Technical Information
Defi nition of Screw Thread Tolerances
It is generally recognised that, in mass production, it is impossible
to reproduce in exact detail the theoretically perfect product
as laid out on the drawing board. The allowed slight variation between
the theoretically perfect product drawing and each unit of the actual
product is called the tolerance.
Allowance
nut
An intentional difference in correlated dimensions of mating parts.
It is the minimum clearance or maximum interference between
such parts.
Angle of Thread
major diameter
The angle included between the fl anks of the thread measured
in an axial plane.
pitch diameter
Half Angle of Thread
lead angle
minor diameter
The angle included between a fl ank of the thread and the
normal (90°) to the axis, measured in an axial plane.
A-thread
angle (whole)
Lead of Thread
_
The distance a screw thread advances axially in one turn. On a
single-thread screw, the lead and pitch are identical. On a double
thread, the lead is 2x pitch; on a triple thread, the lead is 3x pitch, etc.
A
pitch
_
a – half angle
Major Diameter
The largest diameter of a straight-screw thread.
Minor Diameter
screw
The smallest diameter of a straight-screw thread.
Pitch
The distance from a point on a screw thread to a corresponding point on
the next thread measured parallel to the axis.
The pitch in inches =
1
number of threads per inch
Pitch Diameter
Y100
=
width of
space
P.D.
On a straight-screw thread, the diameter
of an imaginary cylinder that would pass
through the threads at such points as to
make equal the width of the threads and
the width of the spaces cut by the surface
of the cylinder.
widia.com
pitch
width of
thread