Roof Design

ROOF DESIGN:

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 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.

SOFTWARE USED: NX 12.0

• To design the Sheetmetal ROOF in NX by using some Solid/Surface features.
• NX Software is a flexible and powerful integrated solution that helps you deliver the products faster and efficiently.
• NX Cad software is widely used automotive, aerospace, thermal power station. Because of it is having modelling, drafting and manufacturing platform.

INTRODUCTION :

Design of any product starts from an requirement . The design engineer sketches the product based on the the requirement . Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD software's . The another important thing in the design is that, every product that is designed should be able to manufactured. So the role of the design engineer is to design the product that can be easily manufactured and with less cost effective. This is the place where the Design for manufacturing [DFM] and Design of assembly [DFA] takes a role while developing any product. The process of development of design carries on until the design satisfies the customer needs .Here in this project the product that we are going to develop is a car roof.

CAR ROOF :

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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow

WHY REINFORCEMENTS :

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

DIFFERENT TYPES OF REINFORCEMENTS THAT THE CAR ROOF HAS ARE:

• Front roof rail
• Bow roofs
• Central roof rail and
• Rear roof rail

FRONT ROOF RAIL :

The front roof rail is located on the front of the roof where the windshield joins with the roof. The visibility criteria and the headroom clearance of the passenger are taken into consideration for the bottom flange. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.

REAR ROOF RAIL :

This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail. Here one end is joined using spot welding while the other end is joined using mastic sealants.

CENTRAL ROOF RAIL :

Central roof rail helps in providing effectively 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 the strength to the roof during the rool- over test.

BOW ROOFS :

The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure . The number of bow roof present is depends on the overall size of the roof . Presently in this project two bow roofs are added.

JOINING PROCESS :

• Mastics are introduced between the roof outer panel and the reinforcements for joining. Where ever the mastic are introduced there is remarkable improve of the strength.
• Spot welding is also introduced to join the reinforcements and the outer panel.

DESIGN CONSIDERATION OF ROOF:

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

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.

DESIGN SPECIFICATION FOR THE AUTOMOTIVE ROOF COMPONENT:

Master section data for front roof rail, rear roof rail, bow roof front, bow roof rear, Center roof rail and reinforcement. (*Master sections are only use in reference purpose only).

STYLING SURFACE FOR ROOF:

ROOF:

We can design the Roof, front roof rail, rear roof rail, bow roof, center reinforcement rails are creating with help of styling surface for roof.

MASTER SECTION FOR FRONT ROOF RAIL:

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, We have to create the extension sheet with minimum 15mm, and give the positive draft angle 7 degree considering the following drawing operation.

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

MASTER SECTION FOR REAR ROOF RAIL:

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.

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)

MASTER SECTION FOR BOW ROOF RAIL:

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.

BOW ROOF REAR:

It is also same to the Front bow roof but the positioning bow roof only change. 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.

MASTER SECTION FOR REINFORCEMENT CENTER ROOF RAIL:

REINFORCEMENT CENTER ROOF RAIL:

• Reinforcement Center roof rail 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 rails are fixed at the center of the roof for connecting to the B – Pillar support structure, it will used for roll-over test.
• Center bow roof and reinforcements are placed at the center of the roof structure. They two are connecting to help of spot - welding operation. Thickness of the Reinforcement center roof rail is 0.75mm.

MASTIC SEALANTS 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 60-80mm.

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.

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 cannot be used for structural application. This study is useful to find 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)/t] + [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

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.

R_X & R_Y (X & 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.

R_X Value:

R_Y Value:

FINAL RESULTS OF HEAT DISTORTION:

We have 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)/t] + [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:

Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed.

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.

R_X & R_Y (X & 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.

R_X Value:

R_Y Value:

L_X Values:

L_Y Value:

MOMENT OF INERTIA:

BOW ROOF FRONT:

Therefore, we take the Minimum moment of inertia value is I_Y1 = 1282.94mm⁴.

BOW ROOF REAR:

We take the Minimum moment of inertia value is I_Y2 = 1232.89mm⁴.

CENTER REINFORCEMENT RAIL:

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 I_Y3 = 1672.22mm⁴.

FRONT ROOF RAIL:

We take the Minimum moment of inertia value is I_Y2 = 911.62mm⁴.

REAR ROOF RAIL:

We take the Minimum moment of inertia value is I_Y2 = 6800.32mm⁴.

FINAL RESULTS OF SNOW LOAD CRITERIA:

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

If roof structure pass to the snow load condition. They, must have Q_r values from above 3.1.

Q_r > 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.

SECTION MODULUS:

The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object, which is collectively termed as flexural rigidity (EI). Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.

Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.

There are two types of section modulus we have,

• Elastic section modulus
• Plastic section modulus

Elastic modulus is defined as,

S = I/Y

S – Section modulus,

I – Second moment of area

Y – distance from the neutral axis to outer most fiber of the object.

FRONT ROOF RAIL:

Moment of inertia (Max), Imax = 1.621 x 10⁴ mm⁴

Moment of inertia (Min), Imin = 911.62 mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 78 mm

For maximum MOI,

S = 1.621 x 10⁴ / 78

S_MAX = 207.82 mm³

For minimum MOI,

S = 911.62 / 78

S_MIN = 11.6874 mm³

REAR ROOF RAIL:

Moment of inertia (Max), Imax = 9.705 x 10⁴ mm⁴

Moment of inertia (Min), Imin = 6.800 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 103 mm

For maximum MOI,

S = 9.705 x 10⁴/103

S_MAX = 942.23 mm³

For minimum MOI,

S = 6.800 x 10³/103

S_MIN = 66.019 mm³

REINFORCEMENT CENTER ROOF RAIL:

Moment of inertia (Max), Imax = 8.391 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.672 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 43.5 mm

For maximum MOI,

S = 8.391 x 10³/43.5

S_MAX = 192.896 mm³

For minimum MOI,

S = 1.672 x 10³/43.5

S_MIN = 38.436 mm³

BOW ROOF FRONT:

Moment of inertia (Max), Imax = 4.033 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.2829 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 33 mm

For maximum MOI,

S = 4.033 x 10³/33

S_MAX = 122.212 mm³

For minimum MOI,

S = 1.2829 x 10³/33

S_MIN = 38.8757 mm³

BOW ROOF REAR:

Moment of inertia (Max), Imax = 4.7791 x 10³ mm⁴

Moment of inertia (Min), Imin = 1.2328 x 10³ mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 28.7 mm

For maximum MOI,

S = 4.7791 x 10³/ 28.7

S_MAX = 166.519 mm³

For minimum MOI,

S = 1.2328 x 10³/ 28.7

S_MIN = 42.954 mm³

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 RAIL:

RE - INFORCEMENT CENTER ROOF RAIL:

BOW ROOF REAR:

REAR ROOF RAIL:

ROOF VIEWS:

FRONT VIEW:

TOP VIEW:

RIGHT SIDE VIEW:

BACK VIEW:

ISOMETRIC VIEW:

CONCLUSION:

• I did the My roof design and check heat distortion and snow load condition for bow roofs are placed at right place or not. So, after finishing my design I did the heat distortion and snow load condition.
• After, finishing the curvature study. Then go to check the section modulus. 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.