Lec 8 Gear.ppt

7/29/2019 Lec 8 Gear.ppt

1/10

1

Rolling Cylinders Rolling Cones Rolling Hyperboloids

Gears are from

Spur gears Bevel gears Hypoid gears


2/10

2

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


3/10

3

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


4/10

4

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


5/10

5

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


6/10

6

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


7/10

7

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


8/10

8

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


9/10

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


10/10

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

T

dp

Lec 8 Gear.ppt

Documents

Transcript of Lec 8 Gear.ppt