Roof Design

ROOF DESIGN

OBJECTIVES

• The objectives of this project is to design car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rails and rear roof rail for the given styling surface.
• Perform a curvature study on the roof and perform the Calculations for the heat disortion and snow load criteria to determine whether the bow roofs are at 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-CAD

INTRODUCTION:

Design of any products 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 design 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 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 inforcements to provide the strength to the overall structure and to with stand the overhead forces like Snow.

WHY REINFORCEMENTS?

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

Different types of inforcements 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 upon the master section given.

REAR ROOF RAIL:

Rear Roof Rail is one which joints the back door and Body side Outer.

CENTRAL ROOF RAIL:

Central roof rail helps in providing effectively Support to the flat area of the roof as it is more susceptible to the failure under the action of load. Generally Central roof rail is placed at the centre 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 rool-over test. The thickness of the central roof rail is 0.75 mm.

BOW ROOF RAILS:

The bow roof rails are gives to improve the torsional stiffness and bearing capacity of the roof structure.

JOINING PROCESS:

• Mastics are introduced between the roof outer panel and the reinforcements for joining. wherever the mastics are introduced there is remarkable improve of 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 disortion 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 upon this test. For this they consider two points while giving the rating.

• How fast the car can reform 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 air bags can do their 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 raer 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 requirement 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 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 limt 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 positon which helps to strengthen the roof.

BOW-ROOF PREDICTION FORMULA:

W = [1.73×10^-3×L] + [1.85×10^-8× (R²)/t] + [1.10×10^-3×I]-2.68

Where,

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

R= Roof curvture

R = 2(R_x×R_y) \ (R_x+R_y)

R_x= X Curvature

R_y= Y Curvature

t= Roof plate thickness [mm]

I= Bow Roof Span [mm]

Judgement 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:

Roof Rail	Rx (mm)	Ry (mm)	Rx×Ry (mm)	Rx+Ry (mm)	R (mm)	L (mm)	t (mm)	I (mm)	W (mm)	Result
FR-BR1	5589.35	3572.46	19967729.3	9161.81	4358.9	2030.19	0.75	685.53	1.99	PASS
BR1-CRR	4701.72	7319.79	34415603.04	12021.51	5425.67	2030.19	0.75	243.95	1.84	PASS
CRR-BR2	4216.23	2109.71	8895022.59	6325.94	2812.24	2030.19	0.75	243.95	1.24	PASS
BR2-RR	3435.59	1723.59	5919727.705	5159.18	2294.83	2030.19	0.75	696.44	1.67	PASS

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 in 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 removed the roof should go it its original position. This is the basic requirements for snow load criteria.

Q_r = [I_y × t²] / [∝ × S × [(R_x+R_y)/2]²×10^-8]

where,

∝ = M_y × L ×2×10^-12,M_y = Y(L_y-Y)

Judgement Condition = Q_r ≥ 3.1

250 ≤ S ≤ 380

t = Roof plate thickness [mm]

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

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

L_x = 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 for Front Roof Rail to Roof BOW[mm]

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

S = L₁/2 + L₂/2

I_y = Geometrical Moment of Inertia of Roof BOW ( Y cross-section)[mm⁴]

R_x = Lateral direction curvature radius of Roof Panel Y cross-section on Roof BOW[mm]

Roof Panel curvature Radius of the Length L_x in Front View.

R_y = Longitudinal direction curavture 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:

BOWS	Iy [mm4]	My	∝	Lx (mm)	Ly (mm)	Y (mm)	S	Rx (mm)	Ry (mm)	t (mm)	Qr	OK/NG
FR - BOW1	448.67	831678.14	1	1100.1	1898.72	685.53	464.74	4048.01	2033.87	0.75	5.87	PASS
BOW1 - CC	284.03	403681.14	0.45	1111.9	1898.72	243.95	243.95	3859.69	1930.83	0.75	17.36	PASS
CC - BOW2	223.6	403681.14	0.49	1100.75	1898.72	243.95	470.195	4136.71	9648.48	0.75	1.15	FAIL

SECTION ANALYSIS:

Section moment of inertia is used to predict the amount of deformation and deflection of part. 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 increase the section inertia of the part. Here the mster sections of the Roof and Roof Rails and their Section analysis are given.

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 =8.92×104 mm⁴

Moment of Inertia Mimimum,(MIN) I_min = 448.68 mm⁴

BOW ROOF 2:

Moment of Inertia Maximum, (MAX) I_max = 4.39×10⁴ mm⁴

Moment of Inertia Minimum, (MIN) I_min = 223.59 mm⁴

CENTRAL ROOF RAIL:

Moment of Inertia Maximum, (MAX) I_max = 7.91×104 mm⁴

Moment of Inertia Minimum, (MIN) I_min =284.04 mm⁴

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 Colour ranges identifying zones on analyzed elements where the deviation from the draft direction at any point,corresponds to specified values.

Minimum Draft Angle of 7º is considered for analysis. Green Colour infers on the parts that all face along the Tooling direction has positive draft angle greater than 7º and passed in Analysis.

DRAFT ANALYSIS FOR FRONT ROOF RAIL:

DRAFT ANALYSIS FOR BOW ROOF RAIL-1 AND BOW ROOF RAIL-2:

DRAFT ANALYSIS FOR CENTRAL ROOF RAIL:

DRAFT ANALYSIS FOR REAR ROOF RAIL:

CONCLUSION:

Hence the Roof,Front Roof Rail,Bow Roof Rail,Central Roof Rail and 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.