Roof Design

AUTOMOBILE SHEETMETAL ROOF DESIGN USING NX CAD

INTRODUCTION :

An automobile roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Though it just looks like a styling surface that protects from sun, wind and other elements, various environmental conditions are taken into account for designing of the same. Roof should be provided with proper reinforcements for strengthening. Vehicle roof can be subjected to high heat conditions or load during snowy conditions. To make the roof withstand these conditions, various calculations are done. This ensures the safety of the passengers in the vehicle.

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.

FACTORS INFLUENCING ROOF DESIGN :

• Visibility criteria
• Head room clearance
• Curvature study
• Bow roof position
• Heat distortion and Snow load criteria
• Draft analysis
• Saftey parameters

COMPONENTS TO BE DESIGNED :

• Roof outer panel with necessary flanges and ditch area
• Front roof rail
• Rear roof rail
• Centre roof rail
• Bow roofs

Roof outer panel with necessary flanges and ditch area

An outer panel is the outermost sheet metal of the roof in which there are aerodynamic cut-outs or sunroof cut-outs or etc. And there can be some fixtures or carriages to carry luggage on top of the car.

The thickness of the sheet metal of the outer panel should be around 0.75 mm and the flange area for the ditch should be minimum 15 mm.

Material – SP121AQ (Cold Rolled Mild Steel)

Front Roof Rail -

A front roof rail is a reinforcement placed on the front section of the roof. This reinforcement connects with side sections of the car and it also attaches with the front windshield of the car. With the available master section of the front rail. To increase the strengthens an emboss operation is also performed while modelling.

A model is designed with the following specifications-

Material – SP783AJQ (Terrace Zinc Galvanizing)

Thickness – 0.75 mm

Draft Analysis -

In the real-world application of manufacturing of sheet metals with some operations like hot and cold rolling, punching etc. The angle modelled in the CAD software for this component should be applicable for the real-world manufacturing processes also.

So, in the CAD software, some analysis can be made to precheck the operations that can be performed in Manufacturing industries. Draft analysis is one of the analyses that check the angle between the components which are operated in sheet metals.

Section Inertia Analysis -

Section modulus is a geometric property for a given cross-section used in the design of beams or any component with flexural properties. Other geometric properties used in the design include an area for tension and shear, the radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness.

So, for the section of the front roof rail component a section inertia modulus analysis made to check the elastic properties or the deformation properties on the component.

Bow Roof Rails -

The front area for the bow roof rail is placed between front roof rail and centre roof rail reinforcements. This component main purpose is to strengthen the flattest surface on the roof available. so, wherever there is a flat surface on the roof rail reinforcement can be introduced. This bore roof rail front is fixed with both sides of the car.

Material – SP121BQ (Cold rolled steel coil)

Thickness – 0.75 mm

Mastic sealent area -

Mastic area is the area where the mastic sealant is applied to Unite the outer panel and the ball roof. If we do weld to combine both these components it may cause the overall visual and mechanical properties on the roof.

Draft Analysis -

In the real-world application of manufacturing of sheet metals with some operations like hot and cold rolling, punching etc. The angle modelled in the CAD software for this component should be applicable for the real-world manufacturing processes also.

So, in the CAD software, some analysis can be made to precheck the operations that can be performed in Manufacturing industries. Draft analysis is one of the analyses that check the angle between the components which are operated in sheet metals.

Section Inertia Analysis -

Section modulus is a geometric property for a given cross-section used in the design of beams or any component with flexural properties. Other geometric properties used in the design include an area for tension and shear, the radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness.

So, for the section of the front roof rail component a section inertia modulus analysis made to check the elastic properties or the deformation properties on the component.

Reinforcement Roof centre -

This component is also one of the bow rails that protect the car and as well as also the passengers from the roll-over accident or test. This component has also some of the Properties that can be attached to this reinforcement centre rail Like clamp for the compartment inside or attaching a fan etc.

This particular roof rail is important because a roof’s flattest area can we identify at the centre so for this reinforcement, there should be some importance given to it. so that's why it has two reinforcement attached to the outer panel.

Material – SP153 – 980 PQ (Cold Rolled JIS Steel)

Thickness – 0.75 mm (both)

Spot Welds -

Draft Analysis -

Section Inertia -

Bow Roof Rail -

The back area for the bow roof rail is placed between back roof rail and centre roof rail reinforcements. This component main purpose is to strengthen the flattest surface on the roof available. so, wherever there is a flat surface on the roof rail reinforcement can be introduced. This bore roof rail front is fixed with both sides of the car.

Material – SP121BQ (Cold rolled steel coil)

Thickness – 0.75 mm

Draft Analysis -

Section Inertia Analysis -

Rear Roof Rail -

Back roof rail is placed on the rear side of the roof. This component or reinforcement has a different master section because this reinforcement has some properties like, to fix the back door to the roof a hinge is placed inside the reinforcement and the size of this reinforcement should be considered regarding with the head clearance area from rear seat compartment of the car.

Material – SP783AJQ (Terrace Zinc Galvanizing)

Thickness – 0.75 mm

Draft Analysis -

Section Inertia -

Roof Assembly with Reinforcement -

Calculation of Bow Rood Rail Predictions :

Heat Distortion 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]

the curvature design criteria are OK if ‘W’ is: OK< 2.7> 3.1

Heat distortion Calculation table -

Snow Load Prediction Criteria

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

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 centre point between Roof rail Front /Rear as the reference point of the front and the rear.

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

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

ALL VIEWS -

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 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 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

RESULTS :

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.

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.

CONCLUSION :

• The results obtained after doing a curvature study on the roof was adequate.
• The position of the bow roofs can be arranged better (over the flat areas) which helps in increasing the strength of the roof.
• Embossments are added to improve the strength & dynamics of the roof but can be avoided if not necessary as it increases the cost of manufacturing.