The Reliability in Action department features case studies submitted by our readers. To have your case study considered for inclusion in an upcoming issue, e-mail it to firstname.lastname@example.org or mail it to Reliable Plant, P.O. Box 87, Fort Atkinson, WI 53538. If we publish your case study, we’ll send you an official Reliable Plant T-shirt.
Here are this issue’s contributions:Of Mice and Men
“We were having intermittent trips with a static var unit for our rolling mill drives. The unit would suddenly stop and indicate it was turned off by the operating panel. This went on for several weeks and we couldn’t locate the problem. When we went to restart the unit, it would not start from the regular operating panel. It had to be started at the backup panel.
“On the next downturn, we decided to investigate further to see if we could find the ‘ghost’ causing us problems. As we opened the cabinet door, we heard a sizzling noise coming from the adjacent cabinet. Further checking revealed the source of the problem to be a mouse that crawled into the 480-volt terminal area. The mouse was still cooking.
“As we continued to open the panduit covers for the wire trays, we located the source of the ‘ghost’ trips. Mice chewed up more than 150 feet of control cable and two special I/O flat cables. They found the blue wire to be the best tasting.
Mice and electrical wires just don’t mix.
“We installed traps and bait boxes. Within the first two days, the mice ate every bit of the bait box material. In the first week, we removed 12 dead mice from the cabinets and traps.
“Since the problem became bad as the outside weather got colder, we established a routine PM every fall to open the panduit covers and check for any mouse-related damage. In the meantime, we maintain the bait boxes and traps.
“If anyone has invented a better mouse trap, I’d like to hear from you.”
“Last July, I was engaged in a routine infrared shoot of our electrical components. The photo above is an untouched image of the inside of a 480-volt disconnect. I opened this disconnect for a routine shoot and found a bat pinned across two legs of this fused disconnect. There were no fuses blown and no apparent detrimental results.
One reason to plug electrical box holes.
“Upon inspection of this disconnect, I noticed a knock-out hole on this box was open. The bat apparently entered in an attempt to find a dark place to sleep.
“The moral of the story is that holes on electrical boxes left unplugged not only can be dangerous to humans, but can be an invitation to creatures seeking shelter. This situation could have shorted out the disconnect, which could have led to equipment downtime or even a fire.”
“Last July 26, vibration readings on an extruder drive motor exceeded the fault alarm on three parameters: vHFD, pk-pk waveform and max-pk waveform. The waveform revealed spikes showing up once per every revolution of shaft speed.
“These alarms are good indications of bearing problems. The spikes occurred just under the running speed, pointing to inner race defects. Confirming this in a vibration spectrum was next. In Peakvue, clear peaks existed at running speed and ball pass frequencies inner race (BPFI). In a normal velocity spectrum, the fundamental bearing frequency doesn’t normally show up. To match bearing frequencies, the bearing sometimes rings at its natural frequency and we look for sidebands of defect frequencies.
“How long will the bearing last? Looking at the trend pk-pk waveform, there was a big jump in a 69-day period. I knew from experience it wasn’t an emergency, so I wrote a work request on July 26. The planner scheduled a time convenient with production to allow for the bearings changeout. On July 29, the motor was taken out of service for fixes. One important thing I noticed was that by turning the shaft by hand, it turned easily and I felt no defects.
“I requested that the old bearings be saved for visible inspection. I took photos of the inner and outer race, which do have clear defects. There were no visible defects on the balls. How long would this bearing last? A week or several weeks? I had tracked this bearing defect since March (four months). The visible defect looks relatively minor, but was impacting in the load zone. Trying to predict how long a defected bearing will last is a guess. Knowing the defect is there is the best reason for action.
“There are different ways to determine how long a bearing defect will run until failure. Load and speed are the most common factors to consider. You can also track a defect through the four stages of bearing failure. But trying to track a bearing to near-failure is risky. Once you determine the bearing is more than a minor defect, change it. As described here, alert and fault levels also help.”