Design a crankshaft for a vehicle Tavera Cheverolet engine and elaborate manufacturing process for same

 

Engine specifications of Chevrolet Tavera Car  

Engine Type	2.5L Direct Injection Turbo Diesel
Displacement (cc)	2499
Max. Output Ps/rpm	80 / 3900 (Approx.60KW@3900rpm )
Max. Torque kgm/rpm	19 / 1800
Bore Diameter(D)	93mm
Stroke length(L)	92mm
Compression Ratio(r)	18.5 : 1
Fuel Tank Capacity (liter)	55
No. of Valves/OHC	8

Table-1 Specifications of Engine

Source: www.chevrolet.co.इन

Crankshaft Nomenclature

1.  

DESIGN CONSIDERATIONS

Parameter	Theoretical Relation	Dimension in mm
Cylinder bore diameter	D	93.00
Cylinder centre distance	1.20 D	111.6
Big-end journals diameter	0.65 D	60.45
Main-end journal diameter	0.75D	69.75≈70
Big-end journal width	0.35 D	32.55
Main-end journal width	0.40 D	37.20
Web thickness	0.25 D	23.25
Fillet radius of journal and webs	0.04 D	3.72

Table-2 Theoretical relations

 Materials

              Crankshafts materials should be readily shaped, machined and heat-treated, and have adequate strength, toughness, hardness, and high fatigue strength. The crankshaft is manufactured from steel either by forging or casting. The main bearing and connecting rod bearing liners are made of Babbitt, a tin and lead alloy.

               Forged crankshafts are stronger than the cast crankshafts, but are more expensive. Forged crankshafts are made from SAE 1045 or similar type steel. Forging makes a very dense, tough shaft with a grain running parallel to the principal stress direction. Crankshafts are cast in steel, modular iron or malleable iron. The major advantage of the casting process is that crankshaft material and machining costs are reduced because the crankshaft may be made close to the required shape and size including counterweights. Cast crankshafts can handle loads from all directions as the metal grain structure is uniform and random throughout.

            Counterweights on cast crankshafts are slightly larger than counterweights on a forged crankshafts because the cast metal is less dense therefore somewhat lighter. Generally automobile crankshafts were forged in past to have all the desirable properties. However, with the evolution of the nodular cast irons and improvements in foundry techniques, cast crankshafts are now preferred for moderate loads. Only for heavy duty applications forged shafts are favored. The selection of crankshaft materials and heat treatments for various applications are as follows.

The crankshafts are subjected to shock and fatigue loads. Thus material of the crankshaft should be tough and fatigue resistant. The crankshafts are generally made of carbon steel, special steel or special cast iron.

Industrial engines: Crankshafts are commonly made from carbon steel such as 40 C 8, 55 C 8 and 60 C 4

Transport engines: Manganese steel such as 20 Mn 2, 27 Mn 2 and 37 Mn 2 are generally used for the making of crankshaft.

Aero engines: Nickel chromium steel such as 35 Ni 1 Cr 60 and 40 Ni 2 Cr 1 Mo 28 are extensively used for the crankshaft.

The crankshaft is subjected to shock and fatigue loads. So, the material should be tough and fatigue resistant. Commonly used materials for crankshaft are alloy steels, special steels or special cast iron. SAE 1541, 1548, 4340, 4310, VANARD 925 are some of the materials generally used.

Material Selection

Material chosen for crankshaft manufacturing is SAE 1045

Chemical proportion

C	Si	Mn	S	P	Fe
0.45	0.21	0.74	Max. 0.05	Max. 0.4	Balance

Table-3 Chemical composition of Material

Mechanical Properties

Yield Strength (Mpa)	430
Tensile Strength (Mpa)	781
Elongation	17%
Reduction of Area	37%
Density	7.85 g/cc
Poisson Ratio	0.29                                       0.29
Hardness	179BHN
Shear Modulas	80Gpa
Youngs modulas	205Gpa

Table-4 Mechanical properties of Material

DESIGN CALCULATIONS

Assumption: Let us assume,

ᶯmech = Mechanical efficiency = 0.80

ᶯmech =        

BP=Brake power (KW) i.e. Maximum power

IP= Indicated power in KW

IP = 60/0.8 = 75KW

IP =  

Where,

Pi=Indicated mean effective pressure

L=Stroke length

D=Bore diameter

K= No. of cylinders

n = N/2 for 4-stroke

N=Speed in rpm

Pi = 

Pi =      (75000 x 60000) / [ 92 x ᴨ/4 x 93²  x  (3900/2) x 4 ]

Pi =      0.93 Mpa

At the TDC of the piston, the volume will be reduced by the compression. At this moment, the maximum pressure inside the cylinder will be,

                        Max. Pressure = B.M.E.P x Compression ratio

                                                 = 0.93 x 18.5

                                                 = 17.205 MPa

Now, this value of B.M.E.P acts on the piston head, and the whole force is transmitted to the crankpin through the connecting rod. This force is the most critical in the design of the crankshaft and the design in done on the basis of the above mentioned force.

To find the force exerted on the crankpin by the piston:

Piston force, F (kN)    =   cylinder bore area (mm²) x B.M.E.P

                        F          = 116.87 kN

Piston force will act at the middle of the crankpin, and it will be balanced by the reactions from the bearings at either side of the crankpin. Let the reactions be R₁ and R₂.

Considering the crankpin as a simply supported beam, we will see that

R₁ + R₂ = F     and     R₁ = R₂

Therefore, we get that             R₁ = R₂ = F/2 = 58.435 kN

Maximum bending moment (M) on the crank pin is given by M = R1 × b

            Where, b is the distance from the centre of the bearing to the centre of connecting rod

Assuming b = 1.2 x D = 111.6 i.e. b = 112mm.

