Chapter 11 Threaded Fasteners• 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

SINGLE THREAD

LEAD = 1P

THREADED FASTENERS

DOUBLE THREAD

LEAD = 2P

THREADED FASTENERS

TRIPLE THREAD

LEAD = 3P

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

Thread Class1A,2A,3A External1B,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)• 4=nominal diameter• 8=number of threads/inch• N=American Standard• P=pipe• S=straight pipe thread• T=taper pipe thread