Gear Metrology
Introduction Gears are mainly used for transmission of power or motion
from one shaft to another, without slip. Thus they are positive
in action and provide constant velocity ratio.
Terminology of Gear tooth
Addendum:- This is the radial distance from the pitch circle to
the tip of the tooth. Its value is equal to one module.
Clearance:- This is the radial distance from the tip of a tooth
to the bottom of a mating tooth space when the teeth are
symmetrically engaged. Its standard value is 0.157 m.
Dedendum:- This is the radial distance from the pitch circle to
the bottom of the tooth space.
Dedendum = Addendum + Clearance = m+0.157 m=1.153m.
Face of Tooth:- It is that part of the tooth surface which is
above the pitch surface.
Flank of Tooth:- It is that part of the tooth surface which is
lying below the pitch surface.
Tooth thickness:- The thickness of tooth, measured along the
pitch circle.
Whole depth= Addendum + Dedendum
Working depth= Addendum + Dedendum - Clearance
Pitch circle:- In every pair of
gears in mesh, the two plain
wheels in contact are assumed
to exist; which are known as
pitch circle.
Pitch point:- It is the point of
contact between the pitch circle
of two gears in mesh.
Circular pitch:- It is the arc distance measured around the
pitch circle from a point on one tooth to a corresponding point
on the adjacent tooth.
Circular pitch = πd/T = πm
d = pitch circle diameter
T = no. of teeth
m = module = d/T
Diametral pitch = Number of teeth
Pitch circle diameter =
T
d
Module = (1/ Diametral pitch)
Back lash = The diameter through which a gear can be
rotated to bring its non-working flank in contact with the teeth
of mating gear.
Pressure angle or angle of obliquity:- It is the angle which
common normal to the two teeth at the point of contact makes with common tangent to the two pitch circles at the pitch point.
Tooth thickness measurement:- Since tooth thickness is
defined as length of an arc, it is not possible to measure it
directly. There are various methods of measuring the gear
tooth thickness.
(i) Measurement of tooth thickness by gear tooth vernier
calliper (Chordal thickness method)
(ii) Constant chord method
(iii)Base tangent method
(iv)Measurement by dimension over pins.
Question :-
Calculate the setting of vernier for a straight spur gear having 40 teeth of
module 3.