ROOF DESIGN

l Objective -

• This project aims to design a car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, central roof rail, and rear roof rails for 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 a moment of inertia and section modulus for front roof rail, bow roofs, central roof rail, and rear roof rails.
• Perform the draft analysis for each reinforcement.

CAD Software used :

SIEMENS NX CAD 12.0.7

Introduction :

The design of any product starts from a requirement. The design engineer sketches the product based on the requirement. Then the 2D sketch with the proper dimensions is converted into the 3D model using the CAD software. Another important thing in the design is that every product that is designed should be able to manufacture, so the role of the design engineer is to create a product that can be easily manufactured and cost-effective. This is the place where the design for manufacturing and design of assembly takes a role while developing any product. The process of development of design carries on until the design satisfies the customer's needs.

Car Roof :

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. The roof is the structure above the windshield glass having the reinforcements to provide strength to the overall structure and to withstand overhead forces like snow.

Reinforcement :

If the roof is flat, it is very weak and is not able to withstand the forces like snow, so to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail, and bow roof rail are introduced and joined to the flat surface of the roof with the different joining process.

Types of Reinforcement :

• Front Roof Rail
• Bow Roofs
• Central roof rail
• Rear Roof Rail

Front Roof Rail :

The front roof rail is the one that joins the windshield glass, body side outer, and inner panel. Here the front roof rails are designed based on the master section given.

Rear Roof Rail :

The rear Roof Rail is the one that joins the back door and the body side outer.

Central Roof Rail :

The central roof rail helps in providing effective support to the flat area of the roof as it is more susceptible to failure under the action of load, generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding strength to the roof during the roll-over test. The thickness of the central roof rail is 0.75mm.

Central Roof Rail Reinforcement

Bow Roofs :

The Bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roofs presents depends on the overall size of the roof. Presently in this project, two bow roofs are provided.

Joining Process :

Mastic is introduced between the roof outer panel and the reinforcement for joining. Where ever the mastic is introduced there is a remarkable strength increase.

Spot welding is also introduced to join the reinforcements and outer panel.

 Design Considerations for a roof :

• Visibility criteria
• Head Clearance
• Curvature study
• Heat distortion and snow load criteria
• Draft Analysis

Roof Crush Test :

The roof crush test is the design given by the IIHS (Insurance Institute of Highway Safety). After analyzing many accidents IIHS has then fixed some regulations in performing crash tests.

IIHS gives the rating based on this test. For this, they consider two points while giving the rating.

• How fast the car can reform to its original shape after the rollover and
• How deep is the protrusion after the rollover?

 The goal of the Roof Crush Test :

To minimize the protrusion into the occupant compartment, so that seat belts and airbags can do their job protecting the occupant

Conditions to follow for Roof Crush Test :

Two conditions 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 5 degrees is tilted from the front end to the rear end and the load will be 1800 mm in length and 750 mm in width. The load is applied at the position of 25 degrees from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crushing speed at which the load is applied is 13 mm/sec or less than that.

Results :

If the displacement goes above the required level then we have to increase the bow roof thickness or add more mastic points to increase the strength. The stronger the roof the better it can protect the occupants in roll-over crashes.

Heat Distortion study on the Roof :

The heat distortion study plays a major role in sheet metal urges. Heat distortion temperature is a temperature limit above which the material cannot be used for structural applications. This study is used to predict the heat distortions temperature at where the material starts to soften when exposed to a fixed load at an elevated temperature. To avoid bending or damage to 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 ] + [ 1.10 X 10^(-3) X1] -2.68

Where

L = Roof length in X -Direction (mm) (Roof dimension in 0-Y)

R = Roof Curvature

Where R = 2 ( Rx*Ry)/ (Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = roof plate thickness (mm)

I = Bow roof span (mm)

Apply the values in the formula :

To find the heat distortion apply all the necessary values from the roof to the given formula.

Calculations

From the above table, it can be concluded that all values of w<2.7 and also not w>3.1 so thus infer that the current positioning of the bow row is good in the state as per the design and found ok.

Snow Load Criteria:

This test is done to know-how is the roof is behaving when there is 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 removed the roof should go to its original position. These are basic requirements for the snow load criteria.

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

 

 

Where

α= My x L x 2 x 10-12,

My = Y (Ly - Y )

Judgement Condition = Qr ≥ 3.1

250 ≤ s ≤ 380

t = roof thickness [mm]

Ly = Distance between the front and rear roof rails on the vehicle along the OY [mm]

Length of Roof panel with the center point between roof rails 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]

 The width of the roof panel is exposed on the surface.

Y = Distance of 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]

Apply the values in the formula :

To find the snow load criteria apply all the necessary  values from the roof to the given formula

Calculations :

Section Modulus :

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in the design include an area for tension and shear, the radius of gyration for compression, and the moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below.

The elastic section modulus is defined as S = I/y

Where,

S = Section modulus

I = Moment of inertia

y = distance between the neutral axis and the end of the object

It is also often used to determine the yield moment (M y) such that

M y = S X σ _y

Where

σ _y = yield strength of the material

M _y = yield moment

Ʃ _y for stainless steel is 520MPA

Calculation of the section modulus :

Bow Roof 1

Moment of inertia minimum ( I _min ) = 2.4856X10³ mm⁴

Bow Roof 2

Moment of inertia minimum ( I _min ) = 2.74108x10³ mm⁴

Central Roof Rail :

Moment of inertia minimum ( I _min ) = 8.367x10³ mm⁴

Front Roof Rail

 Moment of inertia maximum (I _max  ) = 8.5303x10³ mm⁴

Rear Roof Rail :

Moment of inertia minimum ( I _min ) = 5.515x10³ mm⁴

Draft Analysis :

The Draft analysis command enables you to detect if the part you drafted will be easily removed. 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.

Draft Analysis for Front Roof Rail :

Draft Analysis for Bow Roof Rail 1

Draft Analysis for Central Roof Rail

Draft Analysis for Bow Roof Rail 2

Draft Analysis for Rear Roof Rail

Roof Images :

Top View :

Side View :

Isometric View :

 Conclusion :

Hence the roof, front roof rail, bow roof rails, central roof rails, and rear roof rails are designed by following the master sections, and the moment of inertia, section modulus, and draft analysis is carried out to check manufacturability.