Page 311 - Intro Predictive Maintenance

302 An Introduction to Predictive Maintenance

14.2.1 Bearings: Rolling Element

Bearing defects are one of the most common faults identified by vibration-
monitoring programs. Although bearings do wear out and fail, these defects are
normally symptoms of other problems within the machine-train or process system.
Therefore, extreme care must be exercised to ensure that the real problem is identi-
fied, not just the symptom. In a rolling-element, or anti-friction, bearing vibration
profile, three distinct sets of frequencies can be found: natural, rotational, and defect.

Natural Frequency

Natural frequencies are generated by impacts of the internal parts of a rolling-element
bearing. These impacts are normally the result of slight variations in load and imper-
fections in the machined bearing surfaces. As their name implies, these are natural
frequencies and are present in a new bearing that is in perfect operating condition.

The natural frequencies of rolling-element bearings are normally well above the
maximum frequency range, F MAX , used for routine machine-train monitoring.
As a result, predictive maintenance analysts rarely observe them. Generally, these
frequencies are between 20KHz and 1MHz. Therefore, some vibration-monitoring
programs use special high-frequency or ultrasonic monitoring techniques such as
high-frequency domain (HFD). Note, however, that little is gained from monitoring
natural frequencies. Even in cases of severe bearing damage, these high-frequency
components add little to the analyst’s ability to detect and isolate bad bearings.

Rotational Frequency
Four normal rotational frequencies are associated with rolling-element bearings: fun-
damental train frequency (FTF), ball/roller spin, ball-pass outer-race, and ball-pass
inner-race. The following are definitions of abbreviations that are used in the discus-
sion to follow:

BD = Ball or roller diameter

PD = Pitch diameter
b= Contact angle (for roller = 0)
n = Number of balls or rollers
f r = Relative speed between the inner and outer race (rps)

Fundamental Train Frequency. The bearing cage generates the FTF as it rotates
around the bearing races. The cage properly spaces the balls or rollers within the
bearing races, in effect, by tying the rolling elements together and providing uniform
support. Some friction exists between the rolling elements and the bearing races, even
with perfect lubrication. This friction is transmitted to the cage, which causes it to
rotate around the bearing races.