Roof challenge
ROOF DESIGN
AIM :
- The main aim is to determine the heat distortion criteria, Snow load criteria and Section Modulus of the roof and the reinforcements and to check if they are qualifying the given criteria.
THEORY
Roof:
- An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants form sun, wind, rain and other external elements.
- Safety point of view vehicle roof plays a very crucial role especially in rollover kind of accidents, vehicle rollover crashes are the cause for several head,neck and spine injuries for this reason stiffness and strength of any vehicle is crucial, in order to achieve this embosses with reinforcements are provided on the surface of the roof.
Heat Distortion Criteria:
- During summers in the middle East, the heat radiating from the sun causes the roof of the car to change in the metallic regions.Heat distortion criteria indicate what temperature materials start to "soften" when exposed to a fixed load at elevated temperatures.
w=[1.73*10^(-3)*L]+[1.85*10^(-8)*(R^2)/t]+[1.10*10^(-3)*l]-2.68
Where,
L =Roof Length in X-direction (mm)[Roof Dimension with OY]
R =Roof Curvature = 2(Rx +Ry)/(RX+Ry)
t =Roof plate thickness (mm)
l =Bow roof thickness (mm)
Snow Load Criteria :
- During winter season the snow gets clogged over the Top areas of the car and the snow has some mass on it. The roof can fail if the snow load exceeds the capacity of the roof was designed to sustain.
Qr =[Iy*t^2] / [g*s*[(Rx+Ry)/2]^2*10^(-8)]
Where, My =y[ly-Y] & g= My*Lx^(2)*10^(-12)
t= Roof Plate Thickness (mm)
Lx = Distance between the left and right end of the roof on the roof bow(mm).
Ly = Distance between the front and rear roof rails on the vehicle along with OY(mm).
Y = Distance from front roof rails to Roof Bow (mm)
S = Distance for which roof bow bears divided load = (L1\2)+(L2/2)
Iy = Geometric moment of inertia of roof bow (mm)
Rx= Laternal direction curvature radius of the roof panel Y cross-section on roof bow (mm)
Ry= Longitutional direction curvature radius of the roof panel X cross-section on roof bow (mm)
Section Modulus:
- Section modulus is a geometric property for a given cross-section used in the design of beams.Other geometric properties used in design include area of tension, radius of gyration for compression and moment of inertia for stiffness.
- The section modulus is defined as S= I/Y.
where, S= Section Modulus , I= Moment of inertia
Procedure:
Checking the class-A surface :
- After the file is opened, the roof part is not joined together but as the individual surfaces.It is also seen as the surface is perfectly symmetrical in shape which helps us to work on the half of the portion and reflect it which creates the whole roof.
COMPONENTS OF ROOF :
FRONT ROOF RAIL:
- The front roof rail is located on the front of the roof where the wind shield joins with the roof. The visibility criteria and the head room clearance of the passenger are taken into considerations for the bottom flanges. One end of the rail is joined using spot welding while the other end is joined with the roof using mastic sealants for increased strength.
Bow Roof :
- Bow roof is an additional siffening member for the roof to increase the strength and so that it can satisfy the norms. Both the ends are joined together using mastic sealants to increase strength and also since this would not affect the styling of the roof.
CENTER ROOF RAIL:
- Center roof rail or the reinforcement centre is located at the center of the roof.The cross section of this is more since it needsto provide more strength to the roof.Here both are ends are joined together using mastic sealants.
BOW ROOF 2:
- This bow roof also have the same purpose as that of the first one.This one is located between the center bow roof and rear roof rail.
Rear Roof Rail :
- This roof rail joins with the back door and the roof. Inputs from the backdoor are taken into consideration for designing this roof rail. Here one end is joined using spot welding while the other end is joined using mastic sealants.
- We then also make the ditch area around the roof. Ditch area the extended portion of the main roof concealed by other BIW panels.
CALCULATIONS:
BOW - ROOF PREDICTION
| BOW POSITION | RX | RY | R | l | L | T | (R^2)/2 | W | OK/ NG |
| FR and 1st | 5711.602 | 2745.7198 | 3708.60 | 395.73 | 2025.39 | 0.75 | 6876856.98 | 1.6 | OK |
| 1st and CTR | 4856.77 | 6634.2726 | 5608.04 | 429.57 | 2025.39 | 0.75 | 15725056.32 | 2.06 | OK |
| CTR and 2nd | 3896.69 | 11029.52 | 5758.81 | 414.90 | 2025.39 | 0.75 | 16581082.48 | 2.09 | OK |
| 2nd and RR | 3216.504 | 14820.18 | 5285.79 | 331.44 | 2025.39 | 0.75 | 13969787.96 | 1.70 | OK |
SNOW LOAD PREDICTION
| BOW POSITION | RX | RY | LY | LX | T | Y | S | IY | QR | OK/NG |
| FR and 1st | 3878.75 | 10265.50 | 2023.22 | 1131.84 | 0.75 | 524.42 | 253.92 | 1.58187*10^3 | 6.95 | OK |
| 1st and CTR | 4308.29 | 8013.39 | 2023.22 | 1080.29 | 0.75 | 507.85 | 245.58 | 1.39297*10^3 | 16.641 | OK |
| CTR and 2nd | 3793.10 | 6588.32 | 2023.22 | 1053.29 | 0.75 | 491.165 | 253.27 | 5.5145*10^3 | 54.50 | OK |
DRAFT ANALYSIS:
- Before we are give the part for manufacturing we have to some analysis ourself to check all constraints and to check if we have any negative draft on part. We have a tool called draft analysis in NX cad to check the following
BOW ROOF 1
CENTER BOW ROOF
BOW ROOF-2
FRONT ROOF
REAR ROOF
CONCLUSION:
- We were successfully able to design the roof with all the roof rails for better support and strength. We designed it so as to satisfy the norms and regulations by calculating the heat distortion criteria and snow load criteria.
- While doing this we also understood about the methodology behind designing the roof of the car and what all we take into consideration while designing so.