The rated speed of the main propulsion diesel engines on your offshore supply vessel is 900 rpm. The installed centrifugal overspeed trip device similar to the one shown in the illustration is designed to shut down the engine at 110% of rated speed. Upon testing the overspeed trip device, you determine that the actual shutdown occurs at 945 rpm. Which of the following would account for this? Illustration MO-0101
• Centrifugal overspeed trip operation – how the rotating weight (item #10) works against the compression spring (item #12) and linkage to cause shutdown. • Effect of binding in linkage pivots (items #10, 11, 15, 16) on the speed required to move the weight and compress the spring. • How spring preload and the adjusting nut/jam nut (items #13, 14) change the rpm at which the centrifugal force can overcome the spring.
• First compute 110% of the rated 900 rpm and compare that design trip speed to the actual 945 rpm. Is the device tripping too early or too late? • If any pivot or link is binding, does that make it easier or harder for the weight to move outward and compress the spring? Would that raise or lower the actual trip speed? • What happens to the required trip rpm if the spring is given more initial compression (preload) versus if the effective compression is reduced because an adjusting part can back off over time?
• Be sure you know whether the actual trip speed (945 rpm) is above or below the intended 110% speed and what that implies about spring force vs. centrifugal force. • Trace the force path from the counterweight through the links to the spring and decide whether added friction increases or decreases the trip rpm. • Confirm which option would effectively reduce the opposing spring force at a given rpm, causing the mechanism to trip at a lower speed than intended.
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