Ah, to be able to predict the winning numbers in a lottery – who hasn’t had that dream! Life would be so easy and carefree with a crystal ball. And, would not your maintenance job be so much simpler if you could just identify machine damage in time and predict the point at which repairs really become necessary?

According to DIN 31051, condition-oriented maintenance is not limited to inspection, servicing, determination of the machine condition and maintenance scheduling. It also encompasses predicting the length of service life remaining for machines and systems. This means it is not always necessary to replace a rolling bearing when an inspection measurement reveals inner raceway frequencies.

This article describes how it was possible to keep a damaged bearing running for several months until the scheduled annual outage came around – naturally under close observation. Periodic measurements had shown that there had only been a slight change in the envelope spectrum. But one thing at a time …

In January, a contractor was awarded a contract for the mobile condition measurement of a double-shaft extruder gear. So, before the diagnostic specialist carries out measurements on location, the gear construction data is established by way of the factory number and the bearing frequencies are calculated for the entire drive (Figure 1).

Figure 1

For this purpose, the contractor used a computer program that recognizes almost all gear types, including variable speed drives. Having calculated the excitation frequencies, the diagnostic specialist visits the customer to determine the current condition of the drive – which in this case was already 7 years old – and to find any deviations. Beginning bearing damage was identified during control measurement as a result of the distinct, albeit not yet critical, vibrations of an inner raceway frequency that were picked up in the input shaft area. The gear condition was recorded and the recommendation to procure replacement parts for the intermediate shaft was made to the operator. Repeated monthly measurements were recommended to assess whether the bearing needed to be replaced.

Figure 2 shows two envelope spectra that were recorded three months apart at the same measuring point.

Figure 2. Measured Values Recorded at a Three-month Interval

Because changes were only slight, an immediate replacement of the bearing was not deemed necessary, and the decision was made to continue operation until the scheduled annual outage. The bearing was exchanged five months later.

Figure 3. Dismantling the Intermediate Shaft

Upon dismantling the bearing, the damage diagnosed on the inner raceway could be confirmed as expected – nine months after initial diagnosis. Pittings are clearly visible on the inner raceway.

Figure 4. Inner Raceway with Diagnosed Pitting

The extruder was back in operation two days later as planned.

In hindsight, both the diagnosis and the predicted remaining service time proved to be correct and economically wise. Expensive “crash repair” was unnecessary in this case. And, avoiding downtime was almost like picking a “winning number” in the lottery.

About the author:
Mickey Harp is a vibration application engineer for Ludeca Inc. Ludeca is the exclusive distributor and factory-authorized service and training center for PRUFTECHNIK alignment systems and condition monitoring products in the United States, the Caribbean and Venezuela. It is also the manufacturer and distributor of DotLine Laser and SheaveMaster Pulley alignment tools. The author can be reached at 305-591-8935 or Mickey.Harp@ludeca.com. For additional information, visit www.ludeca.com.