INTRODUCTION Crankshaft is a central component of any internal combustion engine and is

used to convert reciprocating motion of the piston into rotary motion through connecting rod and vice versa.

Crankshaft consist of three portions:• Crank pin • Crank web• Shaft

Types of Crankshaft Side or overhung crankshaftSide crankshaft has only one crank web & require only two bearing for support.Used in medium size engine and large size horizontal engines. Centre crankshaftCentre crankshaft has two crank web and three bearing for support.Used in radial aircraft engines, marine engines and more popular in automotive engines.

Depending upon no. of crank pin used in assembly : Single throw crankshaft Multi throw crankshaft

Types of Crankshaft

Material Selection Crankshaft should have sufficient strength to withstand bending and twisting

moment to which it is subjected.The crankshaft is subjected to fluctuating stresses and it should have

sufficient endurance limit stress.Crankshaft are commonly made by drop forging process.Popular material used for crankshaft are plain carbon steel and alloy steel.• Plain carbon steels include 40C8, 45C8 and 50C8.• Alloy steels used for making crankshaft are nickel chromium steels such as 16Ni3Cr2, 35Ni5Cr2 and 40Ni10Cr3Mo6.

Force Imposed On CrankshaftCrankshaft subjected to bending and torsion force due to following three forces.i. Force exerted by connecting rod on crank pinii. Weight of flywheel (W) acting downward in vertical direction

iii. Resultant belt tension acting in horizontal direction (P1+P2)

In the design of crankshaft two cases of crank position are considered.Case I: Crank is at top dead centre position and subjected to maximum bending moment and no torsional moment.Case II: Crank is at any angle with the line of dead centre position and subjected to maximum torsional moment.

Crankshaft at top dead centre

Pp = force on crank pinD = diameter of piston

pmax = max. gas pressure inside the cylinder W = weight of flywheel

P1 = tension on tight side

P2 = tension on slack sideb = distance between main bearing 1 &2c = distance between main bearing 2 &3

R1, R2, R3 are the reactions at the bearings 1, 2 and 3

Crankshaft at top dead centre (Conti..)i. Bearing reaction :Thrust on connecting rod

Pp = (πD2/4)pmax

Crankshaft is simply supported between bearing 1 &2 and subjected to force Pp

Pp*b1 = (R2)v*b or (R2)V = (Pp*b1)/b

Similarly, Pp*b2 = (R1)v*b or (R1)V = (Pp*b2)/bCrankshaft is simply supported between bearing 2 & 3 and subjected to vertical force W and horizontal force (P1+P2)

W*c1 = (R3’)v*c or (R3)’v = (W*c1)/c

W*c2 = (R2’)v*c or (R2)’v = (W*c2)/c

(P1+P2)*c1 = (R3’)h*c or (R3’)h = {(P1+P2)*c1}/c

(P1+P2)*c2 = (R2’)h*c or (R2’)h = {(P1+P2)*c2}/c

Crankshaft at top dead centre (Conti..)Resultant reaction at the bearings are as follows:

R1 = (R1)v

R2 = √ [(R2)v+(R2’)v]2 + [R2’)h]2

R3 = √ [(R3’)v]2 + [(R3’)h]2

ii. Design of crank pin : The crank pin is subjected to max. bendingmoment at central plane.

(Mb)c = (R1)vb1

I = (πdc4/64) ; y=dc/2 ; σb = {(Mb)c*y}/I

(Mb)c = (πdc3/32)*σb

Diameter of crank pin is determined by the above equation.

Crankshaft at top dead centre (Conti..)The length of crank pin is determined by bearing consideration.

pb = allowable bearing pressure at crank pin

pb = Pp/dclc

iii. Design of Left hand Crank Web :From empirical relationships

t = 0.7dc and w = 1.14dc

The left hand crank web is subjected to direct compressive stress and bending stress due to eccentricity of reaction (R1)v.

σc = (R1)v/wt

Mb = (R1)v [ b1-lc/2-t/2]

I = (wt3/12) ; y = (t/2) ; σc = Mby/I

σb = {(R1)v [ b1-lc/2-t/2](t/2)} / (wt3/12)

σb = 6(R1)v [ b1-lc/2-t/2] / (wt2)

Crankshaft at top dead centre (Conti..)The total compressive stress is given by:

(σc )t = σc + σb

The total compressive stress should be less than the allowable bending stress.iv. Design of Right hand crank Web : The right hand and left hand crank webs should be identical from balancing consideration.v. Design of Shaft Under Flywheel :The central plane of the shaft is subjected to maximum bending moment.Bending moment in the vertical plane due to wt. of flywheel

(Mb)v = (R3’)vc2

Bending moment in horizontal plane due to resultant belt tension

(Mb)h = (R3’)hc2

Crankshaft at top dead centre (Conti..)Resultant bending moment

Mb= √ (Mb)v2 + (Mb)h

2

= √ [(R3’)vc2]2 + [(R3’)hc2]2

Similarly ,

Mb = (πds3/32)*σb

ds is the diameter of shaft under flywheel