• RESPECTED FACULTY- VATSAL SHAH
BHATT
KALPESH J.
130460119
009
BHALODIA
KRUNAL
130460119
007
AHIR
RAVI
130460119
001
ACHARYA
BHUSAN
1304601190
13
BELANI
HARSH
1304601190
06
Introduction Of Gears
Classification Of Gears
According To Toothed Wheels
Classification Of Gears
Planetary (OR Epicyclic)
Gears
Nomenclature Of Gear Teeth
Advantages & Disadvantages
Gear Box Design
 Gears are used to transmit motion from one
shaft to another or between shaft and a slide.
This is achieved by successfully engaging teeth.
 Gears used no intermediate links or connector
and transmit the motion by direct contact.
 The surface of two bodies make a tangential
contact.
 The two bodies have either a rolling or a
sliding motion along the tangent at the point of
contact.
i. Parallel
ii. Intersecting
iii. Non-Intersecting & Non-Parallel
i. Low velocity
ii. Medium velocity
iii. High velocity
i. External gearing
ii. Internal gearing
iii. Rack & Pinion
i. Straight
ii. Inclined
iii. Curved
Fig.:-Various Types Of Gears
Spur gears are the most commonly used
gear type. They are characterized by teeth which are:
-- perpendicular to the face of the gear. Spur gears
are by far the most commonly available, and are
generally the least expensive. The basic descriptive
geometry for a spur gear is shown in the figure
below.
Spur gears generally cannot be used
when a direction change between the two shafts is
required.
: Spur gears are easy to find,
inexpensive, and efficient.
Fig.:- Gear Box Of Motor Cycle Using Spur Gears
: Helical gears are similar to the spur
gear except that the teeth are at an angle to the shaft,
rather than parallel to it as in a spur gear. The
resulting teeth are longer than the teeth on a spur
gear of equivalent pitch diameter. The longer teeth
cause helical gears to have the following differences
from spur gears of the same size:
• Tooth strength is greater because the teeth are
longer,
• Greater surface contact on the teeth allows a helical
gear to carry more load than a spur gear,
• The longer surface of contact reduces the efficiency
of a helical gear relative to a spur gear.
Fig.:-Helical Gear
• Helical gears may be used to mesh two shafts that are not parallel,
although they are still primarily use in parallel shaft applications. A
special application in which helical gears are used is a crossed gear mesh,
in which the two shafts are perpendicular to each other: The basic
descriptive geometry for a helical gear is essentially the same as that of
the spur gear, except that the helix angle must be added as a parameter.
: Helical gears have the major disadvantage that they
are expensive and much more difficult to find. Helical gears are also
slightly less efficient than a spur gear of the same size (see above).
Helical gears can be used on non parallel and even
perpendicular shafts, and can carry higher loads than can spur gears.
• To avoid axial thrust, two helical
gears of opposite hand can be
mounted side by side, to cancel
resulting thrust forces
• Herringbone gears are mostly used
on heavy machinery.
• Axial Thrust is a force that is
generated in an axial direction which
is along the shaft.
Bevel gears are primarily used to transfer
power between intersecting shafts. The teeth of these gears are
formed on a conical surface. Standard bevel gears have teeth
which are cut straight and are all parallel to the line pointing the
apex of the cone on which the teeth are based. Spiral bevel gears
are also available which have teeth that form arcs. Hypocycloid
bevel gears are a special type of spiral gear that will allow nonintersecting, non-parallel shafts to mesh. Straight tool bevel
gears are generally considered the best choice for systems with
speeds lower than 1000 feet per minute: they commonly become
noisy above this point.
:
Limited availability. Cannot be used for
parallel shafts. Can become noisy at high speeds.
:
systems.
Excellent choice for intersecting shaft
Fig.:-Straight Bevel Gears
Fig.:-Spiral Bevel Gear
Worm gears are special gears that resemble
screws, and can be used to drive spur gears or helical
gears.Worm gears, like helical gears, allow two nonintersecting 'skew' shafts to mesh. Normally, the two shafts
are at right angles to each other. A worm gear is equivalent to
a V-type screw thread. Another way of looking at a worm
gear is that it is a helical gear with a very high helix angle.
Worm gears are normally used when a high gear ratio is
desired, or again when the shafts are perpendicular to each
other. One very important feature of worm gear meshes that
is often of use is their irreversibility : when a worm gear is
turned, the meshing spur gear will turn, but turning the spur
gear will not turn the worm gear. The resulting mesh is 'self
locking', and is useful in racheting mechanisms.
Fig.:-Worm & Worm Gear
• Worm gears are normally used when a high gear ratio is desired, or again
when the shafts are perpendicular to each other. One very important feature
of worm gear meshes that is often of use is their irreversibility : when a
worm gear is turned, the meshing spur gear will turn, but turning the spur
gear will not turn the worm gear. The resulting mesh is 'self locking', and is
useful in racheting mechanisms.
•
Low efficiency. The worm drives the drive gear
primarily with slipping motion, thus there are high friction losses.
•
Will tolerate large loads and high speed ratios. Meshes
are self locking (which can be either an advantage or a disadvantage).
Rack and pinion is a type of linear actuator
that comprises a pair of gears which convert rotational
motion into linear motion. The circular pinion engages teeth
on a linear "gear" bar–the rack. Rotational motion applied to
the pinion will cause the rack to move to the side, up to the
limit of its travel. For example, in a rack railway, the
rotation of a pinion mounted on a locomotive or a railcar
engages a rack between the rails and pulls a train along a
steep slope.
