THERMO Spoken Here! ~ J. Pohl © ~ 2019 (C8200-184)

Ship's Reduction Gear

The marine gas-turbine of a destroyer can produce 5000 horsepower at its output shaft with that shaft rotating at 25,000 revolutions per minute (rpm). The ship's turbine does not drive the ship through the water; that is what the ship's propellers do. To work well propellers turmn solwer, they turn at say 180 rpm.

Marine power engineers must design a reduction gear for each ship. The power of the turbine must be pass from the gast turbines (at the turbine rotational speed) through a reduction gear (a set of lubricated, cooled gears) to arrive at the propeller shaft with its power intact but a greatly reduced rotational speed. The gear changes the characteristics of power from "low-torque at high rpm" to "high-torque at lower rpm." The rotation rate is reduced and the torque of rotation is increased. These gear boxes are essential but inefficient. Our sketch shows the reduction gear of a gas-turbine destroyer in operation. Use the information of the sketches to answer:

a) Were the gear to operate ideally, what would be the output torque?

b) For the actual gear, calculate: i) the torque, ii) the heat rate and iii) the efficiency delivered to the prop shafts?

Problem for later: To maintain constant speed, an airline pilot sets the engines to 85% power then tips the nose up 5 degrees.

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