The bearing units, now installed at Intalco’s Ferndale, Wash. plant on more than half the plant’s baghouse fans, maintain a nearly 360-deg shaft fit, alleviating the effects of imbalance and contamination. The plant is a primary production facility, processing alumina ore from Australia. Aluminum is cast into a variety of sizes and shapes, including billets, slabs, and tee ingots. Plant production capacity is about 272,000 metric ton/yr.
Four baghouses serve as dust collectors for Potlines B and C. Each baghouse contains 22 overhung air-handling fans, driven by 60-hp motors at 1,440 rpm. Each fan, Figure 1, has two bearing pillow blocks, which support the 27/16-in. diameter fan shaft.
The Pop-Release bearing units, Figure 2, by SKF USA Inc., King of Prussia, Pa., “...have been an ideal solution to several recurring problems,” says Intalco Maintenance Specialist Fred Hull. “We wanted a better fit between bearing and shaft, and Pop Release provides that. In addition, it creates a more balanced arrangement than the standard bearings we had been using exclusively.”
Fan maintenance costs
Fan operation has traditionally been costly. In 1990, for example, before the first sleeve-mounted unit was installed, 45 sets of bearings failed and had to be replaced. In 22 instances, the shaft journal was damaged and the complete fan assembly had to be removed from the baghouse and repaired. The cost, in labor and components, was more than $88,000 in 1990 alone.
Many problems were directly attributable to the spherical roller bearings then in use on all 88 fans. Each unit was held in place by two setscrews 90 deg apart on the shaft circumference. The setscrews tended to gouge the shaft and often threw the entire arrangement out of balance.
“When you tighten setscrews on a standard bearing, you push the shaft against the far side of the bearing bore,” says Mr. Hull. “This slightly offsets the shaft and unbalances the arrangement. As a result, there’s more vibration, which can limit bearing life. Balancing in the field was often required to reduce vibration after a bearing replacement.”
Also, when a standard bearing is mounted in this manner, there is a slight gap between shaft and bearing bore, leading to fretting corrosion. This damages the shaft journal and causes shaft and bearing to adhere, making dismounting difficult.
Continuous improvement
Over the last 10 years, Intalco implemented a series of measures to remedy these and other bearing problems and to improve overall fan performance. As part of predictive maintenance, a 5-person team monitors vibrations on the plant’s 4,000 bearings every month, scheduling periodic shutdowns and making small repairs to head off major work. Lubrication practices have been upgraded, and fan cleaners have been installed to remove dust buildup from fans without shutting them down.
But the most important strategy may have been a decision made during a consultation with Homer Andres, local representative of Bearings Inc., a bearing and power transmission distributor. It was decided to start equipping the fans with the multi-taper-sleevemounted units. Unlike a standard bearing in which setscrews hold the inner ring against only part of the shaft, this unit has a step-sleeve arrangement, Figure 2, to tighten the bearing bore and sleeve around the entire shaft circumference. During mounting, setscrews gradually force the inner ring up a series of inclined planes, compressing the sleeve and correctly centering the bearing on the shaft.
In June 1991, as a test application, 12 of the tapered-sleeve bearings were installed on 6 air-handling fans. Vibrations were monitored during the experiment. Lower readings demonstrated to Intalco technicians that the installations were successful in better balance.
“We didn’t even have to wait till the end of the test period,” says Mr. Hull. “After a few days, we were pretty confident that Pop Release was superior to what we had, and we began to switch.”
The payoff
To date, Intalco has equipped 48 of its 88 air-handling fans with the new bearing units, and plans call for the rest to be upgraded as their bearings fail. The plant has also begun installing the new bearings, with new bearing shafts, on other overhung fans where shaft damage has been a problem. The changeover — along with the predictive maintenance program — has enabled Intalco to reduce baghouse fan maintenance costs from $88,000 in 1990 to $15,000 in 1993. The average annual repair cost per fan dropped from a high of $1,001 in 1990 to repjust $176 in 1993.
The decline in the number of fan assembly repairs was particularly noteworthy. Because the new units cause no shaft gouging, fewer assemblies must be replaced. As more bearing units were installed, assembly repairs steadily decreased — from 22 in 1990, to 17 in 1991, 6 in 1992, and just 2 in 1993.
The same period also saw fewer bearing repairs. Failure rates declined by nearly 70%, from 45 bearing set repairs in 1990 to 15 in 1993.
With the new units, “...we have a better fit between bearing and shaft,” says Mr. Hull. “And because of the better fit, we don’t lose as many shafts and assemblies. These are improvements in fan performance that we can measure.”
For more about Pop Release spherical roller bearing units by SKF USA Inc., King of Prussia, Pa., circle 404 on the reader service card.
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