Design of BIW Component Roof in NX CAD

Automotive Design of Car Roof using NX CAD

Objective:

• The objective of this project is to design car roof with  essential flanges and reinforcements  like ditch area, front roof rail, bow roofs, central roof rail and rear roof rail for the given styling surface

• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct  positions.

• Finding moment of inertia and  section modulus for Bow roofs, central roof rail. front roof rail and rear roof rail.

• Perform the draft analysis for each reinforcements.

Software used:

                       NX 12.0

Introduction:

          Design of any product starts from a requirement. The design engineer sketches the product based on the requirement. Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD software's. The another important thing in the design is that, every product that is designed should be able to manufactured. so the role of the design engineer is to design the product that can be easily manufactured and with less cost effective. This is the place where the Design for manufacturing [DFM] and Design of assembly [DFA] takes a role while developing any product. The process of development of design carries on until the design satisfies the customer needs.

          Here in this project the product that we are going to develop is a car roof:

Car Roof:

            An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow

Why reinforcements:

            If the roof is flat it is very weak and cannot able to with stand the forces like snow, so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process

Different types of reinforcements that the car has are:

• Front roof rail

• Bow roofs

• Central roof rail and

• Rear roof rail

Front roof rail:

           Front roof rail is the one which joins the wind shield glass, body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given.

Rear roof rail:

          Rear roof rail is the one which joins the back door and the body side outer.

Center roof rail:

           Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load. generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding the strength to the roof during the roll over test. The thickness of the central roof rail is 1.5 mm.

Bow roofs:

          The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roof present is depends on the overall size of the roof. Presently in this project two bow roofs are added.

Joining process:

           Mastics are introduced between the roof outer panel and the reinforcements for joining. where ever the mastic are introduced there is remarkable improve of the strength.

Spot welding is also introduced to join the reinforcements and the outer panel.

Design considerations for a roof:

• Visibility criteria

• Head clearance

• Curvature study

• Heat distortion and snow load criteria

• Draft analysis.

Roof crush test:

          Roof crush test is the design given by the IIHS (insurance institute of highway safety). After analyzing the many accidents IIHS has then fixed some regulations, that how to perform the crush test.

            IIHS gives the rating based up on this test. For this they consider two points while giving the rating

• How fast the car can reforms to its original shape after the roll over and

• How deep is the protrusion after the roll over.

Goal of roof crush test:

          To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in protecting the occupant

Conditions to follow for roof crush test:

There are two conditions that should be followed for the roof crush test, they are

Initial condition:

• Close and lock all the doors.

• Close all the windows.

Second condition:

          Fix the body sill or chassis frame on a hard lever surface horizontally.

Method of applying the force:

          The load is applied in such a way that 50 is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width. The load is applied at the position of 250 from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that.

Results:

           If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength. the stronger the roof the better it can protect the occupants in the roll over crashes.

HEAT DISTORTION STUDY ON THE ROOF:

           The heat distortion study plays a major role in sheet metal usage. heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid bending or damages on the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.

Bow – roof prediction Formula

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8)  x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Where

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

R = Roof curvature

R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Judgment Condition: OK< 2.7> 3.1

Apply the values in the formula:

          In order to find the heat distortion apply all the necessary values from the roof to the given formula.

Calculations:

From the above table it can be concluded that all values of w<2.7 and also not w>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok.

SNOW LOAD CRITERIA:

          This test is done to know how is the roof behaving when there is a snow over. normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position. This is the basic requirements for snow load criteria.

Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]

Where

α = My x Lx2 x 10-12 , My = Y(Ly-Y)

Judgment condition = Qr ≥ 3.1

250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Length of Roof panel with the center point between Roof rail Front /Rear as the reference point of the front and the rear.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

Width of the roof panel exposed on the surface.

Y = Distance front Front Roof Rail to Roof BOW[mm]

s = Distance for which Roof BOW bears divided load [mm]

s = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof BOW (Y cross-section )[mm4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Roof BOW [mm]

Roof panel curvature Radius of the Length Lx in Front view

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Apply the values in the formula:

In order to find the snow load criteria apply all the necessary values from the roof to the given formula.

