BLACK SCAB or WIRE WOOL DAMAGE

A large dirt particle (probably not less than 1mm across) carried into the clearance space by the lubricating oil, and becoming embedded in the bearing may form a hard scab of material by contact with the steel journal or thrust collar. This scab will then cause very severe damage to the mating steel surface which is literally machined away with the formation of the so-called “wire wool”. The action is self-propagating once started and susceptibility to scab formation appears to depend upon the nature of the lubricant and the composition of the steel of the rotor shaft or collar. Steels containing chromium or manganese in excess of 1% appear to be particularly susceptible to scab formation, especially in high speed machines with bearing surface speeds of over 20 metres per second.

JOURNAL BEARINGS
Fig. 15 - Whitemetal-lined compressor bearing with formation of “Black scab”. 
Fig. 16 - 13% chromium steel journal running in bearing shown in Fig. 15 showing severe machining “damage”. 

TILTING PAD THRUST BEARINGS 
Fig. 17 - “Black Scab” formation on whitemetal-lined thrust pad. 

RECOMMENDED ACTION 
Scrap bearings. Pay particular attention to cleanliness during assembly, taking care to avoid contamination of bearing surface and oil ways with swarf etc. 

Investigate possibility of changing journal or collar surface material by sleeving with mild steel, or hard chrome plating.

Changing the bearing alloy is unlikely to be effective.

Scrap damaged pads. Clean out lubricating system, change oil and fit new pads.