Roof Design

ROOF DESIGN

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 the front roof rail, bow roofs, central roof rail, and rear roof rails.
• Perform the draft analysis for each reinforcement.

CAD Software used :

1. SIEMENS NX CAD 12

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 a 3D model using 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 where the design for manufacturing and assembly design takes a role while developing any product. The design development process 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 with reinforcements to strengthen 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 :

1. Front Roof Rail
2. Bow Roofs
3. Central roof rail
4. 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.

The Center 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 centre 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

The Bow Roof Rail :

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 panels.

Design Considerations for a roof :

1. Visibility criteria
2. Head Clearance
3. Curvature study
4. Heat distortion and snow load criteria
5. Draft Analysis

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 distortion 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)

Calculation:

• 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 show that the roof behaves when there is snow over it. 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.

Formula

• 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)
• Judgement 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 the Roof panel with the centre point between the 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] 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]

• Here we have to find out the RX and Ry values. To find that

  • Create a sketch on the top roof and draw a horizontal line at the centre.
  • Offset this line to 100mm on both sides. Convert all these lines into reference lines
  • Draw the vertical lines at the middle of each reinforcement and offset it to 100 mm on both sides.
  • Intersection point is created where the lines are met.
  • Project these points to the outer roof panel.
  • Now draw the arc by connecting three points and measure the min radius value
  • the result is mentioned below:

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 :

Front roof rail :

Rear Roof Rail:

Bow Roof Rail 1 :

Bow Roof Rail 2 :

Center Bow Roof Rail :

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 colour ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Viwe:

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.