Crankshaft Design

CRANKSHAFT DESIGN

One of our member wanted to know about Crankshaft design, So here we have compiled the basic method of a Crankshaft Design.

Crankshaft in an IC Engine is used to convert the reciprocating motion of Piston into rotary motion (or vice versa while cranking the Engine). The crankshaft main journals rotate in a set of supporting bearings (main bearings), causing the offset rod journals to rotate in a circular path around the main journal centres, the diameter of which is twice the offset of the rod journals. The diameter of that path is known as Piston stroke. The big ends of the connecting rods contain bearings which ride on the offset rod journals.

Components of Crankshaft

There are many components in a crankshaft, each for a particular function. Figure-1 shows the components but while designing the crankshaft for an engine, designing each of the component is not needed. Main components focused for designing are Main Journals, Crankpin Journals & Crank webs are designed and rest other components are calculated on the basis of Automotive Industry Norms & Design relations which has been produced by years of Research & Development.

Figure-1 : Components of Crankshaft

Design Procedure

      

      1.     Material Selection

First & the important step in any designing process. Material should be selected on the basis of loads & condition to which the component is subjects. The Material should have following properties:

·         Enough strength to withstand high tensile & bending forces.

·         Rigidity to avoid distortion.

·         Minimum weight (Specially in aero engines).

In industrial engines, 0.35 Carbon steel of ultimate tensile strength 500MPa to 525 MPa and 0.45 Carbon steel of ultimate tensile strength of about 627 to 780 MPa are commonly used.

In transport engines, alloy steel e.g. manganese steel having ultimate tensile strength of about 784 to 940 MPa is generally used.

In aero engines, nickel chromium steel having ultimate tensile of about 940 to 1100 MPa is generally used.

The heat treatment for this steel consists in normalizing at 871-927 degC, annealing to the desired structure or machinability; heating to 788-816 degC, quenching in oil, and tempering at 483 degC.

      

      2.     Design Concept

Based on the material properties, we will now decide the dimensions which will be calculated from the loads and conditions. The crank shaft is designed considering two positions of the crank:

a)     When Crank is at Dead centre (Maximum Bending Moment).

b)     When Crank is at angle where Twisting Moment is maximum.

a)   When Crank is at dead centre

Stepwise procedure:

ü  Draw a Free Body Diagram of the Crankshaft with various horizontal and vertical forces.

ü  Calculate the piston force. (We know Maximum Piston pressure, It can be assumed according to the industry norms as 200 bar for Diesel & 180 bar for SI Engines). Piston force is Max. Piston pressure * Area of piston.

ü  Industry assumptions while calculation of forces in FBD.

ü  Find all the horizontal & vertical reactions.

(i)               Design of Crank Pin

Crankpin is also subjected to shear stress due to twisting moment. Thus we can calculate bending moment at centre of crankpin and twisting moment on crank pin and the resultant moment.

Stepwise procedure:

ü  Calculate Bending Moment at the centre of Crank Pin(from FBD).

ü  Equate the BM to (MOI*Bearing stress) for crank Pin (Sigma-b)

ü  Solve and find Diameter of Crank Pin.

ü  Solve FBD for length.

(ii)            Design of Crank Web

The crank web is designed for eccentric loading. There will be two stresses acting on the crank web, one is direct compressive stress and the other is bending stress due to piston gas load (Fp).

Industry Assumptions:

ü  Thickness of crank web Tst = 0.65 *dc + 6.35 (dc = Dia. Of Crank pin)

ü  Width of crank web is, w = 1.125 * dc +12.7

Stepwise procedure:

ü  Calculate the Bending Moment from FBD.

ü  Check if BM is positive or negative. If Negative the increase the crank pin diameter and solve again. If positive then your design is safe.

(iii)          Shaft under the Flywheel

The total bending moment at the flywheel location will be the resultant of horizontal bending moment due to gas load and belt pull and the vertical bending moment due to the flywheel weight.

Then you can find the diameter by using the Moment equation. M=(MOI*Sigma-b).

b)   When the crank is at an angle of maximum twisting moment

The twisting moment on the crankshaft will be maximum when the tangential force on the crank (FT) is maximum. The maximum value of tangential force lies when the crank is at angle 30º to 40º for constant pressure combustion engines (i.e. diesel engines).

When the crank is at angle at which the twisting moment is maximum, the shaft is subjected to twisting moment from energy or force stored by flywheel. The above design parameters can be cross checked for the factor of safety while designing by considering the crankshaft at an angle of maximum twisting moment.

If the factor of safety is more than 1 then the design is safe. Considering this, we have to various forces acting on crankshaft at different twisting angles.

This is a basic design concept used in the industry for designing Crankshafts for various IC Engines, but there are various parameters & relations which are only known to the industry and is their copyright. Thus for studying you can refer to various design data handbooks available in the market for Machine design.