Beginners GuideDefinition:A simple definition of the purpose of
a gearbox may be stated as: " A Gearbox is a unit which receives
the primary input speed - driving speed source and alters it to a
different outputspeed - driven speed source using a series of
gears.When a gearbox alters the driving speed, it is said to create
a "mechanical advantage"Thus the fundamental purpose of a geardrive
is to transmit uniform motion between two shafts at a constant
ratio.The basic law of gearing states that to acheive this, the
common normal to the tooth profiles at all points must passthrough
a fixed point called the " Pitch Point"The various types of gears
that are used are:Spur - the axes of the shafts connected by the
gears are PARALELL.Bevel- the axes of the shafts INTERSECT.Spiral-
the axes of the shafts are NON-PARALELL and NON-INTERSECTING.
Helical- A type of spur gear in which although the axes of the
shafts are paralell the teeth are cut on helices instaed ofparalell
to the axis(i.e. straight across the wheel).Worm- basically a form
of spiral gearing in which the axes of the driving and driven
shafts are usually, though notinvariably, at right angles and the
velocity ratio is high, the driving wheel being of a small
diameter.Gearbox ratios and Gear Ratios:Gearbox ratio
definition:The ratio of the number of revolutions per minute of the
input source in relation to the number of revolutions per minute
ofthe output source is known as the ratio of the gearboxIn the
Materials Handling environment, the main driving source is usually
a standard electric motor, and the gearbox isutilised to reduce the
input speed of the electric motor to produce a lower output speed
from the gearbox.This reduction ratio of output speed vs. input
speed is known as the reduction ratio of the gearbox.Gear ratio
definition:Gearboxes generally contain a series of toothed gears
connected together as previously explained.Bevel and spiral bevel
gears are used as a pair of gears and the ratio of the gears can be
obtained by dividing the numberof teeth in the smaller gear into
the number of teeth in the larger gear. e.g. 48/17 = 2.82:1
RatioSpur gears can be used as a single pair or more than one pair
and the overall ratio of this latter arrangement can beobtained as
follows:Graphic Under Constructione.g. 27/12 X 36/15 X 44/17 =
13.98:1 Ratio
7/30/2019 Beginners Guide GEar Ratio
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This overall ratio of all the gears contained within a gearbox
is known as the gearbox ratio.The same comments apply to helical
gears,whether they be single or double helicals.A combination of
bevel or spiral bevel gears can be used in conjunction with spurs
or helicals in a gearbox.This provides a right-angled
drive.Wormgears are usually used as a single reduction gearbox, but
a gearbox can utilise two sets of worms and wheels, or,withone pair
of gears with either spur, helical or bevel helical gears to
provide a double reduction and a right angled drive.It should be
noted that ratio's given for wormgear units are usually nominal and
not exact.to obtain the exact gearbox ratio , divide the number of
starts in the worm into the number of teeth in the wheel e.g. 481/8
= 5.125:1 or 5:1 Nominal Ratio
