Threads and Fasteners

Screw Threads Threads act as an incline plane wrapped around a surface They work by forcing objects back and forth by rotating an internal thread through an external thread Most basic threads follow the “American National Standard for Unified Screw Threads” UN- Other forms are Worm thread or Acme Thread

Terminology Actually, the crests and roots are flattened or rounded during forming process

Metric M and MJ threads Metric is similar to UN series threads but uses all metric sizing, each type is not interchangeable H=0.5p/tan 300 (0.25p)/(0.5H)=tan 300

Vocabulary Pitch- Distance between each thread or P=1 / no. of threads per inch Lead- Distance the threaded part moves against a fixed part in one full revolution Root- Bottom of the Thread Crest- Top of the thread Nominal Dia.- “Major” outside diameter of thread Minor Dia.- Diameter from root to root Thread angle- angle of thread between crests Depth- perpendicular distance from crest to root

Types of thread drawing Schematic Simplified Detailed

Detailed Pictorial display of threads Applications Sales catalog Display drawing Time consuming to draw

Schematic Not commonly used in industry Require extra drawing time

Simplified Preferred method of thread representation Describe threads clearly Easy and quick to draw Versatile Can be used in all situations

Thread standards - 6 types UNC or NC- Unified coarse thread, general use UNF- Unified Fine thread, automotive or aircraft UNEF- Unified Extra Fine, aircraft or other applications where very fine threads are needed. Sizes based on inches and have a standard thread count depending on type

Reading threads Classes of fit 1, 2, 3 – 1 is least accurate Direction- A is external, B is internal Example 7/8”-9 UNC 3” -2B (RH) .875 - is the major dia. 9 - is the threads per inch UNC - is the standard 2 - is the class of fit B - is the direction 3” is the length in inches RH- right handed threads, assumed

Bolts and Nuts

Bolts and Nuts

Thread Profiles General use Power screws & machine tool threads Thread profiles. (a) ACME; (b) UN (unified; American National ) Text Reference: Figure 15.3, page 668

(a) Single-, (b) double-, and (c) triple threaded screws. Threads l=p l=2p l=3p l; lead p: pitch (a) Single-, (b) double-, and (c) triple threaded screws. Multiple threaded screws Text Reference: Figure 15.2, page 667

Types of Threaded Fasteners Three types of threaded fastener. (a) Bolt and nut; (c) Cap screw; (c) stud. Text Reference: Figure 15.8, page 679

F or W= width across flats = 1.5 D H= Height of Bolt head = 2/3 D Threaded Fasteners TL Hexagon head bolt F or W= width across flats = 1.5 D H= Height of Bolt head = 2/3 D T = Height of nut = 7/8 D TL=thread length=2 x D + ¼”

(a) Fillister head (b) flat head (c) hexagonal socket head

As bolt holes may have burrs or sharp edge after drilling that can increase stress concentration, so use washer under the bolt head, and also under the nuts

Hexagonal nuts (a) end view, (b) washer faced regular nut, (c) regular nut chamfered on both sides, (d) jam nut with washer face, (e) jam nut chamfered on both sides During tightening, the first thread of the nut tends to take the entire load. But yielding occurs, with some strengthening due to cold work that takes place, the load is eventually divided over three nut threads. Don’t’ reuse nuts; it can be dangerous

Screw CAD Steps- Side view Draw Major dia Draw Head Width Dia, 1.5 x MD Draw polygon head shape- circumscribed Vertical flats – hex head Angled flats - square head Transfer screw head intersections

Screw CAD Steps- Front (Bolt Head) Create head Height- 2/3 MD Trim shape from transferred screw head intersections Create tangent arcs from 60* intersections on bolt faces Draw 30* chamfers on outside corners line up with head diameter Trim outsides

Screw CAD Steps- Front (Bolt Threads) Carry over MD and create screw length Determine thread depth 2 x MD + ¼” Offset 1 pitch= 1/TPI Draw 60* thread to create mD depth, carry to side view. Mirror thread to other side Shift thread left or right ½ P for direction Connect crest and root Array # of threads to fit thread depth Chamfer first thread 45*