Dr Jie ‘Jack’ Zhou identifies the key contributing causes to wire wool damage and how to prevent it
Turbomachinery reliability is directly affected by its bearings. Therefore, when bearing damage occurs, corrective action is typically necessary. Proper identification of the damage as well as its causes is key to implementing an effective solution.
Bearing damage is frequently the visible evidence of a problem elsewhere in the system, or the result of several factors combined. One severe type of damage seen on babbitt-lined tilt pad thrust and journal bearings, as well as shafts, is wire wool damage, also called black scab, high-chromium damage, and machining-type wear.
The damage process
Wire wool damage occurs when a relatively large (probably not less than 1 mm [0.04-in] across) hard particle becomes firmly embedded into the bearing material and continuously rubs the shaft sleeve or thrust collar.
The combination effect of a particle, tight tolerance and high shaft speed leads to localised heat and high temperature. Under these conditions, in the presence of a hydrocarbon oil, chromium and/or molybdenum in the steel is converted to hard carbides. Hard chromium carbide then embeds in the bearing material, creates a hard scab and acts as a cutting tool, which leads to extensive grooving and scabbing on the bearing. The process continues with embedded carbides and propagates to the whole bearing, potentially resulting in catastrophic damage.
The key elements in wire wool damage are particles, chromium/molybdenum, oil and a soft runner.
The particles initiate the damage. Particle sources can be contaminants in oil lube systems, such as dust, rust, wear products, oil oxidisation, fine particle accumulation, and silt forming at the gap between the bearing and its runner.
Chromium and/or molybdenum, with the help of oil, is the elemental source of carbides, which in turn cut the runner into wire wools. Runners made of high chromium content materials (1.3% or greater) are more sensitive to wire wool damage than those made of low chromium content materials.
Oil also contributes to the damage. Chlorinated (EP) additives in lubrication oils increase the chance of wire wool damage.
Finally, the runner can only be cut into wire wool if it is softer than the particles and carbides created.
To prevent wire wool damage, each of these contributors should be addressed. As a precautionary measure, maintain a high level of cleanliness during assembly, and avoid contamination during operating.
Runners should be made of low chromium/molybdenum content material, or mild steel if possible. Alternatively, harden runners by surface modification technologies such as nitriding, carbonitriding and carburising, or by surface coating methods such as hard chrome plating.
For lubrication, avoid chlorinated additives. Runners made of low chromium content materials may run smoothly in oils without EP additives. The use of water as lubricant could also be an option. A runner made of high chromium content material was tested using water as lubricant for a few hundred hours without any sign of wire wool damage.
Dr Jie ‘Jack’ Zhou is senior development engineer, Waukesha Bearings.