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(Demanding question) It is required to power the propeller shaft of a ship using two identical high-speed diesel engines, each with a crankshaft which rotates in the clockwise direction when viewed from the power output end. The propeller shaft is to rotate at 125rev/min when each diesel engine is at its maximum power (and torque) condition of 4.9MW at a crankshaft rotational speed of 1,800 rev/min. The diesel engines are to be mounted side by side with a 3.80 m lateral spacing between their two crankshaft axes. The propeller shaft is to rotate in the clockwise direction when viewed from the stern of the ship. Sketch a layout for the gearing that would satisfy the above requirements and calculate suitable dimensions (diameters and widths) for the gears. Use a nominal surface stress of 618×106 Pa throughout. The elastic coefficient of the gear material should be taken as 1.91×105 (Pa)0.5 and the pressure angle as 20∘. Apply an application factor of 1.4 for the primary gear set and of 1.2 for any secondary gears. The following formula should be used:

Nominal surface stress =−Cp√2 Ft(R+1) /cosϕsinϕdpR

where the symbols take their usual meanings.

Answer :

A suitable layout for the gearing to power the propeller shaft of the ship can be achieved by using a primary gear set with a 1:1 gear ratio, followed by a secondary gear set with a reduction ratio of 125:1800.

The primary gear set should have gears with suitable dimensions to handle the maximum power and torque conditions of the diesel engines, while the secondary gear set should be designed with dimensions that can accommodate the desired propeller shaft speed.

To meet the requirements of powering the propeller shaft, a two-step gear system is proposed. In the first step, a primary gear set is used to establish a 1:1 gear ratio. This means that the rotational speed of the crankshaft of the diesel engines will be directly transferred to the first set of gears.

The primary gears should be designed with dimensions that can handle the maximum power (4.9MW) and torque conditions of the diesel engines. The application factor of 1.4 is applied to ensure reliability and account for any potential variations in operating conditions.

In the second step, a secondary gear set is introduced to achieve the desired reduction ratio. The reduction ratio is calculated as the ratio of the desired propeller shaft speed (125 rev/min) to the crankshaft rotational speed of the diesel engines (1800 rev/min).

By selecting appropriate gear sizes in the secondary gear set, the rotational speed can be reduced while transmitting power efficiently. The application factor of 1.2 is used for the secondary gears, considering the reduced load and power requirements compared to the primary gear set.

The dimensions of the gears, such as diameters and widths, are determined based on the nominal surface stress formula provided. The formula takes into account factors such as the contact pressure, the tooth form factor, and the pressure angle.

By substituting the appropriate values into the formula, suitable dimensions can be calculated for the gears in both the primary and secondary gear sets.

Learn more about Reduction ratio

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