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Published byLucy Dixon Modified over 7 years ago
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Chapter 3 Fasteners
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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.
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Objectives (Cont.) Identify locking nuts. Identify types of screws.
Identify types of washers. Identify various nonthreaded fasteners.
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Fasteners Simple devices that hold parts together or in place
Fasteners connect Metal to metal Plastic to metal Plastic to plastic
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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
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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
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Linear Movement and Rotation
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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
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External Threads Formed on outside of fastener
Allow fastener to be screwed into threaded hole or nut
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Internal Threads Formed inside of hole All nuts have internal threads
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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
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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
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Bolts vs. Screws Screws Bolts
Fasteners with thread diameters of less than 1/8″ Bolts Fasteners with thread diameters larger than 1/8″
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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
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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
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Tapping (Cutting Threads)
Process of cutting threads in hole Tap tool (tap) is used to cut internal threads
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Bolts and Machine Screws
Head and body Body is divided into threaded portion and shank No shank if threads go completely to head
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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)
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Bolt and Screw Designations
Designated by one or more Thread form nomenclature Type of head Type of head drive system Special features
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Thread Form Nomenclature
Identifies fasteners by Diameter Threads per inch Length Conforms to either US customary thread form system Metric thread form system
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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
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Bolt and Machine Screw Identification
US customary bolts and machine screws are identified by Diameter Threads per inch (tpi) Length Format Diameterthreads per inch × length
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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
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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
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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
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Bolt and Screw Identification
Type of head, drive system, or combination Head shape determines type of head Hexagonal (hex)-shaped Square Pan-shaped Flat
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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
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Hex Flange (Washer) Head Fasteners
Washer integrated with head Increases bearing surface Distributes torque load over larger surface area than standard hex head
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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
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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
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Types of Screw Heads
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Drive Systems Hexagon socket head Hex head, socket head
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Drive Systems (Cont.) Torx-type (hexalobular)
Size designated by “T” followed by number External (reverse) Torx fastener Size designated by “E”
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Drive Systems (Cont.) Cross-recessed drive Phillips-type JIS
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Drive Systems (Cont.) Combination drive systems
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Special Feature Fasteners
Threaded fastener with unique design Intended to be used for specific applications
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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
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Stud Bolt Head is anchored in part
Threaded end protrudes to act as stud
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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
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Hex Socket Head Shoulder Screw
Hex socket recess in head Round shoulder shank diameter is larger than fastener thread diameter
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SEMS Fastener Washer is permanently attached to screw, bolt, or nut
Fastener and washer are installed in one step
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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
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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
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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
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Sheet Metal Screw Fastens thin metal sheets or light-duty plastics
Self-threading Internal threads remain after screw installed
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Thumb Screw Screw with special designed heads
Tightened and loosened by hand
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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
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Typical Nuts Hex nuts Square nuts Locknuts Finger-tightened nuts
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Hex Nut Six-sided nut that screws onto bolts Identified by thread form
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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
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Locknut Nut deformation or insert keeps nut from spinning freely and backing off
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Wingnut Designed to be installed and tightened by hand
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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
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Washers (Cont.) Lock washer Keeps fasteners from loosening
Can be used to improve electrical connections
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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)
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Adhesive Fasteners Threaded fastener with correct amount of adhesive preapplied When fastener is installed, friction from tightening releases adhesive Also called microencapsulated fasteners
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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
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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
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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
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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
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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
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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
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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
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Torque Specification Torque specification must account for
Elastic limit Tensile strength Torque value cannot be greater than elastic limit
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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
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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
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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
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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
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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
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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
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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
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Wire Clamp and Spring Clamp
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Rigid Clamp and Flexible Hose Clamp
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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
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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
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Retainers Keep a component in place
Either lock onto something or lock in place inside something
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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
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Clips Hairpin clip R-clip
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Snap Rings External Internal Fit into grooves to retain parts on shaft
Installed into grooves inside a bore
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Circlips Circlips (hookless snap rings)
Assembled into an internal groove in a bore Keep wrist pins in place
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Roll Pins Spring steel rectangles rolled into cylinders
Driven in place; held in place by spring pressure Used to lock parts in place
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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
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