30-05-2012, 01:21 PM
MECHANISM
MECHANISM.ppt (Size: 635.5 KB / Downloads: 56)
Nomenclature of gear
Pitch circle: A circle the radius of which is equal to the distance from the gear axis to the pitch point.
Addendum circle: A circle bounding the ends of the teeth, in a right section of the gear.
Root (or dedendum) circle: The circle bounding the spaces between the teeth, in a right section of the gear.
Addendum: The radial distance between the pitch circle and the addendum circle.
Dedendum: The radial distance between the pitch circle and the root circle.
Clearance: The difference between the dedendum of one gear and the addendum of the mating gear
Gear trains
Combination of two or more gears when they made to mesh each other to transmit power from one shaft to another.
TYPES OF GEAR TRAIN
Simple gear train
Compound gear train
Reverted gear train
Epicylic gear train
Reverted gear train :
When the axes of the first driver and the last driven are co-axial, then the gear train is known as reverted gear train.
In a reverted gear train, the motion of the first gear and the last gear is same
Epicyclic gear train :
Epicyclic means one gear revolving upon and around another. The design involves planet and sun gears as one orbits the other like a planet around the sun.
A small gear at the center called the sun, several medium sized gears called the planets and a large external gear called the ring gear.
Advantages
Simple gear train :
To obtain desired direction of motion of the driven gear ( CW or CCW)
and to obtain high speed ratio.
Compound gear train :
A much larger speed reduction from the first shaft to the last shaft can be obtained with small gear.
If a simple gear trains used to give a large speed reduction, the last gear has to be very large.
Reverted gear train :
The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in clocks (where the minute and hour hand shafts are co-axial).