Gears

Gear drives

Gears are defined as toothed wheels or multi-lobed cams, which transmit power and motion from one shaft to another by means of successive engagement of teeth.

Advantages of gear drives over belt drives

1. It is a positive drive and the velocity ratio remains constant.
2. The center distance between the shafts is relatively small, which results in compact construction.
3. It can transmit very large power, which is beyond the range of belt or chain drives.
4. It can transmit motion at very low velocity, which is not possible with belt drives.
5. The efficiency of gear drive is very high, even up to 99% in case of spur gears.

Classification of Gears:

Spur Gears

In spur gears, teeth are cut parallel to the axis of shaft. As the teeth are parallel to the axis of the shaft, spur gears are used only when the shafts are parallel. The profile of the gear tooth is in the shape of the involute curve and it remains identical along the entire width of the gear wheel. Spur gears impose radial loads on the shafts.

Helical gears

The teeth of helical gears are cut at an angle with the axis of the shaft. Helical gears have involute profile similar to that of spur gears. However, this involute profile is in a plane, which is perpendicular to the tooth element. The magnitude of the helix angle of pinion and gear is same. A right-handed pinion meshes with a left handed gear and vice-versa. Helical gears impose radial and thrust loads on shaft.

Herringbone gear

It is a special type of helical gear, consisting of two helical gears with the opposite hand of helix. The construction results in equal and opposite thrust reactions, balancing each other and imposing no thrust load on the shaft. Herringbone gears are used only for parallel shafts.

Bevel gears

Bevel gears have the shape of a truncated cone. The size of the gear tooth including the thickness and height, decreases towards the apex of the cone. Bevel gears are normally used for shafts, which are at right angles to each other. This, however, is not a rigid condition and the angle can be slightly more or less than 90 degrees. The tooth of the bevel gears can be cut straight or spiral. Bevel gears impose radial and thrust loads on the shafts.

Worm gears

Worm gears consist of a worm and a worm wheel. The worm is in the form of a threaded screw, which meshes with the matching wheel. The threads on the worm can be single or multi-start and usually have a small lead. Worm gear drives are used for shafts, the axes of which do not intersect and perpendicular to each other. The worm imposes high thrust load, while worm wheel imposes high radial load on the shafts. Worm gear drives are characterized by high speed reduction ratio.