Roof challenge

ROOF DESIGN CHALLENGE

OBJECTIVE:

• To design the automotive roof based on the given styling surface and analyze the prepared model to check the manufacturability.
• 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.
• To perform the section modulus study for the master sections and create for the Front roof rail, Rear roof rail, Bow roofs and center roof rail.

ROOF:

• The Automobile roof consists of the following parts:

• The outer roof panel.
• Front roof rail.
• Bow roof rail.
• Centre roof rail.
• Rear roof rail.

TO CREATE THE ROOF GIVEN DATA IS:

ROOF SKIN:

FRONT ROOF RAIL MASTER SECTION;

 

REAR ROOF RAIL MASTER SECTION:

ROOF OUTER PANEL:

• Roof outer part is the major styling portion of the assembly made up of sheet metal.
• Roof outer panel mainly protects from external objects and abnormal environmental conditions. All the pillars are spot welded around the roof & roof rails.
• Welding on upper surface is avoided for the best aesthetic look.

FRONT ROOF RAIL:

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

BOW ROOF RAIL – 1 & 2:

• Bow roofs are used in the roof assembly to improve the strength at a specific region. These bow roofs are joined directly to the roof with the Mastic Sealants.
• It can be created in any numbers depending upon the roof length but the position of the bow roofs is to be justified by curvature study.

CENTER ROOF RAIL:

• Center roof rail or reinforcement center is used as a cross member between bodyside left hand and right hand respectively.
• In some cases, the reinforcement center is not joined with the roof and in some other cases, it is made to join in both roof and bodyside.
• The reinforcement center and Bow roof are joined either spot weld or by mastic sealants.

REAR ROOF RAIL:

• The rear roof rail acts as support at the rear end of the roof. It is joined to the back door.
• The rear roof rail is designed according to the inputs taken from the back door. So that it should have headroom clearance and rear visibility criteria.

ASSEMBLY OF ROOF:

HEAT DISTORTION CALCULATION:

• Do a curvature study on the roof and perform calculations to determine the position of the Bow-roof
• Heat Distortion study plans vital role in sheet metal. Heat distortion temp is known as the temperature limit above which the material can't be used for structural applications.

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]

TABLE – 1

BOWS	L(mm)	RXmm	RYmm	RX*RY	RX+RY	R	t	I	W	RESULT
FR -BOW 1	2033.70	5700.38	2980.34	16989144.47	6808.736	3914.21	0.75	520.94	1.78	ok
BOW 1 - CR	2033.70	4812.37	6839.83	32915832.42	11652.207	5649.71	0.75	356.44	1.23	ok
CR= BOW 2	2033.70	3982.49	10555.58	42037550.70	14538.080	5783.09	0.75	352.53	2.05	ok
BOW2 - RR	2033.70	3322.49	14311.16	47548747.52	17633.65	5392.95	0.75	376.28	1.95	ok

Here we can see that the roof is completely satisfying the conditions of heat distortion.

MOMENT OF INERTIA {Ly}

• Section moment of Inertia used to predict the amount of deformation or deflection of the designed part across the section.
• Higher moment of inertia is preferred as it reduces the amount of defection occurred by the applied force.
• The moment of Inertia (IY) for all the rail roof is calculated across the section by using a command called section analysis in NX Cad.

SECTION MODULUS:

• Section modulus is a geometric property for a given cross – section used in the design of beams and moment of inertia for striffness.
• The section modulus is defined as

S = L/Y

            S = Section modulus

.           l = moment of inertia.

            neutral Y = distance from the axis to any given part.

FRONT ROOF RAIL:

BOW ROOF RAIL:

CENTER ROOF RAIL:

BOW ROOF RAIL 2:

REAR R0OF RAIL:

SNOW LOAD CALCULATION:

• Due to snowfall, snow will get collected on the top of the roof which will tend to bend the roof. The bow roof will be in such a position that will provide more strength to the roof against bending.

 

SNOW LOAD PREDICATION FORMULA:

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

α = My x Lx2 x 10-12, My = Y(Ly-Y)

Judgment 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 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] Roof panel X curvature radius of length s in Side view.

TABLE -2

 

• It is observed that the roof is satisfying the condition only in the area between bow roof 1 and centre roof. in the other areas it is not satisfying the condition. So in order to strengthen the roof at these areas embosses are created on the outer roof.

CONCLUSION:

• Finally, the roof of the hatchback car has been successfully designed with considering all the engineering aspects, and draft, heat distortion, and snow load analysis were done.