Tooth Tips: Andy Milburn, P.E.

Think like a detective when it comes to determining the cause of equipment failure… keep an open mind, and be sure to preserve all of the evidence. It’ll pay off in the long run.


When failures occur, especially on a piece of critical equipment, the first priority is usually to get the equipment operating again. Unfortunately this can result in the loss of valuable information as to the cause of the failure and lead to more failures down the road.

During the investigation, remember that pitting or bending failures occur because the material strength is too low or the applied local stress is too high. Wear failures are generally caused by lubrication problems. Take lots of photos and notes regarding the condition of components and avoid jumping to conclusions too early. Sometimes what looks like an obvious reason for the failure is really secondary damage caused by the initial failure. If the situation warrants, make a cause-and-effect or "fishbone" diagram to guide you during the investigation.

Each situation is unique and will dictate the exact procedure, but some general guidelines are as follows:

1. As soon as possible after a gearbox failure, visit the site to start the inspection.

• If the gearbox is still installed, make sure that it's locked out and can't rotate unexpectedly, and be sure to follow all factory safety procedures.

• Record all of the information from the nameplates of the prime mover, gearbox, and driven equipment.

• Examine the outside of the gearbox for evidence of loose bolts, oil leaks, overheating, or a broken housing.

• Check the alignment of the high-speed and low-speed shafts with the mating equipment and the condition of shaft couplings.

• Check for the proper installation and condition of all monitoring equipment, such as temperature and vibration.

• Check the oil level and for the proper installation and operation of any external lubrication equipment.

• If the gearbox has already been disassembled, collect all of the failed components and the breather, oil filter, and as much of the debris as possible. Don't clean anything or touch fractured surfaces. It is also important to prevent the parts from being contaminated or corroded.

• Collect several oil samples in clean oil-sample bottles.

2. Collect information regarding the operation of the gearbox prior to the failure.

• Determine the time of the failure and all operating parameters such as temperature, vibration levels, speed, and applied load.

• Interview operators and maintenance personnel for their observations and comments regarding unusual noises, vibration, or other conditions prior to the failure.

• Collect all maintenance records that show previous repairs, alignments, inspections, oil changes, and oil analysis.

3. If possible, move the gearbox to a clean and well-lighted facility for disassembly and a detailed inspection of the components.

• Prior to and during disassembly, mark all components with identifying information such as the gearbox serial number using a vibrating pencil. Mark the position and orientation of all components in relation to one another prior to disassembly, including the orientation of bearing inner and outer races with their mating parts.

• Measure and record the torque required to loosen all fasteners.

4. During and after disassembly, visually examine all of the components using a bright light and a 10X magnifying glass. Inspect and document the condition of the parts prior to any cleaning and then, if necessary, clean the parts for further inspection.

• Record identifying numbers on all components.

• Thoroughly examine all of the gearbox components for areas of pitting, fracture, polishing, fretting, and abrasive wear.

• Determine the failure and wear modes. Several helpful references are:
A) "Failure Atlas for Contact Machine Elements" by T. E. Tallian, 2nd Edition 1999
B) ANSI/AGMA 1010-E95 "Appearance of Gear Teeth-Terminology of Wear and Failure" 1995
C) Gear Failure Atlas by GEARTECH -Townsend, MT c1999

5. Determine if the failed components were manufactured properly by sending them out for inspection.

• A metallurgical lab can examine the failed parts using high magnification to determine the failure initiation points and more detailed information on wear and failure modes.

• Have them check the material composition, hardness, microstructure, and cleanliness to determine if it met original specifications.

• Failed gears can be sent to a gear lab to determine their dimensions and accuracy.

• Send oil samples out for analysis to determine the condition of the oil and levels of wear metals and oil additives.

After test results and observations are completed, select the most probable cause of the failure and take steps to prevent future occurrences.