Case Histories

Manufacturing

Cavitation Damage

Cavitation damage of injection piston
Region of cavitation damage on a cast iron injection housing
SEM of cavitation damage of injection piston
Scanning electron microscopy image of a typical “cavity” caused by cavitation damage

Specimen: Cast iron injection housing

Material: ASTM A 536 Grade 65-45-12 ductile iron

Background: The cast iron injection piston housing had been subject to approximately 1 million cycles in a plastic injection moulding system. Seven identical failures of localized damage to the ring seal on identical cast iron injection piston housings had occurred.

Service Life: Approximately 1.5 years, or 1 million cycles

Findings: Visual examination of the region of localized damage revealed areas of large “craters” surrounded by smaller craters or micro-pits. Such damage is typical of cavitation damage. Cavitation damage is defined as the wearing away of metal through the formation and collapse of cavities in a liquid, subject to rapid and intense pressure changes. In this situation, the liquid was the oil used to lubricate the piston and housing. The very rapid movement of the piston through the housing caused the rapid and intense pressure changes in the oil, thus causing cavitation damage.


Failure of Wire Rope

Failure Analysis of a Wire Rope - Multiple strand failure
Failure Analysis of a Wire Rope – Multiple strand failure
Failure Analysis of a Wire Rope
Failure Analysis of a Wire Rope

Specimen: 9/16-inch diameter; 8 x 26 Cushion Pac 8 High Performance Wire Rope

Material: Improved Plowed Steel

Background: The wire rope was used on a hoist cable from a 110-foot crane in the shipping area of a manufacturing plant. The wire rope fractured while lifting bundles which weighed only approx. 28% of the maximum recommended load capacity of the crane.

Service Life: 6 Months

Findings: The majority of the breaks in the wire rope occurred in a similar region of the strand’s helix. This would indicate that the fracture had initiated by bending fatigue which would load the wires repeatedly in one plane. Multiple fatigue breaks eventually lead to the ultimate catastrophic failure of the wire rope.

Gear Tooth Failure on a Triple Reduction Gearbox

Overall view showing the two fractured gear teeth. For the areas of localized spalling, the spalling appears to start at the pitch circle and move upwards to the crest of the gear tooth.
Overall view showing the two fractured gear teeth. For the areas of localized spalling, the spalling appears to start at the pitch circle and move upwards to the crest of the gear tooth.
A Magnaflux test is performed on the gear in order to visualize the cracking by employing small magnetic particles and a fluorescent dye. Any cracks or fractures will become visible. The Magnaflux is considered non-destructive testing (NDT) as the specimen is not damaged after testing.
A Magnaflux test is performed on the gear in order to visualize the cracking by employing small magnetic particles and a fluorescent dye. Any cracks or fractures will become visible. The Magnaflux is considered non-destructive testing (NDT) as the specimen is not damaged after testing.
Polished and etched transverse section of two gear teeth. Etching has revealed the case hardened layer (appears light grey in color). Two areas of the damage to the flank of the 2 adjacent gears (as indicated by the arrows). The subsurface cracking is visible at both locations.
Polished and etched transverse section of two gear teeth. Etching has revealed the case hardened layer (appears light grey in color). Two areas of the damage to the flank of the 2 adjacent gears (as indicated by the arrows). The subsurface cracking is visible at both locations.

Specimen: Two fractured gear teeth of a gear with 185mm face width.

Background: A triple reduction gearbox had failed after 2 years of service. The gearbox has an impact of 700 HP @ 1780rpm on an expeller and is one of many other similar gear boxes used. Analysis will be performed in order to determine the root cause for the gear failure to provide information whether gear failure in the other gear boxes may be imminent.

Service Life: Approximately 2 years

Findings: Microscopic examinations of cross-sections of the gear teeth show the presence of subsurface cracking at the location of the pitch circle where the initial point contact occurs between meshing gears. Subsurface cracking is indicative of excessive loading. The presence of subsurface cracking is a characteristic of rolling contact fatigue. The gear teeth are not able to transmit a load of 700 HP. The other gear boxes should be inspected for the early stages of rolling contact failure of the gear teeth.