SCREW FASTENERS

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as a male thread (external thread) or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.

A screw will almost always have a head on one end which contains a specially formed shape that allows it to be turned, or driven, with a tool. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws often have a head smaller than the outer diameter of the screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.The distance between each thread is called the "pitch".

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey." Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to counter 8clockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. The left side pedal of a bicycle has a left-hand thread.

TYPES OF SCREW FASTENERS

Fastener Categories

Wood ScrewsScrews with a smooth shank and tapered point for use in wood. Abbreviated WS

Machine ScrewsScrews with threads for use with a nut or tapped hole. Abbreviated MS

Thread Cutting Machine ScrewsMachine screws with a thread cutting (self tapping) point.

Sheet Metal ScrewsFully threaded screws with a point for use in sheet metal.Abbreviated SMS

Self Drilling SMSA Sheet metal screw with a self drilling point.

Hex BoltsBolts with a hexagonal head with threads for use with a nut or tapped hole.Abbreviated HHMB or HXBT

Carriage BoltsBolts with a smooth rounded head that has a small square section underneath.

Lag BoltsBolts with a wood threadand pointed tip. Abbreviated Lag

Flange BoltsFlange bolts have a flange on the bottom of the head that distributes the load like a washer.

Socket Screws Socket screws, also known as Allen head are fastened with a hex Allen wrench.

Eye BoltsA bolt with a circular ring on the head end. Used for attaching rope or chain.

Eye LagsSimilar to an eye bolt but with wood threads instead of machine thread.

U-BoltsBolts in U shape for attaching to pipe or other round surfaces. Also available with a square bend.

J-BoltsJ shaped bolts are used for tie-downs or as an open eye bolt.

Shoulder BoltsShoulder bolts (also known as stripper bolts) are used to create a pivot point.

Elevator BoltsElevator bolts are often used in conveyor systems. They have a large, flat head.

Sex BoltsSex bolts (a.k.a. barrel nuts or Chicago bolts) have a female thread and are used for through bolting applications where a head is desired on both sides of the joint.

Mating ScrewsMating screws have a shoulder that matches the diameter of the sex bolts they are used with.

Hanger BoltsHanger bolts have wood thread on one end and machine thread on the other end.

Set Screws Machine screws with no head for screwing all the way into threaded holes.

Head Styles

FlatA 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

HexA hexagonal Head.Abbreviated HH or

HX

Hex WasherA hex head with

builtin washer.

Slotted Hex Washer

A hex head with built in washer and

a slot.

   

Socket CapA small cylindrical head using a socket drive.

ButtonA low profile rounded head using a socket drive.

   

Drive Types

PhillipsThe standard drive type for most screws.Abbreviated PH

FrearsonSimilar to Phillips. Requires a Frearson driver for installation.

SlottedA slot in the head.

Abbreviated SL

CombinationA combination of

slotted and Phillips drives.

Abbreviated Combo

Socket, Hex or Allen

A hexagonal hole for use with an Allen wrench.

One WayInstalls with a normal slotted

driver but can not be removed

without special

SquareAlso known as Robertson drive.Abbreviated SQ or SD.

StarAlso known as Torx®. A six-pointed star pattern, specifically

tools. designed to prevent cam-out and stripped heads.

Nut Types

HexA 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

JamA hex nut with a reduced height.

Nylon Insert Jam Lock

A nylock nut with a reduced height.

WingA nut with 'wings'

for hand tightening.

CapA nut with a

domed top over the end of the

fastener.

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.

Prevailing torque lock

A non-reversible lock nut used for high temperature

applications

K-Lock or KepA nut with an attached free-

spinning external tooth lockwasher.

2-Way Reversible LockA lock nut that can be installed either side up. Often used in high temperature applications.

CouplingCoupling nuts are long nuts used to connect pieces of threaded rod or other male threaded fasteners.

SlottedSlotted nuts are used in conjunction with a cotter pin on drilled shank fasteners to prevent loosening.

CastleCastle nuts are similar to slotted nuts but with the slots in a rounded section above the main nut.

Washer Types

FlatA flat washer used

to distribute load.

Available in SAE, USS, and other

patterns.

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 head screws.

Split LockThe 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.

SquareA square shaped

washer.

DockDock washers have a larger outside diameter and are thicker than standard.

   

OgeeThick, large diameter, cast iron washers with a curved or sculpted appearance. Typically used in dock and wood construction

SealingA neoprene washer bonded to a metal backing. Used to seal out air/water or dampen noise and vibration.