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CHAPTER 4

RE-DESIGN AND ANALYSIS OF AN EXISTING CHASSIS

4.1 INTRODUCTION

A vehicle chassis carries heavy load. The truck has a box type rail

structure to prevent an overloading during coal transportation. The maximum

permissible load is 2% of the body payload. The average loading of the truck

was 54,500 kgs. After 10,341 Hrs usages of the truck chassis, fracture

occurred at the rear rail structure of the chassis. Existing chassis has to be

redesigned.

4.1.1 Field Failure of Chassis

A prototype vehicle was submitted to the durability test, off-road at

a proving ground test track. Failures of rear rail structure, horse collar portion

and suspension mounting points were identified. Cracks were noticed at the

rear side of the truck chassis. These cracks have grown leading to fracture of

the chassis. Major cracks were observed at nearly the rear rail structure and

suspension mounting points. Loads are transmitted by wheels to the body of

the vehicle through the suspension components.

During vehicle running condition chassis crack is propagated at the

end of approximately 10,128 field hour. Crack is propagated at the beginning

of the rear rail structure and horse collar portion, this is due to the torsional

load case as shown in Figures 4.1 and 4.2 to determine the stress critical area

on the chassis for further to do the localized system FE analysis.

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Figure 4.1 Rear rail structure (next to the horse collar)

Figure 4.2 Field Failure of the Chassis (crack at the horse collar portion)

Failure area

Failure area

Failure areaFailure area

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4.1.2 Design Calculation to Modify the Chassis

Rail width and thickness calculation.

Tensile Yield Strength b/a =1 b/a = ½ b/a =0

40,000 PSI 76.7 63.2 58

Depth of rail b/t < 63.2

b/t = 21(ratio)

b = 21×t (t=2mm)

b =21×2

b = 42 in b - Width of the chassis

b = 42 × 25.4 t - Thickness of the chassis

d - Height of the chassis

Assume b/t ~ 1067/20 and b/t~1067/25

Therefore, the thickness of the plate t = 20 mm and t1 = 25 mm.

4.1.3 Torsional Resistance Calculation

Minimum cross section of the rear rail structure:

b = 1067 mm

b

d

b

dt = 20mm

t1 = 25mm

b

d

t

t1

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Torsional Resistance:2 2

1 12 2

1 1

2tt (b t) (d t )Rbt dt t t

where b = 178mm

D = 470 mm

T = 20mm

t1 = 25mm 2 2

2 22 20 25 (178 20) (470 25)R

(178 20) (470 25) (20) (25)

1000 (24964)(198025)R3560 11750 400 625

124.9 10R11536 544

124.9 10R14285

Total Resistance = RL + RR

= 343017150 + 343017150

Shear Stress = 1 1

T2t (b t)(d t )

R=343017150 mm4

4

R total = 686034300mm4

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Torsion (T) for left ramping:

T (Torsion) = FT ×1/6 GVW

T = 3480 × 1/6 × 961380

Left-Right Ramping =2T

Short Side = 1 1

T / 2 per side2t (b t)(d t )

=811.2 e / 2

2 25(178 20)(470 25)

=85.6e

40 158 445

=8

65.6e2.8e

L R L R RL

1/6 GVW 1/6 GVW 1/6 GVW 1/6 GVW 1/3 GVW 0

FT

T = 5.6 × 108 N-mm

Torsion (T) = 11.2e8 N-mm

= 200 MPa or 29007.5 PSI

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4.1.4 Modification of Critical Area of the Chassis

In order to benefit from using high strength steel, a total redesign is

recommended in this area. Some design changes are discussed below. Two

approaches that are recommended for good design practice

1. Re-design the location of the weld at the rear rail structure.

2. Reduce stresses in the weld by adding web-stiffeners and / or

increase sheet thickness in the rail structure.

The effect of varying width and thickness of the chassis analysis

has been done. This modification shows better design as shown in Fig 4.3,

maximum shear stress and fatigue life of the bending load occur at the rear

rail structure. A more optimized geometry of the transition from full rail

structural section width and thickness would be most desirable for best fatigue

performance. Since introducing this web-stiffener also introduces more welds,

which should be avoided if possible. However, this minor modification at

least shows that the critical area can be improved.

Figure 4.3 Rear Rail Structure (Box section modulus calculation)

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4.1.5 CAD Model of the Chassis

Figure 4.4 CAD Model of chassis for localized system analysis

4.2 LOAD DISTRIBUTION ON CG POINTS OF THE CHASSIS

Whilst adequate durability under dynamic conditions is a design

requirement for the vehicle structures, the static load cases cannot be

disregarded. The values for the individual load cases are taken from the

expected service conditions of the off road vehicle. The worst-case loading

conditions (distribution of the load) as well as simply support is considered

for the static load case. The factors are usually applied to the static load case,

especially for those vehicles with long simply supported containing

concentrated loads (e.g. Dumper). Such loads result in high bending moments

over the rear axle. The various dynamic conditions considered here for

determining tire reaction loads are shown in Figure 4.4.

