Using a step-by-step analysis process, engineers correctly diagnose many gear failures and, more importantly, develop solutions that prevent such failures from happening again. The basic steps in this process, which a previous article (“How to analyze gear failures”) describes in detail, include:

• Inspect failed parts.
• Determine type of failure.
• Conduct tests and perform calculations.
• Form and test conclusions.

The following examples demonstrate how engineers have used this basic approach to solve gear failures in various applications — wind turbines, waste water treatment plants, hydroturbines, and tunneling machines.

Wind turbine gears

Two years after a wind turbine farm began operating, technicians found that six of the 24 gearboxes for the wind turbines were noisy and one of these had jammed so that it couldn’t rotate. The remaining units continued to operate smoothly and quietly.

Investigators first interviewed the owner and service technicians to obtain operating histories of the wind turbines, then they inspected the six noisy gearboxes plus two others that appeared to be operating normally.

The exteriors of the gearboxes showed no signs of oil leaks, overheating, or unusual contamination. After opening the inspection ports, the six noisy gearboxes were found to have broken teeth on the low-speed pinions. The one jammed gearbox had broken teeth on both pinion and gear. Gears from the two units that operated normally had no apparent damage.

After disassembling the eight gearboxes, investigators inspected, documented, and photographed all components.

The oil sump of each damaged gearbox yielded broken teeth, which were collected for later inspection. The jammed gearbox contained so much damage, it would have been difficult to analyze. So attention shifted to the other five damaged gearboxes.

Each of the five pinions had bending fatigue fractures on two adjacent teeth, Figure 1. In each case, beach marks on the fracture surface showed that the fatigue crack originated in the root fillet at the end of the leading side of the tooth, which is in tension, Figure 2.

The inspection revealed several facts:
• On each pinion, the first broken tooth (first one to be loaded as the gear rotates) had a smooth fracture surface, which is characteristic of slow crack growth, whereas the trailing tooth had a rougher surface, indicating faster crack growth, Figure 2.
• Teeth adjacent to the fracture had extensive macropitting, whereas teeth away from the fractures were undamaged, except for mild abrasion.
• Working surfaces of the teeth that were retrieved from the oil sumps had no pitting or other damage.
• Color photos of the failed pinions disclosed copper plating on the tooth ends and in the adjacent radius with the integral shaft, commonly known as the grinding relief radius for the bearing journal, Figure 1. The copper plating was at the same end of the teeth, and in the same area, where the fatigue cracks originated. On the two undamaged pinions, copper plating was present in the grinding relief radius, but stopped short of the tooth ends.
• Gears from the two gearboxes that operated normally had no apparent damage. Contact patterns taken from these gears with marking compound were centered, showing that the teeth were properly aligned.

These facts led to the following conclusions:
• The primary mode of failure was bending fatigue originating at the tooth ends that were copper plated.
• The lead tooth failed first, by slow crack growth. As this tooth cracked, it transferred load to the following tooth. This following tooth cracked more rapidly, because it was overloaded due to transferred load from the lead tooth.
• Macropitting on teeth adjacent to the fractured teeth was a secondary failure mode that was caused by overloading due to loss of load sharing. This conclusion was substantiated by the lack of pitting on teeth located away from the fractures and on the broken teeth retrieved from the oil sumps.
• The gears were properly aligned. Normally, misalignment is suspected when fractures originate at the ends of gear teeth. In this case, however, the wear patterns, tooth contact patterns, and bearing conditions all indicated good tooth alignment.
• Based on these conclusions, investigators formed a hypothesis that the pinion teeth failed, because the copper plating prevented their ends from being properly hardened.

Copper plating is used to mask areas that shouldn’t be hardened. The copper prevents carburizing during heat treatment so these areas develop low hardness. It is likely that the bearing journals of the pinions were plated (to minimize hardness and ease machining) by lowering them into the plating solution. Apparently, some pinions were lowered too far into the solution so the tooth ends were inadvertently plated.

A metallurgical laboratory then tested the gear teeth to confirm (or disprove) the hypothesis. A scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) revealed striations on the tooth fracture surfaces, proving that the fractures were caused by fatigue. An EDX analysis confirmed the presence of copper plating at the fractures.

Metallurgical sections were prepared by cutting and polishing the ends of teeth from both damaged and undamaged pinions. Hardness surveys on these sections confirmed that the copper plating prevented carburization of the ends of the fractured teeth, so that hardness was limited to 40 HRC. On the undamaged teeth, however, the hardness gradient ranged from 60 HRC at the surface to a core hardness of 40 HRC, which is normal.

Based on these tests, it was concluded that the failures were caused by improper heat treatment due to copper plating on the tooth ends. The resulting low hardness and low strength allowed the bending fatigue failures to occur.

The wind turbine owner subsequently found copper plating on the tooth ends in four more gearboxes. Technicians replaced these pinions two years ago, and the gears have operated with no further failures.

Waste water aeration blower

At a waste-water-treatment plant, there are two blower drives, each using a diesel engine operating through a speedincreasing gearbox to drive a roots-type aeration blower. Each gearbox has double helical gears on parallel shafts and converts the 900-rpm engine speed to 3,600 rpm at the blower.

After only 3 months operation, both gearboxes were replaced because of noise. The replacement gearboxes also became noisy after only 8 weeks, prompting the operators to investigate.

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