Wicket gate lever redesign
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Date
2012-06-11
Authors
Beatty, Jamie
Schnaider, Christopher
Jozwiak, Sebastian
Cortens, Leo
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Abstract
The purpose of this report is to provide analysis and redesign details on a mechanical component which our client, Manitoba Hydro, has noted are prone to failure. The component in question, a wicket gate lever, serves as a linkage that controls the flow of water in a hydroelectric generating station. After roughly forty years of operation, large cracks have developed in these components. Manitoba Hydro requires a new lever design which will not crack and will remain serviceable until the station is decommissioned.
Strain gauges placed on the wicket gate lever and surrounding components show that the stress at the crack location does not exceed 2.6 ksi. This is in reasonably close agreement with the finite element analysis and hand calculations performed on the part, which evaluated the stress at the crack location as 2.1 ksi and 2.7 ksi respectively. All of these stresses are well below the estimated endurance strength of 8.16 ksi for the ASTM A27 steel used in the wicket gate’s construction, which should indicate an infinite fatigue life of the part.
After performing the above analysis and examining the wicket gate levers, our team has determined that the failure of the wicket gate levers cannot be due to excessive stress, and the only other reasonable cause of failure is poor material quality and casting. Surface defects, large deviations in the as-cast part, and large voids would aid in crack initiation and propagation in the wicket gate levers. These flaws are therefore the only plausible cause of failure.
In order to rectify these problems, the material, manufacturing process, and geometry of the part have been changed. Cold wrought ASTM A36 plate steel will be used in order to facilitate the machining and welding processes to create part. Geometric changes have been made to reduce stress concentrations and provide weld surfaces. The machining process will provide a smooth surface finish, allow for very consistent and precise geometry, and eliminate any voids which would occur during the casting process.
Since the strain gauge analysis indicates low stress levels throughout the wicket gate lever relative to the yield stress, and since the new design improves upon the material qualities, cracking will not occur in the part. Overall, the new wicket gate lever is expected to remain in service after it is built, and will last until the station is decommissioned.
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Keywords
wicket, gate, lever, redesign, Manitoba Hydro