Chapter 12 pt. 3

Chapter 11 pt. 3Types of Threaded Fasteners1BoltsUsed to hold parts togetherClearance holes in partsNut on other endCan also be used with a threaded hole

2Machine ScrewsUsed for assembly of machine partsCoarse & fine threadsDiameters from .06 to .50Lengths from .125 to 3Several head sizes available

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4Cap ScrewsFine finished machine screwsGenerally threaded into a tapped holeDiameters from .06 to 4Variety of head types

5Set ScrewsUsed to clamp parts on a shaft (pulleys, gears)Ensure that the part stays on the shaft and rotates with itcan have several drive typesVariety of point types for different applications6

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8Captive ScrewsHave a mechanism to keep them attached to a panel when unthreadedKeeps the screw from being lost when the panel is removedMakes it easier to reinstall the panelDeveloped by the military

9Design of Threaded Holes2 types of holesThru holes-go thru the partBlind holes-dont go thruThru holes are easier to perform tapping operation (cut chips can fall out other side)Blind holes require special designHole has to be made deeper than the threads will go10

11Design of External ThreadsMust be designed with allowance for incomplete threads from tool runoutExternal threads should have a lead in chamfer

Tool Runout-distance the tool can go beyond the required full thread length12

13Prevailing Torque LocknutsHave a special locking mechanism in itCan be a plug or ring of soft metal or plasticSpins freely until locking mechanism is engaged

14Jam NutThin nut used with a standard nutFriction between them locks them into placeNot used much (older practice)

15Slotted Castle NutSlots around the top of the nut to accept a cotter pinPin goes through a hole in the fastener

16Lag Screws & Wood ScrewsDesigned to attach things to woodPilot hole must be drilled first

17Self Tapping ScrewsCut their own threads in a holeNo tapping operation requiredUsed in variety of materials from plastic to metalSpecial design at the bottom of the thread does the cuttingSome can also drill hole (thin material)18

19Threaded InsertsHelically formed coils of diamond shape wireInserted into a threaded holeUsed to repair damaged or worn out threaded holes

20Some are used to provide stronger threaded hole in softer material

21Fasteners on DrawingsUsually only shown on assembly drawingsHex shapes drawn across corners

Fasteners should be drawn all in same orientationSlotted fasteners should be drawn with drive feature horizontalFasteners are not sectioned in section views

22WashersUses for washers:Span oversized holesDistribute loads over a larger areaLocking deviceseal23Flat WashersPlain flat shapeSpan oversized holesMade from many materials

24Helical WashersTrapezoidal wire formed in a helix (looks like one coil of a spring)Free height is about 2X the thicknessUsed as a locking deviceSpring pressure increases the friction between mating threads

25Tooth Lock WasherHas teeth that dig into the face of the part and the head of the fastener to keep it from looseningMade from hardened carbon steel

26Special Purpose WashersMolded or stamped nonmetallic washersAvailable in many materialsUsed as:Protect surface of a partElectrical insulatorSeals

27TEST REVIEWTerminology:PitchLeadThread formTap drillDieSelf tapping screwSet screw3 types of thread representation (which one is common)Thread notes (metric & inch)How to note left handedThread classes (metric & inch) (what they control and the types)3 basic locking methods2 functions of washers28