Roof Design

AIM:-

For the given roof styling, develop the essesntial flanges and reinforcements, provide appropriate dimensions and check for draft analysis and to perform roof curvature study using NX-CAD

Introduction:

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.

The roof is a styling surface part which is fixed with the BIW body through the rails and panels inside the upper roof to provide it strength and stiffness.

Ditch area plays an important role here to fix the reinforcement on the component.

The rails and reinforcements are joined to the ditch area with a spot weld where it's not the styling part.

The required tool opening angle is minimum 3 Degrees.

Factors which is considered before creating Ditch area,

• Headroom clearance
• Visibility criteria
• Back door assembly
• Windshield assembly

SOFTWARE USING NX CAD:-

CLASS A STYLING INPUT:-

• After the file is opened, the roof part is not joined together but as the individual surfaces.
• It is also seen as the surface is perfectly symmetrical in shape which helps us to work on the half of the portion and reflect it which creates the whole roof. We work on the half of the surface and mirror to complete the rest.

COMPONENTS OF THE ROOF:-

• Front roof rail
• Rear roof rail
• Bow roof
• Centre roof rail
• Ditch area.

FRONT ROOF RAIL:-

It is created using the Given reference master section.

The front roof rail is used to provide strength to the roof panel front and it also acts as the link between windshield glass, roof panel, and headliner. Spot welds are applied at the windshield end of the rail and the other end is joined with the roof using mastic sealant which also increases the strength.

REAR ROOF RAIL:-

It is created using the Given reference master section.

DESIGN:-

The rear roof rail is again used to strengthen the roof and it acts as a link between the back glass and the roof which is connected to the backdoor of the car. Spot welds are applied at the back door end of the rail and the other end is joined with the roof using mastic sealant.

CENTER ROOF RAIL AND BOW ROOF RAIL:-

Center roof rail will be at the center of the roof to provide strength and stability to the roof. Here the center roof rail is joined with the roof through mastic sealant.

The bow roof is used for strengthening purposes and is also attached to the upper roof surface with mastic sealants.

DITCH AREA:-

Ditch area is an extended portion of the main roof concealed by other BIW panels. The rails support of the roof is attached to the ditch area using spot weld.

SECTIONAL VIEW:-

Note: 5mm distance is given between outer panel and the roof rail for mastic sealants.

FINAL BODY:-

Note: All the components above has created with thickness of 0.75mm

CURVATURE STUDY ON THE ROOF:-

After getting the final body it has to be checked and justify that it passes the standards.

Analysis:

It is done to justify the position of the bow roofs.

1.Roof crash test.

2.Snow load test.

3.Heat distortion criteria

HEAT DISTORTION CRITERIA:-

Heat distortion temperature is a temperature at which the metal softens and deform under a specific load.This study is done to check the ability of the roof to withstand high temperature with respect to the position of the reinforcement which contribute to the increase strength.

• To calculate the heat distortion criteria by the bow roof position formula,
• We need to create a sketch , draw a line on the center of the roof on OY axis (mid horizontal axis)
• Next draw a line from the center line by a 300mm distance
• After that draw a line from 100mm distance on both sides of the 300mm line
• Then draw a line between the center of roof rails and draw the lines from the center line by the distance of 100mm on both sides.
• Atlast make an intersection on these lines from a coincident line to get points.

• Roof curvature (Rx & Ry) radius is measured by creating a 'Three point arc'

Prediction formula:W=[1.73×10^-3×L] + [1.85× 10^-8×R²/t]+[1.10×10^-3×l]-2.68

Judgemental condition:Ok<2.7>3.1

Where

L=Roof length in X-direction in mm (Roof dimension in O-Y)

R=Roof curvature

R=2(R_X×R_Y)/(R_X+R_y)

R_X=X Curvature

R_y=Y Curvature

T =Roof plate thickness in mm

L =Bow roof span in mm

BETWEEN	L	Rx	Ry	R=(Rx*Ry)/(Rx+Ry)	t	l	R^2/t	W	OK/NG
Front Roof Rial-Bow Roof 1	2136	5724.89	2738.49	3704.79	0.75	465.34	18300625.25	1.85	Ok
Bow Roof1-Center Roof Rail	2136	4900.71	6481.06	5581.16	0.75	330.92	41532462.59	2.13	Ok
Center Roof Rail-Bow Roof2	2136	4029.42	10313.26	5794.79	0.75	428.10	44772788.19	2.3	Ok
Bow Roof2-Rear Roof Rail	2136	3279.82	14584.14	5355.29	0.75	349.50	38238841.31	2.09	Ok

The test value infer that the design is good and under the limit of standards.

SNOW LOAD CRITERIA:-

Snow load criteria study is done to check the ability of the to withstand snowfall and bounce back to its orginal shape without any permenant deformation once the load is removed.

• To calculate the snow load condition, we have a snow load formula
• To measure the required values
• Created a sketch and draw a line on OY axis from the center of the roof
• From that line draw a parallel line distance of 100mm on both sides
• Then make intersection points of these lines and draw a three point arc .

• Roof curvature (Rx & Ry) radius is measured by creating a 'Three point arc'

Snow load prediction formula :Q_r =[(I_y × t²)/(α × s×(R_X+R_Y/2)² × 10^-8)]

Where

α = M_y× L_x²× 10^-12 ,M_y=Y(L_y -Y)

Judgement Condition:Q_r≥3.1

t=Roof plate thicness in mm

L_Y =Distance between Front and rear roof rail on vehicle O-Y in mm i.e length of the roof panel with the centre point between roof rail FR/RR as the referance point of the front and rear

Y=Distance from front roof rail centre to bow roof in mm

S - Distance for which BOW bears divided load in mm

S= L₁/2+L₂/2

I_y=Geometrical moment of inertia of roof BOW (Cross-section)mm⁴

R_x=Lateral direction curvature radius of roof panel Y cross-section on Roof ROW in mm

R_y =Longitudinal direction of the curvature radius of roof panel X cross-section on BOW roof (mm).

BETWEEN	Iy	α	Lx	Ly	Y	S	Rx	Ry	t	Qr	Ok/NG
Front Roof Rail-Bow Roof1	113.93	0.95	1133.10	1905.27	545.17	472.97	4399.30	2800.21	0.75	0.42
Bow Roof1-Center Roof Rail	116.77	1.06	1084.54	1905.27	945.96	449.39	3781.47	6419.71	0.75	0.19
Center Roof Rail-Bow Roof2	114.70	0.74	1059.79	1905.27	1443.96	479.65	4251.24	10769.25	0.75	0.08
Bow Roof2-Rear Roof Rail		0	1044.55	1905.27	1905.27		4227.97	16267.67	0.75

S value is not fit in the criteria i.e 250 ≤ s ≤ 380

The Qr value requirement for snow load criteria is Qr ≥ 3.1 which is not satisfied in any of the cases as per my calculation.So the surface is not good for the snow load criteria.

SECTION INERTIA ANALYSIS:-

BOW ROOF 1:-

CENTER ROOF RAIL:-

BOW ROOF 2:-

DRAFT ANALYSIS:-

It is done to check if the features designed clear the minimum draft 5 degree for manufacturabililty. 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.

It is seen that the features designed clear the minimum draft angle which is denoted in 'green'

CONCLUSION:-

• Curvature study on the roof was analyzed with heat distortion and snow load standards
• Draft analysis was done to check toolablilty.
• This is how the roof is designed.

THANK YOU...