REDUCTION GEARS AND COUPLINGS

 

Steam turbines must operate in a relatively high rpm range for greatest efficiency while propellers operate most efficiently in a relatively low rpm range. One means of accomplishing the necessary step down from high turbine shaft rpm to lower propeller shaft rpm is by use of an electric drive. The most common means of obtaining the required propeller rpm for ship propulsion is by use of reduction gears.

The use of reduction gears is by no means limited to ship propulsion. Other steam turbine-driven machinery such as ship’s service generators, air compressors, and various pumps also have reduction gears. In these, as in ship propulsion units, turbine operating efficiency requires a higher rpm range than that suitable for the driven unit.

Reduction gears are classified by the number of steps used to bring about the speed reduction and the arrangement of the gearing. There are single and double reduction gears, although double reduction is more usual.

Single reduction gear

This arrangement consists of only of one pair of gears. The reduction gear box consists of ports through which the propeller shaft and engine shaft enters the assembly. A small gear known as a pinion is driven by the incoming engine shaft. The pinion directly drives a large gear mounted on the propeller shaft. The speed is adjusted by making the ratio of the speed reduction to the diameter of pinion and gear proportional. Generally, a single gear assembly has a gear double the size of a pinion.

Double reduction gear

Double reduction gears are generally used in applications involving very high speeds. In this arrangement the pinion is connected to the input shaft using a flexible coupling. The pinion is connected to an intermediate gear known as the first reduction gear. The first reduction gear is then connected to a low speed pinion with the help of one more shaft. This pinion is connected to the second reduction gear mounted directly on the propeller shaft. Such arrangement facilitates the reduction of speed to a ratio as high as 20: 1.

Single or double reduction systems may be used, although double reduction is more usual. With single reduction the turbine drives a pinion with a small number of teeth and this pinion drives the main wheel which is directly coupled to the propeller shaft. With double reduction the turbine drives a primary pinion which drives a primary wheel. The primary wheel drives, on the same shaft, a secondary pinion which drives the main wheel. The main wheel is directly coupled to the propeller shaft. 

All modern marine gearing is of the double helical type. Helical means that the teeth form part of a helix on the periphery of the pinion or gear wheel. This means that at any time several teeth are in contact and thus the spread and transfer of load is much smoother. Double helical refers to the use of two wheels or pinions on each shaft with the teeth cut in opposite directions. This is because a single set of meshing helical teeth would produce a sideways force, moving the gears out of alignment. The double set in effect balances out this sideways force.

Double helical type gears are capable of transmitting large power loads smoothly, and do not impart axial thrust to either the driving member or the driven member.

In most reduction gears, all pinions are completely machined out of specially heat-treated nickel-steel forgings. The gear wheels are generally of built-up construction, with the teeth cut in forged steel bands which are welded to steel webs. The first reduction gears are generally welded on their respective shafts. The bull gear is usually pressed on the shaft against a locating shaft shoulder, secured by one or more keys. Lubrication of the meshing teeth is from the turbine lubricating oil supply. Sprayers are used to project oil at the meshing points both above and below and are arranged along the length of the gear wheellocked, with a locknut, on the shaft.

Coupling

Since the shaft (rotor) is made in small parts due to forging limitations and other technological and economic reasons, the couplings are required between any two rotors. This coupling permits angular misalignment, transmits axial thrust and ensures axial location. The couplings are either rigid or semi flexible. The former neither permits angular nor lateral defection while the later permits only angular defection. Number of critical speeds depends upon the modes of vibrations and hence the type of coupling provided between rotors.

These couplings must be capable of transmitting heavy loads and in some turbines are required to accommodate for axial expansion and contraction. The types of couplings generally employed in power plants are:

· Flexible coupling

· Solid shaft coupling

Where axial shaft movement is required a flexible coupling is employed and these are either: 1. Sliding claw (or tooth)

      2. Flexible connection (between the two flanges)

    Flexible couplings provide longitudinal and radial flexibility between the input shaft and the pinion. This permits each shaft to be adjusted axially and radially to its proper position without imposing excessive forces.

In most installations the flexible couplings are of the gear type. Power is transmitted through a floating intermediate member with external teeth that mesh with the internal teeth of the shaft rings (sleeve) mounted on the driving and driven shafts.

Turning gear

All geared turbine installations are equipped with a turning gear. The turning gear on a turbine installation is a reversible electric motor driving a gearwheel which meshes into the high-pressure turbine primary pinion.

The unit is used for turning the main engine during warming-up and securing periods so that the turbine rotor may heat or cool evenly. (The rotor of a hot turbine or of one that is in the process of being warmed up, with gland sealing steam cut in, will become bowed or distorted if left stationary even for a few minutes.) The turning gear is used for other routine purposes such as for turning the engine in order to bring the reduction gear teeth into view during routine inspection. In addition, the turning gear is used for the required daily jacking of the main engines.

 

EXERCISES

 

⇐ Предыдущая567891011121314Следующая ⇒

Поделиться: