In the cement manufacturing world, the raw mill is the quiet workhorse—grinding limestone, clay, and other ingredients into raw meal, the critical precursor for clinker production. Clinker is the hot, marble-sized product discharged from the kiln and ultimately ground into cement, making the raw mill’s performance foundational to the entire plant. If the raw mill falters, the kiln starves—and clinker output drops with it.
At Safwa Cement Company in Jeddah, Saudi Arabia, that risk became very real when a single, hidden component deep inside the dynamic separator began to fail.
The raw mill under study is not lightweight. At 5.8 meters (19 feet) in diameter and 12 meters (39 feet) long, this mill pushes 400 tons per hour—9,600 tons per day—of raw material through the system. To control the fineness of this raw meal before it proceeds to the kiln, a dynamic separator continuously separates fine meal from oversized particles. Correctly sized material moves forward in the process toward pyroprocessing, where it will ultimately become clinker. Oversized particles are thrown back to the mill for regrinding.
Figure 1: Exterior of dynamic separator.
The separator’s vertical drive shaft depends on two bearings: an external oil-lubricated bearing in clean air, and an internal grease-lubricated bearing buried inside the separator housing—where temperatures reach 120–130°C (248 – 266 °F) and the bearing itself typically runs at 105–125°C (220 – 257 °F). If it hits 140°C (284°F), the system automatically trips to protect the bearing. In an environment this punishing, lubrication isn’t just maintenance—it’s survival.
Figure 2: Bearing housing arrangement inside the separator
The First Red Flag: Rising Vibration
The earliest warning didn’t come from temperature at all, but from vibration. Over the span of just one month, the separator gearbox vibration increased steadily from 2.1 mm/s to 4.6 mm/s.
Technicians suspected the usual culprits: gearbox gears or bearings, damaged coupling inserts, the upper bearing, or even rotor imbalance. But oil analysis, vibration diagnostics, coupling inspections, and rotor checks all came back clean.
With every other component ruled out, attention shifted toward the one item no one could inspect during normal operation—the internal lower bearing.
The Root Cause Hidden Inside the Separator
A 12-hour planned shutdown gave access to the separator interior, and the core issue revealed itself immediately.
The grease supply line feeding the lower bearing and its dust seals was damaged. Although technicians had been applying grease, none of it was reaching the bearing. The dust seal—designed to form a grease barrier keeping contaminants out—had also been left dry and ineffective.
Inside the bearing housing, the grease had hardened into a thick, dust-packed sludge.
Figure 3: Hardened grease coating the separator bearing
The rollers were so stiff they barely moved within the raceways. The vibration detected at the gearbox wasn’t a gearbox problem at all—it was the echo of a bearing slowly seizing in the heart of the separator, transferring stress up the entire shaft.
Remarkably, the bearing itself had not yet suffered permanent damage.
The Recovery: Cleaning, Repacking, and a Dramatic Drop in Vibration
Technicians thoroughly cleaned the bearing, repaired the grease line, restored proper lubrication routing, and repacked the housing with fresh grease. Upon restart, gearbox vibration dropped from 4.6 mm/s to 1.8 mm/s—less than half the original reading.
The intervention prevented not only bearing failure but a potential production crisis. With clinker output dependent on steady raw meal feed, a failure inside the separator would have directly threatened kiln stability and clinker production rates.
What Was at Risk
Had the bearing failed fully, the consequences would have been severe. The 22344K C3 bearing alone carries an estimated replacement cost of roughly USD $5,000—not counting labor, not counting lost production. More critically, the raw mill would have been forced into a prolonged outage, starving the kiln and reducing clinker output for as long as the repair took.
Key Lessons from the Case
1. Lubrication delivery is just as important as lubrication type.
A blocked or damaged grease line can defeat even the best grease selection.
2. Dust seals must be fed with grease to function.
Without grease flow, they cannot create the positive barrier needed to repel contaminants.
3. Vibration doesn’t always point to the component that appears to be failing.
A distressed internal bearing can transmit misleading symptoms upstream into the gearbox.
4. Early detection prevented a clinker-impacting outage.
Had the lower bearing seized, the entire upstream portion of the clinker production chain would have been disrupted.
A Small Problem With Big Consequences
Clinker production depends on stable, steady, finely controlled raw meal feed. This case study shows how a simple lubrication line failure—buried inside a separator and operating in 120°C (248°F) dust-laden gas—nearly jeopardized a 400-t/h raw mill and everything downstream of it.
This near-miss story ends with a recovered machine, not a shutdown kiln. But it also serves as a sharp reminder: in cement plants (and in any plant’s critical machinery), even the smallest lubrication oversight can echo all the way down to product quality, production rates, and the bottom line.
