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 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

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) 4=nominal diameter 8=number of threads/inch N=American Standard P=pipe S=straight pipe thread T=taper pipe thread