4.12 DESIGN PROCEDURE FOR KNUCKLE JOINTThe basic procedure to determine the dimensions of the knuckle joint consists of the following steps:(i) Calculate the diameter of each rod by Eq. (4.26a).D= 4Ptps(ii) Calculate the enlarged diameter of each rod by empirical relationship using Eq. (4.26b).D1= 1.1 D(iii) Calculate the dimensions aand bby empirical relationship using Eqs (4.26m) and (4.26n).a= 0.75 Db= 1.25 D(iv) Calculate the diameters of the pin by shear consideration using Eq. (4.26c) and bending consideration using Eq. (4.26g) and select the diameter, whichever is maximum.d= 2Pptor dPbab=¥+ÈÎÍ˘˚˙322433ps(whichever is maximum)(v) Calculate the dimensions doand d1by empirical relationships using Eqs (4.26j) and (4.26o) respectively.do= 2dd1= 1.5d(vi) Check the tensile, crushing and shear stresses in the eye by Eqs (4.26h), (4.26e) and (4.26i) respectively.stPb dd=-()0scPbd=t=-Pb dd()0(vii) Check the tensile, crushing and shear stresses in the fork by Eqs (4.26k), (4.26f) and (4.26l) respectively.stPa dd=-20()scPad=2t=-Pa dd20()The application of the above mentioned procedure is illustrated in the next example.

100Design of Machine ElementsExample 4.5It is required to design a knuckle joint to connect two circular rods subjected to an axial tensile force of 50 kN. The rods are co-axial and a small amount of angular movement between their axes is permissible. Design the joint and specify the dimensions of its components. Select suitable materials for the parts.SolutionGiven P= (50 ¥103) NPart ISelection of materialThe rods are subjected to tensile force. Therefore, yield strength is the criterion for the selection of material for the rods. The pin is subjected to shear stress and bending stresses. Therefore, strength is also the criterion of material selection for the pin. On strength basis, the material for two rods and pin is selected as plain carbon steel of Grade 30C8 (Syt= 400 N/mm2). It is further assumed that the yield strength in compression is equal to yield strength in tension. In practice, the compressive strength of steel is much higher than its tensile strength. Part IISelection of factor of safetyIn stress analysis of knuckle joint, the effect of stress concentration is neglected. To account for this effect, a higher factor of safety of 5 is assumed in the present design. Part IIICalculation of permissible stressesstytSfs===()4005802N/mmscycytSfsSfs====()()4005802N/mmt====SfsSfssyyt().(). ()0 50 5 4005402N/mmPart IV Calculation of dimensionsThe dimensions of the knuckle joint are calculated by the procedure outlined in Section 4.10.Step IDiameter of rodsD= 4Ptps= 4 50108028 21303()().¥=pormm Step IIEnlarged diameter of rods (D1 )D1= 1.1 D= 1.1(30) = 33 or 35 mmStep IIIDimensions a and b a= 0.75 D= 0.75(30) = 22.5 or 25 mmb= 1.25 D= 1.25(30) = 37.5 or 40 mmStep IVDiameter of pind= 2Pp t= 2 50104028 21303()().

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Term

Spring

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takahashi

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