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 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 -CAD

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 has are :

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

FRONT ROOF RAIL :

Front  roof rail is the one which joins the wind shield glass , body side outer and the inner panel  also. Here the front roof rail is designed based up on the master saection given 

REAR ROOF RAIL :

Rear roof rail is the one which joins the back door and the body side outer

 

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. The thickness of the central roof rail is 1.25 mm .

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 CONSIDERATIONS FOR A ROOF :

• Visibility criteria
• Head clearance
• Curvature study
• 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.

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 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

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 and 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)/2] + [1.10 x 10^(-3) x I]-2.68 

Apply the all the values,   

Roof Rails	RX [mm]	RY[mm]	RX*RY	RX+RY	R[mm]	L	t[mm]	I[mm]	W	Result
Front Roof to Bow Front	5740.99	2462.41	`14136671.18	8203.4	3446.53	2035.17	0.75	383.5677	1.372645334	OK
Bow Front to Center roof	2462.41	6161.75	15172754.81	8624.16	3518.66	2035.17	0.75	431.1567	1.429640426	OK
Center roof to Bow Rear	3976.82	10684.46	42490174.21	14661.28	5796.24	2035.17	0.75	431.3996	1.626150343	OK
Bow Rear to Rear Roof	3293.52	14500.47	47757587.95	17793.99	5367.83	2035.17	0.75	331.0644	1.47154073	OK

 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 and R_Y Value:

L_Y Value:

L_X1Value:

L_X2Value:

L_X3Value:

MOMENT OF INERTIA:

BOW ROOF FRONT:

          Therefore, we take the Minimum moment of inertia value is I_Y1 = 4155.14mm⁴.

BOW ROOF REAR:

   We take the Minimum moment of inertia value is I_Y2 = 4432.73mm⁴.

CENTER ROOF RAIL:

Minimum moment of inertia for center reinforcement rail is I_Y3 = 5366.05mm⁴.

FINAL RESULTS OF SNOW LOAD CRITERIA:

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

Roof Rails	IY[mm^4]	LX[mm]	LY[mm]	L1[mm]	L2[mm]	RX[mm]	RY[mm]	t[mm]	y[mm]	S[mm]	My[mm]	((RX+RY)/2)^2	α	Qr	Result
Front Roof to Bow Front	4155.14	1143.05	2035.17	470.13	509.99	4583.1327	2526.9808	0.75	470.13	490.06	735772.25	12638428.5	0.9613330208	39.25	OK
Bow Front to Center Roof	4432.73	1090.65	2035.17	509.99	510.29	3763.0076	6069.3413	0.75	980.13	510.14	1034076.35	24168771.22	1.230051835	16.44	OK
Center Roof to Bow Rear	5366.05	1061.65	2035.17	510.29	458.75	4276.0079	11086.5511	0.75	1490.42	484.52	811906.295	59002054.76	0.9151001717	11.53	OK

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 analysis:

                     Section moment of inertia is used to predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.

BOW ROOF

Create an intersection curve with the help of the plane and select the bow roof 1

FRONT BOW ROOF:

REAR BOW ROOF:

CENTRAL ROOF RAIL:

FRONT ROOF RAIL:

REAR  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 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 of 7⁰ is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 7⁰ and passed in analysis

DRAFT ANALYSIS FOR FRONT ROOF RAIL :

DRAFT ANALYSIS FOR FRONT BOW ROOF RAIL:

DRAFT ANALYSIS FOR RARE BOW ROOF RAIL:

DRAFT ANALYSIS FOR CENTRAL ROOF RAIL :

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 rail and the rear roof rail is designed by following the master sections and the moment of inertia and draft analysis are carried out to check feasibility manufacturing.