From the field · Case 06

Yaw Bearing Wear — The Metal in the Purged Grease

What makes this case different from the others is that there was no alarm. The turbine was running, SCADA was clean, nobody had complained. We caught the fault during a routine greasing job, while looking at the colour of the old grease coming out. Enercon E92, scheduled annual service.

Symptom

• Alarm: none. The turbine was producing normally.
• First sign: some of the old grease purging out during lubrication was dark grey with a metallic glitter. Grease from the other points looked normal.
• Yaw motor current: average value within normal limits in SCADA, but peak values had crept up over recent months.
• Sound: an audible friction noise as the yaw passed through one particular position.

First assessment

Metal in grease is not on its own a reason to panic; a new bearing sheds some metal during run-in. What stood out here was not that metal was present but how it was distributed. If every greasing point on the slewing ring had produced metal, that would be general ageing. Metal from only some points said the damage was in a specific region of the bearing, not across it.

There are two classic reasons for damage to concentrate in one region of a slewing bearing: either that region is carrying disproportionate load, or that region is not being lubricated. The second is far more common, and the reason is almost always the same — a blocked line feeding it.

Step-by-step diagnosis

1. Collected the purged grease separately, point by point. Putting it all in one container would have killed the case. I collected the grease from each greasing point separately, labelled with its number. That became the basis of every step that followed.
2. Spread each sample thinly on a white surface. Simple but effective: spread as a thin film, metal particles in the grease become visible to the naked eye. I also checked with a magnet — the particles being magnetic supported bearing steel as the source.
3. Drew a sector map. I marked how much metal came from which point around the circumference of the bearing. The pattern was unmistakable: metal was concentrated at three adjacent greasing points and effectively absent at the rest. The damage sat in roughly a quarter of the ring.
4. Checked the automatic lubrication system. The pump was running and the reservoir was full — no problem at first glance. But following each line out of the distributor, I found a hardened grease plug at the connection feeding the sector that was shedding metal.
5. Confirmed it by opening the line. With the pump running, grease came out of the other lines and not out of the blocked one. The diagnosis was settled at this step: that sector had not been fed for some time.
6. Measured ring gear backlash. To see whether the wear had spread from the bearing into the gear. Backlash was within the manufacturer's tolerance — good news; the damage had not reached the gear.
7. Located the noise by watching the yaw move. The friction noise occurred exactly as the nacelle passed through the sector that was shedding metal. Three independent pieces of evidence — grease, lubrication line, noise — all pointed at the same region.

Root cause

A blockage in the automatic lubrication line had left one sector of the slewing bearing without grease. That region had started running dry, and wear and light scoring had formed on the raceway. The metal particles it shed then became a second factor accelerating the wear — it had entered a self-feeding loop.

How long it took to develop cannot be said precisely, but the hardness of the old grease and the level of wear suggested a process measured in months. The reason it produced no alarm is simple: the yaw motor can absorb rising friction for a long time by drawing more current, and that rise never reaches the alarm threshold.

Fix

1. Clearing the lubrication line (2 hours): the blocked line was removed, cleaned and its fitting replaced. With the pump running, grease delivery was then verified line by line.
2. Heavy greasing of the bearing (3 hours): the starved sector was greased in stages until the old grease and metal particles had been pushed out. Continued until the purged grease came back clean.
3. Short-interval follow-up (3 months): the normally annual grease check was done at three months for this turbine, to see whether the metal was reducing.
4. Result: at the second check the metal had clearly reduced; by the third it was at normal levels. The wear had stopped. No bearing replacement was needed.

What to look for in purged grease

Preventive maintenance

• Collect purged grease point by point. This whole case rests on that single habit. Put it all in one container and the pattern disappears; you are left with "there is some metal".
• Keep a sector map. Mark what came from which point around the ring and keep the record. Being able to compare at the next service is worth many times a one-off observation.
• Do not accept "the pump is running" as lubrication. A running pump does not mean grease is reaching every point. Verify delivery at each line outlet.
• Watch the peak values of yaw motor current. While the average still looks normal, a climb in the peaks is the early sign of rising friction.
• Record where the noise occurs. "There is a noise when the yaw turns" is an inadequate record. Write down the yaw angle it happens at — it takes the diagnosis straight to that region.