VGO Testing & Inspection Engineers was retained to conduct a condition assessment of the gears from 12 spillway gate hoist system gearboxes from three, regional, flood control dams. The dams had been constructed in the mid-50’s to mid-60’s and were still using the original gearboxes. Thus, the gear boxes were on the order of 45 to 55 years old, which raised concerns about their continued serviceability. To address these concerns, a contract was awarded to the gearbox manufacturer for inspection and rehabilitation of the gearboxes.
When the manufacturer disassembled the gearboxes and inspected the internal components, numerous cracks and/or other defects were reportedly found in the gears, leading the manufacturer to recommend complete replacement of all the gears.
However, due to the long lead time of the proposed replacement, there were concerns that the new gears would not be available by the start of the flood season, at which time the dam spillways would definitely need to be operational. Consequently, VGO was retained to examine the gears to 1) categorize the types of defects, 2) determine the potential mode of failure that could be caused by the defects, and 3) assess the relative level of risk for failure on each of the gears, such that a decision could be made about which gearboxes needed to be rebuilt first and which ones could be re-used temporarily until new components became available.
To accomplish this, VGO engineers traveled to the gear manufacturer’s facility and inspected the gears by wet fluorescent magnetic particle testing. Based on the visual appearance of the crack indications, VGO engineers concluded that the vast majority of them were clearly due to casting defects—primarily shrinkage cracks and shrinkage cavities. However, by the time VGO engineers were on site, it had been determined that new gears could indeed be procured prior to the start of flood season. Thus, VGO’s assignment was altered to focus on assessing the types of defects in these 12 gearboxes, with a goal of providing the client with information that could be used to evaluate the condition of other gearboxes of similar vintage currently in use.
Subsequently, five of the gears were sent to VGO for laboratory analysis to more fully identify the nature of the flaws. VGO’s analysis confirmed that the flaws were indeed original manufacturing flaws, primarily in the form of shrinkage voids and hot cracks, which occurred during the casting of the gears.