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Aim : For the Given Roof styling, Develop the essential flanges and reinforcements, Provide appropriate dimensions and check for Draft analysis, and submit your Section Inertia report on the 0-Y section. Start with the creation of a roof ditch area the tool opening angle is only 3 degrees. Do a curvature study on the roof…
abhijeet dhillon
updated on 18 Nov 2021
Aim :
Solution:
First we will discuss some rules regarding automotive safety :
There are two types of safety systems which are used :
1.Active Safety Systems : These systems are used to prevent any hazards from happening via the help of electronics etc .It consits of the following :
a.Pretensioner Restraint System : It is used to restrain the body with help of a seatbelt which controlled with help of a pretensioner at the backside of the seat.
b.Head Restraint System : It is used to restrain the movement of the head during a front collision by moving the head restraint by 25 or 30 mm so that fatal neck injuries are avoided .
c.Active Rollover Protection System : This system stops a rollover of the vehicle by momentarily braking the system with the help of sensors and shift the inertial force to the front system so that the roll is transmitted equally.
d.Pre Safe Breaking :In this system braking is done in such a manner with or without the intervention of the driver the brakes will be applied via sensors so that accidents can be avoided.
2.Passive Safety Systems : These systems are used to prevents any damages via designing structural components etc.
Now we will discuss the terms related to automotive roof design :
The roof has reinforcements attached to it which are discussed below :
1. Front Roof Rail : It is used for supporting the front lamps as they are mounted on the front rail.The front rail is kept open for 3 degrees so that the manufacturing method of deep drawing can be done easily.
2.Centre Roof Rail
3.Bow Roof 1
4.Bow Roof 2
5.Rear Roof Rail
According to the size of the vehicle , the roof is divided into number of reinforcement that varies according to the vehicle. The bow roofs are positioned on the most flat areas of the roofs so that the attachment can be made easier. The bow roofs are introduced on the flat reasons to strengthen the flat areas from failure.
The rear roof rail attaches to the backdoor and outer body whereas the front roof rail joins with the windshield glass .
The radius should be minimum of 3 mm and for thickness downward it should be added with the thickness value while for downward direction it should be reduced.The head room should be minimum of 85 mm from the top of the roof so that proper space is there. Whenever there is a reinforcements applied on the roof a mastic is applied between the outer panel and it so that the rigidity improves .
Now we will talk about the position of the bow roofs
The roof can either consists of 4 bow roof or 3 bow roofs , if it it has 4 bow roofs than the centre bow roof is not attached to the roof while in the other case if it has 3 than the centre inforcements has been attached to the roof and has mastic points on it .
The position of the bow roof can also depend on the curvature of the roof , if the bow roof has higher curvature in the front and flatness at the rear , than the centre reinforcement is done inb the front while the bow roofs are placed in the rear .
The heat distortion in the roof is given by the formula :
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(Rx*Ry)/(Rx+Ry)
Rx = X curvature
Ry = Y curvature
t = Roof plate thickness [mm]
l = Bow Roof Span [mm]
Snow load is the load, in pounds per square foot, placed on the exterior of a structure by snow accumulation. The snow load capacity for each structure must be determined on a case-by-case basis and is determined by the building materials used and the style of construction, as well as roof surface area.
The snow load formula for the roof is given by :
Snow load Prediction formula
Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 10-8]
Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.
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 front and rear reference point.
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]
Roof panel X curvature radius of length s in Side view
These formulas are explained by the following diagrams as shown below :
If these condition are not good than the styling team changes the radius of curvature otherwise embossments are created so that the strength increases in these parts .
Now we will design a roof using a class A surface
We have extracted the upper surface of the roof as shown below :
We have untrimmed the surface and removed the holes as shown below :
We will take reference from the master section and create it :
\
We have created an offset surface at 32 mm as shown below and extended the sheet in order to create the flange for the windshield :
Now we will create the flange as shown below :
Now we will create flanges using law extend and offset command as shown below :
We will create a flange on the side as shown below :
We will create a plane and using this plane take an offset as shown below :
We have created the offset on the surface as shown below :
We will use the trim sheet option as below :
We have created an offset surface as shown below :
We have trimmed this sheet as well :
Now we have created two flanges using trim sheet as shown below :
We have trimmed the front roof rail as shown below :
We have applied blends on the following :
Now we will be working on the bow roof which has the following master sections as follows :
We have created the plane and taken its intersection curve as shown below :
Now we will offset the curve and the surface as shown below :
We have trimmed the offset surface with the help of the trim sheet and kept one target body as shown below :
Now we will follow the master section and create it by using offset and trim command from the following curves as shown below :
Now we have offsetted two surfaces at 3mm and 5 mm respectively and later trimmed them as shown below :
These are in accordance to the master section as shown in the picture
Now we will apply face blends as shown below :
We have applied the blends as shown above.
