Reliability of roller-mounted spillway gates

Roller-mounted spillway gates were introduced for Reclamation projects in the 20's and 30's. This was the era of innovative gate designs, and a number of distinctive spillway gate types were designed. These included: radial/tainter gates; drum gates; roller-mounted gates; rolling gates; and crest gates. We have found that with time, some of these gate types are no longer produced. Roller-mounted gates are one of these now obsolete types. The reason for their demise is due to two factors: costly manufacturing and maintenance compared to the other types. We still have a number of these gates in operation, and the challenge, until they are replaced with another type of gate, is to keep them operational. The roller gate has the potential of malfunctioning if not maintained adequately. This is a result of the rollers, pins, and links, which make up the roller assemblies becoming corroded. If the pins seize on the rollers, the motion changes from rolling to sliding. When the links corrode and break the rollers can come off of the tracks. The change in motion of the rollers has a very adverse affect on the forces to move the gate. The friction increases by a couple of orders of magnitude, when going from rolling to sliding motion. A thorough inspection of the roller assemblies are required to determine their condition. This inspection may be time consuming and difficult to perform. By the nature of their design, to observe the rollers functioning may not be practical, especially when they are submerged. Reclamation has a number of installations with roller gates, and we are concerned about their operability under emergency conditions. They may function adequately under normal loads, but under conditions when the reservoir's elevation increases into an emergency situation the gates may fail to operate. This was the situation at a project in California in 1995, where the gate's shear pins were failing under an emergency situation. The shear pins function was to protect the gate and hoisting system if loads became excessive. Fortunately no major damage was done due to the high discharges. At another installation the results may not have been so benign. The project has three, 30-foot by 30-foot spillway gates, located on a 45 degree incline on the upstream face of a concrete buttressed dam. These gates have required two major refurbishments in their service life since the early 30's. Inspection of the rollers is practically impossible, because of the installation arrangement. Even using climbers to inspect the downstream side of the gates left inconclusive evidence on whether the rollers were still functioning correctly. To overhaul the roller assemblies is very expensive, as the gates have to be jacked off the tracks to work on the assemblies. Selecting the time for an overhaul is critical, if performed too early, funds are used inefficiently, too late and they may not function under high heads. In the Spring of 2003, we performed tests on two of the gates to determine whether the rollers were functioning correctly. This entailed installing strain gages on the lifting stems and operating the gates a couple of feet. Analysis of the data reveled that two out of the three carriages per side were sliding, indicating that the gates should be overhauled in the near future. A combination of testing, maintenance and inspections are necessary to ensure roller-mounted spillway gates will operate reliably under all conditions. 8 pp.