Bearing Quality, Service Life and Fatigue Life

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These are the questions that a bearing manufacturer needs to address to satisfy customer requirements.

What does a machine designer want?
Very often the machine designer is governed by a customer or a technical society’s specification, that defines a calculated life of the machine being designed. For the bearing in that machine, this specification asks for “L10 life” of so many hours (revolutions), are achieved.

L10 life is a calculation that uses the applied load from the application and another ISO governed value, the Bearing Dynamic Capacity (Cdyn) to determine the number of cycles until the onset of sub-surface fatigue.

The difficulty with using the L10 life and dynamic capacity to determine a service life, is that it does not include important factors that influence service life. For the basic “life” calculation, there is no adjustment for the delivered quality of lubrication and cleanliness of the application. There has been “Adjusted L10” calculations that do address these factors, but despite being around for over 30 years, a basic L10 life calculation is still often asked for in machine design.

In terms of the calculation of Cdyn, there are several manufacturing qualities benefits that are not quantitatively defined, and are not equally expressed in the bearing Cdyn tables published by manufacturers.

Meaning a machine designer or governing body who does not take into consideration an adjusted L10 life, is missing an opportunity for more efficient machine design. Also, selecting a Cdyn only, may exclude manufacturers who have a better product for their application.

What does a machine user want?
Machine users want the longest service life between breakdowns or a longer running time before a planned maintenance shutdown. In some cases, the machine users will ask for a bearing (brand) by name that they have had positive experience with, and have a perception of reliable rotation.

In these cases, they are looking for a long service life. Unfortunately, service life is not expressed by an L10 formula, but by a “modified” L10 formula that considers the lubrication conditions including contamination.

What have some bearing manufactures done to bring this to the attention of the bearing users? They have created marketing tags for their products to indicate that these bearings have improved features. These tags are names such as “Explorer” and “X-Life” and are unique to specific brands.

What are the properties that these tags could define?
Cleaner steel – over the past few decades there has been a reduction of the percentage of oxygen in some manufacturers bearing steel. With less oxygen, there is fewer carbides that build up on the grain boundaries of the steel structure. It is these anomalies that serve as the initiation point for sub-surface fatigue, which is the failure mode that L10 life predicts. However, there is no standard by which steel cleanliness is factored into the Cdyn formula.

Rolling element diameter tolerance – element to element – another feature that extends service life is the tolerance of the actual difference between the diameter of each rolling element relative to each other in a single bearing. When there is differences in rolling element diameter, then the load is not shared evenly. The larger rollers will carry a higher load than the smaller rollers. This leads to a higher stress under the largest roller and it is the magnitude of this load that governs how quickly the fatigue will initiate.

Cage design – the design of the cage can play a number of roles including proper guidance, creating a reservoir for grease, avoidance of lubrication being scraped off the loaded area of the roller, and the ability to accommodate rolling element speed differences in different places in the bearing at any instant.

Some in the industry may argue that there is a performance factor that can be adjusted when publishing a dynamic capacity, and this is true; however, this value does not have a defined method of calculation, and some companies may take liberties at determining this value in their calculation of Cdyn.

Misuse of the quality perception
One misuse of the quality levels of bearings is the marketing of “skate wheel” (roller blade) bearings. Some companies have targeted the aftermarket with “ABEC 5” quality bearings. The problem with the ISO and ABEC tolerance levels is to call the bearing an ABEC 5 or P5 means all dimensions are to that precision level, including the bearing width. In many applications, the precision of the width does nothing, so why make a bearing P5 width tolerance when it does not need it.

As a result, one cannot get “ABEC 5” roller blade bearings from the major manufacturer, as they realize that to call a bearing P5 ISO or ABEC 5, the width tolerance has to be held to a tight tolerance.

What is more important is the running accuracy of the bearing, i.e. the run-out of the ring. What has happened is that skateboard / roller blade makers have tried to capture the market by selling “ABEC 5” bearings as better bearings, but in reality that much accuracy in a skate wheel bearing is wasted. The major manufactures do make this size bearing with tighter tolerances, but only where useful such as the bore, OD and radial runout. They do not hold an “ABEC 5” width tolerance, so they cannot be labelled as such. Also, major manufactures focus on true quality factors, and raceway smoothness, which is not specified with “ABEC 5.”

Not all bearings are made the same. Some meet the most basic ISO or ABEC standard, others exceed this standard with features that are not part of the standard but directly address service life, as opposed to a simple L10 life formula. Some manufacturers will tag their products to show consumers that this is truly a better bearing.

In the strive for better service life, one must go beyond the bearing capacity and L10 life and understand how the added features from premium manufacturers can provide the longest service life of the bearing and the machine that contains it. MRO