prev

next

out of 51

Published on

13-Nov-2014View

4.178Download

3

Embed Size (px)

Transcript

<p> Suez Canal University Faculty of engineering Port-Said Production Engineering and mechanical Design Department Keys and Key Keys and Key Keys and Key Keys and Keyways ways ways ways Prepared By Dr. Eng. Mohammed A. Soliman Associate Prof. Of Machine Design And Tribology Fundamental Problem in Fundamental Problem in Shaft DesignShaft DesignHow do I connect stuff to the shaft?How do I connect stuff to the shaft?Interference Fits Interference FitsKeys & Keys & Keyseats KeyseatsSplines Splines/Polygons /Polygons Integral Shaft Integral ShaftPins PinsHubs/Collars Hubs/CollarsInterference FitsInterference FitsInterference Fits Interference Fits Hole is undersized and part is heated to allow it to slide over shaIt. Compressive interIace pressure develops when part cools. Reference Lecture 15 Notes Reference Lecture 15 Notes.M ott, Figure 11-1Keys are used to transmit torque Irom a component to the shaIt.Keys and Keys and KeyseatsKeyseatsKeyseatTypes of Types of KeyseatsKeyseatsO livo, Fig. 40-3Keyseats Keyseatsare classified according to are classified according to the process by which they are m ade. the process by which they are m ade.Keyway Fabrication M ethodsKeyway Fabrication M ethodsEnd M ill on Vertical End M ill on Vertical M illing M achine M illing M achineKey Cutter on Horizontal Key Cutter on Horizontal M illing M achine M illing M achineChang, Fig. 5.8, M ott, Fig. 12-6Fillet Radii and Key Cham fersFillet Radii and Key Cham fersGeneral Practice: Zero General Practice: Zero root fillet and cham fer root fillet and cham ferNot to ScaleStandard contains recom m ended fillet radii and key cham fer com binations to provide lower stress concentration factors.Keys and Keys and Keyseats Keyseats, ANSI , ANSI Standard B17.1 Standard B17.1- -1967 1967.Better Practice Better Practice45ocham ferFillet Radii FabricationFillet Radii FabricationR0.3125A Bull` end mill can be used to machine Iillet radii in keyways.Bull End MillM SC Catalog, Fanfara, Figure 3-6M ott, Figure 11-1The hub is slightly larger than the shaIt and key to allow it toslide over the shaIt during assembly. The set screw is used to take up the slack. The resulting Iriction is used to provide resistance to axial motion. Thread adhesive may be required to ensure that vibration doesn`t cause the set screw to loosen.Square and Rectangular Square and Rectangular Parallel KeysParallel KeysSquare and Rectangular Square and Rectangular Key Geom etryKey Geom etryKeys and Keys and Keyseats Keyseats, ANSI , ANSI Standard B17.1 Standard B17.1- -1967 1967.Width is approximately / the diameter oI the shaIt.Standard contains tables oI recommended key sizes versus shaIt diameter.Set ScrewsSet ScrewsFlat Point Flat PointCup Point Cup PointOval Point Oval PointCone Point Cone PointHalf Half- -dog Point dog PointShigley, Fig. 8-26Holding Power Holding PowerResistance to axial or rotary motion oI the hub or collar relative to the shaIt.Holding power is a Iunction oI Iriction between contacting portions oI hub or collar and shaIt and any penetration oI the setscrew into the keyway or shaIt.Representative Holding Representative Holding Power ValuesPower ValuesShigley, Table 8-13Based on alloy steel screw against steel shaIt, class 3A coarse or Iine threads in class 2B holes, and cup-point socket setscrews.Tapered KeysTapered KeysM ott, Figure 11-3Designed to be inserted Irom the end oI the shaIt aIter the hub is in position. The taper will impart a compressive contact pressure between the hub and the shaIt. Friction will help transmit torque and provide resistance to axial motion oI the hub relative to the shaIt. Tapered keys do not require set screws. Access to both ends oI tapered keys are required so that the key can be inserted and driven out when the key is being removed.GibGibHead KeysHead KeysInstallation is similar to standard tapered keys. The extended head provides a holding method Ior removing the key by pulling instead oI driving it out.M ott, Figure 11-3, Shigley, Fig. 8-28W oodruff KeysW oodruff KeysM ott, Figure 11-3ANSI Standard B17.2 ANSI Standard B17.2- -1967 lists recom m ended dim ensions 1967 lists recom m ended dim ensions for W oodruff Keys. for W oodruff Keys.Circular groove in shaIt holds the key in place while the hub is slid over the shaIt. The WoodruII key will have less shear strength than a rectangular or square key.Circular (Pin) KeysCircular (Pin) KeysSigniIicantly lower stress concentration Iactors result Irom this type oI key as compared to parallel or tapered keys. A ball end mill can be used to make the circular key seat.Fanfara, Figure 3-6, M ott, Figure 11-3 Tapered BushingsTapered BushingsM ott, Fig. 11-10, www.em erson-ept.comTapered hub causes split bushing to be drawn down on shaIt. Higher strength alternative to set screws.Key is used to transmit torque, Iriction keeps system Irom sliding