Now we will create side supports for the bow roof as shown :
Now we will trim so that surfaces can be created smoothly as shown below :
Now we will create guide curves as shown below and create a lofted surface as shown :
Now we have created the side as shown below :
Now we will be creating the mastics as shown below :
First we will mirorr the following as shown below :
We have used mirror geometry as above.
Now we will work on the rear roof rail based on the master section shown below :
We have created the first surface as shown below and created the it according to the master section as shown below :
We have created the second step at depth of 5 mm as shown below according to the master section :
We have offsetted the following surface at a depth of 70 mm as shown below and extended
After trimming we get the following :
Now we have compeleted the rear rail roof as shown below :
Now we will design the reinforcement center according to the following master section :
We have offsetted the surface as shown here :
Through studio curve we have connected them .
Now we have connected the following as shown below :
Therefore we have created all sections as shown below :
We have thickened it and now we will do the section analysis as shown below :
Section Modulus :
Section modulus is the direct measure of the strength of the steel. Bending a member that has a larger section modulus than another will be stronger and harder to bend. Section modulus is a geometric property for a given cross-section used in the design of flexural members.
There are two types of section moduli, the elastic section modulus (S) and the plastic section modulus (Z). In the case for bending a steel section it is important to calculate ‘S’ by taking the moment of inertia of the area of the cross section of a structural member – divided by the distance from the neutral axis to the furthest point of the steel section. This is where the steel will bend first. The bending moment that it takes to yield that section equals the section modulus times the yield strength.
In simple terms, the section modulus is the ratio of bending moment to bending stress for steel. If your steel has a high section modulus it will be harder to bend and can withstand a high moment without having high bending stress. You need to divide the maximum bending moment by the section modulus to get the bending stress and then compare the bending stress to the allowable tensile stress to see if the steel can take that much moment.
Now the section for the following roof is as shown :
For this section we get :
Now we will get the section of the following as shown below :
From the section we get the following as shown below :
Simiarly from the sections from the front roof rail we get the following :
We get the following section as shown below :
Now we will calculate the snow load and roof load according to the following formula :
Bow–roof prediction Formula
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(Rx*Ry)/(Rx+Ry)
Rx = X curvature
Ry = Y curvature
t = Roof plate thickness [mm]
l = Bow Roof Span [mm]
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)
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 front and rear reference point.
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]
Now we will use the method below to calcuate the radius of curvatures as shown below :
After creating the following we can project the points on the surface to calculate the radius as shown below :
We have projected them on the surface as shown below :
Now using the 3 point arc we will connect three points as shown below :
We get the radius as shown above Rx , simiarly for the other section we get :
Now for the radius across the y section we have :
The next section has :
Now we will appy the above formula to calculate the Bow roof load critera :
Bow Position | Rx | Ry | R | I | L | t | R^2/2 | W | OK/NG |
Section 1 | 4060 | 11021 | 5933 | 327 | 2033 | 1.5 | 17600244 | 1.53 | ok |
Section 2 | 3761 | 5309 | 4402 | 450 | 2033 | 1.5 | 46033215 | 1.77 | ok |
Now we will calculate the snow load condition for the roof :
Bow Position | Iy | Alpha | Lx | Ly | Y | S | Rx | Ry | t | Qr | Determination |
Section 1 | 167 | 1.05 | 1119 | 2033 | 569 | 449 | 4105 | 15113 | 1.5 | 0.84 | N.G |
Section 2 | 267 | 1.21 | 1081 | 2033 | 979 | 464 | 3794 | 10534 | 1.5 | 2.09 | N.G |
Hence we have the following :
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