Threaded Fasteners

Nomenclature and DefinitionsFastener Types Head StyleDrive TypeThreadsIdentification GradeNuts and WashersThread RepresentationMiscellaneous

Fastener TypesBolt

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 Not really a screw by definitionno chamfer

Head StyleFlatA countersunk head with a flat top.Abbreviated FH

OvalA countersunk head with a rounded top.Abbreviated OH or OV

PanA slightly rounded head with short vertical sides.Abbreviated PN

TrussAn extra wide head with a rounded top. RoundA domed head.Abbreviated RH

Head Style (contd)HexA hexagonal Head.Abbreviated HH or HX

Hex WasherA hex head with built in washer.

Slotted Hex WasherA hex head with built in washer and a slot.

Socket Head CapA small cylindrical head using a socket drive.

ButtonA low profile rounded head using a socket drive.

Drive TypePhillips An X shaped drive.Abbreviated - PH

SlottedA slot in the head.Abbreviated - SL

CombinationA combination of slotted and Phillips drives.Abbreviated - Combo

Socket, Hex or AllenA hexagonal hole for use with an Allen wrench.

ThreadsThread 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 1.5mm. 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 TypesUnified Thread

UNC Unified National CoarseUNF Unified National FineAmerican National ThreadSharp-V ThreadMetric ThreadWhitworth ThreadSquare ThreadAcme ThreadButtress ThreadRolled ThreadPipe Thread

NPT National Pipe TaperNPS National Pipe Straight

Unified Threads (detail)P = Pitch = 1/Number of threads per inch (TPI)H = Angular Depth = 0.866025 x Pd = Actual Depth = 0.54127 x P or 5/8 HHn = Basic height of Internal Thread = 0.54127 x P or 5/8 HHs = Basic height of External Thread = 0.61344 x PH/8 = Shortening of major dia. = 0.108253 x PH/4 = Shortening of minor dia. = 0.216506 x P

Thread Detail contd

Crest Width = 1/8 PitchRoot Width = Pitchr = Radius at the Root = 0.1443 x P

Internal and External ThreadsThread fit comparison

Thread Representations

Detailed - Pictorial, Time Consuming to Draw

Schematic

Simplified Preferred method, versatile

Thread Representations

IdentificationIn 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 IdentificationProper annotation

-20 x 2.5 UNC 2BClass Fit

1-Loose2-Standard3-CloseInternal or external

A-External threadsB-Internal threads

Identification (contd.)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 = 2D+1/4Over 6 bolt = 2D+1/2Bolts too short for the formula are threaded up to the head.Bolts can also be designated full thread.

Grade

Grade 5 fasteners are 120 ksi fasteners. This means the tensile strength is 120,000 lbs per square inch. Grade 8s 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

Hex

A six sided nut. Also referred to as a finished hex nut.

Nylon Insert LockA 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 (contd.)AcornAcorn nuts are a high crown type of cap nut,

used for appearance.

FlangeA nut with a built in washer like flange.

TeeA nut designed to be driven into wood to create

a threaded hole.

SquareA four sided nut.

K-Lock or KepA nut with an attached free-spinning external

tooth lock washer.

Nuts and Washers (contd.)FlatA flat washer used to distribute load.

Split LockThe most common style of washer used to

prevent nuts and bolts from backing out.

External Tooth LockA washer with external 'teeth' Used to prevent

nuts and bolts from backing out.

Internal Tooth LockA washer with internal 'teeth' Used to prevent

nuts and bolts from backing out.

FenderAn oversize flat washer used to further distribute

load especially on soft materials.

FinishingA washer used to obtain a 'finished' look.

Usually used with oval or flat head screws.

MiscellaneousLH / RH ThreadSet ScrewShoulder BoltCarriage BoltCounter sinkCounter boreThru HoleTapDieRivetPinKeyRetaining Ring