Spur gears are used to transmit motion and power between parallel shafts. Their teeth are straight and run parallel to the shaft axis.
Number of teeth.- The total in a complete circle.
Pitch circle diameter.- The basis of measurement of gears; the size of a gear is its pitch circle diameter, called the pitch diameter. The pitch circles of mating gears are tangential.
Circular pitch.- The distance from the centre of one tooth to the centre of the next tooth measured round the circumference of the pitch circle.
Diametral pitch.- The number of teeth to one inch of the pitch diameter; e.g if a gear has 40 teeth and the pitch diameter is 4 inches, it has 10 teeth to each inch of its pitch diameter and is called 10 Diametral Pitch or 10 D.P.
Module or metric pitch.- The pitch circle diameter of a gear in millimetres divided by the number of teeth; e.g. if a gear has a pitch diameter of 240 millimetres and 60 teeth, it, is then called 4 module.
Outside diameter.- The diameter over the top of the teeth.
Addendum.- The distance from the pitch circle diameter to the top of the teeth.
Dedendum.- The distance from the pitch circle diameter to the root of the teeth.
Pressure angle.- The angle of the line of pressure or action.
Base circle diameter.- The circle from which the line of pressure or action is a tangent.
Face width.- The length of teeth in an axial direction.
Hunting tooth.- If a pair of spur gears have 40 teeth and 20 teeth after, every two revolutions the same pair of teeth engage. If however 41 teeth and 20 teeth are used, one will rotate 41 times and the other 20 before the same pair of teeth engage again. This extra tooth is called a Hunting Tooth and it serves to distribute wear more evenly, but the consequent alteration in Velocity and Gear ratios must be acceptable.
Gear and pinion.- When two gears are in mesh the smiler is called the pinion-and the larger the gear.
Gear ratio.- The number of teeth in the gear divided by the number of teeth in the pinion or the ratio of the Pitch Circle Diameters.
Velocity ratio.- The angular velocity of the driver divided by the angular velocity of the driven gear. For spur gears this ratio varies inversely as the pitch circle diameter and as the tooth numbers. Most spur gears have teeth of involute curve shape generated from the base circle. This shape ensures uniform velocity between mating gears.
Materials.- Spur gears are usually manufactured from cast iron, mild steel, bronze or brass, but on occasions fibre or nylon gears are used for silent runnings but obviously these cannot be used to transmit high load.
Mechanical Engineering Design Topics Page - Gearing Selection Page
Pages created by David Price - 9/08/95
This Information is an educational resource and is not to be reproduced (other than for private study) without
written consent of the University of Southampton. copyright © Dave Whatley 1995
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