Roof Design
ROOF DESIGN
INTRODUCTION : Automobile roof is the upper portion of an automobile which protects the passengers from sun, wind, rain. Various environmental conditions are taken into account for designing of the same. Even though it serves as styling surface roof should be provided with proper reinforcements for safeguarding passengers. Vehicle roof are vulnerable to high heat conditions & moderate load during snowy conditions.Heat load calculation & snow load calculation are done to get a proper design of roof rails. This ensures the safety of the passengers in the vehicle.
OBJECTIVE
The main objective of the project is to design a roof and associated reinforcements from the existing styling surface. heat load and snow load calculations will be done on the section of the roof for validating effectiveness of roof reinorcement.Perform the draft analysis for each reinforcements to asses the manufacturability.
DESIGN PARAMETERS
For designing roof we have to create no of roof 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. And front, rear bow roof and center roof rails are fixed to roof using mastic sealants. Gap between roof rails and bow roof should be less for Better NVH. Following criterias are taken into account for roof rail design.
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Visibility criteria
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Head clearance
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Curvature study
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Hear distortion and snow load criteria
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Draft analysis
Master sections :These are the crosssectional data for 3D model. These master sections are widely used in OEMs and in automotive industries to convey the functional requirements of product to the design engineer. It gives the parameters like length, width, height, radius and clearance should be considered while designing the particular part or component.
WHAT IS ROOF IN AUTOMOBILES?
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 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
RESULT :--
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.
Now lets see what are all the parts are there in roof:--
- OUTER ROOF
- FRONT ROOF RAIL
- BOW ROOF 1
- CENTER ROOF
- BOW ROOF 2
- REAR ROOF RAIL
- OUTER 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.
- 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 section given
Front Roof Rail Master section
The front roof rail is used as the reinforcement to the roof surface to increase the strength and also used to mount the brackets for some features such as room lamp and rear view mirror. The width of the front roof rail is decided based on the visibility criteria. The depth of the front roof rail is decided in such a way that there should be enough headroom clearance between the head of the driver and the end surface of the roof rail to avoid clostrophobic efffect to the person.
front roof design as per master section
CENTER ROOF RAIL
Center roof rail imparts strength for the center portion of outter roof pannel which is supported by B pillar at both ends.
Center roof rail as per master section
BOW ROOF RAILS
These are used between center roof rail and front roof rail or center roof rail & rear roof rail to impart additional support. The number of bow roof rail depends on the length of roof pannel.
REAR ROOF RAIL
It is the reinforcement provided at rear side of the roof panel. Backdoor and bodyside panels are attached with this part.Provision for back door opening is also provided on rear roof rail.
DRAFT ANALYSIS
- DRAFT ANALYSIS OF FRONT ROOF RAIL
- DRAFT ANALYSIS OF CENTER ROOF RAIL
- DRAFT ANALYSIS OF BOW ROOF RAILS
4.DRAFT ANALYSIS OF REAR ROOF RAIL
5.DRAFT ANALYSIS OF ALL ROOF RAIL
6.DRAFT ANALYSIS OF OUTER ROOF
FINAL ASSEMBLY
Front View:
Top View:
Side View:
HEAT LOAD CALCULATION
The heat distortion study 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 where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending, damage of the roof based on the heat distortion temperature.This study will help to locate the position of the bow roof rails to give enough strength.
Bow – roof prediction Formula
W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68
Where
L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)
R = Roof curvature
R = 2(Rx*Ry)/(Rx+Ry)
Rx = X curvature
Ry = Y curvature
t = Roof plate thickness [mm]
l = Bow Roof Span [mm]
Judgement Condition : W<2>
CALCULATIONS
From the above table it can be concluded that all values of 'W' is less than 2.7,which implies the position of roof rails are as per standard.
SNOW LOAD CALCULATION
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.
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 Roof panel with the center point between Roof rail 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]
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]
CALCULATIONS
From above calculations the value of 'Q' obtained is always greater than 3.1
.
CONCLUSION
Design of roof was succsessfully completed using SIEMENS NX as per the given master sections.The position of bow roof rails are as per the given standards.Draft analysis & section analysis ensure the manufacturability of every components.