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GLOSSARY OF TERMS
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Definitions of 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.
- 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 a 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 of 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 of the threads and the width of the spaces cut by the surface of the cone.
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- 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 between 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 as 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 cress-sectional area computed from the mean of the basic pitch and minor diameters.
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- 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
- Half Angle of Thread
- The angle included between a side of the thread and the normal to the axis, measured in an axial plane.
- Angle of Thread
- The angle included between 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 the 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 threaded 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.
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Terms Relating to Classification and Tolerances
- Allowance
- An intentional difference in the dimensions of mating parts. It is the minimum clearance of the maximum interference which is intended between mating parts. It represents the condition of the tightest permissible fit, or the largest internal member mated with the smallest external member.
Examples:
A 1/2-13 American National Coarse Thread.
Minimum pitch diameter of a nut (2B)............. 0.4500
Maximum pitch diameter of a screw (2A).......... .4485
Allowance.................................................. 0.0015
- Tolerance
- The amount of variation permitted in the size of a part.
Example:
A 1/2-13, 3A American National Coarse Thread Series.
Maximum pitch diameter........... 0.4500
Minimum pitch diameter............. .4463
Tolerance............................... 0.0037
- Basic Size
- the theoretical or nominal standard size from which all variations are made.
- Crest Clearance
- Defined on a screw form as the space between the crest of a thread and the root of its mating part.
- Finish
- the character of the surface on a screw thread or other product.
- Fit
- The relation between two mating parts with reference to the conditions of an assembly. For example: Wrench fit; close fit; medium fit; free fit; loose fit. The quality of fit is dependent upon both the relative size and the quality of finish of the mating parts.
- Neutral Zone
- A positive allowance (see "Allowance.")
- Limits
- the extreme permissible dimensions of a part.
Example:
1/2-13, Class 3A, American National Coarse Thread.
The limits are:
Maximum pitch diameter of a (3A)............. 0.4500
Minimum pitch diameter of (3A).......... .4463
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