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!


Upcoming Events

August 9 - August 11 2022

MaximoWorld 2022

View all Events
banner
80% of Reliabilityweb.com newsletter subscribers report finding something used to improve their jobs on a regular basis.
Subscribers get exclusive content. Just released...MRO Best Practices Special Report - a $399 value!
DOWNLOAD NOW
Conducting Asset Criticality Assessment for Better Maintenance Strategy and Techniques

Conducting an asset criticality assessment (ACA) is the first step in maintaining the assets properly. This article addresses the best maintenance strategy for assets by using ACA techniques.

Harmonizing PMs

Maintenance reliability is, of course, an essential part of any successful business that wants to remain successful. It includes the three PMs: predictive, preventive and proactive maintenance.

How an Edge IoT Platform Increases Efficiency, Availability and Productivity

Within four years, more than 30 per cent of businesses and organizations will include edge computing in their cloud deployments to address bandwidth bottlenecks, reduce latency, and process data for decision support in real-time.

MaximoWorld 2022

The world's largest conference for IBM Maximo users, IBM Executives, IBM Maximo Partners and Services with Uptime Elements Reliability Framework and Asset Management System is being held Aug 8-11, 2022

6 Signs Your Maintenance Team Needs to Improve Its Safety Culture

When it comes to people and safety in industrial plants, maintenance teams are the ones who are most often in the line of fire and at risk for injury or death.

Making Asset Management Decisions: Caught Between the Push and the Pull

Most senior executives spend years climbing through the operational ranks. In the operational ranks, many transactional decisions are required each day.

Assume the Decision Maker Is Not Stupid to Make Your Communication More Powerful

Many make allowances for decision makers, saying some are “faking it until they make it.” However, this is the wrong default position to take when communicating with decision makers.

Ultrasound for Condition Monitoring and Acoustic Lubrication for Condition-Based Maintenance

With all the hype about acoustic lubrication instruments, you would think these instruments, once turned on, would do the job for you. Far from it!

Maintenance Costs as a Percent of Asset Replacement Value: A Useful Measure?

Someone recently asked for a benchmark for maintenance costs (MC) as a percent of asset replacement value (ARV) for chemical plants, or MC/ARV%.