Gears:Definition and Types

INTRODUCTION

·         Gears are mechanical devices with teeth like projections on its peripheral and they are used to transmit power from a driving shaft to a driven shaft with constant velocity ratio (so gears are positive drives).In simple words, gears are used to transmit speed from a prime mover( like an electric motor) to machine with increment or decrements as required. A limitation of gears is that they can’t be used for large distance power transmission and they cost damn high!( velocity ratio is also known as gear ratio or speed ratio of gear ).

·         There are different methods and techniques for transmitting power are by using belt, chain, rope etc but problems with these techniques are: they don’t provide required or desired velocity ratio and they can’t be used for very small distance like in watches.

·         There are different types of gears are available in nowadays.Spur gears are first gears to be designed besides these different gears available in the market now are helical gear, bevel gear, double helical gear( herringbone gear ), worm and worm gear, rack and pinion etc.

·         In gear pair smallest gear is known as the pinion.

What are Different Types of Gears?

SOME BASICS ABOUT DEVELOPMENT OF GEARS

v  As we know spur gears are the first one to be made when there was needed to transmit power with required or desired velocity ratio. Teeth of spur gears are straight, spur gear pair can be used for low-speed power transmission but at high speed, they fail because of slapping action produce between teeth of spur gear when they mesh so they are not desirable at high speed.

                So we were in need of improved gear, these improved gears are known as helical gears. Helical gears are same as spur gear but they have inclined teeth instead of straight teeth due to this when two helical gear mesh engagement of their teeth is gradual. Because of this advantage of spur gear they can be used for the high-speed application. But a small disadvantage is that axial thrust is produced by the inclination of a tooth which causes failure of bearing. A cost of the bearing is low and failure of it depends on the speed of shaft so it is not a major problem until the shaft speed is too much high like in a racing car.

                 In a very high-speed application like in racing cars, it is not advisable to use helical gear because of axial thrust. If helical gears are used then bearing will fail in seconds because of the high speed of the shaft. Here we need to eliminate this axial thrust by equal and opposite thrust. Herringbone or double helical gears are designed based on this principal. In herringbone gear, there are two set of inclined teeth which inclination are opposite to each other because of this axial thrust produced by both sets are equal and opposite so bearing will be saved.

Herringbone or double helical gear can be used only for very high-speed applications. It  can’t be used for all applications because of this:

Cost of spur gear> Cost of helical gear>> Cost of herringbone gear

Terminology of Gears: Pitch, Pitch Circle, Module &Others

HELICAL GEAR PAIR

SPUR GEAR PAIR

HERRINGBONE OR DOUBLE HELICAL GEARS