but this fix could certainly help some of those blower and fan applications in question.

General Maintenance & Lubrication Basics


Commonly made mistakes include not using the most suited lubricant, relubrication intervals and/or quantity required for specific applications.  This is often due to misinformation or simply an attempt to utilize the grease or oil that is currently stocked in the store room for other equipment.  While it is a good practice to consolidate some lubricants to streamline inventory, it is crucial that this is done correctly since it can cause major problems if the improper lubricant finds its way into equipment.  It is important to note, however, that there is some equipment that requires a specialized lubricant with certain properties due to the nature of the application, and these lubricants should not be consolidated.

Particular attention needs to be given to the type of lubricant required for a specific application, even though a stocked lubricant may meet the oil viscosity requirements.  It may have an additive package that was not developed specifically for that application.  For example, I have seen a motor oil utilized in a pump application.  Although it did have the correct base oil viscosity requirements, the additive package was developed primarily for automobile engines. Interestingly enough this oil was recommended from a vender as a substitute for the OEM specified lubricant.  While this lubricant worked, it probably didn’t provide the best properties for the application or provide the optimum service life for the equipment.  Care should be taken, especially during a lubrication consolidation program, to assure that any new lubricant will not only have the proper viscosity requirements, but is the specified lubricant from the OEM manufacturer. 

In applications that utilize grease, attention should be given to thickener types since they are not all compatible.  This is also an important issue to consider in a consolidation process since incompatible greases will result in a mixed lubrication regime, which can dramatically alter grease thickener properties.  This can result in a softer thickener which will lower the grease’s resistance to slumping (the greases ability to stick to a surface and provide lubrication).

For instance, the most common greases found in fan and blower applications utilize a Polyurea, Lithium or Lithium Complex thickener.  Depending on the manufacturer of the grease, these may or may not be compatible.  When in doubt, you can be 100 percent assured of success if you don’t mix them.  When in question, there are grease compatibility charts that can help you determine what thickeners and base oils will work together if cleaning out existing grease from a bearing housing is not an option.

Another common maintenance malpractice is the overgreasing of bearings in any application.  Contrary to certain beliefs, more is not always better!  Using too much grease can drastically increase bearing operating temperatures by pressurizing the housing.  In addition, forcing extra grease into the bearing rolling elements creates added friction/drag and possibly too much oil bleed.  It is no surprise that over greasing can indeed lead to premature failure. 

It is a good practice to ask the equipment manufacturer for the type of grease, quantity and lubrication interval recommended for a particular application.  Guessing should not be an option since it can be based on false assumptions and improper facts.  This simple measure can save thousands of dollars in equipment downtime, repairs, replacement parts and even yearly bonuses.  Relubrication intervals and quantity formulas can also be found in catalogs of some bearing manufacturers.  There are also online programs, like SKF Dialset, that will calculate relubrication intervals and quantity.  You do need specific bearing application information including: speed, load, duty cycle, bearing size/type, environmental conditions and temperature etc.    The correct relubrication intervals are key in flushing out oxidized lubricant, moisture and particle contamination.  This practice is one of the most vital variables in maximizing bearing service life. 

Pillow block housing designs that utilize open labyrinth seals are very popular in blower/fan applications.  However, in extreme dusty/dirty environments they allow for a greater amount of fine particle contamination to get into the housing.  This is especially true when the bearing becomes idle and the block cools down, which induces a negative draft drawing air into the pillow block.   Some of these labyrinth seal designs do, however, incorporate a contact seal as an option up to a certain speed limit.  This will help greatly if the pillow block is outside and subjected to heavy dust/dirt environment, for example, in a cement plant.  Even if the seal face wears down and no longer makes contact, it can still provide an added barrier to hold grease and help block out contaminants.

Grease serves multiple functions:

1)  It helps seal out contaminants 
2)  It helps to transfer heat from the bearing
3)  It provides lubrication by bleeding oil
4)  It isolates water from the metal surfaces
5)  It provides antioxidants to fight oxidation
     at higher temperatures
6)  “Extreme-Pressure” EP additives
     chemically bond themselves to the metal
     surfaces to avoid metal-to-metal contact,
     especially when shaft speed is slow or
     idle and has no hydrodynamic film.

