Chapter 11 Threaded Fasteners
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Two Basic types of fasteners: Permanent & removable
Permanent- Examples: Rivets & welds
Removable- Examples: bolts, screws, nuts, pins, rings keys
Standardization reduces the costs of fasteners
• 3 purposes
– To hold parts together
– To adjust
– To transfer power
Screw Threads
• Definition: a SCREW THREAD is a ridge of uniform section in the form of a
helix on the external or internal surface of a cylinder
• Definition: the PITCH of a thread P is the distance from a point on the
thread form to the corresponding point on the next form, measured
parallel to the axis
• Definition: the LEAD L is the distance the threaded part would move
parallel to the axis during one complete rotation in relation to a fixed
mating part (the distance a screw would enter a threaded hole in one
turn)
THREADED FASTENERS
SCREW THREAD TERMS
THREADED FASTENERS
THREAD FORMS
THREADED FASTENERS
THREAD FORMS
Three types of thread representation
used on working drawings
• Simplified
• Detailed
• Schematic
Right- and Left-Hand threads
• Threads are assumed to be right hand unless
otherwise designated
• A bolt being threaded into a tapped hole
would be turned in a right-hand (clockwise)
direction
THREADED FASTENERS
RIGHT- AND LEFT-HAND THREADS
THREADED FASTENERS
LEAD = 1P
SINGLE THREAD
THREADED FASTENERS
LEAD = 2P
DOUBLE THREAD
THREADED FASTENERS
LEAD = 3P
TRIPLE THREAD
Inch Threads
• Definition: PITCH =1/#threads per inch
• Definition: THREAD SERIES are for fasteners
with different diameters and the same
#threads per inch
• UNC Unified Course Thread Series
• UNF Unified Fine Thread Series (Table
appendix)
• UNEF extra fine thread series
THREADED FASTENERS
BASIC THREAD CALLOUT
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Thread Class
1A,2A,3A External
1B,2B,3B Internal
1A,1B: loosest fit
2A,2B: ordinary good grade of commercial
product
3A,3B: High-grade commercial products
Example: .750-10 UNC-2A
class of fit
• Nominal Diameter
thread series
#threads/inch
(Diameter, #threads/inch, thread form & series, class of fit)
THREADED FASTENERS
EXTERNAL THREADS
THREADED FASTENERS
BLIND HOLE
INTERNAL THREAD
Metric Threads
• Definition: Pitch for metric threads is the distance between corresponding
points on adjacent teeth.
• Course-thread is intended for general engineering work and commercial
applications
• Fine-thread series is for general use. Compared with coarse, a fine-thread
screw is stronger in both tensile and torsional strength
• Definition: the FIT of a screw thread is the amount of clearance between
the internal and external threads when they are assembled
• Tolerance grades: the larger the number, the larger the tolerance
– Tolerance position e (large allowance)
– Tolerance position g (small allowance)
– Tolerance position h (no allowance)
– For internal threads:
• Tolerance position G (small allowance)
• Tolerance position H (no allowance)
THREADED FASTENERS
BASIC THREAD CALLOUT
METRIC THREADS
ISO Metric Screw Thread Designation
• Nominal size (basic major diameter) and pitch
expressed in millimeters.
• Example: M16 x 1.5 – 5g6g
Metric designation Nominal diameter Pitch
Tolerance position & grade
• Example: M10 x 1.5 x 25
Ф10mm
1.5 pitch
25 mm long
• Example: M16 (pitch may be omitted for coarse threads)
Metric Threads continued
• For external threads, the length of the thread
may be given as a dimension of the drawing.
• For threaded holes that go all the way through
the part, the term THRU is sometimes added
• If no depth is given, the hole is assumed to go
THRU
• For threaded holes that do not go all the way
thru, the depth is given in a note (minimum
depth of full thread) Ex. M12 x 1.75 x 20
Pipe Threads
• Inch: 4-8NPT or 4 NPT
• Metric: 4 IN. – 8 NPT or 4 IN.-NPT
• Nominal diameter and pipe thickness specify
inch-sized pipe (typical)
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4=nominal diameter
8=number of threads/inch
N=American Standard
P=pipe
S=straight pipe thread
T=taper pipe thread