Reduction gears and couplings (c.93-94)

On practically all turbine ships, main turbines drive the propellers through mechanical reduction gears or by electri­cal transmission. When reduction gears are used, flexible couplings are employed between the turbines and the gear sets.

Gears or toothed wheels are used to transmit rotary motion and torque from one shaft to another which may have a dif­ferent speed or different direction of rotation or both.

Most present-day merchant marine turbines operate at speeds ranging between 3,000 and 10,000 r.p.m. and are connected to large, slow-turning propellers which operate at anywhere from 80 to 100 r.p.m. Consequently, a double re­duction main propulsion gear unit is required to provide the necessary speed reduction. Conventional practice employs two compound turbines, high pressure and low pressure, which connected through the gearing, drive a single propeller. One type of double reduction gear unit is known as the articulated.

To minimize the effects of hull distortion and provide for slight thermal expansion of various parts, turbine and pinion shafts are connected through couplings.

Turbine couplings may be of the solid or flexible type.

Solid couplings are generally used for small units, where the turbine and the driven member are mounted on a common bed or foundation.

For large turbines driving the propeller through a reduc­tion gear or turbines driving auxiliaries, where the driving and driven units each have their own two bearings, a flexible coupling is necessary. Flexible couplings permit some slight misalignment after installation, such as caused by pipe strains, expansion due to heat and others. In the propulsion turbines the claw and gear tooth types are used for rotors and pinions connection.

Words to be Learnt

toothedзубчатый wheelколесо torqueвращающий момент consequentlyкак следствие conventionalтрадиционный articulatedцепная (передача) distortion деформация

pinion shaft ведущая шестерня to permit позволять misalignment несоосность strain напряжение expansion расширение claw кулак, выступ

Notes

flexible coupling упругая муфта, соединение

gear set узел редуктора

to drive through a reduction gear приводить в движение через редуктор

double reduction gear двухступенчатый редуктор

speedreductionпонижение скорости (частоты вращения)

solidcouplingжесткая муфта

drivinganddrivenunitприводящий и приводимый в действие агрегат

claw coupling кулачковая муфта

Starting (c.98-99)

Before starting a unit certain precautions ought to be taken to make sure that the unit is ready for operation. The unit should be absolutely clean inside and out, and all parts securely fastened.

The following procedure for starting the unit should be observed: measure clearances where indicators are installed. See that the turbine rotors and gears move freely. This may be done by turning the units by means of a motor-driven turning gear.

All valves and cocks for draining water from the main steam pipe, manoeuvring valves and turbine casings should be opened.

All steam valves at the manoeuvring gear and about the turbines should be closed.

The cocks to the pressure and vacuum gauges on the tur­bine and condenser should be open.

Determine that there is a supply of circulating water through the oil cooler by turning on the water and then turn­ing it off.

Inspect the lubricating oil system carefully. It sometimes happens that the oil in the system becomes cold and viscous because of the weather or of the location of the oil tanks. Therefore, before starting the equipment it may be necessary to heat the oil in the tanks sufficiently to allow it to flow freely. The initial temperature of the oil should not be gen­erally lower than 90°F.

Warm up turbine by running slowly, alternately ahead and astern. It is desirable to allow a reasonable time for warm­ing up, say for one hour. If conditions are such as to pre­clude alternate running ahead and astern, it is preferable to omit the warming operation entirely. In this event it is espec­ially important that the first opening of the throttle valve be sufficient to start the turbine turning at once, to avoid unequal heating of the rotor and consequent warping. If the unit has been shut down for a few hours only, and the turbines are still warm, it is essential that the rolling alternately ahead and astern be carried out before bringing the unit up to speed.

Start first stage of the air ejectors, bringing vacuum up to normal.

Unit is now ready to manoeuvre or operate.