Grease is made up of three basic ingredients:  thickener, base oil and additives. Think of the thickener as a sponge.  While a sponge absorbs and holds water, the thickener holds oil and clings to bearing components while bleeding small quantities of oil.  The bleed rate increases with temperature.  As the grease ages and goes through multiple temperature cycles, it will eventually bleed itself out of oil and become a hard, stiffened thickener.  If left unattended, the grease will no longer provide lubrication, which is one reason why relubrication intervals, and flushing out contamination and spent oxidized lube, are so important. 

In actuality, most bearings don’t need very much lubricant, a thin layer of Elasto-Hydrodynamic oil film during operation can be approximately 200 times thinner than a sheet of paper and yet provide an adequate film to keep the contact surfaces separated.  Hydrodynamic lubrication is a term used to describe the lubricant’s ability to increase film thickness and pressure with increasing speed of the rolling element.  This is similar to a car tire hydroplaning on water and lifting away from the road surface.

This behavior also explains how different lubricants, like mineral-based paraffinic oils and synthetic oils, have different pressure coefficients and film thickness variations with respect to speed.  For example, synthetic and ester-based lubricants don’t tend to build up as much pressure and film thickness with increasing speeds as do mineral oils.  For these reasons synthetic oils have been known to conserve energy, lower friction and temperature and are also used in high-speed bearing  applications.

Overfilling of Oil Housings


When equipment utilizes oil as the lubricant, beware of overfilling.  This can be as dangerous as too little lubrication since it will churn excess oil between the bearing rolling elements and raceways.  When the oil film builds up, it creates a high-pressure zone between the rolling element and raceway which adds friction and shear within the lubricant.  This will elevate bearing temperature, lower oil viscosity and can result in bearing failure. 

When installing a pillow block, it can be hard to establish a proper oil level.  You may not be able to clearly determine if the oil level is at the middle of the lowest roller, which is the proper setting, in certain pillow block housings.  I observed this firsthand when installing a new pillow block in a blower application.  Although the bearing catalog provided an oil level dimension from the base, it was still somewhat difficult to set the automatic oil leveler with great accuracy.  It is also important to note that the volume of oil in this housing was relatively small and provided little margin for oil level deviation.  In an application such as this, there is little or almost no room for error, which makes it risky and unrealistic from a maintenance standpoint. 

If conditions permit, utilizing grease in an application like this can help make life simpler, as well as provide better protection from outside contamination, especially with open labyrinth seal designs.  The grease will also act as a heat sink to help dissipate heat through the pillow block.  In fact, after utilizing grease in this blower application, we observed lower temperatures and have had no other issues. 

If you ever experience hot running pillow blocks and are unsuccessful in lowering pillow block temperatures despite utilizing different greases or oils, there is a possible solution.  There is a type of pillow block housing that utilizes a slinger ring just like the ones used in most API pumps and smaller steam turbines.  Utilizing a slinger ring type pillow block bearing housing will help bring down the bearing operating temperature.  This is due to a thinner oil film created from oil splashing, which minimizes oil churning and added friction.  Some of these pillow block housings also have an option to use internal water cooling jackets for added cooling capacity.  If the budget permits, oil mist lubrication has proved itself as one of the most reliable means of lubrication in the industry, since it provides a constant source of fresh uncontaminated oil and maintains a light film that promotes cooler operation and added bearing life.

Each specific application will dictate which type of solution is most appropriate, since it is based on operating speeds, loads, temperature etc.  However engineers involved with the designing or maintaining of equipment need to familiarize themselves with basic lubrication theory since it explains how these lubricants actually work and provides more insight into their limitations.  Also knowing the basic characteristics of both mineral and synthetic oils will help determine which is best suited for the application, since they both have limitations and there is no such thing as a “one for all” solution like some individuals will try to sell you.

After graduating with an Associates Degree in Applied Science from Camden County College in 1989, Charles Kropewnicki began working as an AutoCAD Draftsman and Designer.   He decided to go back to college full-time and graduated from Widener University with a BSME in 1998.   After graduation, he utilized his CAD skills and worked as a Designer/Engineer with Pro-Engineer 3-D CAD.   After a couple years, he accepted a position with SKF as an Application Engineer.  At SKF, he assisted customers by giving bearing presentations and wrote bearing failure analysis reports with recommendations; this provided insight to causes of shortened bearing life.  Charles also received training from SKF College in Lubrication Tribology, which provided a theoretical basis for grease and oil behavior within a bearing.  In looking to expand his knowledge regarding rotating equipment, he accepted a position with Western Refining in Gallop, NM and is currently working at Western as a Project Engineer.  Charles can be reached by phone at 610-316-9841 or via e-mail at ukpcak1@yahoo.com