TOOTHED WHEELS OR GEARS : Classification

In precision machine, in which a definite velocity is of importance, the only positive drive is by means of gears or toothed wheels. It is also provided, when the distance between the driver and the follower is very small.
Classification:

1. According to the position of axes of the shafts.

1. Parallel
2. Intersecting
3. Non-intersecting and non-parallel

• The two parallel and co-planar shafts connected by gears are called spur gear and this arrangement is known as spur gearing. These gears are parallel to the axis of the wheel.
• Another name given to the spur gearing is helical gearing, in which the teeth are inclined to the axis45°.
• The double helical gears are known as herringbone gears.
• The two non-planar or intersecting, but co-planar shafts connected by gears are called bevel gears and the arrangement is known as bevel gearing.
• Bevel gears, like spur gears may also have their teeth inclined to the face of the bevel, in which case they are known as helical bevel gears.
• The two non-intersecting and non-parallel i.e., non-coplanar shafts connected by gears is known as skew bevel gears or spiral gears and the arrangement is known as skew bevel gearing or spiral gears.
• This type of gearing also have a line contact, the rotation of which about the axes generates the two pitch surfaces known as hyperboloids.

2. According to the peripheral velocity of the gears.

1. Low velocity – less than 3m/sec
2. Medium velocity – between 3 and 15m/sec
3. High velocity – more than 15m/sec

3. According to the type of gearing

1. External gearing
   
2. Internal gearing
   
3. Rack and Pinion

• In external gearing, the gears of the two shafts mesh externally with each other. The larger of these two wheels is called spur wheel and the smaller wheel is called pinion. In an external gearing, the motion of the two wheels is always unlike.
• In internal gearing, the gears of the two shafts mesh internally with each other. The larger of these two wheels is called annular wheel and the smaller wheel is called pinion. In an internal gearing, the motion of the two wheels is always like.
• Sometimes, the gear of a shaft meshes externally and internally with the gears in a straight line. Such a type of gear is called rack and pinion. The straight-line gear is called rack and the circular wheel is called pinion. With the help of a rack and pinion, we can convert linear motion into rotary motion and vice versa.

4. According to position of teeth on the gear surface. The teeth on the gear surface may be straight, inclined or curved.

• The spur gears have straight teeth where as helical gears have their teeth inclined to the wheel trim. In case of spiral gears, the teeth are curved over the rim surface.