This document discusses different types of gears used in mechanical systems. It describes gears based on the position of their axes, including parallel axes like spur gears and intersecting axes like bevel gears. It also discusses gear dimensions, terminology, and specifications. Key gear elements that are defined include pitch diameter, diametral pitch, module, addendum, dedendum, circular pitch, and pressure angle. Different gear tooth profiles and their applications are also outlined.

1. Gears
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2. Gears
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3. Friction wheels or discs
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4. Friction wheels or discs
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5. Toothed Gears
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6. 6

7. Based on the position of axes of shafts
 Parallel axes (e.g. Spur Gears)
 Teeth of spur gears are parallel to shaft axis
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8. Based on the position of axes of shafts
 Parallel axes (e.g. Spur Gears)
 Teeth of spur gears are parallel to shaft axis
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9. Based on the position of axes of shafts
 Parallel axes (e.g. Helical Gears)
 Teeth of helical gears are inclined to shaft
axis
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10. Based on the position of axes of shafts
 Parallel axes (e.g. Helical Gears)
 Teeth of helical gears are inclined to shaft
axis
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11. Based on the position of axes of shafts
 Parallel axes (e.g. Double Helical Gears or
herringbone gears)
 Teeth of double helical gears are also
inclined to shaft axis
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12. Based on the position of axes of shafts
 Parallel axes (e.g. Double Helical Gears or
herringbone gears)
 Teeth of double helical gears are also
inclined to shaft axis
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13. Based on the position of axes of shafts
 Intersecting axes (e.g. Bevel Gears)
 Teeth of bevel gears may either be parallel
to shaft axis (simple bevel gears) or inclined
to shaft axis (named as helical bevel gears).
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14. Based on the position of axes of shafts
 Intersecting axes (e.g. Bevel Gears)
 Teeth of bevel gears may either be parallel
to shaft axis (simple bevel gears) or inclined
to shaft axis (named as helical bevel gears).
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15. Based on the position of axes of shafts
 Non-parallel and Non-intersecting axes (e.g.
Spiral Gears)
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17. Based on the position of axes of shafts
Mitres
 When equal bevel gears (having equal teeth)
connect two shafts whose axes are mutually
perpendicular, then the bevel gears are
known as mitres.
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21.  Hypoid bevel gears also have a curved-tooth
shape but are designed to operate with
nonintersecting axes.
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Rear-drive automobiles use hypoid gears in
the rear axle so that the drive-shaft axis can
be below the axis of the axle and thus
permit a lower floor height.

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23.  For two gears to operate properly, their pitch
circles must be tangential to each other.
 The point at which the two pitch circles are
tangent, at which they intersect the
centreline connecting their centres of
rotation, is called the pitch point.
 The common normal at the point of contact
of mating teeth must pass through the pitch
point.
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25.  The basic size of gear teeth may be
expressed in two ways.
 The common practice, especially in the
United States and England, is to express the
dimensions as a function of the diametral
pitch (DP).
 DP is the number of teeth (N) per unit of
pitch diameter (PD); thus (PD) N/PD.
Dimensionally, DP involves inches in the
English system and millimetres in the SI
system, and it is a measure of tooth size.
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26.  Metric gears use the module system (M),
defined as the pitch diameter divided by the
number of teeth, or M = PD/N.
 It thus is the reciprocal of diametral pitch
and is expressed in millimetres.
 Any two gears having the same diametral
pitch or module will mesh properly if they
are mounted so as to have the correct
distances and relationship.
 The important tooth elements can be
specified in terms of the diametral pitch or
the module and are as follows:
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27.  Addendum: the radial distance from the
pitch circle to the outside diameter.
 Dedendum: the radial distance from the
pitch circle to the root circle. It is equal to
the addendum plus the clearance, which is
provided to prevent the outer corner of a
tooth from touching against the bottom of
the tooth space.
 Circular pitch: the distance between
corresponding points of adjacent teeth,
measured along the pitch circle.
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28.  Tooth thickness: the thickness of a tooth,
measured along the pitch circle. When tooth
thickness and the corresponding tooth space are
equal, no backlash exists in a pair of mating
gears.
 Face width: the length of the gear teeth in an
axial plane.
 Tooth face: the mating surface between the
pitch circle and the addendum circle.
 Tooth flank: the mating surface between the
pitch circle and the root circle.
 Pressure angle: the angle between a tangent to
the tooth profile and a line perpendicular to the
pitch surface.
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31.  Four shapes of gear teeth are used in the
United States:
 14 -1/2° pressure angle, full depth (used
most frequently)
 14 -1/2° pressure angle, composite (seldom
used)
 20° pressure angle, full depth (seldom used)
 20° pressure angle, stub tooth (second most
common)
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39.  Special type of compound gear train used in
automible transmission systems!
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