Roof challenge

Roof Design Report

AIM :-

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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.

Introduction of roof :-

The conventional task of the roof panel is that of compartment shielding, passenger protection and connection of body sides, front and roof cross members. In case of roll over, compartment integrity is provided by the compartment frames, whilst the roof target is only to avoid the ejection of passengers, providing protection against hard local contact.

Correspondingly a wide selection of suitable materials are available: Steel, aluminium, thermosetting plastic sheet, glass. In case of steel grade FEP04, the usual thickness is in the range of 0.65 to .85 mm. In all cases, any roof panel material requires braces to prevent panel flexing and vertical waviness, commonly known as oil canning: for this purpose light steel bows are introduced, which are welded or riveted or screwed to the body side and bounded to roof panel by smooth flexible adhesives such as butyl mastic (in order to avoid sink marks on roof surface), mainly providing a damping effect.

Another task of roof bows is to support the interior roof liner which requires a number of fittings both in central and boundary areas to avoid deflections and vibrations. Fittings are usually plastic clips or velcro strips. Some roof liners, mostly on less expensive vehicles are bonded directly to roof panel.

Objective :-

• For the given Roof styling and master section, develop the essential flanges and reinforcements, provide appropriate dimension and check for Draft analysis and also submit section inertia analysis report on 0-Y section.
• Creation of roof ditch area the tool opening angle is only 3 to 5 degree.
• Design and develop roof rails as per the given master sections.
• Perform section modulus study for the Master sections created for the Front roof rail, Rear roof rail, Bow Roof Rail and Rear Roof Rail.
• To do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.
• To perform Heat distortion criteria and Snow Load criteria tests on the designed roof and do optimization if necessary.

Roof Design :-

• Thickness of roof=0.65mm
• Draft angle=3deg-5deg

Front roof rail :-

It is the support which is joined with the Wind Shield Glass, Body Side Outer and the inner panels. It is usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm. A draft angle of 3-5 degrees is provided keeping in mind the deep drawing operation. The Wind Shield end flange is the exact offset of the roof surface. The bottommost flange is decided after taking into consideration the visibility criteria and the Headroom Clearance of the passenger. Spot Welds are applied at Wind Shield end of the rail and the other end is joined with the roof using Mastic Sealants which also contributes to increased strength.

Front Roof Rail Master section:-

Rear roof rail :-

It is the support which is joined to the Back Door and the Body Side Outer and the inner panels. It is also usually made from Body Side Outer scrap and therefore the thickness of the rail is 0.75 mm. Similar to the Front Roof Rail design, we have to take into consideration the rear Headroom Clearance and the rear visibility criteria while designing the Rear Roof Rail. Inputs from the backdoor design is also taken to accommodate the required clearance. Spot Welds are applied at the Back doorend of the rail and the other end is joined with the roof using Mastic Sealants.

Master section of the Rear Roof Rail :

Reinforcement Roof Centre(RRC) :-

Similar to other Rails design, we have to take into consideration the Headroom Clearance and the visibility criteria while designing the Centre Roof Rail and Bow Roofs. But unlike the Front and Rear Roof Rails, Spot Welds are not applied for joining these rails to the roof. Instead Mastic sealants are introduced at both the ends since Centre Roof Rail and Bow Roofs are fully positioned under the Styling surface of the Roof. For increasing the strength of the rails, embossing is done. To reduce the weight of the material used, a different styling has been followed with a thickness of 0.75 mm.

Master section of Center Reinforcement Rail :

Bow roof:-

Bow roofs are placed on the flat regions of the roof which provides strength in bigger curvature region and in between Front roof rail,centre roof rail and rear roof rail.Bow roofs are also design based on the design consideration similar to Front Roof Rail(FRR),please refer the master section attached below.

Master section of Bow Roof :

Bow roof shown in cross-section.

Roof rails are designed according to master section provided.

Thickness of each roof rail = 0.65mm

Section Inertia Analysis on roof rails :

Section Inertia Analysis on Bow roof rail - 1

Minimum Moment of Inertia Imin = 19145.69 mm4

Section Inertia Analysis on Center Bow Roof Rail:

Minimum Moment of Inertia Imin = 20621.21 mm4

Section Inertia Analysis on Bow Roof Rail-2:

Minimum Moment of Inertia Imin = 23093.41 mm4

Draft Analysis of Roof and Roof Rails :-

Draft analysis is carried out to obtain visual feedback on back draft condition of the part and an optimum die press direction is defined that maximises back draft. This analysis helps us to avoid die-lock situations, make decisions on an appropriate parting line, obtain suggestions on cutter sizing for machining and other purposes.

Draft Analysis of Roof:

Draft Analysis of Front Roof Rail:

Draft Analysis of Bow Roof Rail-1:

Draft Analysis of Center Roof Rail:

Draft Analysis of Center Bow Roof Rail:

Draft Analysis of Rear Roof Rail:

Analysis-

1. Curvature Study

It is done to justify the positioning of the Bow Roofs. There are mainly three criterion to fulfill

1. Roof Crush test

2. Heat Distortion Criteria

3. Snow Load Criteria

Roof Crush Testing is done only after the part or prototype is manufactured in physical conditions.

Heat Distortion Criteria:

Heat distortion test is a type of static load test which plays an important role in maintaining roof strength. Heat distortion temperature is the temperature at which metal softens and deforms under a specified load. From Heat distortion criteria, roof strength can be enhanced by appropriate positioning of bow roof rails.

Bow – roof prediction Formula :

W = (1.73 x 10^-3 x L) + (1.85x10^-8 x (R^2)/t + 1.10 x 10^-3 x l) - 2.68

where,

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

R = Roof curvature = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Judgement Condition : 2.7<W<3.5

From above results it can be observed that the designed roof passes Heat Distortion Criteria.

Snowed Criteria:

The snow load test is a type of static load test used to verify that roof assembly exhibits no permanent deflection due to a distributed load representing snow load: the distributed pressure applied is in the range 10-15 N/dmᶺ2.

Snow Load Prediction Formula

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)

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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

From the above results it can be seen that the designed roof passes Snowed Criteria.

CONCLUSION :-

The roof and ditch area is successfully designed, along with the heat distortion test and snow load test. All the requirements are calculated and gets passed through them, so it could be sent to further steps of production.