The importance
of metric gears are slowly gaining in prominence. In fact all over the world
with perhaps a few exceptions use metric gears. Though it must be
appreciated that in terms of gear theory there is no difference between the
inch and metric systems. However the formulae for metric gearing has a
different set of symbols. There are many metric gearing standards followed by
countries like Germany, Japan, Australia and the international ISO standard.
Let's see what metric gearing is all about. Metric gears are typically
defined by a module system, which is as follows:
(Images of the nomenclatures used in a typical metric gearing system)
- Module: The length measured in mm of pitch circle diameter
per tooth. As represented by MOD = PCD / N
- Number of Teeth: Number of teeth on a gear. Represented by
N = PCD / MOD
- Pitch Circle Diameter: The diameter of a pitch circle.
Represented by PCD = N x MOD
- Outside Diameter: The outside diameter of a gear. Represented
by OD = (N + 2) x MOD
- Center Distance: The distance measured between axes of two
gears in a mesh. Represented by C = PCD(gear) + PCD(pinion) 2
- Circular Pitch: The distance between two adjacent teeth that
is measured along the arc at the pitch circle diameter. Represented by
CP = p x MOD
- Circular Tooth Thickness: The width of the tooth which is
measured along the arc at the pitch circle diameter. Represented by CTT
= CP / 2
- Addendum: The height of a tooth above a pitch circle
diameter. Represented by A = MOD Dedendum:The depth of a tooth which is
below the pitch circle diameter. Represented by D = H - A
- Whole Depth: The total depth of the space found between two
adjacent teeth. a)For Finer than 1.25 MOD: H = 2.4 x MOD b)1.25 MOD and
coarser: H = 2.25 x MOD