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4.2.1 Loads on the Engine CG

The engine weight acts as on the neutral axis of the chassis. Weight

acts on the CG point of the engine and load transfer to the engine mounting

bracket on the chassis structure. For the engine taking moment about

mounting bracket with respect to engine CG of the neutral axis, we get,

Me × L = Me Cos × Le. Me Sin × H

where, M – moment at the mounting bracket

L – Length from the neutral axis to mount

H – height of the front axle axis and the ground

The above equation gives the weight of the engine on grades,

through which increase in weight ( Mg) on the engine due to grade can be

calculated. Similarly, the loads shift on negative grade can also be calculated.

4.2.2 Loads on the Body CG

The body weight acts at the neutral axis of the chassis. Weight will

be acting on the CG point of the body with payload and load transfer to the

rear rail structure of the chassis.

Me × L = Me Cos × Le. Me Sin × H

where, M – moment at the rear rail structure

L – length from the neutral axis to chassis

H – height of the body CG to the chassis

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The above equation gives the weight of the body on CG, through

which increase in weight ( Mg) on the engine due to grade can be calculated.

Similarly the loads shift on negative grade can also be calculated.

4.3 FE MODEL OF THE CHASSIS

In the finite element model, shell elements have been used for the

longitudinal members and cross members of the chassis. The advantage of

using shell element is that the stress details can be obtained over the

subsections of the chassis as well as over the complete section of the chassis.

Beam elements also used to simulate various attachments over the chassis,

like fuel tank mountings, engine mountings, transmission mounting, etc.

Spring elements have been used for suspension stiffness of the vehicle. The

vehicle model is fixed at the wheels. The model is tested with the

experimental results determined for the opposite wheels at the bumps. The

diagonally opposite wheels of the vehicle are lifted to full deflection of

suspension and the stress is measured at six locations. The measured stresses

and the stresses calculated from ANSYS for the vehicle model at these six

locations are almost similar.

4.3.1 Boundary Conditions

The punishing treatment received by the vehicle bodies in service,

together with the great variety of use and abuse, means the combinations of

many load cases have to be considered for find the stress distribution. Before

moving directly to these conditions, it will be a fair practice to visit the

simplified representation of the structure, and the actual representation of the

load over it. The structure can be simply assumed as a beam, with uniformly

distributed load due to body & frame over its length, and the point loads at the

engine, transmission and various heavily loaded accessories.

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4.3.2 Diagonally Opposite Wheels on Bumps

The cross members and side members are severely stressed when

the diagonally opposite wheels of the vehicle are on the bump. Here, the

suspensions, whose wheels are on bumps are given the maximum

displacement (permissible for the suspension) in the vertical direction. The

non-bump wheels spring mounting locations are face constrains in the vertical

direction. Heights of the bumps are taken such that the diagonally opposite

suspensions undergo the maximum deflection. The maximum stress is occures

near the fuel tank mountings, as the fuel tank adds to the rigidity due to its

own section modulus. The diagonally opposite bumps will twist the chassis,

due to which the rear most cross member undergo high torsional stresses.

4.3.3 Combination Load

The combination of various load cases over the roads punishes the

vehicle very hard. One such condition occurs when the vehicle moves over

the grade of 12%, with an acceleration.

Mrr = {( Mg + Ma + Mr) × Cos × Tl + ( Mg + Ma + Mr)

× Sin × H}/T + Fzrr

Similarly weights on the other wheels can also be calculated.

4.3.4 Road Bump Load

The impact forces experienced by the vehicle at the road bumps can

be determined by mounting an accelerometer at the front and the rear axles

such that it measures the vertical acceleration. Then, the vehicle is passed

over the bumps at a speed range between 10-20 km/hr. It can be noticed that

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the Gross vehicle weight load 1/3 acts at the front wheel of the vehicle and

2/3 at the rear wheel of the vehicle.

4.4 LOCALIZED SYSTEM LOAD CONDITIONS

Localized system analysis is carried out for the chassis of dump

truck at zero inclination. Let us consider the CG points of the sub systems like

engine, transmission hydraulic tank, fuel tank and tractive effort etc… the

weight of the individual sub system can be calculated as shown in Table 4.1

Load acting points at the Front and rear suspension mounts locations will be

constrained and reaction forces acting on the chassis. Weights of sub systems

applied on each CG points of the systems like engine, transmission, fuel tank,

hydraulic tank, body and tractive effort are shown in Figure 4.5. Link element

RBE 3 is also connected to suspensions mounting location and to the CG

points of an individual system.