•
The rack and pinion arrangement is commonly found in the
steering mechanism of cars or other wheeled, steered
vehicles. This arrangement provides a lesser mechanical
advantage than other mechanisms such as recirculating ball,
but much less backlash and greater feedback, or steering
"feel".
•
Enclosed steering rack in an automobileFor every pair of
conjugate involute profile, there is a basic rack. This basic
rack is the profile of the conjugate gear of infinite pitch
radius.
Fig.:-Lathe carriage drive mechanism showing rack and pinion arrangement.
Fig.:- Spur tooth rack and pinion
Fig.:- Helical tooth rack and pinion
• Gears whose centers can move.
• Used to achieve large speed reductions
in compact space.
• Can achieve different reduction ratios
by holding different combinations of
gears fixed.
• Used in automatic transmissions of
cars.
Fig.:-Planetary OR Epicyclic Gear
Components Of A Planetary Gear
Planet
Carrier
Input shaft
Sun gear
Ring gear
• They have higher gear ratios.
• They are popular for automatic
transmissions in automobiles.
• They are also used in bicycles for
controlling power of pedaling automatically
or manually.
• They are also used for power train between
internal combustion engine and an electric
motor.
A Variant Of A Planetary Gear
Clearance
Fillet radius
Pitch circle
gear diam.
Base Circle
• 1. Pitch Circle : - It is an imaginary circle which by pure rolling action, would give
the same motion as the actual gear.
• 2. Pitch Circle Diameter : - It is the diameter of the pitch circle.
• 3. Pitch Point : - It is a common point of contact between two pitch circles.
• 4. Pitch surface : - The surface of the imaginary rolling cylinder (cone, etc.) that the
toothed gear may be considered to replace.
• 5. Addendum circle : - A circle bounding the ends of the teeth, in a right section of
the gear.
• 6. Root or Deddendum circle : - The circle bounding the spaces between the
teeth, in a right section of the gear.
• 7. Addendum : - The radial distance between the pitch circle and the addendum
circle.
• 8. Deddendum : - The radial distance between the pitch circle and the root circle.
• 11. Flank of a tooth : - The part of the tooth surface lying inside the pitch surface.
• 12. Circular thickness ( also called the tooth thickness ) :- The thickness of
the tooth measured on the pitch circle. It is the length of an arc and not the length of a straight
line.
• 13. Tooth space : - The distance between adjacent teeth measured on the pitch circle.
• 14. Backlash : - The difference between the circle thickness of one gear and the
tooth space of the mating gear.
Backlash = Space width – Tooth thickness
• 15. Circular pitch (Pc) : - The width of a tooth and a space, measured on the
pitch circle or it is the distance measured on the circumference of the pitch circle
from a point of one tooth to the corresponding point on the next tooth.
• 16. Diametral pitch (Pd) : - The number of teeth of a gear per inch of its pitch
diameter. A toothed gear must have an integral number of teeth. The circular pitch, therefore,
equals the pitch circumference divided by the number of teeth. The diametral pitch is, by
definition, the number of teeth divided by the pitch diameter.
• 17. Module (m) : - Pitch diameter divided by number of teeth. The pitch diameter is
usually specified in inches or millimetres; in the former case the module is the inverse of
diametral pitch.
1.
2.
3.
4.
It transmit exact velocity ratio.
It has high efficiency.
It has reliable service.
It may be used to transmit large power.
1. The manufacture of gears requires special
tools and equipments.
2. The error in cutting teeth may cause
vibration & noise during operation.
Types
Features and
Applications
Precision Rating
Comments
Regarding
Precision
Spur
Parallel Shafting.
High speeds and
loads highest
efficiency
Precision Rating is
excellent.
Applicable to all
types of trains
and a wide range
of velocity
ratios.
Simplest tooth
elements offering
maximum
precision. First
choice,
recommended for
all the gear meshes,
except where very
high speeds and
loads or special
features of other
types, such as right
angle drive, cannot
be avoided.
Helical
Parallel Shafting. Very
high speeds and loads.
Most applicable to
high speeds and
Equivalent quality to
spurs, except for
Intersecting
shafts,
High speeds,
High loads.
Precision Rating
is fair to good.
Suitable for 1:1
and higher
velocity ratios
and for rightangle meshes
(and other
angles)
Good choice for
right angle drive,
particularly low
ratios. However
complicated both
form and fabrication
limits achievement
of precision. Should
be located at one of
the less critical
meshes of the train.
Worm & Worm Right-angle skew
High velocity
ratio
Angular meshes
High loads.
Worm can be made
to high precision,
but worm gear has
inherent limitations.
To be considered for
average precision
meshes, but can be
of high precision
with care. Best
Bevel
shafts,
High velocity
ratio,
High speeds and
loads, Low
efficiency, Most
designs
Types
Specials Face, Spiroid,
Helicon,
Beveloid.
Features and
Precision
Rating
Applications
Intersecting and Special cases
skew shafts.
Modest speeds
and loads.
Precision Rating
is fair to good
Comments
Regarding
Precision
To be avoided as
precision meshes.
Significant nonconjugate action
with departure from
nominal center
distance and shaft
angles. Fabrication
needs special
equipment and
inspection is
limited.
GEAR BOX
Stick shift
Synchronizers
The gear box is in first gear, second gear
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