Roof Design
Roof Design
OBJECTIVE-
- The objective of this project is to design a 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 a 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 reinforcement.
SOFTWARE USED-
NX CAD
INTRODUCTION-
The design of any product starts from a requirement. The design engineer sketches the product based on the requirement. Then the 2D sketch with the proper dimensions is converted into a 3D model using advanced CAD software. Another important thing in the design is that every product that is designed should be able to manufacture. so the role of the design engineer is to design a 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 the design carries on until the design satisfies the customer's 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. The roof is the structure above the windshield glass having the reinforcements to provide strength to the overall structure and to withstand the overhead forces like snow.
WHY REINFORCEMENT:
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.
TYPES OF REINFORCEMENT:
- Front roof rail
- Bow roofs
- Central roof rail and
- Rear roof rail
FRONT ROOF RAIL:
The front roof rail is the one that joins the windshield glass, the body side outer, and the inner panel also. Here the front roof rail is designed based upon the master section given
REAR ROOF RAIL:
The rear roof rail is the one that joins the back door and the body side outer
CENTER ROOF RAIL :
central roof rail helps in providing effective support to the flat area of the roof as it is more susceptible to failure under the action of load. generally, the 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 strength to the roof during the roll- over the test. The thickness of the central roof rail is 1.25 mm.
BOW ROOFS :
The bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roof present 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's outer panel and the reinforcements for joining. where ever the mastic is introduced there is a remarkable improvement in the strength
Spot welding is also introduced to join the reinforcements and the outer panel.
DESIGN CONSIDERATION
- Visibility criteria
- Head clearance
- Curvature study
- Heat distortion and snow load criteria
- Draft analysis
ROOF CRUSH TEST;
The 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 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 rollover and
- How deep is the protrusion after the rollover.
THEME OF ROOF TEST
To minimize the protrusion into the occupant compartment, the seat belts and airbags can do their job in protecting the occupant.
CONDITIONS TO FOLLOW FOR THE 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 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 crushing speed at which the load is 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 rollover crashes.
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 the 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.
REAR ROOF RAIL
Create an intersection curve with the help of the plane and select the bow roof 1
Moment of inertia maximum,( MAX) I max =9.12578*10^3( mm4)
Moment of inertia maximum ,( MIN) I min =6.7792*10^3 ( mm4)
BOW ROOF 1
Moment of inertia maximum,( MAX) I max =2.7604*10^3( mm4)
Moment of inertia minimum,( MIN) I min =95.69757 ( mm4)
CENTER ROOF RAIL
Moment of inertia maximum,( MAX) I max =4.596*10^3( mm4)
Moment of inertia minimum ,( MIN) I min =114.4025( mm4)
BOW ROOF 2
Moment of inertia maximum,( MAX) I max =2.826*10^3( mm4)
Moment of inertia minimum,( MIN) I min =99.46( mm4).
FRONT ROOF RAIL :
Moment of inertia maximum,( MAX) I max =1.76712*10^3( mm4)
Moment of inertia minimum ,( MIN) I min =1.09862*10^3( 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.
A minimum draft angle of 70 is considered for analysis. green color infers on the parts that all face along the tooling direction has a positive draft angle greater than 70 and passed in analysis.
DRAFT ANALYSIS FOR FRONT ROOF RAIL :
DRAFT ANALYSIS FOR BOW ROOF RAIL - 1
DRAFT ANALYSIS FOR CENTRAL ROOF RAIL
DRAFT ANALYSIS FOR BOW ROOF RAIL - 2
DRAFT ANALYSIS FOR FRONT REAR ROOF RAIL
ROOF IMAGES
FRONT VIEW :
TOP VIEW :
ISOMETRIC VIEW :
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 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 an elevated temperature. In order to avoid bending or damage 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
In case -1 (from front roof rail to roof rail 1)
i)Ry = 5735.79mm
ii)Rx = 2275.319mm
iii)L = 2033.70MM
iv)I = 310.1773mm
v)R = 3258.1589mm
by substituting all the values in the W we get,
W = 1.45 PASSED
In the same way, do it for all four cases,
Case 2 ( from bow roof rail 1 to center roof rail )
W = 2.6035 PASSED
CASE 3 ( From center roof rail to bow roof rail 2 )
W = 2.13254 PASSED
CASE 4 ( From bow roof rail 2 to rear roof rail)
W = 1.2655 PASSED
From the above table, it can be concluded that all values of w<2.7 so thus infer that the current positioning of the bow row is good in the state as per the design and found ok.
SNOW LOAD CRITERIA :
This test is done to know how is the roof behaving when there is snow over it. 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 removed the roof should go it its original position. These are 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 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]
APPLY THE VALUES IN THE FORMULA :
In order to find the snow load criteria apply all the necessary values from the roof to the given formula,
IN CASE 1 ( From front roof rail to bow roof rail 1 )
Qr = 32.478 PASSED
IN CASE 2 (From bow roof rail 1 to center roof rail )
Qr = 21.56985 PASSSED
IN CASE 3 (From center roof rail to bow roof rail 2)
Qr = 18.264 PASSED.