axially along shaIt.Stress Analysis of Stress Analysis of Parallel KeysParallel KeysA key has two Iailure mechanisms: 1) it can be sheared oII, and 2) it can be crushed due to the compressive bearing Iorces.TFFShear Plane Shear PlaneBearing Surface Bearing Surface( )( )DWL2TWL2DTAFWL A2DTFavesaves== ===Shear Stress Analysis of Square Shear Stress Analysis of Square and Rectangular Parallel Keysand Rectangular Parallel KeysM ott, Fig. 11-4(b)Required Key LengthRequired Key Length(Shear) (Shear)sIytallyt ysN0.5S0.5S S==DW S4TNLN0.5SDWL2TytIssIsytave== =From Maximum Shear Stress Failure Theory, the shear yield strength is given by: The minimum length oI the key can be Iound by setting the average shear stress equal to the allowable shear stress.Bearing Stress: Square and Bearing Stress: Square and Rectangular Parallel KeysRectangular Parallel KeysM ott, Fig. 11-4(a) 2HLA2DTFAFbbb===1.5 K 1.0NKSIsycall b, =Triaxial Triaxial Stress Factor Stress FactorDH KS4TNLycIsb =Com parison of Shear and Com parison of Shear and Bearing Length EquationsBearing Length EquationsDW S4TNLytIss =DH KS4TNLycIsb =M inim um Required M inim um Required Length to Prevent Length to Prevent Shear Failure Shear FailureM inim um Required M inim um Required Length to Prevent Length to Prevent Bearing Failure Bearing FailureII K1, these equations give the same result Ior a square key. In general K will be greater than 1.0 and more shear Iailures will be observed in the Iield. Keys are generally designed to Iail beIore overloads can cause damage to the shaIt or attached component. In this respect they act like a mechanical Iuse.Stress Analysis of Stress Analysis of W oodruff KeysW oodruff KeysShear Area2Bsdsd2B22s2A d2B2B+ =AShear Analysis of Shear Analysis of W oodruff KeysW oodruff Keys2Bsdsd2B( )( )saves s ss s22s2DA2Td B d 2W Area Shear Ad B d AA d2B2B= = =+ =AIsytall s,N0.5S =Shear PinsShear PinsThe strength analysis oI shear pins is similar to that used to Iind the strength oI a Iastener. We`ll deIer the strength analysis until we cover Iasteners.M ott, Fig. 11-9Stress Concentration FactorsStress Concentration FactorsR.E. Peterson, Stress Concentration R.E. Peterson, Stress Concentration Factors, W iley, New York, 1974. Factors, W iley, New York, 1974.Key seats create stress concentrations in the shaIt. There are diIIerent stress concentration Iactors Ior bending and torsional loads. Peterson contains a compilation oI stress concentration Iactors that includes key seat geometries. For Ilat end mills, Peterson gives Kt2.14 Ior bending and Kt2.62 Ior torsion. These may be reduced by using key seats made with bull end mills. The stress concentration Iactor Ior a sled runner key seat is signiIicantly lower than Ior a proIile key seat. A circular key and keyseat will have lower stress concentration Iactors any other key geometry.Assignm entAssignm ent1. Determine the length oI a parallel key Ior a gear to be mounted on a shaIt with a 2.00 in-diameter shaIt. The key is made Irom AISI 1020 cold-drawn steel. The gear transmits 21,000 lb-in oI torque and has a hub length oI 4.00 inch.2. A V-belt sheave transmits 1,112 lb-in oI torque to a 1.75 in-diameter shaIt. The sheave is made Iorm ASTM class 20 cast iron and has a hub length oI 1.75 in. Design a parallel key and key seat. The key material is AISI 1020 cold-drawn steel. Create an AutoCAD drawing that would enable a machinist to make the key seat.SplinesSplinesFundamental Problem in Fundamental Problem in Shaft DesignShaft DesignHow do I connect stuff to the shaft?How do I connect stuff to the shaft?Interference Fits Interference FitsKeys & Keys & Keyseats KeyseatsSplines Splines/Polygons /Polygons Integral Shaft Integral ShaftPins PinsHubs/Collars Hubs/CollarsSplinesSplinesMott, Fig. 11-6GSplines can be thought of as a series of axial keyways with mating keys machined onto a shaft.GThere are two major types of splines used in industry: 1) straight-sided splines, and 2) involute splines.GSplines provide a more uniform circumferential transfer of torque to the shaft than a key.Splined Splined Shaft and HubShaft and Hubwww.advanceadapters.comExternal External Spline SplineInternal Internal Spline SplineSpline Spline StandardsStandardsGANSI B92.1-1970 (R1982), Involute Splines, American National Standards Institute.GANSI B92.2-1980, Metric Module InvoluteSplines, American National Standards Institute.GSAE Straight Tooth SplinesStraightStraight--Tooth Tooth Spline Spline GeometryGeometryGSAE straight-tooth splines usually contain 4,6,10, or 16 splines.GParameter dimensions are controlled by the fit needed for a particular application.Mott, Fig. 11-4StraightStraight--Tooth Tooth Spline Spline StrengthStrengthSplines have the same failure mechanisms as keys: 1) shear or 2) bearing.GThe torque capacity per unit length of an SAE spline is based on a 1,000 psi bearing stress on the sides.GDepending on the class of fit, a spline is able to accommodate axial movement