Calculations:

From the above table Snow load conditions the curvature values are below the range so that We added embosses in the Roof outer panel to strengthen the Roof Curvature.

SECTION MODULUS :

           Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below.

There are two types of section modulus:

The elastic section modulus and

The plastic section modulus.

The elastic section modulus is defined as

S = I / y

where:

S= Section modulus

I = second moment of inertia (or area moment of inertia, not to be confused with moment of inertia)

y = the distance from the neutral axis to any given fibre. It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre

Bow roof 1:

Create an intersection curve with the help of the plane and select the bow roof 1

Moment of inertia maximum,(MAX) I max = 2.7868 × 10^4(mm4)

Moment of inertia maximum,(MIN) I min =190.452 (mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fiber

S = I / y

Y = 29.9 mm

FOR MAXIMUM MOI:

S= 2.7868 × 10^4(mm4) / 29.9 mm

S max = 932.04 mm3

FOR MINIMUM MOI:

S= 190.452 (mm4) / 29.9 mm

S min = 6.36 mm3

Bow roof 2:

Moment of inertia maximum,(MAX) I max =2.8132 × 104 (mm4)

Moment of inertia maximum,(MIN) I min =220.396 (mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fiber

S = I / y

Y = 29.9 mm

FOR MAXIMUM MOI:

S= 2.8132× 104 (mm4) / 29.9 mm

S max = 940.86 mm3

FOR MINIMUM MOI:

S= 220.396 (mm4)/ 29.9 mm

S min = 7.37 mm3

Bow roof 3:

Moment of inertia maximum,(MAX) I max =2.8153 × 104 (mm4)

Moment of inertia maximum,(MIN) I min =221.3351 (mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 29.9 mm

FOR MAXIMUM MOI:

S= 2.8153 × 104 (mm4) / 29.9 mm

S max = 941.57 mm3

FOR MINIMUM MOI:

S= 221.3351 (mm4)/ 29.9 mm

S min = 7.40 mm3

Central roof rail:

Moment of inertia maximum,(MAX) I max =6.5922 × 104 (mm4)

Moment of inertia maximum,(MIN) I min =274.6182(mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 39.9 mm

FOR MAXIMUM MOI:

S= 6.5922 × 104 (mm4) / 39.9 mm

S max = 1652.18 mm3

FOR MINIMUM MOI:

S= 274.6182 (mm4)/ 39.9 mm

S min = 6.88 mm3

Front roof rail:

Moment of inertia maximum,(MAX) I max =1.5480 × 105 (mm4)

Moment of inertia maximum,(MIN) I min =8.1902 × 103(mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 61.9 mm

FOR MAXIMUM MOI:

S= 1.5480 × 105 (mm4) / 61.9 mm

S max = 2500.80 mm3

FOR MINIMUM MOI:

S= 8.1902 × 103(mm4)/ 61.9 mm

S min = 132.31 mm3

Rear roof rail:

Moment of inertia maximum,(MAX) I max =8.70166 × 105(mm4)

Moment of inertia maximum,(MIN) I min =5.0852 × 104(mm4)

SECTION MODULUS

S= moment of inertia /distance from the neutral axis to any given fibre

S = I / y

Y = 114.35 mm

FOR MAXIMUM MOI:

S= 8.7016 × 105(mm4)/ 114.35 mm

S max = 7609.61 mm3

FOR MINIMUM MOI:

S= 5.0852 × 104(mm4)/ 114.35 mm

S min = 444.70 mm3

DRAFT ANALYSIS:

          The Draft Analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

          Minimum draft angle of 70 is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 70 and passed in analysis

Views of the designed roof:

Front view:

Side views:

Top view:

Bottom view:

Back view:

Conclusion:

          Thus the required reinforcements and Ditch Area for the Roof were designed and developed from the given styling. The curvature study was done after performing calculations for the Heat Distortion with the position criterion met for all reinforcements and Snow load conditions the curvature values are below the range so that We added embosses in the Roof outer panel to strengthen the Roof Curvature.

          Further the Section Modulus study was conducted.