Roof Design

ROOF PANNEL DESIGN

AIM:

Create a Roof Pannel Design using Siemens 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 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.

Introduction:

Automobile roof is the upper portion of an automobile which protects the passengers from sun, wind, rain. Various environmental conditions are taken into account for designing of the same. Even though it serves as styling surface roof should be provided with proper reinforcements for safeguarding passengers. Vehicle roof are vulnerable to high heat conditions and moderate load during snowy conditions.Heat load calculation & snow load calculation are done to get a proper design of roof rails. This ensures the safety of the passengers in the vehicle.

DESIGN CONSIDERATIONS FOR A ROOF:

• Visibility criteria
• Head clearance
• Curvature Study
• Heat distortion criteria 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 ROOF CRUSH TEST:

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

INITIAL CONDITION:

• Close all the doors and windows
• We can take only BIW for crush test

SECOND CONDITION:

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

IF THE ROOF IS FAILED IN THE CRUSH TEST,THEN

• We increase the roof thickness
• Increase the reinforcements
• Increase the thickness of reinforcements
• We can change the curvature of roof
• Some time increasing the mastic and spot welds also increase the strength

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,

RESULT:

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.

COMPONENT (REINFORCEMENT) OF ROOF:

• Bow Roof Rail
• Center Roof Rail
• Rear Roof Rail

STYLING ROOF:

ROOF OUTER:

FRONT ROOF RAIL:

• Front roof rail is joining the wind shield glass with roof. It also joins the roof with inner panel and body side outer.
• The depth of the front roof rail is based on the front visibility criteria, and width of the front roof rail is decided according to head room clearance.
• For joining the front roof rail with flange of roof OOFby using spot welding, and we use mastic for joining the front roof rail with the roof.

BOW ROOF:

• The Bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure, and overall increase the strength of the roof.

• The number of bow roofs depends on the overall dimensions of the roof. Longer the roof more the bow roofs are integrated to BIW frame.

• The positioning of bow roofs is based on curvature study. The heat distortion criteria and snow load criteria are checked to analyses the position of the bows in various conditions.
• The bow roofs are attached to the outer panel using mastic sealants. Each mastic point will have up to 80 mm circular effective area.

CENTER ROOF RAIL:

• Centre roof rail placed at the center of the roof and connecting the B pillar support structure.

REAR ROOF RAIL:

• Rear roof rail is joining the back door with roof, and also joins the body side outer.
• The design consideration for rear roof rail is rear visibility criteria and rear head room clearance.
• For joining the rear roof rail with flange of roof by using spot welding, and we use mastic for joining the rear roof rail with the roof.

SECTION MODULUS:

• The section modulus value gives us the strength of the component. Strength basically means that whatever load we apply the resistance provided by the component opposite to the deformation of the component.
• Bending a steel section that has a larger section modulus than another will be stronger and harder to bend.
• In simple terms the section modulus is the ratio of bending moment to bending stress for steel, if your steel has a high section modulus it will be harder to bend and can withstand a high moment without having high bending stress.

THERE ARE TWO TYPE OF SECTION MODULUS:

• Elastic section modulus
• Plastic section modulus

SECTION MOMENT OF INERTIA:

Section moment of inertia is used to predict the amount of deformation and deflection of a 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 master sections of the roof and roof rails and their section analysis are given.

The elastic section modulus is defined as

S = I / Y

S = Section modulus

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

Y = distance from the neutral axis to end most point of the fiber.

SOLID SECTION INERTIA FOR (FRONT ROOF RAIL):

• IMAX = 1.841949911e+05[mm^4]
• IMIN = 7.907712362e+03[mm^4]

SOLID SECTION CRITERIA FOR (BOW ROOF RAIL 1 ):

• IMAX = 3.003748398e+04[mm^4]
• IMIN = 1.435288080e+03[mm^4]

SOLID SECTION INERTIA FOR (BOW ROOF RAIL 2 ):

• IMAX = 3.007359079e+04[mm^4]
• IMIN = 1.279468708e+03[mm^4]

SOLID SECTION INERTIA FOR ( BOW ROOF RAIL 3 ):

• IMAX = 3.004209071e+04[mm^4]
• IMIN = 1.287334829e+03[mm^4]

SOLID SECTION INERTIA FOR (CENTER ROOF RAIL ):

• IMAX = 1.699297109e+05[mm^4]
• IMIN = 1.130959780e+04[mm^4]

SOLID SECTION FOR INERTIA ( REAR ROOF RAIL ):

• IMAX = 3.984733935e+05[mm^4]
• IMIN = 2.655171013e+04[mm^4]

HEAT DISTORTION CRITERIA:

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]

Judgement Condition : OK< 2.7> 3.1

S.NO	BOWS	RX	RY	R	I	L	T	R^2/t	W	OK/NG
1	FR&BR-1	4003.44	2329.79	2945.14	290.5	1919.43	0.75	11564033	1.17	OK
2	BR-1&BR-2	5305.85	4837.13	5060.65	229.9	1919.43	0.75	34146904	1.5	OK
3	BR-2&CTR	3471.86	7357.38	4717.55	233	1919.43	0.75	2673704	1.4	OK
4	CTR&BR-3	3981.18	18022.61	6521.71	355	1919.43	0.75	56710268	2.0	OK
5	BR-3&RR	3297	4571.48	3831	400.1	1919.43	0.75	19568748	1.4	OK

Roof design is passing in heat distortion condition.

SNOW LOAD CONDITION:

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]

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)

Judgement condition = Qr ≥ 3.1

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]

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]

Roof panel X curvature radius of length s in Side view

S.NO	BOWS	α	Lx	Ly	S	Iy	Rx	Ry	Y	T	Qr	DETERMINATION(Qr)
1	FR&BR-1	0.79	1144.9	1942.5	346	1435.2	4732.47	2332.99	389.8	0.75	23.6	OK
2	BR-1&BR-2	1.04	1100.8	1942.5	306	1279.5	2417.46	4738.88	693.7	0.75	17.6	OK
3	BR-2&CTR	1.09	1077.2	1942.5	370	11309.5	9190.53	7354.88	1003.3	0.75	23	OK
4	CTR&BR-3	0.81	1058.3	1942.5	469	1287.3	4272.74	11042.4	1434.7	0.75	3.2	OK
5	BR-3&RR	-	1043.8	1942.5	-	-	3073.61	11306.6	1942.5	0.75	-	-

Roof design is passing in snow load condition.

DRAFT ANALYSIS:

DRAFT ANALYSIS FOR ROOF OUTER PANNEL:

DRAFT ANALYSIS FOR FRONT ROOF RAIL:

DRAFT ANALYSIS FOR BOE ROOF RAIL 1:

DRAFT ANALYSIS FOR BOW ROOF RAIL 2:

DRAFT ANALYSIS FOR BOW ROOF RAIL 3:

DRAFT ANALYSIS FOR CENTER ROOF RAIL:

DRAFT ANALYSIS FOR REAR ROOF RAIL:

CONCLUSION:

Hence the roof , front roof rail, bow roof rails , central 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.