Law of gearingA primary requirement of gears is the constancy of angular velocities or proportionality of position transmission. High-speed gear trains also require transmission at constant angular velocities. Constant velocity is the 'conjugate action' of the gear tooth profile. A common normal to the tooth profiles at their point of contact must pass through a fixed point on the line of centers called the pitch point. Any two profiles engaging each other and satisfying the law of gearing are conjugate curves.
Gear Geometry
The essential features of a gear mesh are:
Center distance: The distance between the centers of two pitch circles.
Pitch diameters: The tangent to two basic circles is the line of contact in gear vernacular. Where this line crosses the line of center establishes the pitch. The ratio of pitch diameters gives the velocity ratio.
Pitch: It is a measure of tooth spacing along the pitch circle. There are two basic forms. Circular pitch is the direct measurement of distance of one tooth center to the adjacent tooth center. It is equal to the pitch circle circumference divided by the teeth. Diameter pitch is the measure of the number of teeth per inch of the pitch diameter. Both the pitches are inversely related to each other and permits an easy transformation from one to the other.
Number of teeth
Pressure angle of the contacting involutes: The angel between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Gears must have the same pitch and the pressure angel to mesh.
Module: It is the ratio of pitch diameter to the number of teeth. It is mainly used for metal gears. A higher module indicates coarser tooth spacing.
Gear train: When two or more gears are meshing, it is called a gear train.
Gear box: It is an automotive assembly of gears and associated parts by which power is transmitted from the engine to the driving axle.
Shafts: Cylindrical rods made of metal used for power transmission, linear motion and various other purposes in industries.
Sprockets: Teeth like projection arranged on a wheel rim to engage the links of a chain.
Pinions: Small tapered gear that meshes with a larger gear or rack.
Kinetics
When two gears are connected they rotate in opposite directions. The gear that does the driving is known as the driver and the other is known as the driven gear. If two gears have the same number of teeth then one turn of driver gear causes the driven gear to turn once. When connected to a power source it applies torque to the input shaft driving it at a considerable speed. For a single pair of gears the output shaft rotates at a different speed as that of the input shaft. The torque applied on the output shaft drives the load. One essential for the proper meshing of the gears is that the size of the teeth on the pinion should be the same as the size of the teeth on the wheel. The module must be common to both the gears. Pitch circles contact one another at the pitch point and the pinion's pitch line velocity must be identical to the wheels pitch line velocity. At the pitch point develops a tangential component of action-reaction due to contact between the gears. When the driver gear is smaller than the driven gear then speed is reduced and it amplifies torque in proportion to their teeth numbers. The only way that the input and output shafts of a gear pair can be made to rotate in the same sense is by interposition of an odd number of intermediate gears. These do not affect the speed ratio between input and output shafts. Such a gear train is called a simple train. If there is no power flow through the shaft of an intermediate gear then it is an idler gear.