Also, we know that 

From the above equation, we get that

 

Where,

                        d = diameter of the crankpin

 = max. bending stress of the material of the crankshaft with suitable factor of safety (350Mpa )

Equating the values of M in the above equations, we can get the value of the crankpin diameter d.

                                   

                                            d  = 58 mm

Length of the crankpin ( L_c)   = F* D/p

  

            Where, P = maximum permissible stress on the bearing, 50MPa,

                                    L_c         =  116870/58*60

                                           Lc = 40 mm

Crank web thickness is given by 0.25D, i.e. = 0.25 x 93 = 23.25 mm

Design of the main journal

The main journal diameter is usually given by 0.75D, where D is the bore diameter. In this case the value of main journal diameter will be 69.75

Now, we have to check if this diameter is sufficient to withstand the torque on the main journal due to the crank. For this, we need to find the thrust (F_Q) on the connecting rod at the time of maximum torque.

Assuming that the torque is maximum when the connecting rod is at 35^o (ɵ) with the line of stroke, and the pressure inside the cylinder at this point is 5MPa,

           

=          33.965 kN

The tangential component of this force will cause the torque on the main journal. This tangential component    F_T                = FQ x cos 35^o,

                                                = 27.822kN

Now, the torque acting will be given by F_T  x r, where r is the crank radius, and is taken as 0.5 times the stroke.

Torque, Ts         = 27.822 x 0.045

                                                = 1252 Nm.

Now, we know that to design the diameter for a given torque, we use

d = 36.45mm

Where  is the maximum shear stress acting, which is taken as 175 MPa.

So, as per the above equation, the diameter of the main journal is obtained as 36.45 mm, which is much lesser than 5 mm, i.e. the design is safe and we can use the main journal diameter as 69.75 mm.

DIMENSIONS CALCUALTED:

Required design parameter	Dimension
Crank pin diameter (d)	58mm
Length of crank pin (Lc)	40mm

Table-5 Required dimensions

Manufacturing Process Chart

Manufacturing of Crankshaft

Usually, there are two kinds of process to produce large-scale crankshafts. One method is to cast crankshafts with nodular cast iron, which has advantages of short production cycle and low cost. The weak point using this process is low intensity and bad toughness of the materials. So this process is only suitable for the low load engines. Another process is to forge the crankshaft with the low alloy steel, which has advantages of high intensity and good toughness. We will use forging for this particular crankshaft.

Forging:

It is manufacturing process where metal is pressed, pounded or squeezed under great pressure into high strength parts known as forgings. Heated metal to be shaped is placed on a mold. Pressure is applied to the metal with the help of a press or hammer and due to this impact the malleable metal conforms to the die cavity shape.

·         Crankshaft is locally heated where web to be formed. Then axial press is used for formation of webs.                                                               

·         Sets of hydraulic presses are used for the formation crankpin journals and webs.

·         This method of forging gives continuous and parallel grain flow. This kind of grain flow increases the fatigue strength of crankshaft.

A)  Heat Treatment Of Crankshaft:

 Heat treatment is done to improve the machinability and to reduce the residual stresses. The typical heat-treating process for carbon-steel alloys is first to transform the structure of the rough-machined part into the face-centered-cubic austenite crystalline structure (‘austenite’) by heating the part in an oven until the temperature throughout the part stabilizes in the neighborhood of 1550°F to 1650°F (depending on the specific material). Next, the part is removed from the heating oven and rapidly cooled ("quenched") to extract heat from the part at a rate sufficient to transform a large percentage of the austenitic structure into fine-grained martensite.                                                                                                       

Crankshaft Machining:

Crankshaft machining contains operations like Turning, Facing, Centering, trimming, Web milling , crankpin milling , drilling of holes, deburring , rough grinding, finished grinding, threading ,super finishing etc.

1) Modern CNCs are used for the crankshaft machining if they produced in mass.

2) Some of the machines used for different operations are listed below

a)  SPM (Special Purpose Machines) :

 Facing, Centering, Crank web milling, journal milling, crankpin milling

Crank Journal Lapping, Crankpin Lapping, Super finishing

b) HMC (Horizontal machining Centre):

Journal Grinding, Main Journal Grinding, Threading

Heat Treatment Nitriding:

There are three common types of hardening processes used on steel crankshafts, and they are induction hardening, tuftriding, and nitriding.

Nitriding is a chemical hardening process in which the part is heated in a furnace, the oxygen is vacuumed out, and nitrogen is introduced which penetrates the entire surface. The depth of hardness is dependent upon the time the crankshaft is exposed to the gas. Typically, a nitrided crankshaft will have a hardness depth of about .010 - .030. The part gains a high-strength, high hardness surface with high wear resistance, and greatly improved fatigue performance due to both the high strength of the case and the residual compressive stress. Nitriding is a low heat process compared to Tuftriding, but it shares the advantage of avoiding the introduction of localized stress zones as in induction hardening.

REFERENCES

www.chevrolet.co.in

www.autoindia.com

www.steel-tube.com

www.the-crankshaft.info

Textbook “Design of Machine Elements” By Gupta and R.S. Khurmi

Team Members:

1. Gajanan Gambhire (35)
2. Anoop Singh Rajput (18)
3. Ankit Jain (16)
4. Adarsh Malu (05)
5. Deepak Dhanotiya (32)

Originally published in Knol under creative commons license http://knol.google.com/k/anoop-singh-rajput/design-a-crankshaft-for-a-vehicle/3ddjl5ukgxdn4/3 by Anoop Singh Rajput as an assignment in my class