Roof Design

OBJECTIVE:

• The objective of this project is to create the Roof design in NX cad, and also designing Front roof rail, rear roof rail, bow roof, center roof rail and Reinforcement are creating to build the strong roof.
• Do curvature study on Roof and performance calculations for Heat distortion and Snow load criteria to determine the position of the Bow roofs.
• Finding the moment of inertia and section modulus for Bow-roofs and center roof rail.
• And last, Analyse the Draft of each part.

INTRODUCTION:

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.

When it is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the design. Because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength.

Front roof and rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don’t want so much gap between roof rails and bow roof area. So, we reducing the NVH issues.

DESIGN CONSIDERATION OF ROOF:

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

DESIGN SPECIFICATION FOR THE AUTOMOTIVE ROOF COMPONENT:

Master section data for front roof rail, rear roof rail, bow roof, center roof rail and the reinforcement.

(Note: Master sections are only use in reference purpose only)

• Given styling surface for Roof:

• Roof Design: Which consist Front roof rail, Rear roof rail, Bow roof and Centre roof reinforcement. It design with the help of master section and given styling surface.

Front Roof Corner (Front Ditch Area):

Front ditch area is creating with help of the dump solid of class A surface developed by considering the parameters to design the front ditch area. Now, we have to create the extension sheet with minimum 15mm (It may vary), and give the positive draft angle 3 degree considering the following drawing operation.

Rear Ditch Area:

It’s same to the front roof ditch area but we have to fix the rear door in this area, when considering the parameters to design the rear roof ditch area.

Front Roof Rail:

Front roof rail is used for supporting the roof for externally applied forces occurring at accidents. When we increasing the front roof rail is good for strength but also it is not for drivers because of it hide visibility of road. When considering the parameters of roof rail, give them 60-80mm length, proper draft angle and also using the embosses for increasing the strength. Front section of front roof rails spot welded to the front ditch area and other end is fixed with help of mastic sealants.

Sheetmetal thickness (t) = 0.75mm

Embosses height = 2 < 3t

Flange length = 13.2 > 3t+R

Radius = (3mm or 5mm) > t

Rear Roof Rail:

Rear roof rail is creating to length 80-100mm with proper draft angle. Give embosses on the sheet to ensure the enough strength for holding the roof structure. When considering the all parameters like length, width, sealants, rubber materials, wind shield to create the design properly.

Head room clearance = 120mm

Sheetmetal thickness = 0.75mm

Embosses height 2 < 3t

Radius = (3mm or 5mm)

Bow Roof 1 (Bow Roof Front):

The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure. Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyze the bow roof position by various condition. We also give the Mastic area for fixing the Bow roof to the roof outer. Each mastic sealants are having 80mm circular effective area of surroundings. Thickness of the Bow roof is 0.75mm.

Centre Roof Reinforcement:

Center roof reinforcement is effective support to the flat area structure roof. Because of center area of roof is more susceptible to failure under action of load. Usually, Reinforcement center roof are fixed at the center of the roof for connecting to the B–Pillar support structure, and it will used for roll-over test. Center bow roof and reinforcements are placed at the center of the roof structure. These two are connecting with the help of spot welding operation. Thickness of the Reinforcement center roof is 0.75mm. And it is wider than Bow Roof.

Bow Roof 2 (Bow roof rear):

It is also same to the front bow roof but the position of the rear bow roof i different. The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure. But, remember we place bow roof in flatter region for enhance strength of the roof.

Mastic Sealant of Roof:

The Mastic sealant or the Mastic point is an elastic compound that behaves like a rubber. The mastic sealants are applied at the many places like roof outer to front roof rail, rear roof rail, bow roof and reinforcement center roofs. They help the roof outer panel to come back to its original shape after it gets deformed. The mastic has an influence area of 80mm. It will give the three methods in one operation, it fills the gap between the two or more substrates, it forms a barrier through the physical properties of the sealant itself, and by adhesion to substrates.

ROOF TESTING:

Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation. The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.

The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.

Thicken assembly of ROOF:

• Outer Roof Panel (Front Roof Corner naming):

• Front Roof Rail:

• Rear Roof Rail:

• Bow Roof front (Bow Roof 1):

• Centre Roof Reinforcement:

• Bow Roof 2 (rear):

• Thicken Assembly

CURVATURE STUDY ON THE ROOF:

• Heat Distortion
• Snow Load

Heat Distortion:

The heat distortion study the plays an important role in Sheetmetal application. Heat distortion is temperature limit above which material can not be used for structural application. This study is useful to fin the where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending, damage to the roof based on the heat distortion temperature. This study will provide the which position of the bow roof give enough strengthen to roof.

