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 considered 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 reinforcement. Perform the draft analysis for each reinforcement to assess 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 centre roof rails are fixed to roof using mastic sealants. Gap between roof rails and bow roof should be less for Better NVH. Following criteria are considered for roof rail design.
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 analysing 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 reform 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 their 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 go 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.
Master sections:
These are the cross-sectional 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.
FRONT ROOF RAIL:
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 claustrophobic effect to the person.
CENTER ROOF RAIL:
Centre roof rail imparts strength for the centre portion of outer roof panel which is supported by B pillar at both ends.
BOW ROOF RAILS:
These are used between centre roof rail and front roof rail or centre roof rail & rear roof rail to impart additional support. The number of bow roof rail depends on the length of roof panel.
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:
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 analysed 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 infers on the parts that all face along the tooling direction has positive draft angle greater than 70 and passed in analysis
DRAFT ANALYSIS OF FRONT ROOF RAIL
DRAFT ANALYSIS OF CENTER ROOF RAIL
DRAFT ANALYSIS OF BOW ROOF RAILS
DRAFT ANALYSIS OF REAR ROOF RAIL
FINAL ASSEMBLY
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<2.7
APPLY THE VALUES IN THE FORMULA:
In order to find the heat distortion, apply all the necessary values from the roof to the give formula
CALCULATIONS:
From the above table it can be concluded that all values of w<2.7 and also not w>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok.
SNOW LOAD CRITERIA:
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 it its original position. This is the basic requirements for snow load criteria.
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 centre 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 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]
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 1:
Bow Roof 2:
Centre Roof Rail:
ROOF IMAGES:
ISOMETRIC VIEW
FRONT VIEW
SIDE VIEW
TOP 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.