By Frank Klein, Senior Engineer, Rexnord Corporation
The following is a summary of a white paper that reviews different methods for calculating bearing life and their implications, the validity of traditional approaches for calculating bearing lives and provides a general overview of bearing life expectations.
Bearing specifications are written by companies for the purpose of defining anticipated machinery performance.
The traditional approach in writing such specifications for anti-friction bearings has been to request “calculated bearing life” for an L-10 life of 50,000 or 100,000 hours, as examples, based on a five- or ten-year operating life before “major repairs” are required. The L-10 bearing life is naturally understood to be a bearing’s defined life expectancy based on a statistical 90 percent reliability.
To achieve such values, calculation methods have become more and more sophisticated. Traditionally, calculating L-10 was a matter of knowing the speed of the bearing (cycles), taking the equivalent load on the bearing and comparing it to the bearing’s dynamic capacity.
Today it is common practice to calculate an adjusted L-10, which takes the traditionally derived results and allows for the influence of various operating parameters.
With the base L-10 calculation, everyone gets the same answer – which is not the case with adjusted L-10 calculations.
The predicament for the specification writer therefore becomes twofold:
Predicament #1: Given that an L-10 values has to be specified, what is a reasonable value?
Predicament #2: Given that adjusted L-10 calculations can sometimes appear to be a function of the wizard behind the curtain, how can one judge the validity of the lives calculated?
Click here for a detailed white paper that addresses the questions in this blog post.
The fact is that simply specifying an L-10 value may not achieve the intended purpose of machine life expectations.
How does one judge which adjusted L-10 calculation is closest to the intent of the specification?
- Ensure that the L-10 requirement is consistent with the rest of the specification. Calling for a gearbox 1.5 service factor, for example, but a minimum 100,000 hour base L-10 means the selection is driven by the L-10 requirement. Unless the item is a specialty design, the features are designed for economic competitiveness, and the bearing life comes along for the ride.
- Make sure the load base is clear by having everyone calculate base L-10 on motor and Normal Running Load (NRL). Consider that the catalogued design non-adjusted L-10 life of most industrial gearboxes is less than 5,000 hours based on a mechanical SF of 1.0. For example, getting 100,000 hours requires an impossible adjustment factor in relation to the 1.5 SF on the gearing.
- Apply limits that can be used for the various factors, as that creates a level playing field for everyone responding to the RFQ. For examples for lubrication (initial oil viscosity and nominal and nominal sump temperature) allows a maximum of, say, 2.5.
- Use a factor of 1.0 for oil cleanliness, as this takes initial and operating cleanliness into account and does not get into discussions of “initial start-up damage” or “filtration healing.”
- Allow a load zone factor only if detailed Finite Element Analysis (FEAs) were performed with conclusive deflection results — especially for prescribed variable loading conditions (histograms). Such an approach will be expensive as the supplier may be exposing proprietary design details. However, interpolations or extrapolations of a general design method would not qualify, as their validity could not be confirmed.
- Make sure that there are features, i.e. physical means, of getting to the assumptions that make sense.
When specifying anti-friction bearing life for any piece of equipment, the writer of the specification assumes a purpose (or purposes) for targeting a particular value. Once a value has been defined, it is of the utmost importance that the specification writer defines the loads, operating conditions, methods of calculations, and even values for particular factors in the calculations, in order to help assure accuracy and integrity of the claimed values that will be offered.
Additionally — and this is the critical overall systems-purpose for the specification — with the interpretive ability to understand what calculated bearing lives can and cannot guarantee, the author of the specification can outline a clear road map for associated functions, such as initial design, manufacturing, maintenance, and warranty, to meet the high expectations of specifying a particular L-10 life for the equipment.
Frank Klein originally delivered a presentation on this topic at the 2014 Society for Mining, Metallurgy & Exploration (SME) Annual Meeting & Exhibit for professionals.