Considerations for Sizing SAF Pillow Block Bearings

Do you need help with spherical bearing sizing for your SAF application? Selecting a properly sized SAF pillow block ensures that the bearing meets its expected service life and keeps your machinery running smoothly. Additionally, knowing what size to select can keep you from over-designing your drive and spending money on unnecessarily-large bearings and housings.

There are three main, intertwining considerations to take into account when selecting a SAF block:

  • Desired bearing life – how long do you want the bearing to last between replacements
  • Imposed load – radial, thrust, shock and other load information
  • RPM of the bearing – how fast the bearing is going to turn

Factoring these three considerations into your decision-making process will guide you to a properly sized SAF pillow block.

Desired Bearing Life

Knowing the life expectancy of a bearing is helpful when selecting a new one for an application. The standard industry measurement is L10 bearing life. This is the number of hours that a group of bearings can run before 10% of them experience failure. About 90% of bearings will outlive their L10 calculations when operating under ideal conditions. The L10 life is commonly considered the minimum amount of hours you can reasonably expect your bearing to operate.

Use the equation below to calculate the L10 bearing life of spherical roller bearings.

A mathematical formula to discern L10 life based on the roller bearing's dynamic capacity, imposed radial load, and speed in RPMs.

To get an idea of how long your bearing will last, you can calculate your own L10 bearing life as long as you know these three things:

  • Bearing dynamic capacity (which can be found in the manufacturer’s catalog or spec sheets) 
  • Imposed radial load
  • Operating speed (RPM) 

A bearing has a set dynamic capacity. In general, larger bearings have larger dynamic capacities. 

You can increase L10 hours by:

  • Lowering imposed load
  • Lowering operating speed
  • Selecting a bearing with a higher dynamic load capacity

The L10 equation above will allow you to determine the life of a bearing in your specific application. However, narrowing down your options may mean a lot of back-and-forth to do calculations.

Cr/P Factor Chart For Spherical Bearing Selection:

If you’re far enough along in your design process to know the load that will be imposed on your spherical bearing, then the Cr/P factor chart (below) can save a lot of time in the selection process. We’ve designed the chart to allow engineers the ability to skip the L10 calculations for common machinery and speeds. This chart will help you determine the dynamic load capacity required of your bearing selection in order to achieve the service life desired.

A chart that shows recommended amounts of service life on the left and RPMs across the top. Inside the chart are the factors to be multiplied by the imposed load on the bearing.

To use the chart, the Cr/P factor should be multiplied by the imposed load on your bearing. The resulting number is the dynamic load capacity required of your bearing selection.

Imposed Loads

Taking imposed loads into consideration when choosing a SAF pillow block bearing is a crucial step in the decision-making process. It is important to select a bearing size that is capable of enduring the typical loads of your application’s operation to avoid premature bearing failure. 

The L10 life calculation specifically takes radial loads operating in ideal conditions into consideration. Since thrust loads and shock loads often reduce the amount of service hours of a bearing, it is recommended that you add a precautionary “safety factor.” 

A safety factor is common for applications subjected to frequent shock loads or other harsh conditions. L10 life calculations are based on optimal operating conditions. If the bearing is subjected to frequent or extreme conditions, a safety factor can be designed into the calculation. Consider increasing the load variable in the calculation if a bearing is going to be subjected to harsh conditions. 

Applications operating in only slightly less than ideal conditions such as rare shock loads or slight operational vibration, may benefit from a 10% safety factor addition to the equation. Using an example: Adding a 10% safety factor to the imposed load of 3,000 lbs would increase the radial load variable of the equation to 3,300 lbs.

Certain types of bearings have higher load ratings. Spherical bearings, for example, have significantly higher load ratings than ball bearings. Because of this, it is important to ensure all other components, such as shafts and mounting surfaces are capable of withstanding imposed loads when sizing spherical roller bearings and bearing blocks.

Please note that spherical bearings typically require radial loads that meet or exceed axial loads to operate properly. A spherical SAF block is typically not compatible with your application if your application has more thrust load than radial load.

Operating Speed

The final sizing consideration to factor into your decision-making process is operating speeds and compatibility with other components of your application.

The desired operating speed (measured in RPM) of the bearing can affect size selection. Speed capacity in a bearing is a fine balance of the construction and size of a bearing vs the imposed load. For example – SAF bearing blocks with spherical bearings are notorious for lower speed capacities so be sure to check their maximum RPM ratings in manufacturer catalogs.

Exceeding speed limitations may cause problems that reduce bearing life expectancy and reliability.

Final Thoughts and Questions to Guide You in the Right Direction 

It is important to take each of the factors we have discussed into consideration when choosing a SAF pillow block bearing. We advise you to ask the following questions during your decision-making process:

  • Will the bearing do the job? (catalog speed and load Capacities)
  • Will the bearing last as long as I want? (L10 or Cr/P Factor Calculations)
  • Will the shafts and other attached equipment do the job? (Check capacities of all components).

Following this guide and asking these questions will help you select the right bearing size and get maximum life in your application.

What Do You Need in an SAF Housing?

Read our article about housing considerations, options, and features.

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