A laser-cladding process developed by Caterpillar Inc. to repair wear and damage to large axels on mining trucks is applicable to worn or damaged main shafts of wind turbines. At a recent O&M Summit, Kristin Schipull, a metallurgist with Caterpillar (www.cat.com), showed how the process works and reported that repaired axels match the performance of new ones. Although Caterpillar did not invent the process, the company is always working to apply the repair techniques to more components.
An axel repair is fairly straightforward. A worn axel from a mining truck is cleaned and inspected for the extent of its damage and an engineer details the steps for its repair. The exact metal deposition can involve a variety of process such as thermal spray, welding, and coating and cladding, providing several ways to remanufacture a worn part. And truck axels are a little like large wind-industry components.
On a wind turbine main shaft, bearing journals, or the coupling end of the shaft, the damage could be the spalling that comes from removing or disassembling it, or from removing a damaged bearing. The used shaft is placed on a lathe and the damaged material removed. A cleaned shaft would then be mounted and slowly turned on a spindle. A robot-controlled laser slowly sweeps the length of the damaged area while powdered metal is fed around the laser beam to melt and fuse it to the shaft surface. Layers are built up to a required thickness and later machined to required specs. “We typically apply 1 to 2 mm per layer and have validated stainless steel coatings up to 15-mm thick,” says Schipull. “Materials include a wide range for the powder process and that gives a lot of flexibility for a repair. Most often, powders are of stainless steel, tungsten carbide – a lot of hard face coatings. Turbine shafts will be repaired with stainless steel.”
Laser cladding provides a metallurgical bond to the surface with a small or minor heat-affected zone. From a performance perspective, the coatings have good microstructure, and perform well in a four-point bending fatigue analysis. The latter test is one technique Caterpillar uses to simulate a load a full component will experience, but on a small scale. These repairs can be applied to most shafts under 20,000 lbs., adds Schipull.
“On steel castings, the coated part outperforms the original casting because the coating structure is superior to that of the original steel,” she says. “Our department motto is: As good as new and strong as ever. Remanufactured parts can meet the same standard as new components.” For the wind industry, Shermco Industries will provide a point of contact while the repair work will be done in Caterpillar’s Fargo, N.D. factory. WPE
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