Chapter 3 Fasteners

Objectives Identify types of threaded fasteners. Identify the parts of a bolt. Identify a bolt by diameter, threads per inch, length. Identify types of bolts. Identify nuts.

Objectives (Cont.) Identify locking nuts. Identify types of screws. Identify types of washers. Identify various nonthreaded fasteners.

Fasteners Simple devices that hold parts together or in place Fasteners connect Metal to metal Plastic to metal Plastic to plastic

Types of Fasteners Threaded fasteners Mechanical fasteners Use screw threads to tighten two components together Mechanical fasteners Use mechanical force or process to hold parts together or make connection with component Retainers limit component’s movement while allowing it to slide, pivot, or rotate

Threaded Fasteners Thread Typical rotation Inclined plane wrapped in helix around central axis Converts rotational movement to linear Typical rotation Clockwise rotation tightens fastener Counterclockwise rotation loosens fastener

Linear Movement and Rotation

Special Applications Some applications require reverse threads Left-hand threads Slope downhill left to right Opposite incline to right-hand threads Tighten by counterclockwise rotation Loosen by clockwise rotation

External Threads Formed on outside of fastener Allow fastener to be screwed into threaded hole or nut

Internal Threads Formed inside of hole All nuts have internal threads

Bolts Externally threaded fasteners Pass through holes in two or more parts Parts are joined when nut is tightened onto bolt Most bolts must be held to tighten nut Consist of threads and shank

Screws Externally threaded fasteners Pass through hole in one part Thread into internally threaded hole (or form their own threads) in second part Tightened or released by rotating head

Bolts vs. Screws Screws Bolts Fasteners with thread diameters of less than 1/8″ Bolts Fasteners with thread diameters larger than 1/8″

Machine Screws Thread into another component Standard coarse and fine thread Can be used with nuts Both made in standard sizes Conform to each other

Non-Machine Screws Threads on screw cut matching threads in material into which it is tightened Used on plastic or soft metal Coarse threads on fasteners provide holding power

Tapping (Cutting Threads) Process of cutting threads in hole Tap tool (tap) is used to cut internal threads

Bolts and Machine Screws Head and body Body is divided into threaded portion and shank No shank if threads go completely to head

Bearing Surface Area underneath head Cap screw (round bearing surface) Usually flat and smooth to reduce friction Applies clamping force to components being tightened Cap screw (round bearing surface) Flat head fastener (tapered bearing surface)

Bolt and Screw Designations Designated by one or more Thread form nomenclature Type of head Type of head drive system Special features

Thread Form Nomenclature Identifies fasteners by Diameter Threads per inch Length Conforms to either US customary thread form system Metric thread form system

Thread Standards Terms to identify conforming threads—Unified National threads, inch threads, English threads Conforming thread standards are set by ASME ANSI IOS Standards set for fastener diameter and threads per inch

Bolt and Machine Screw Identification US customary bolts and machine screws are identified by Diameter Threads per inch (tpi) Length Format Diameterthreads per inch × length

Metric Bolts Identified by Diameter (in millimeters) Pitch Distance from tip of one thread to tip of next Amount of linear distance that fastener moves in one revolution

Metric Bolt Identification Format Format (in this order on head) Capital M Bolt body diameter in millimeters Dash (spoken as “by”) Thread pitch Example: M8–1.25

Metric Bolt Identification Standardized diameters and thread pitches Identified by pitch Smaller number after dash means more threads per bolt If no pitch, default is coarse thread pitch Multiplication symbol and following number indicates bolt length

Bolt and Screw Identification Type of head, drive system, or combination Head shape determines type of head Hexagonal (hex)-shaped Square Pan-shaped Flat

Hexagonal Head Fasteners (Flats) Can be held or rotated using standard wrenches because of standardized head dimensions Distance across two opposite sides of head conforms to standard size established for bolt thread diameters

Hex Flange (Washer) Head Fasteners Washer integrated with head Increases bearing surface Distributes torque load over larger surface area than standard hex head

Serrated (Binding) Head Bolts Raised serrations on bearing surface lock into surface of component Prevents bolt from loosening or backing out Angled serrations keep vibrations from loosening bolt

Square Head Fasteners Four-sided head fits into square opening Not necessary to hold head with wrench while tightening nut Exposed threaded end used as stud

Types of Screw Heads

Drive Systems Hexagon socket head Hex head, socket head

Drive Systems (Cont.) Torx-type (hexalobular) Size designated by “T” followed by number External (reverse) Torx fastener Size designated by “E”

Drive Systems (Cont.) Cross-recessed drive Phillips-type JIS

Drive Systems (Cont.) Combination drive systems

Special Feature Fasteners Threaded fastener with unique design Intended to be used for specific applications

Stud Usually threaded at both ends Different thread type on each end One end installed in part Other end accepts nut Different thread type on each end

Stud Bolt Head is anchored in part Threaded end protrudes to act as stud

Square Shoulder (Carriage) Bolt Shoulder located directly under head Inserted into preformed hole in one component so nut can be applied to threaded end to attach second component

Hex Socket Head Shoulder Screw Hex socket recess in head Round shoulder shank diameter is larger than fastener thread diameter

SEMS Fastener Washer is permanently attached to screw, bolt, or nut Fastener and washer are installed in one step

Set Screw Threaded fastener used to mechanically hold two components in position with each other Usually does not have head Intended to be installed flush with or below surface

Self-Threading (Thread Forming) Screw Special threads and ends displace and reshape material, forming internal threads Normally used in soft metals and plastics Can be used to produce machine threads or screw threads Internal threads formed during installation are permanent

