Roof Design

• ROOF DESIGN:-

• OBJECTIVE :-

• The objective of this project is to design car Roof with important flanges and reinforcements i.e. ditch area, front Roof Rail, Bow Roofs, Center Roof Rail and Rear Roof Rail for the given Roof surface.
• Perform a curvature study on the Roof and perform the calculations of Heat Distortion Criteria and Snowed load Criteria to examine whether the bow Roofs are at the correct positions.
• Finding Moment of Inertia and Section modulus for Bow Roofs, Center Roof Rail, Front Roof Rail and Rear Roof Rail .
• Perform the draft analysis for each Reinforcements.

• INTRODUCTION :-

Design of any product starts through a requirement. The design engineer sketches the product based on the the requirement. Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD softwares. Responsibility of the design engineer is to design the product that have manufacturability and with low cost. This is the place where the Design for manufacturing (DFM) and Design of assembly (DFA) used in development of product. The process of development of design continues 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 :-

A Roof is the portion of an Automobile that positioned above the passenger compartment, protecting the vehicle occupants from wind, sun, rain and other external elements. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the structure and to withstand the overhead forces like snow.

• WHY REINFORCEMENTS :-

If the Roof is flat it is very weak and cannot able to withstand the overhead forces like snow, So to increase the strength of the Roof these reinforcements like Front Roof Rail, Center Roof Rail, Rear Roof Rail and the Bow Roofs are designed and joined to the flat surface of the Roof by different joining process i.e. Spot welding, Mastic etc.

• Types of Reinforcements of Car :-
• Front Roof Rail
• Bow Roofs
• Center 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. Here the front Roof Rail is designed based on given master section.

• REAR ROOF RAIL :-

Rear Roof Rail is the one which joins the back door and the body side outer panel.

• CENTER ROOF RAIL :-

Center Roof Rail helps in providing support effectively to the flat area of the Roof as it is susceptible to failure under the action of overhead load. Center Roof Rail is placed at the center of the Roof which is connected to the B-pillar. This Center Roof Rail helps in adding the strength to the Roof during the Rollover test. The thickness of the Center Roof Rail is 0.75 mm.

• BOW ROOFS :-

The bow Roofs are provided to improve the torsional stiffness and load bearing capacity of the Roof. The number of bow Roof present is depends on the overall length of the Roof. In this challenge two bow Roofs are added.

• JOINING PROCESS :-

Mastics are given between the Roof outer panel and the reinforcements for joining process. Wherever the mastics are given 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 :-
• Head clearance
• Visibility criteria
• Curvature study
• Draft analysis
• Heat distortion and snow load criteria

• ROOF CRUSH TEST :-

Roof Crush test is the regulation given by the IIHS (Insurance Institute of Highway Safety). After analyzing the accidents, Then IIHS has fixed some regulations, that how to perform the crush test of Car.

IIHS gives the rating based on this Crush Test. For this they consider two points while giving the rating.

• How fast the car can regain to its original shape after the rollover is removed.
• How deep is the protrusion will occur after the rollover.

• GOAL OF ROOF CRUSH TEST :-

To reduce the protrusion into the passenger compartment, So that the seat belts and Air bags can do their job to protect the passengers.

• CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :-

1. INITIAL CONDITION:-

• Close and lock all the doors
• Close all the windows

2. SECOND CONDITION :-

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

• METHOD OF APPLYING THE FORCE :-

The load is applied in a way that 5⁰ is tilted from the front side to the rear side and the load will be 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 120 seconds and the crush speed is 13 mm/sec or less at which the load applied.

• RESULTS :-

If the displacements goes above the requirements level then we have to increase the bow Roof thickness or add more mastic points to increase the strength. Roof should be stronger and it can protect the passengers in the rollover collision.

• HEAT DISTORTION STUDY ON THE ROOF :-

The Heat distortion study plays important role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for the vehicle structural usage. This study is used to predict the Heat Distortion Temperature at where the material starts being soften when exposed to a fixed load at elevated temperature. To avoid damages or bending on the Roof, This study will predict the bow Roof position which helps to strengthen the Roof based on the heat distortion temperature.

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]

Judgement Condition : OK< 2.7> 3.1

• APPLY THE VALUES IN THE FORMULA :

In order to find the heat distortion, put all the required values from the Roof in 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 Thus says that current positioning of bow Roofs are good in state as per design and ok.

• SNOW LOAD CRITERIA :

This test is done to know how Roof is behaving when snow is overhead of vehicle. Due to the snow weight, Chance of dent are more. But the Roof should be designed in a way that when the snow is removed, Roof should go to 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)

Judgement 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 :

1In order to find the snow load criteria apply all the necessary values from the Roof to the give formula

• CALCULATIONS :

It is observed from the values that not obtained values of Qr > 3.1

• SECTION MODULUS ANALYSIS:

Section moment of inertia is used to predict the value of deformation and deflection of a component. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses can increase the section inertia of the part. Roof Rails and their section analysis are given.

• BOW ROOF 1

• CENTER ROOF RAIL :

• BOW ROOF 2

• 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

• DRAFT ANALYSIS FOR FRONT ROOF RAIL :

• DRAFT ANALYSIS FOR BOW ROOF - 1:

• DRAFT ANALYSIS FOR CENTER ROOF RAIL :

• DRAFT ANALYSIS FOR BOW ROOF RAIL - 2:

• DRAFT ANALYSIS FOR FRONT REAR ROOF RAIL :

• FINAL DESIGN :

Hence the Roof , front Roof Rail, bow Roof Rails , Center Roof Rail and the rear Roof Rail is designed by following the master sections and the moment of inertia and draft analysis are carried out to check feasibility manufacturing.