Find the root cause of gear failure - part II

The second part of this two-part series describes the specific actions maintenance crew members can take when diagnosing gears. This detailed article is a follow up to the orignial, which explained the makeup of the analysis team and the skills that each member should possess. Read part I of this article.

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By Robert Errichello

PlantServices.com

Part 1 of this article appeared in the March 2005 issue. It explained the makeup of the analysis team and the skills that each member should possess. It discussed getting organized, preparations for the investigation, the possibility of keeping the gearbox operating and some of the initial testing that takes place.

Removal time

The analyst should perform a thorough external examination before the gearbox is removed and disassembled. This involves an inspection form to record important data that would otherwise be lost once disassembly begins. For example, the condition of seals and keyways must be recorded before disassembly. Otherwise, if they get damaged, it will be impossible to determine when it happened.

Before cleaning the gear housing, inspect it for signs of overheating, corrosion, contamination, oil leaks and damage.

Measure shaft coupling alignment before moving the gearbox. Note the condition and loosening torque of fasteners including coupling and mounting bolts. To check for possible twist in the gear housing, install a dial indicator at each corner of the gearbox. Then, measure movement of the mounting feet as bolts are loosened. If there’s no twist, each indicator will record the same vertical movement. If not, calculate the twist from relative movements.

Unless your plant has adequate facilities for disassembling the gearbox, relocate it for disassembly at a fully-equipped gear manufacturing facility. Large gearboxes require a pair of overhead cranes for manipulating and disassembly. Special tools such as bearing pullers or hydraulic presses may be required for removing components. Most importantly, the disassembly should be done in a clean, well-lighted environment.

In transit

Take precautions if you need to ship a gearbox. Fretting corrosion on gear teeth and rolling element bearings is a common problem that may occur during transit. Ship the gearbox on an air-ride truck, and support it on vibration isolators to help avoid fretting corrosion. If possible, ship it with oil. To minimize contamination, remove the breather and seal the opening, secure labyrinth seals with silicone rubber, and cover the gearbox with a tarpaulin. It’s best to inspect the gearbox as soon as possible. If that’s not possible, store it indoors in a dry, temperature-controlled environment.

Disassemble the gearbox

A skilled technician should disassemble the equipment under the analyst’s direction. Otherwise, by disassembling, cleaning or draining oil, a well-meaning technician could inadvertently destroy evidence.

Technicians should understand that failure investigation is different from a gearbox rebuild, and the disassembly must be controlled.

The analyst should verify that gearbox drawings, disassembly tools and adequate facilities are available. Make sure you provide the analyst sufficient privacy to conduct the investigation and access to needed information.

The analyst should explain the objectives to the technician. They will review the gearbox assembly drawings together, checking for potential disassembly problems. The technician should understand that failure analysis must be done slowly and carefully. Normally they’re trained to work quickly.

After the external examination, the technician should clean the exterior of the gearbox thoroughly to avoid contaminating the interior when opening it. Once disassembled, the analyst should inspect the components, both failed and undamaged.

Other points should be obvious. Inspect components before cleaning them. Mark relative positions of components before removing them. Don’t discard any parts. Don’t touch fracture surfaces or try to fit broken pieces together. If fractures can’t be examined immediately, coat them with oil and store the parts to avoid damaging fracture surfaces. Examine functional surfaces of gear teeth and bearings and record their condition. Before cleaning the parts, look for signs of corrosion, contamination and overheating.

After this initial inspection, wash the components with solvent and re-examine them. This examination should be as thorough as possible because it’s often the most important phase of the investigation and may yield valuable clues. A low-power magnifying glass and 30X pocket microscope are helpful tools for the exam.

It’s important to inspect bearing rollers and raceways and mounting surfaces because they might provide clues about the cause of gear failure. For example:

• Wear on bearing rollers, raceways or mounting surfaces can cause excessive radial clearance or endplay that misaligns gears.
• Bearing damage may indicate corrosion, contamination, false brinelling, fretting corrosion, electrical discharge or lack of lubrication.
• Plastic deformation between rollers and raceways may indicate overloads.
• Gear failure often follows bearing failure.

Document observations

Identify and mark each component (including gear teeth and bearing components) to facilitate clearly identified written descriptions, sketches and photographs. It’s especially important to mark bearings, including inboard and outboard sides, to identify their location and orientation.

Describe components consistently. An example includes always starting with the same part of a bearing and progressing through the parts in the same sequence. This helps to avoid overlooking any evidence. Important observations should be described in writing using sketches and photographs where needed. The following guidelines help maximize your chances for obtaining meaningful evidence:

• Concentrate on collecting evidence, not on determining cause of failure. Regardless of how obvious the cause may appear, don’t form conclusions until all evidence is considered.
• Document what is seen. List observations, even if some seem insignificant or if you don’t recognize the failure mode. Remember, there’s a reason for everything you see, and it might become important later when you consider the rest of the evidence.
• Document what you don’t see to help eliminate certain failure modes and causes. For example, if there’s no scuffing, you can conclude gear tooth contact temperature was less than the lubricant’s scuffing temperature.
• Search the bottom of the gearbox. Often this is where you find the best-preserved evidence, such as a fractured gear tooth that falls free without sustaining secondary damage.
• Use time efficiently. Be prepared for the inspection. Plan your work carefully to obtain as much evidence as possible. Don’t be distracted by anyone.
• Control the investigation. Watch every step of the disassembly. Don’t let the technician get ahead of you. Disassembly should stop while you inspect and document the condition of a component, and then proceed to the next component.
• Insist on privacy. Don’t let anyone distract your attention and don’t announce any conclusions until the investigation is complete.