Reduction Gear 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