Bow Roof Prediction Formula:

Heat Distortion Criteria:

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/2] + [1.10 x 10^(-3) x I]-2.68

Judgement Condition: OK<2.7>3.1

Where,

L= Roof Length in X- direction [mm](Roof Dimension in 0-Y)

R = Roof curvature

R = 2(R_X*R_Y)/(R_X+R_Y)

R_X = X Curvature

R_Y = Y Curvature

t = Roof plate thickness [mm]

I = Bow roof span [mm]

Thickness for the Roof(t):

Roof Outer Panel, t= 0.75mm

Front Roof Rail, t = 0.75mm

Rear Roof Rail, t = 0.75mm

Bow Roof Rail Front, t = 0.75mm

Bow Roof Rail Rear, t = 0.75mm

Reinforcement roof rail, t = 0.75mm

`Rx & Ry` `(X and Y` `Curvature)`:

• From 0-Y axis, at -3Y about distance 300mm is drawn parallel on the perpendicular plane. After that 100mm lines are offset to either side of -3Y line. Now center lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch. Make all geometry lines are construction lines except points.
• Project the intersection points on sketch to the Roof surface.
• Join the intersection points in roof surface using the arc curve an measure the radius using the minimum radius comment.

Sketch for base line

Intersection point with projected curve of roof surface

Middle line for crating intersection point

Intersection Point

Project the point on Roof Surface

Using the arc with three point

`Rx` radius value

`Ry` radius value

Roof Length (L):

Bow Roof Span (I):

Here we want to take the distance from front roof rail to bow roof front for finding the bow roof span(I), we want to measure another bow roof span also on.,

Distance between Front Roof Rail to Bow Roof Front, Bow Roof Front to Centre Reinforcement, Centre Reinforcement to Bow Roof Rear and last Bow roof Rear to Rear Roof Rail.

Final Results of Heat Distortion:

Now we have all the necessary values for finding the heat distortion value(W):

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/2] + [1.10 x 10^(-3) x I]-2.68

Apply the all the values,

From the above table we, conclude that all the values are of W< 2.7 and also W> 3.1. So, that all bow roofs areas are placed at the right place.

Snow Load:

This test is done to know how is the roof behaving when there is a 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 removal, the roof should go its original position. This is the basic requirements for snow load criteria.

Snow Load Criteria:

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

Where,

α = M_y x L_X² x10^-12, M_{y =} Y(L_y–Y)

Judgement condition = Q_r > 3.1

250 < s < 380

t = Roof plate thickness[mm]

L_Y = 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 rails front/ rear as the front and rear reference point.

L_X= Distance between the left and right end of the roof on the roof bow[mm],

With of the roof panel exposed on the surface.

Y = Distance between Front roof rails to roof bow [mm],

s = Distance for which roof bow bears divided load [mm],

s = L₁/2 + L₂/2

I_y = Geometrical moment of inertia of roof bow (Y cross – section) [mm4],

R_X = Lateral direction curvature radius of roof panel Y cross section on roof bow [mm4],

Roof panel curvature radius of the length L_X in front view.

R_Y = Longitudinal direction curvature radius of the roof panel X cross section on roof bow[mm4],

Roof panel X curvature radius of length s in side view.

`Rx & Ry` `(X and Y` `Curvature)`:

• At 0-Y axis on perpendicular plane to roof. Draw 100mm lines offset to either side of the of 0Y axis line. Now center lines are drawn between each adjacent roof rail respective. Intersection points are marked on the sketch. Make all the geometry in construction lines except points.
• Project the intersection points in the sketch are projected to roof surface.
• Join the project points using arc curve and measure the radius value using minimum radius comment.

`Rx` radius value

`Ry` radius value

`Ly`Value

`Lx1`value

`Lx2`value

`Lx3`Value

`Lx4` value

Distance between front roof rail to bow roof rail and so on...

MOMENT OF INERTIA:

Bow Roof Front(1):

Therefore, we take the Minimum moment of inertia value is `Iy1 = 1739.45mm^4`.

Centre Roof Reinforcement:

We are taking the Hollow type for finding moment of inertia of the part. Therefore, reason is other two bow roofs are having solid structure. But center reinforcement roofs having the two solid part. So, that why we are joining to them and check for moment of inertia.

Minimum moment of inertia for center reinforcement rail is `Iy2 = 1972.49 mm^4`.

Bow Roof rear (2):

We take the Minimum moment of inertia value is `Iy3 = 1777.78 mm^4`.

Final Results of Snow Load Criteria:

We have the necessary values for finding the Snow load value(W):

If roof structure pass to the snow load condition. They, must have `Qr` values greater than 3.1.

`Qr`> 3.1

So, we done the curvature study on the roof. Heat distortion and snow load condition my roof is pass the two criteria, so position of the bow roofs and center reinforcement roofs placed at right area.

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.

Minimum draft angle is 7 degree is considered for analysis. Green colour show positive draft angle of the part more than 7 degree.

• Roof Outer Panel:

• Front Roof Rail:

• Bow Roof Front:

• Centre Roof Reinforcement:

• Bow Roof Rear:

• Rear Roof Rail:

ROOF VIEWS:

FRONT VIEW:

BACK VIEW:

ISOMETRIC VIEW:

CONCLUSION:

Here, I did the roof design and check heat distortion and snow load condition for bow roofs and centre roof are placed at right place or not. So, after finishing the design I did the heat distortion and snow load condition.

And last, we check the draft analysis. Because, whether part is manufacturable or not. That, why we doing this analysis. That’s all my design pass the all requirements for placing the roof on the car.