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Rolling Cylinders Rolling Cones Rolling Hyperboloids
Gears are from
Spur gears Bevel gears Hypoid gears
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Spur Gears
Teeth are parallel to axis Least expensive type
Noisier High efficiency (99% / set)
Can be disengaged Backlash can be a problem
At the time of engagement of the two gears, the contact extends across the entire width
on a line parallel to the axis of rotation. This results in sudden application of the load,high impact stresses and excessive noise at high speeds
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Helical Gears
Teeth not parallel to axis(but curved )
Mating gears have same helix angle but opposite hands
Parallel or crossed axes More expensive than spur
Quieter than others Axial force component
Stronger tooth section Harder to disengageCrossed axes (e = 50 - 90% )
Parallel axes (e = 96 - 98%)At the beginning of contact only at the point of leading
edge of curved teeth. As the gears rotate, the contact
extends along a diagonal line across the teeth. Thus the
load application is gradual
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Herringbone Gears
Two opposite-hand helicals back to back
High power transmitting capacity
High pitch line velocity
As quiet as helicals
Very expensive to make
Double helical gear
Develop opposite thrust reactions
and thus cancel out the thrust force
within the gear itself. The net axialforce that acts on the bearings is
zero
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Rack and Pinion
One of the mating gear is of infinite pitch circle diameter
Rotary to linear motion
Can be spur, helical, or herringbone
Involute becomes a straight linetrapezoidal teeth
Used in rack and pinion steering
Used as a cutter to make circular gears
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Cone apices intersect Low power capacity
Not involute but octoid
Kinematically the motion is equivalent to the rolling of two cones
Straight bevel: Teeth are straight ,radial to the point of intersection of
the shaft axes and vary in cross section throughout their length.
Gear of same size connecting two shafts at right angles are mitre gears
Spiral bevel: Teeth are inclined at an angle to the face of the bevel.
Gradual load application as in helical gear
Zerol bevel: Curved teeth but with a zero degree spiral angle
Bevel Gear
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Worm has single tooth or thread Teeth are not involutes
Worm is helical with big helix angle Ratio = no. of teeth on wheel
Made and installed as matched pairs Center distance is crucial
Shaft have any angle, normally 90 Very expensive to make
Can design them to prevent backdriving Poor effy (40 - 85%) high sliding and thrust load
High ratios obtainable in single stage High torque capacity
Non-throated Single throated
Double throated
Worm Gear
id
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Hypoid Gears
Meant for nonparallel and nonintersecting axes
Non-involute teeth
High torque capacity
Used in automotive differentials inorder to lower the axis of the driveshaft
below the center of the rear axle to reduce
the driveshafthumpin the back seat
Hypoid pinion is larger and stronger
than the spiral bevel pinion
Shaft can pass each other so that
bearings can be used on both sides of
the gear and the pinionContinuous pitch line contact of the
two mating hypoid gears while in
action
Based on rolling hyperboloids
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Fig. 14.5Pitch cylinders: Imaginary friction
cylinders, which by pure rolling
together, transmit the same motion
as the pair of gears
Pitch circle: Circle corresponding to
the equivalent pitch cylinders
Pitch point: Point of contact of two
pitch circles
Addendum circle: Circle passing
through the tips of teeth
Dedendum circle: Circle passing
through the roots of the teeth
Whole depth: Total radial depth ofthe tooth space
Working depth: Maximum depth to
which a tooth penetrates into the
tooth space of the mating gear
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Circular pitch: Distance
measured along the
circumference of the pitch
circle from a point on one tooth
to the corresponding point on
the adjacent tooth
Width of space: Tooth space
along the pitch circle
Addendum: Radial height of a
tooth above the pitch circle(normally 1 module)
Dedendum: Radial depth of
tooth below the pitch circle(normally 1.25 module)
Clearance: Radial difference between the addendum and the dedendum of a tooth
Tooth thickness: Tooth thickness measured along the pitch circle
Top land: Surface of the top of the tooth
Bottom land: Surface of the bottom of the tooth between the adjacent fillets
Face: Tooth surface between the pitch circle and the top land
Flank: Tooth surface between the pitch circle and the bottom land including filletFillet: Curved portion of the tooth flank at the root circle
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