Self-Threading (Thread Forming) Screw (Cont.) When screw is reinstalled, threads must screw into original internal threads To align screw with original threads Rotate screw backward by hand until it clicks Rotate screw clockwise to tighten

Sheet Metal Screw Fastens thin metal sheets or light-duty plastics Self-threading Internal threads remain after screw installed

Thumb Screw Screw with special designed heads Tightened and loosened by hand

Nut Internally threaded fastener that screws onto externally threaded fasteners Applies holding torque to keep components together under normal vibration Bolts pass through two components and use a nut to keep parts together

Typical Nuts Hex nuts Square nuts Locknuts Finger-tightened nuts

Hex Nut Six-sided nut that screws onto bolts Identified by thread form

Flange Hex (Hex Washer Head) Nut Washer bearing surface on one side of nut Distributes tightening force over larger surface area Nut can be torqued to higher value

Locknut Nut deformation or insert keeps nut from spinning freely and backing off

Wingnut Designed to be installed and tightened by hand

Washers Flat washer Thin disc with hole in middle Used between Surface of components and nut Surface of components and fastener head Allows large clamping force without damage

Washers (Cont.) Lock washer Keeps fasteners from loosening Can be used to improve electrical connections

Thread Adhesive Applied to fastener threads to prevent loosening due to vibration When fastener is reinstalled Remove old adhesive Reapply fresh adhesive with same holding power Color indicates qualities (strength, heat resistance, penetrating ability)

Adhesive Fasteners Threaded fastener with correct amount of adhesive preapplied When fastener is installed, friction from tightening releases adhesive Also called microencapsulated fasteners

Removing Cured Adhesive Dried adhesive must be removed from threaded hole Determine Fastener thread form and diameter Thread pitch Select tap with same thread type Thread tap into hole to remove old adhesive

Thread Repair Restoring threaded hole to its original thread form Metal threaded insert may sometimes be installed To repair threaded holes, follow manufacturer’s repair procedure

Torque System Torque Twisting force that tries to rotate fastener Stress keeps fastener tight (torqued) If torqued too much, fastener could break or components could be damaged If torqued too little, fastener could loosen during operation

Correct Torque Torque must be enough to Stretch fastener without breaking it Hold components together without damage Torque must not exceed strength of fastener Correct process must be used

Need for Torque Vibrations from engine add stress parallel to center of fastener Screw tends to back out of threads Nut tends to unscrew from bolt

Thermal Expansion Components expand when engine heats up and contract when engine cools off, causing fasteners to loosen Thermal expansion and contraction varies in different parts of engine Same fastener used in two locations may require two different torque values Follow manufacturer’s torque requirements

Engineered Stretch Fasteners are able to stretch and remain under tension Fasteners return to original shape when loosened Amount of stretch is engineered to adequately hold each piece of equipment in place Strength of fastener affects amount of torque applied

Torque Specification Torque specification must account for Elastic limit Tensile strength Torque value cannot be greater than elastic limit

Sustaining Torque Ability of threaded fastener to sustain torque through threads depends on Diameter of fastener Number and type of threads Grade of material Tensile strength is determined by Minor diameter of fastener Fastener material

Fastener Grade Indicates metal alloy composition of fastener Composition contributes to elastic limit and tensile strength Higher grade bolt has greater strength but lower elasticity than lower grade bolt

Torque Wrench Torque applied to fastener—a force acting at a distance from the center of the fastener Torque is applied and measured with torque wrench It is important to read and understand torque wrench instructions

Selecting Torque Wrench Identify torque specification units for fastener In-lb (inch-pounds) Ft-lb (foot-pounds) N·m (newton-meters) Select wrench that Can measure needed units Includes torque range to be measured

Tightening in Proper Sequence Follow equipment manufacturer’s torque sequence Some fasteners can be torqued in one step Others must be torqued in stages (especially for high torque values) Install all fasteners before tightening any of them

Nonthreaded Fasteners Connect two parts together to make one assembly Use mechanical holding process other than threads May have threads, but threading is not primary means of connection

Clamps Circular flat metal or wire bands Apply pressure to one component to keep it fastened to another component Types of clamps Wire clamp Spring clamp Rigid clamp Flexible hose clamp

Wire Clamp and Spring Clamp

Rigid Clamp and Flexible Hose Clamp

Spring Safety Hooks (Clasp) Connect parts together loosely Spring keeps it closed when attached Can be quickly taken on and off by Depressing spring Hooking or unhooking clasp

Dowels and Sleeves Dowels (or dowel pins) Solid metal fasteners used to perfectly align two parts Usually slip into mating hole with just enough clearance to enter without friction or looseness Dowel sleeves are similar to dowels, but hollow

Retainers Keep a component in place Either lock onto something or lock in place inside something

Types of Retainers Push-on retainer E-clip retainer Round press-on clip that holds two parts in place When pushed onto shaft, spring-loaded tabs press into surface, locking retainer in place E-clip retainer Slides into grooves on outside of shaft and locks in place

Clips Hairpin clip R-clip

Snap Rings External Internal Fit into grooves to retain parts on shaft Installed into grooves inside a bore

Circlips Circlips (hookless snap rings) Assembled into an internal groove in a bore Keep wrist pins in place

Roll Pins Spring steel rectangles rolled into cylinders Driven in place; held in place by spring pressure Used to lock parts in place

Cotter Pins Used to retain components on a shaft Split pin retainers Inserted through hole in shaft Ends are spread to keep pin from backing out