NES Feature Story Condition Analysis Feature
The Fundamentals of Bearing Wear and Condition Analysis
A customer recently called Napoleon Engineering Services (NES) asking if it could evaluate a bearing which was operating in refrigerant, an apparent ineffective lubrication in conventional terms.
The Olean, New York-based bearing services company understood what was needed:
A condition analysis.
The primary purpose of NES’ condition analysis program is to identify the wear characteristics (and wear rate) of a bearing through both visual and dimensional inspection techniques. In many applications, the lubricant itself, its condition or the application loads and speed create a condition of high-point asperity contact. Bearing manufacturers may optimize the bearing design or incorporate certain surface treatments to reduce wear rate in these applications. But in the end, wear will usually win out and bearing life is compromised. How much so can be determined by a formal condition analysis program.
WEAR, TECHNICALLY, is the loss of material created by the interaction between the internal contacting bodies if operated under marginally lubricated conditions or when bearing kinematics become extreme due to application conditions.
The kinematic conditions within a ball bearing involve the relative motion between the ball and the raceway. Although pure rolling motion is ideal, all ball bearings experience spinning/sliding, gyroscopic motion and a resulting PV (pressure-velocity) within the pressure ellipse – all of which may accentuate bearing wear under certain operational situations. Bearing optimization works to reduce the kinematic effects on wear, but sometimes the lubrication itself may be ineffective at providing adequate component separation and wear rates increase exponentially, shortening the potential service life of the bearing.
“These factors are intended to be controlled by the bearing design,” noted Andrew Hvizdzak, NES’ senior bearing engineer. “We can optimize the bearing, but it might not be given the chance to run that way because of lubrication condition, the temperature of the environment and other variables.
“These can all detract from the system’s ability to absorb that sliding.”
But what do we mean, exactly, by “spinning and sliding?”
SLIDING, SIMPLY, is relative movement over the contact area.
A radially loaded bearing experiences “pure” rolling at two points within the pressure ellipse. The rest of the contact zone experiences sliding or a difference in velocity between the ball and the raceway. Loading and rotational speed will affect the magnitude and location of pure rolling versus sliding in the pressure ellipse. Spinning, meanwhile, is a parasitic motion that occurs when the bearing has a non-zero contact angle. A spin-to-roll ratio is used as a metric to gauge the amount of spinning that occurs in relation to rolling. When the spin-to-roll ratio is high, excessive sliding will occur which will produce additional heat generation due to friction. This could result in the balls slipping as they orbit the raceway. In worst case scenarios, this can lead to wear and early surface-initiated fatigue failure.
Of the pressure-velocity (PV) component, Hvizdzak added: “Pressure is a function of applied load and raceway curvature, and velocity is the function of the sliding. This refers to the sliding velocity, NOT the pure rolling that’s happening. You could have low load (and therefore low pressure) but see high sliding velocity and have a high PV ratio or vice versa. …
“In either case, once you hit a certain threshold, you’ll probably see an acceleration of wear. And when both are up there, you have an even higher probability of wear. And the only way to combat that is through proper lubrication.”
NES possesses the ability, and has extensive experience in identifying those characteristics, tracing causes of bearing failure and establishing if a bearing has remaining useful life after it’s been in operation.
BEARING CONDITION can be evaluated primarily through two different methods.
The first is visual. “We’re not measuring wear depth at this stage,” Chris Napoleon, NES president and chief engineer, said. “We’re looking at what the surface is telling us about the operating environment. It’s a qualitative assessment that shows everything from a clean running surface, to frosting, circumferential banding, fine particle debris denting, foreign particle damage, embedded particles and thermal distress.
“These findings are important to understand what is going on from an operating environment standpoint, but it may not be inclusive of true wear, which is the loss of material.”
The other method is dimensional, which allows NES to complete the full scope of its condition analyses.
“There’s a lot of data that we can use to quantify the level of wear within a ball bearing,” Gus Napoleon, an NES inspection engineer, said. “There are assembly-level inspections of radial clearance, endplay, contact angle and even ring offset for angular contact ball bearings. All of these characteristics are influenced by a loss of material from the raceway ball path or ball diameter when compared as manufactured targets.
“We also perform profilometry inspections of the raceways to determine the location of the ball path, deviation from true form and changes in the surface finish properties. We can quantify the amount of wear resulting from the application or lubrication conditions that the bearing is experiencing and know its impact on overall wear rate for a given amount of time in service.”
There are times, Napoleon said, when NES visually observes evidence of a compromised raceway surface, but their wear measurements indicate very low wear rates. In this case, the bearing is still operating with the same load-carrying capability as originally intended since the raceway curvature isn’t significantly different than its original form. Other times, NES sees heavy banding and large deviation from true form across the raceway due to material loss or wear. In these situations, bearing life will be shortened, and evaluating the wear amount with respect to the time in operation versus the expected service life allows us to provide a good estimate of whether a bearing will meet its intended life.
The combined visual and dimensional assessment in the NES condition analysis program, and the resulting detailed reports, are geared toward providing customers with more accurate information about the condition of their bearing and empirical service life estimate.