15.1.
Introduction
Gears are mainly used for transmission of power and motion. In order that the rotary motion of the driven shaft be perfectly uniform relative to the driving shaft, it is essential that both gears be of perfect geometrical form and be perfectly mounted on perfect shafts running in perfect bearings. It is thus obvious that a big factor which decides the accuracy of gearing is the precision with which gears are manufactured. For closer control over the accuracy of manufacture, precision measurement of gears plays a vital role. Fortunately, developments in manufacture of gear and its inspection techniques have gone hand-in-hand. For measurement of gears, which is rather complex field, it has been possible to follow general rule of metrology, i.e. the accuracy of measurement should be better than the component tolerance by a factor of 10.
The most commonly used forms of gear teeth are (i) involute and («) cycloidal. The involute tooth is derived from the trace of the point on a straight line, which rolls without slipping around a circle, which is the base circle, or it could be defined as the locus of a point on a piece of string which is unwounded from a stationary cylinder. The cycloidal tooth is derived from the curve which is the locus of a point on a circle rolling on the pitch circle of the gear. Here the addendum of the tooth is trace of the point on a circle rolling outside of pitch circle and this is an epicycloidal curve whereas the dedendum portion of the tooth is the trace of the point on a circle rolling on inside of the pitch circle of the gear and is a hypocycloidal curve. Involute gears are also called straight tooth or spur gears and are mainly used for general purpose in precision engineering due to some advantages. The cycloidal gears are not generally used in modern engineering, but used for some crude purposes where heavy and impact loads come on the machine. These along with bevel and spiral gears will not be considered here. The two great advangates associated with involute for the flank curve of gear teeth are;
(i) Variations in the centre distance between two gears have no effect on the velocity ratio between a pair of involute gears.
(ii) The involute rack has straight teeth. Thus the complex involute form on gear can be generated from a simple cutter.
(iii) The involute system has a standard pressure angle which is either 20° or 14.5° whereas on the cycloidal system the pressure angle varies from zero atpitch line to a maximum at the tips of teeth.
(iv) In the cycloidal system for achieving correct meshing the gears must be operated on centre that will maintain theoretical pitch circles in exact contact which is not necessary in involute system.
Various types of gears commonly used are:
Spur Gear : It is a cylindrical gear whose tooth traces are straight lines. Helical Gear : It is a cylindrical gear whose tooth traces are straight helices. Spiral Gear : A gear whose tooth traces are curved lines.
Straight Bevel : A gear whose tooth traces are straight line generators of a cone. It is Gear conical in form operating on intersecting axes usually at angles.
Worm Gear Pair: The worm and mating worm wheel have their axes non-parallel and non-intersecting.
