ABSTRACT
This presentation illustrates gear theory, gear types, and gear nomenclature
and can also aid in gear design
INTRODUCTION
Gears are a means of changing the rate of rotating of a machinery shaft. They
can also change the direction of the axis of rotation and can change rotary
motion to linear motion.
Gear
A gear is a rotating machine part having cut teeth, or cogs,
which mesh with another toothed part in order to transmit torque.
Two or more gears working in tandem are called a transmission and can produce a
mechanical advantage through a gear ratio and thus may be considered a simple
machine. Geared
devices can change the speed,
torque, and direction of a power source. The most common situation is for a
gear to mesh with another gear, however a gear can also mesh a non-rotating
toothed part, called a rack, thereby producing translation instead of rotation.
•
The gears in a transmission are analogous to the wheels in a pulley. An
advantage of gears is that the teeth of a gear prevent slipping.
•
When two gears of unequal number of teeth are combined a mechanical
advantage is produced, with both the rotational speeds and the torques of the
two gears differing in a simple relationship.
•
In transmissions which offer multiple gear ratios, such as bicycles and
cars, the term gear, as in first gear, refers to a gear ratio rather
than an actual physical gear. The term is used to describe similar devices even
when gear ratio is continuous rather than discrete, or when the device does not
actually contain any gears, as in a continuously variable transmission
Types of
gears
External
vs internal gears
An external gear is one with the teeth formed on the outer surface of a
cylinder or cone. Conversely, an internal gear is one with the teeth
formed on the inner surface of a cylinder or cone. For bevel gears, an internal
gear is one with the pitch angle exceeding 90 degrees. Internal gears do not
cause direction reversal
Spur gear
Spur gears or straight-cut gears are the simplest type
of gear. They consist of a cylinder or disk with the teeth projecting
radially, and although they are not straight-sided in form, the edge of
each tooth is straight and aligned parallel to the axis of rotation. These
gears can be meshed together correctly only if they are fitted to parallel
shafts.
Bevel
gear
A bevel gear is shaped like a right circular cone with most of its tip cut off.
When two bevel gears mesh, their imaginary vertices must occupy the same point.
Their shaft axes also intersect at this point, forming an arbitrary
non-straight angle between the shafts. The angle between the shafts can be
anything except zero or 180 degrees. Bevel gears with equal numbers of teeth
and shaft axes at 90 degrees are called miter gears.
The teeth
of a bevel gear may be straight-cut as with spur gears, or they may be cut in a
variety of other shapes. Spiral bevel gear teeth are curved along the
tooth's length and set at an angle, analogously to the way helical gear teeth
are set at an angle compared to spur gear teeth. Zerol bevel gears have
teeth which are curved along their length, but not angled. Spiral bevel gears
have the same advantages and disadvantages relative to their straight-cut
cousins as helical gears do to spur gears. Straight bevel gears are generally
RACK AND
PINION
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A rack is a toothed bar or rod that can be thought of as a sector gear
with an infinitely large radius of curvature. Torque can be converted to linear
force by meshing a rack with a pinion: the pinion turns; the rack moves in a
straight line. Such a mechanism is used in automobiles to convert the rotation
of the steering wheel into the left-to-right motion of the tie rod(s). Racks
also feature in the theory of gear geometry, where, for instance, the tooth
shape of an interchangeable set of gears may be specified for the rack
(infinite radius), and the tooth shapes for gears of particular actual radii
then derived from that. The rack and pinion gear type is employed in a rack
railway.
THE WAY
GEARS WORK
Gears are
very versatile and can help produce a range of movements that can be used to
control the speed of action.
In basic
terms, gears are comparable to continuously applied levers,as one tooth is
engaging,another is disengaging.The amount of teeth each gear wheel has effects
the action on the gear wheel it engages or meshes with.
The gear
wheel being turned is called the input gears and the one it drives is called
the output gear.
Gears
with unequal numbers of teeth alter the speed between the input and output.
This is referred to as the following example shows how the ratios are
calcuated,
If the
input gear(A) has 10 teeth and the output gear (B) 30 teeth,then the ratio is
termed 3 to 1 and is written down as 3:1
Ratio = number
of teeth on the output gear B(30)
number of teeth on the input gear A(10)
The first figure(3) refers to how many turns the input gear(1) must turn in
order to rotate the output gear 1 full revolution.
The
principle behind gears is also very simple.in the above example,for every
complete revolution of the input gear the out put turns 1/3 of the way round.
General
Gear Nomenclature
Rotation
frequency, n
Measured
in rotation over time, such as RPM.
Angular
frequency, ω
Measured
in radians per second. 1RPM = π / 30 rad/second
Number of
teeth, N
How many
teeth a gear has, an integer. In the case of worms, it is the number of thread
starts that the worm has.
Gear,
wheel
The
larger of two interacting gears or a gear on its own.
Pinion
The
smaller of two interacting gears.
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Path of contact
Path followed by the point of contact between two meshing gear teeth.
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Line of action, pressure line
Line along which the force between two meshing gear teeth is directed. It has
the same direction as the force vector. In general, the line of action changes
from moment to moment during the period of engagement of a pair of teeth. For
involute gears, however, the tooth-to-tooth force is always directed along the
same line—that is, the line of action is constant. This implies that for
involute gears the path of contact is also a straight line, coincident with the
line of action—as is indeed the case.
•
Axis
Axis of revolution of the gear; center line of the shaft.
•
Module, m
A scaling factor used in metric gears with units in millimeters whose effect is
to enlarge the gear tooth size as the module increases and reduce the size as
the module decreases. Module can be defined in the normal (mn), the transverse
(mt), or the axial planes (ma) depending on the design approach employed and
the type of gear being designed.Module is typically an input value into the
gear design and is seldom calculated.
Addendum,
a
Radial
distance from the pitch surface to the outermost point of the tooth. a = (Do −
D) / 2
Dedendum,
b
Radial
distance from the depth of the tooth trough to the pitch surface. b = (D −
rootdiameter) / 2
Whole
depth, ht
The
distance from the top of the tooth to the root; it is equal to addendum plus
dedendum or to working depth plus clearance.
Clearance
Distance
between the root circle of a gear and the addendum circle of its mate.
Working
depth
Depth of
engagement of two gears, that is, the sum of their operating addendums.
CONCLUSION
With these paper presentation we can now differentiate types of gears and where
each type will be used and also know the principle of gear.
04:23:00
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PHYSICS
,
THE PRINCIPLE OF A GEAR
