Reduction Gear Unit

Single Reduction Gear	Double Reduction Gear	Types
Why Use Reduction Gears	Epicyclic Reduction Gear unit	Worm Gear Reduction Unit

One of the most simplest and common types of Reduction Gear Units is the use of worm gear, applied in auxiliary machinery. While single as well as double helical gears are applied in ships with more complex type mechanism of propulsion machinery. Main Reduction gears are generally identified by the series of steps undertaken for bringing in speed reduction as well as the arrangement of the gearing. On that basis a classification is made about the type of Reduction Gears. There are following types of Reduction Gear.

Single Reduction Gear
In case of Single Reduction Gears, only a solitary pair of gears are used. A small gear called pinion that is driven by the input shaft is made to drive a big(bull) gear directly on the propeller shaft. In case of Single Reduction Gear, the ratio of speed reduction to diameter of the pinion and the gear is made proportional. To elucidate the point, in a 2:1 single reduction gear, the diameter of the driven gear is double of driving pinion.

(Cross section of single reduction marine gear)
Double Reduction Gear
In case of Double Reduction Gear units, a pinion having a high speed is connected to an input shaft. This is done by using a flexible coupling. This high speed pinion is used for driving an intermediate or first reduction gear. This first reduction gear is connected to the low speed pinion via a shaft. This in turn drives the bull gear or the second reduction gear that is mounted on a propeller shaft. To make things clear a 20 to 1 speed reduction can be achieved by having a 2:1 ratio between high speed pinion and first reduction gear. While 10:1 ratio is achieved between the low speed pinion located on the first reduction shaft and second reduction gear positioned on the propeller shaft. The following image would further clarify.

Further the Double reduction Gear Unit is further subdivided into the following types:

Types of Double Reduction Gears

Articulated
An Articulated type of gearing normally has more bearings and takes up more longitudinal space in comparison to the nested or locked train type of double reduction gearing. An Articulated propulsion reduction gears consists of:
- Two first reduction pinions (high speed).
- Two number first reduction gears.
- Two number second reduction pinions (slow speed).
- Main gear.
Nested type of Double Reduction Gearing
Nested gearing can be easily said to be the least complex of all the double reduction gears. Feature included:
- No use of quill shafts
- Minimum number of bearings
- Minimum flexible couplings
Locked Train
In the case of Locked Train type of double reduction gears, each of the pinions having high speed engages with or drives two of the first reduction gears. This way it is able to transmit power effectively to two second reduction pinions. The Locked Train type of Double reduction gears offer a distinct advantage over other types of main reduction gearing having similar dimensions. Here, the power path between the main gear and each of the first reduction pinion gets split in two ways in place of the usual one way. Thus the impact of load on a gear teeth under a said power condition is substantially less.

Why Use Reduction Gears?

One of the best ways to know why Reduction Gear Unit are used or why so much importance is attached to Reduction Gears is to understand the operation of Reduction Gears in terms of the engines used to drive a ship. In a ship, for greater efficiency steam turbines are required to operate with a relatively high range of rpm. While for an effective functioning of propellers a relatively low rpm range is needed. The Reduction Gear Unit accomplishes perfectly the task of obtaining the necessary propeller rpm for ship propulsion. A few popular types of reduction gear units are discussed here.

Types of Reduction Gear Unit