THREADS AND FASTENERS - Goodheart-Willcox

preceding the designation, such as 8UN. This series
of threads is for special purposes, such as highpressure
applications. Constant-pitch threads are
also used for large diameters where the other
thread series do not meet the requirements.
Thread Classes
After classification by form and series,
threads are further classified by manufacturing
tolerance. These classes are 1A, 2A, and 3A for
external threads and 1B, 2B, and 3B for internal
threads. On some older drawings, classes 2 and
3 may appear without a letter designation.
Classes 1A and 1B replace the older
American Standard class 1. They are used in
applications requiring minimum binding and
allow for frequent and quick assembly or disassembly
of parts.
Classes 2A and 2B are threads with tighter
tolerances. They are used for general purposes
such as for nuts, bolts, screws, and normal applications
by mass production industries.
Class 3A and 3B are threads with very stringent
and close tolerances. They are used for
applications in industries requiring tighter tolerances
than the preceding classes of 1A and 1B, or
2A and 2B.
Multiple Threads
Threads can have two or three ridges running
side-by-side. These are called double threads or
triple threads. It may help to think of rope
wrapped around a pole. If one rope is wrapped, it
is like a single thread. If two ropes are wrapped,
they are like double threads. Three ropes
wrapped around the pole are like triple threads.
Simply looking at the thread generally will
not indicate if it is single, double, or triple.
However, a close inspection should reveal a
greater slope to double and triple threads than
that found on single threads. Due to the greater
slope angle of the thread, the “holding power”
of a double or triple thread is less.
Remember, the lead of a double thread is
twice the pitch. One revolution of the threaded
Unit 11 Threads and Fasteners 135
cylinder advances two pitches because there are
two ridges. Likewise, a triple thread advances
three pitches.
Left-Hand Threads
Threads are most commonly created so that
clockwise revolutions advance the threaded
parts together. Common nuts and bolts tighten
with clockwise turns. These threads are called
right-hand threads. However, some applications
require threads which tighten with counterclockwise
movement. These threads are referred
to as left-hand or reverse threads.
Left-hand threads are usually found on
items where right-hand threads may produce
unwanted loosening, perhaps due to motion.
Examples include bicycle pedal threads, threads
on a flush handle, arbor nut on a table saw
blade, and similar applications. Left-hand
threads are indicated in a print using a callout.
Detailed representations should also reflect a
slope angle opposite that of right-hand threads.
See Figure 11-6.
CLOCKWISE
RIGHT–HAND THREAD
LEFT–HAND THREAD
Figure 11-6.
Right-hand and left-hand threads.
COUNTERCLOCKWISE
136 Print Reading for Industry
Specification of Screw Threads
A screw thread is specified on a drawing
using a standard note with a leader and an
arrow pointing to the thread. The note contains
specific information for the specific thread. The
note in Figure 11-7 is interpreted as follows.
A) Nominal size (major diameter or screw
number)
B) Number of threads per inch
C) Thread form
D) Thread class number or symbol
E) Left-hand (LH), double threads (DOUBLE)
Threads are right-hand and single lead
unless otherwise specified. The letters LH after
the class symbol specify the thread as left hand.
The word DOUBLE indicates double thread. The
word TRIPLE indicates triple thread. Single
threads and right-hand threads do not require
RH or SINGLE to be specified.
Schematic and simplified thread representations
are not impacted by the thread form, the
pitch, left-hand, or multiple thread status. The
schematic spacing is usually just a factor of the
major diameter, and nothing more.
MAJOR DIAMETER IN INCHES
NUMBER OF
THREADS PER INCH
THREAD
FORM
THREAD
CLASS
1/2 – 13 UNC 2A
Figure 11-7.
A screw thread is specified in a local note.
Other specifications for threads may be
given. These additional specifications may
include thread length, hole size (for internal
threads), or chamfer. See Figure 11-8.
If the tolerance of the thread pitch diameter
is given, it is placed at the end of the specification
for the thread as follows.
3
− 10UNC − 2B PD 0.6850 TO 0.6927
4
The specification for a constant-pitch series
thread with a tolerance for the pitch diameter
would be listed as follows.
1
− 8UN − 3A PD 2.1688 TO 2.1611
4 2
Metric Threads
Metric threads are graphically represented
in the same manner as Unified and American
National Standard. However, the format of the
associated note is different. In a note for metric
threads, the letter M is followed by the diameter
and pitch. The M designates the thread as a
metric series. Thread designation is for a
coarse thread unless otherwise noted. Metric
A B C D E
1 – 5 ACME – 2A LH DOUBLE