MECHANICAL DRAWING Chapter 11: Threads Fasteners and Springs

Objectives of threads and fasteners 1. To hold parts together (Assembly). 2. To provide for adjustment between parts (Drum). 3. To transmit power

Define and label the parts of a screw thread Chapter Objectives Define and label the parts of a screw thread Identify various screw thread forms. Draw detailed, schematic, and simplified threads. Define typical thread specifications Draw various screw head types. Draw springs

Screw Thread Nomenclature External thread: a thread on the outside of a member (shaft). Internal thread: a thread on the inside of a member (hole). Major diameter: the largest diameter of the screw thread. Minor diameter: the smallest diameter of the screw thread. Pitch: the distance from a point on a screw thread to the corresponding point on the next thread measured parallel to the axis (US units = 1/ number of threads per inch). Pitch diameter: the diameter of an imaginary cylinder passing through the threads where the widths of the threads and the width of the spaces are equal

Screw Thread Nomenclature Lead: the distance that the screw advances in one turn. Angle of a thread: the angle between the sides of the thread. Crest: the outer surface of the thread Root: the inner surface of the thread. Side: Axis of the screw: From of the thread: the cross section of thread cut by a plane containing the axis Series of thread: the standard number of threads per inch for various diameters.

Standardization of screw thread is very important Important notes: Thread Form: International standard form (SI metric). Most widely used. United States forms (English units). USA, Great Britain and Canada. Standardization of screw thread is very important

Different number of threads per inch for various standard diameters Screw Thread Forms Sharp V thread: high friction because of full thread face. American national: flatten roots and crests. General use. Replace by the following thread. Unified thread (N): US standard thread (replaces the previous thread). Root is rounded, crest (rounded/flat). Old standard US threads: (still included in the new standard - 11 series) (UN or N); general. (UNC or NC); course thread series. (UNF or NF); fine thread series. (UNS or NS); special series. ●) Unified extra fine series (UNEF) Different number of threads per inch for various standard diameters

Screw Thread Forms Metric thread (M): Standard (international screw thread fasteners ISO). Normally flat crest and root, could be rounded in external thread. Similar to the unified thread (lower depth). Course thread (general purpose) Fine thread Square thread; power transmission (difficult to fabricate, and other problems). ACME thread is more used. ACME thread: largely replaces the square thread. ●) Worm standard thread; similar to the ACME thread but deeper.

Screw Thread Forms Whitworth thread: replaced by the unified thread (angle=55) Knuckle thread: usually rolled from sheet metal. (Ex. Electrical bulbs and sockets, bottle tops, …etc). Buttress thread: transmits power in one direction only. High strength requirements. ASME standards == American Society of Mechanical Engineers.

Right and Left-Hand Threads Thread Pitch, Right and Left-Hand Threads & Single and Multiple Threads Thread Pitch: Right and Left-Hand Threads Single and Multiple Threads

Drawing a Thread

Drawing a Thread Simplified and Schematic Internal Thread symbols

Tables for Screw threads American and Metric

Tables for Screw threads American and Metric

Tables for Screw threads American and Metric

Detailed representation: External Metric, Unified, & American National Threads (DRAWING)

Simplified External and Internal Threads (DRAWING)

Schematic External and Internal Threads (DRAWING)

American National Thread Fits A ≡ external thread B ≡ internal thread Class 1 fit: Clearance between mating parts is essential for rapid assembly, (1A, 1B). Class 2 fit: High quality of commercial thread product, (2A, 2B). Class 3 fit: Very high quality of commercial thread product (high cost), (3A, 3B) Metric and Unified Thread Fits g ≡ external thread H ≡ internal thread General-purpose application: (6H for internal, 6g for external) ≡ (2A, 2B). Closer fit: (6H for internal, 5g6g for external)

Thread Notes

Thread Notes

Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING) Note that slope line is in the opposite direction of the external thread as you see internal threads!

Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING) Note that slope line is in the opposite direction of the external thread as you see internal threads!

Detailed External Square Thread

Detailed External Square Thread Use the same table for square and acme threads

Detailed Internal Square Thread Lines from crest to crest Lines from root to screw axis

Detailed External ACME Thread

Phantom Lines and Threads in Assembly For long threads use phantom (hidden) lines Threads in Assembly Stud, bolt heads, and nuts are not sectioned External and internal threads are shown in V-shapes

One of the parts is threaded (no nut) Bolts, Studs, and Screws Threaded at both ends Greater thread length One of the parts is threaded (no nut)

Standard Bolts and Nuts Inch and metric sizes chamfered at 15-30o Regular Heads & Bolts (W = 1½D, H = ⅔D, T =⅞ D) Heavy Heads & bolts (structure) (W = 1½D + ⅛" (3mm) H = ⅔D, T = D) See Table in Appendix 20 for standard bolts and nuts dimensions (no standard for bolt lengths) Inch sizes only Chamfered at 30o (W = 1½D, H = ⅔D, T =⅞ D) Thread Length: Screws up to 6" length = 2D+¼" (6mm) Screws over 6" length = 2D+½" (12mm)

Standard Bolts and Nuts Across Corners Across Flats

Hexagonal Bolts, Cap Screws, and Nuts (DRAWING)

Square Bolts, Cap Screws, and Nuts (DRAWING)

Standard Cap Screws All screws except (e) can be drawn directly as in figures. For exact dimensions see table (A21)

Standard Cap Screws

Standard Machine Screws (Smaller, Thread all the length) All screws can be drawn directly as in figures. For exact dimensions see table (A21)

Keys, Pins, and Rivets (Keys)

Keys, Pins, and Rivets (Keys) Square or Flat Keys Gib Head Key Woodruff Keys and Key Slot Cutter Pratt & Whitney Key

Keys, Pins, and Rivets (Keys Drawing) Square and Flat Keys

Keys, Pins, and Rivets (Keys Drawing) Gib Head Keys

Keys, Pins, and Rivets (Keys Drawing) Pratt and Whitney Keys

Keys, Pins, and Rivets (Keys Drawing) Woodruff keys

Keys, Pins, and Rivets (Keys Drawing) Woodruff key solt

Keys, Pins, and Rivets (Machine Pins) Taper Pin: Used to fasten collars to shaft Collars See table A31 for standard diameters for given lengths

Keys, Pins, and Rivets (Rivets) Usage: Permanent fastening used to hold sheet metal or rolled steel together. Made of wrought iron, carbon steel, copper, or other metals. Hole diameter is slightly larger than the diameter of the rivet. Rivet diameter d = 1.2 to 1.4 t: thickness of the sheet A riveting machine is used to drive the rivet and forms the head on the plain end. This causes the rivet to swell and fill the hole tightly. wrought iron = حديد مطاوع Swell = become larger

Keys, Pins, and Rivets (Rivets) Riveted Joints :

Springs (Types)

Springs (Representation/Drawing) Schematic:

Springs (Representation/Drawing) Detailed: