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Screw and Thread Terms
(Tech Tip #8)
Can you explain some basic screw and thread terms?
- Screw Thread
- A ridge or uniform section in the form of a helix on the external or internal
surface of a cylinder, or in the form of a conical spiral on the external or internal
surface of a cone.
- External Thread
- An external thread is a thread on the outside of a member.
- Internal Thread
- An internal thread is a thread on the inside of a member.
- Major Diameter
- The largest diameter of the thread of the screw or nut. The term "major
diameter" replaces the term "outside diameter" as applied to the thread of a screw and also
the term "full diameter" as applied to the thread of a nut.
- Minor Diameter
- The smallest diameter of the thread of the screw or nut. The term "minor
diameter" replaces the term "core diameter" as applied to the thread of a screw and also
the term "inside diameter" as applied to the thread of a nut.
- Pitch Diameter
- On a straight screw thread, the diameter of an imaginary cylinder, the surface
of which would pass through the threads at such points as to make equal the width the
threads and the width of the spaces cut by the surface of the cylinder. On a taper screw
thread, the diameter, at a given distance from a reference plane perpendicular to the axis
of an imaginary cone, the surface of which would pass through the threads at such points as
to make equal the width the threads and the width of the spaces cut by the surface of the
cone.
- 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 divided by the Number of threads per inch
- Half Angle of Thread
- The angle included bewteen a side of the thread and the normal to the axis,
measured in an axial plane.
- Angle of Thread
- The angle included bewteen the sides of the thread measured in an axial
plane.
- Helix Angle
- The angle made by the helix, or conical spiral, of the thread at a pitch
diameter with a plane perpendicular to the axis.
- Crest
- The surface of the thread corresponding to the major diameter of the screw and
the minor diameter of the nut.
- Lead
- The distance a screw thread advances axially in one turn. On a single threaded
screw the lead and pitch are identical; on a double threaded screw the lead is two times
the pitch; on a triple headed screw the lead is three times the pitch, etc.
- Root
- The surface of the thread corresponding to the minor diameter of the screw and
the major diameter of the nut.
- Side or Flank
- The surface of the thread which connects the crest with the root.
- Axis of a Screw
- The longitudinal central line through the screw.
- Base of Thread
- The bottom section of the thread; the greatest section between the two adjacent
roots.
- Depth of Thread
- The distance between the crest and the base of the thread measured normal to
the axis.
- Number of Threads
- The number of threads in one inch of length.
- Length of Engagement
- The length of contact bewteen two mated threaded parts measured
axially.
- Depth of Engagement
- The depth of thread contact of two mated parts, measured radially.
- Pitch Line
- An element of the imaginary cylinder or cone specified under "Pitch
Diameter".
- Thickness of Thread
- The distance between the adjacent sides of the thread measured along or
parallel to the pitch line.
- Mean Area
- The term "mean area of a screw," when used in specifications and for other
purposes, designates the cross-sectional area computed from the mean of the basic pitch and
minor diameters.
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