Threaded Fasteners Nomenclature and Definitions u u u

Threaded Fasteners

Nomenclature and Definitions u u u u u Fastener Types Head Style Drive Type Threads Identification Grade Nuts and Washers Thread Representation Miscellaneous

Fastener Types u Bolt • Bolts are defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification such that they can accept a non-tapered nut. • Screws are defined as headed, externally-threaded fasteners that do not meet the above definition of bolts. Screws always cut their own internal threads when initially installed, as there is generally no tool meeting the arbitrary specification of their threads to tap out the internal threads beforehand. Also, they are generally tapered to a sharp point. • Machine Screw u u Not really a screw by definition no chamfer <ؽ” < 3. 0” in length

Head Style • Flat A countersunk head with a flat top. Abbreviated FH • Oval A countersunk head with a rounded top. Abbreviated OH or OV • Pan A slightly rounded head with short vertical sides. Abbreviated PN • Truss An extra wide head with a rounded top. • Round A domed head. Abbreviated RH

Head Style (cont’d) • Hex A hexagonal Head. Abbreviated HH or HX • Hex Washer A hex head with built in washer. • Slotted Hex Washer A hex head with built in washer and a slot. • Socket Head Cap A small cylindrical head using a socket drive. • Button A low profile rounded head using a socket drive.

Drive Type u u Phillips An X shaped drive. Abbreviated - PH Slotted A slot in the head. Abbreviated - SL Combination A combination of slotted and Phillips drives. Abbreviated - Combo Socket, Hex or Allen A hexagonal hole for use with an Allen wrench.

Threads u u u Thread Count (TPI) TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. In general smaller fasteners have finer threads, so the thread count is higher. Thread Pitch Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1. 5 means that the distance between one thread and the next is 1. 5 mm. In general smaller fasteners have finer thread so they have lower thread pitch. Lead – Distance a bolt will travel during one complete revolution. (Equals pitch. )

Thread Types u Unified Thread • UNC – Unified National Coarse • UNF – Unified National Fine u u u u u American National Thread Sharp-V Thread Metric Thread Whitworth Thread Square Thread Acme Thread Buttress Thread Rolled Thread Pipe Thread • NPT – National Pipe Taper • NPS – National Pipe Straight

Unified Threads (detail) u u u u P = Pitch = 1/Number of threads per inch (TPI) H = Angular Depth = 0. 866025 x P d = Actual Depth = 0. 54127 x P or 5/8 H Hn = Basic height of Internal Thread = 0. 54127 x P or 5/8 H Hs = Basic height of External Thread = 0. 61344 x P H/8 = Shortening of major dia. = 0. 108253 x P H/4 = Shortening of minor dia. = 0. 216506 x P

Thread Detail cont’d • Crest Width = 1/8 Pitch • Root Width = ¼ Pitch • r = Radius at the Root = 0. 1443 x P

Internal and External Threads u Thread fit comparison

Thread Representations • Detailed - Pictorial, Time Consuming to Draw • Schematic • Simplified – Preferred method, versatile

Thread Representations

Identification u u In the United States, the convention for describing threads is to give the length of the bolt, followed by the nominal diameter and then the number of threads per inch. So, for example, one might ask for a “ 2 -inch quarter-twenty bolt, ” which would be • 2 inches long, • have a nominal diameter of a quarter of an inch, and • have twenty threads to the inch. If the bolt is smaller than a quarter-inch in diameter, the gauge number will precede the TPI instead of the diameter. A typical smaller fastener could be “a 1 -inch #8 -32, ” “ 8” being a gauge number and “ 32” the number of threads per inch.

Thread Identification u Proper annotation • ¼-20 x 2. 5 UNC 2 B u Class Fit • 1 -Loose • 2 -Standard • 3 -Close u Internal or external • A-External threads • B-Internal threads

Identification (cont’d. ) u u Bolt length • For fasteners where the head usually sits above the surface, the measurement is from directly under the head to the end of the fastener. • For fasteners that are designed to be countersunk, the measurement is made from the point on the head where the surface of the material is, to the end of the fastener. Thread length – • • Up to 6” bolt = 2 D+1/4” Over 6” bolt = 2 D+1/2” Bolts too short for the formula are threaded up to the head. Bolts can also be designated “full thread”.

Grade u Grade 5 fasteners are 120 ksi fasteners. This means the tensile strength is 120, 000 lbs per square inch. Grade 8’s are 150 ksi fasteners meaning the tensile strength is 150, 000 lbs per square inch. Also, the ultimate shear strength of a fastener is typically about 60% of its ultimate tension strength. So given a certain diameter (cross-sectional area) and strength rating, someone can figure out how much load that fastener can carry in both tension and shear.

Nuts and Washers u u u Hex A six sided nut. Also referred to as a finished hex nut. Nylon Insert Lock A nut with a nylon insert to prevent backing off. Also referred to as a Nylock Jam A hex nut with a reduced height. Wing A nut with 'wings' for hand tightening. Cap A nut with a domed top over the end of the fastener.

Nuts and Washers (cont’d. ) Acorn nuts are a high crown type of cap nut, used for appearance. u u u Flange A nut with a built in washer like flange. Tee A nut designed to be driven into wood to create a threaded hole. Square A four sided nut. K-Lock or Kep A nut with an attached free-spinning external tooth lock washer.

Nuts and Washers (cont’d. ) u u u Flat A flat washer used to distribute load. Split Lock The most common style of washer used to prevent nuts and bolts from backing out. External Tooth Lock A washer with external 'teeth' Used to prevent nuts and bolts from backing out. Internal Tooth Lock A washer with internal 'teeth' Used to prevent nuts and bolts from backing out. Fender An oversize flat washer used to further distribute load especially on soft materials. Finishing A washer used to obtain a 'finished' look. Usually used with oval or flat head screws.

Miscellaneous u u u u LH / RH Thread Set Screw Shoulder Bolt Carriage Bolt Counter sink Counter bore Thru Hole Tap Die Rivet Pin Key Retaining Ring