Roof Design

DESIGN A ROOF USING NX CAD

OBJECTIVE:

Given Car Roof stlying design develop a BIW Roof of a car with Front Roof Rail, Bow Roof Rail, Center Roof Rail, and Rear Roof Rail by following the engineering standards. The roof must be subjected to Draft Analysis for verifying whether it satisfies the manufacturing requirements. Curvature Study and Section Modulus Study needs to be carried out for the designed roof.

INTRODUCTION:

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.

It also acts as a cross member between body sides of the car body and provides provisions for front windshields, backdoor. Roof assembly also one among the parts which provide safety for rollover accidents.

DESIGNING THE REINFORCEMENTS:

Ditch Area:

The Ditch Area of the Roof was designed by taking the considerations provided by the styling team. The ditch area was designed to have only 3 degree of maximum draft angle and 0.75mm thickness. The ditch area is the extended portion of the main roof concealed by other BIW panels. The rails of the roof were attached by using Spot welds and Mastic Sealants. Visibility Criteria, Head Room Clearance, windshield clearance and back door clearance was also taken into the design consideration of the Roof.

Front Roof Rail:

The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into consideration for the bottom flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

Rear Roof Rail:

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail. Here one end is joined using spot welding while the other end is joined using mastic sealants.

Bow Roof:

Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strength and also this would not affect the styling of the roof.

Reinforcement Center:

Centre Roof rail or the reinforcement centre is located at the centre of the roof. The cross section of this is more since it needs to provide more strength to the roof. Here both are ends are joined together using mastic sealants.

Roof assembly:

ANALYSIS:

Heat Distortion Criteria:

Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperature with respect to the positioning of the reinforcements which contribute to increased strength.

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]

Snow load prediction:

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

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where,

• α = My x Lx2 x 10-12 , My = Y(Ly-Y)
• 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 front and rear reference point.

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

The width of the roof panel is 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]

• From the above results, these values for the snow load values it does not satisfy the condition.
• To satisfy this condition we make the roof stronger we create embosses on the roof to strengthen the roof and increase the thickness of Bow.

Section modulus:

Bow roof:

Reinforcement center:

Draft Analysis:

• While designing a component a design engineer must make sure that the component can produced by any means of manufacturing methods with least amount of time, material, manpower and power. So that we can reduce the overall cost of car.
• In NX draft analysis can be done after modelling so that we can verify whether the product is manufactured by pressing or not. One side of the surface is highlighted by green and other side by Red colour means that the model can be manufactured. Result of the draft analysis is shown in the figure below.

1.Ditch area:

2.Front roof rail:

3.Rear roof rail:

4.Bow roof-1:

5.Bow roof-2:

6.Center rainforcement:

CONCLUSION:

• Thus the required Ditch Area, Bows and Reinforcements for the Roof were designed and developed from the given styling.
• Calculations for the Heat Distortion criteria and Snow load predictions were also performed.
• The BIW roof of a car was designed and achieved. For a better result applying Thickness in Bows and create Embosses over the Roof.