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PdM TIPS


Lubrication, Condition Monitoring, and Diagnostics of Conveyor Bearings


ALAN F. GAMBSKY, PdM SUPERVISOR, AARROWCAST INC., SHAWANO, WISCONSIN


Tis is the first of a column series focusing on predictive maintenance in metalcast- ing facilities.


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ltrasound may be used to de- termine when and how much a bearing needs to be lubri-


cated. Tis technology can also give the earliest possible indication of when a bearing has entered an irreversible failure mode. To illustrate this, con- sider the comparison of two identical bearings in a conveyor belt application, on opposite sides of a tail pulley. Each “bin” in Fig. 1 peaks at 10.84


Hz, which is the ball spin frequency of the bearing with 30 roller elements (15 on each side of a double row arrange- ment) running at 87 rpm. Temperature is 79F (26C). Te rms dB value is 11. Fig. 2 shows the same type of bearing on the other side of the tail pulley. Temperature is 70F (21C). Notice the peak values are similar, but the “haystack” (width of the combined bins) is narrower. Te rms dB level is 3. Ultrasound is used at Aarrowcast


Inc. to monitor the bearing condi- tion to make sure the proper amount of lubricant is being delivered to the bearing. Ultrasonic noise analysis also can be used to determine if the bearing is entering a fault condition in stage 1 (lubrication deficiency) or later stages, when the application of lubricant no longer restores the noise level to base- line (new) levels. A rise of 6-8 dB is an indication a bearing is in need of lubrication. How


well does the bearing respond to the application of the lubricant? When the noise level is reduced back to base- line and it stays there, then normal operation has been re-established. If the noise and temperature level rise as soon as more grease is applied, then the bearing is over lubricated. If the bearing noise level is reduced


by the application of grease, but returns to the warning or alarm level, then the bearing is permanently damaged and no additional amount of grease will prevent the failure progression. If this is detected, ultrasound can help determine what stage of failure and possibly what is failing inside the bear- ing. Te dominant frequencies of the bearing noise can be analyzed using the diagnostic tools provided by the data collector. Te peak value of the noise levels divided by the rms value can be used to indicate the severity of the damage in the crest value. Some have used this to assign some level of criticality or urgency, where the higher the crest value (cF), the more damage the bearing has sustained. As the bearing progresses from ear-


ly failure (stage 2) to more advanced stages, a dominant noise frequency may show in the noise frequency spec- trum, which is relative to the compo- nent within the bearing that is at fault. Tis frequency can be identified if you know or can estimate with reasonable accuracy, the rpm of the shaft at which the bearing is operating, and the number of rolling elements inside the


bearing. In the frequency spectrum, the dominant frequency and harmon- ics will show as peaks, with harmon- ics and sidebands arranged at regular intervals. As the failure advances, the sum of these frequencies grow larger in height and width and the entire mass of the haystack will begin to move to the left. In late stage 2 and early stage 3 of the bearing failure, the frequency is easily detected by vibration analysis. An experienced mechanic can detect bearing failure as it enters stage 3. In stage 4, the bearing is generating sig- nificant heat, and may fail at any time. Temperature is used to know when a


bearing has received too much lubri- cant. Te rolling elements have to plow through the excess grease. Te rolling element may even be forced into direct contact by the excessive pressure being developed inside the bearing. Both situations cause an unhealthy rise in temperature. Excess lubricant may be relieved from the bearing to restore normal operation when proper lubrica- tion techniques are used. If proper lube levels have been established, temperature rising higher than 8–12 degrees on similar bearings in the same application and similar environmental factors, can indicate a diagnosis of the bearing condition. Even if the compared bearings have similar noise levels, but significant dif- ferences in temperature alone, you can assume the higher bearing has entered a more advanced stage of failure than the lower temperature bearing.


Fig. 1. Ultrasound analysis of a B8 pillow-block bearing from a conveyor belt is shown for one side of a tail pulley.


40 | MODERN CASTING December 2016


Fig. 2. Ultrasound analysis for the B8 pillow-block bearing from a conveyor belt on the other side of the tail pulley shows different results.


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