Roof Design
OBJECTIVE
Desidn a Roof, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, Boe Roof 1, Bow Roof 2 with given master section and also check for the heat distortation and snow load condition.
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 .
The Roof consists of the following parts
- Front Roof Rail
- Centre Roof Rail
- Rear Roof Rail
- Bow Roof 1
- Bow Roof 2
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.
1.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.
2. Centre 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.
3. Rear Roof Rail
It is created on the rear end of the roof and while designing we have to consider the tail gate position as well.
4. Bow Roof 1
It is crated in between the front roof rail and centre roof rail
5. Bow Roof 2
It is created in between the centre roof rail and rear roof rail.
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.
HEAT DISTORTION STUDY ON THE ROOF :
The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications . This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature . In order to avoid bending or damages on the roof, based on the heat distortion temperature , this study will predict the bow roof position which helps to strengthen the roof .
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]
Judgment Condition
OK< 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 .
Snow Load Prediction Formula
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)
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 front and rear reference point.
Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]
The width of the roof panel is 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]
Roof panel X curvature radius of length s in Side view
Judgment condition
Qr ≥ 3.1
APPLY THE VALUES IN THE FORMULA
In order to find the snow load criteria apply all the necessary values from the roof to the give formula
CALCULATIONS
It is observed from the values that obtained values of QR > 3.1
SECTION ANALYSIS
Section analysis is a geometric property for a given cross-section used in the design of beams or flexural members. Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question.
Bow roof 1
Moment of inertia maximum,(MAX) I max =2.06 × 104 ( mm4)
Moment of inertia maximum ,(MIN) I min =68.56 ( mm4)
Bow roof 2
Moment of inertia maximum,(MAX) I max =2.06 × 104 ( mm4)
Moment of inertia maximum ,(MIN) I min =67.50 ( mm4)
Central roof rail
Moment of inertia maximum,(MAX) I max =2.5315 × 104 ( mm4)
Moment of inertia maximum ,(MIN) I min =71.92 ( mm4)
Front roof rail
Moment of inertia maximum,(MAX) I max =1.727× 105 ( mm4)
Moment of inertia maximum ,(MIN) I min =893.4889 ( mm4)
Rear roof rail
Moment of inertia maximum,(MAX) I max =1.9424× 106 ( mm4)
Moment of inertia maximum ,(MIN) I min =3.8827 × 104( mm4)
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 REAR ROOF RAIL
DRAFT ANALYSIS FOR BOW ROOF RAIL 1
DRAFT ANALYSIS FOR BOW ROOF RAIL 2
DRAFT ANALYSIS FOR CENTRE BOW ROOF RAIL
DIFFERENT VIEWS FOR ROOF
FRONT VIEW
TOP 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 , section modulus and draft analysis are carried out to check feasibility manufacturing.