Technologic outline of the practical Lesson 10 gears a gear is a toothed wheel

Technologic outline of the practical Lesson 10

GEARS

A gear is a toothed wheel used to transmit rotary motion from one shaft to another. If power is transmitted between the two shafts, the angular velocity ratio of these two shafts is constant and the driving shaft and the driven shaft rotate at a uniform rate. Shafts may be parallel, intersecting, and non-coplanar. Types of gears may be diverse depending upon the above positions of the shafts. Gears may be classified according to their shape and according to the position which the teeth occupy respectively to the axis of rotation. The teeth cut on the face of a gear may be curved, straight or helical.

The main types of gears are: bevel gears, eccentric gears, helical or spiral gears, herringbone gears, screw gears, spur gears and worm gears.

Bevel gearing is used to transmit power between two shafts, which lie in a

common plane and whose axes intersect each other. The axes may be inclined to each other at any angle, although 90° is the most common one. The teeth of bevel gears may be either straight or spiral. In the straight bevel gears the elements of teeth converge to a common point called the “apex”.

Eccentric gears operating on parallel shafts are used to transmit a varying angular velocity either continuously or for a portion of revolution.

Helical or spiral gears operate on parallel shafts at high speeds, providing maximum strength of gear teeth for a given width of face.

Such gears are heat- treated and then ground to accurate shape and size, necessary for smooth and quiet running at high speeds. The teeth of helical gears, having been cut on a conical surface, curve continually toward or away from the apex of the cone upon which they are cut. These gears closely resemble bevel gears and are frequently called spiral bevel gears.

Similarly to helical gears, herringbone gears also operate on parallel shafts. Herringbone gears have helical teeth radiating from the centre of the face towards the sides of the gear body. They are used where high speeds and high gear ratios are necessary.