along the shaft and still transmit torque.StraightStraight--ToothToothSplineSplineStrengthStrength(Continued) (Continued)( )( ) = + = =+= + = =8d DN 1,000 Td D214d DN 1,000 Td D21h4d D2d2D21Rh R N 1,000 T2 2T = Torque per unit lengthN = Number of teethD = Major spline diameterd = Minor spine diameterd = f (D) Torque Capacity CurvesTorque Capacity Curves(SAE Straight (SAE Straight- -Tooth Tooth Splines Splines) )Mott, Fig. 11-7Note that an involute spline has a higher torque capacity than does a straight-tooth spline of the same major diameter.InvoluteInvoluteSplinesSplinesInvolute splines generally have a 30opressure angle.Mott, Fig. 11-8Standard Standard Diametral Diametral Pitches Pitches and Lengthsand LengthsThere are seventeen diametral pitches in common use:2.5 3 4 5 6 8 1012 16 20 24 32 40 4864 80 128Standard Lengths Standard LengthsDiametral Diametral Pitches PitchesCommon designs use spline lengths of 0.75 D to 1.25 D, where D is the pitch diameter of the spline. When these standard lengths are used, the shear strength of the splines will exceed that of the shaft from which they are made.Spline Spline Manufacturing MethodsManufacturing Methodswww.drivetraindirect.comForged blank is rolled under tons of pressure prior to heat treating.The finished spline is more accurate and stronger (35%) than cut spines.Splines are either cut (machined) or rolled. Rolled splines are stronger than cut splines due to the cold working of the metal. Nitriding is common to achieve very hard surfaces which reduce wear.Rolled Rolled Spline Spline Process ProcessSplineSplineFailure ExampleFailure Examplewww.4wdonline.comNote the yielding of the shaft outside of the engagement area due to a torsional load. The mating internal spline forced the external slines to remain parallel. In this case the spline is stronger than the shaft.Splined Splined Linear BearingLinear Bearingwww.tsubaki.comCircular shaped splines have been combined with ball bearings to create linear bearings that can resist a torsional load.PolygonsPolygonswww.generalpolygon.comAn alternative to splines that has significantly lower stress concentration is the polygon. Four and three lobed polygons are shown.Design information on polygons is available from General Polygon.Retaining RingsRetaining RingsGRetaining rings are used on shafts to maintain the axial position of components.GThere are many types of retaining rings. In general, they may be classified as: 1) internal and 2) external.External ExternalInternal Internalwww.rotorclip.comDifferent Types of Retaining Different Types of Retaining RingsRingswww.mdmetric.comSpring Loaded Retaining RingsSpring Loaded Retaining RingsGBowed retaining rings provide restoring forces to the components being held.GFlat retaining rings allow small amounts of axial motion of the held component.Bowed Internal Bowed Internal Retaining Ring Retaining RingBowed External Bowed External Retaining Ring Retaining Ringwww.rotorclip.comSmalley Compression Spring Smalley Compression Spring Retaining SystemRetaining Systemwww.smalley.comHigher restoring forces can be obtained using compression rings manufactured by Smalley.Retaining Ring Stress Retaining Ring Stress ConcentrationsConcentrationsG External retaining rings used on shafts require that grooves be cut into the shaft.G The grooves generally have sharp corners or very small fillet radii which result in significant stress concentration factors.Mott, Fig. 11-5Retaining Ring Stress Retaining Ring Stress Concentration FactorsConcentration FactorsG The high stresses at the root of the retaining ring groove will be highly localized and will not significantly effect the static strength of a shaft made from a ductile material.G The stress concentration factors will be important in determining the life of the shaft and must be included in life calculations.Shigley, Fig. A15-14 & 15Retaining Ring DesignRetaining Ring DesignDimensions and design guidelines for retaining rings are contained in catalogs and literature published by retaining ring manufacturers.Rotoclip Rotoclip, Inc. , Inc. Waldes Truarc Waldes Truarc, Inc. , Inc. Smalley SmalleyDesigns that use retaining rings must take into account how the rings will be installed and make sure that sufficient assembly clearance is provided.Integral ShaftsIntegral ShaftsG An alternative to attaching components to shafts is to machine the components directly onto the shaft.G This higher priced approach is often the only approach available when tight space constraints exist.G Complex combinations of components can be obtained using modern CNC turning centers.www.astas.co.za/shafts.htmlAssignmentAssignment1) Make a drawing of an SAE straight-tooth- 4-spline connection having a major diameter of 1.5000 in and a class A fit. Show all critical dimensions. What is the torque capacity of the spline?2) Identify two applications of retaining rings used in mechanical equipment. D...</p>

Recommended

View more >