Table 4.1 Weight distribution of the vehicle

Weight distribution S.No. Systems Weight (N)

1. Engine CG 203562. Transmission CG 103993. Hydraulic Tank CG 55924. Fuel Tank CG 93205. Body CG 5542656. Tractive Effort 339436

The frame consists of different casting and plates which are welded

to form the chassis. This geometry model has been built with solid works and

has been imported into hyper mesh to build the finite element model with

shell elements. In addition, the spring elements are used to simulate the

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suspension property and the lower node is restricted. At the same time, the

weight of the assemblies can be replaced with mass elements.

Finally, the finite element analysis model is shown in Figure 4.5

with nodes and elements. The displacement plot and von misses stress contour

plot were arrived after processing. According to the results, the high stress

areas are located at the horse collar portion and rear rail structure and the

maximum value is 307MPa. The high stress of the chassis area of rear rail

structure is mainly caused by the dynamic and fatigue condition of the

chassis.

Figure 4.5 Mesh model with loading condition

6.59e+03 1.0e+04

Force 9.32e+04

2.65e+03

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4.5 RESULTS AND DISCUSSION

4.5.1 Total Displacement in Bending and Torsion Loads of Chassis

The location of total displacement is maximum at the middle of the

rear rail structure and the numerical value of maximum displacements

3.97mm. As shown in Figure 4.6. It was reduced 50% of the value with

compared to linear static analysis.

Figure 4.6 Maximum Displacement of combined bending and torsion

4.5.2 Von-misses Stress in Torsion Case

The location of Maximum Von-misses stress occurred at the rear

rail structure of the chassis, as shown in Figure 4.7. The stress magnitude

noticed at critical point at rear rail structure is 137.2 MPa. The formula given

below can be used to calculate the von misses stress and shear stress of the

chassis structure.

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2 2 2max 1 2 2 3 3 1Von MisesStress ( ) ( ) ( ) / 2

21 y x y 2

1,2 xy

(2 2

2 21 a c a b c

E E( ) ( ) ( )1(1 ) 2(1 )

2 2max a b b c

2E ( ) ( )2(1 )

Figure 4.7 Von-misses Stress of Bending and Torsion

Von-mises

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According to the literature survey and vehicle standards, the ratio of

yield stress to the working stress (Factor of safety) for an off-high way

vehicle should be the numerical value of 4 in a static condition and 1.5 in a

dynamic condition. The vehicle chassis will be satisfied the factor of safety at

the critical area of the chassis.

4.5.3 Principal Stress in Torsion Case

The Maximum principal stress occurs at the bottom of the rear rail

structure, as shown in Figure 4.8.The stress magnitude noticed at critical point

at rear rail structure is 115 MPa and total displacement at the middle of the

rear rail gives the critical area. Stress value of the chassis with in that standard

and recommended limit.

Figure 4.8 Maximum Principal Stress in Bending and Torsion

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4.5.4 Shear Stresses in Torsion Case

The shear stress distribution provided from the FE analysis. Results

shows, that the tensile stress concentrated in the region of torque tube and

horse collar of the chassis. Due to the dynamic effect of torsion stress

distribution at the location of torque tube on the chassis. The critical regions

and the premature fatigue failure occur on the same area of the chassis as

shown in Figure 4.9.

Figure 4.9 Maximum Shear Stress in Torsion Load

The maximum shear stress is in direction of parallel and

perpendicular to the neural axis of the chassis. The value of average shear

stress is 72.2 MPa, which is 1.5 times more than the yield strength of the

chassis. This means that chassis rail structure satisfies the safety factors and

conditions for maximum loading, as shown in Figure 4.10.

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Figure 4.10 Shear Stresses and Normal Stresses of Torsion

4.6 CONCLUSION

Based on the FE results of the modified chassis, it is clear that the total deformation and Von-misses stresses were to meet the standard and recommended value. The overall chassis performance significantly increased strength of the chassis and with stand the payload with 2% of additional payload and net vehicle weight. The highest stress occurred is 137 MPa by FE analysis.

The calculated maximum principal stress is 115.2 MPa. The maximum displacement of numerical simulation result is 7.0294 mm. The result of numerical simulation is higher than the result of linear static analysis, which is 3.85 mm. The difference is caused by simplification of model and uncertainties of numerical calculation. The modified chassis was able to withstand the higher ton load capacity and also increase the performance of the chassis.