Roof challenge
AIM: Roof design using NX 12 and its optimization by calculating heat distortion criteria,snow load criteria and section inertia analysis.
OBJECTIVE:
- To develop essential flanges and reinforcements.
- Provide appropriate dimensions and check for draft analysis and perform section modulus on the O-Y section.
- To do a curvature study on the roof and perform calculations to determine the position of bow roof.
INPUTS:
Master section of rear roof rail:
Master section of front roof rail:
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. Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.Today, a lot of cars come with a sunroof which allows us to direct breeze into the car from the top. ... These features can also be used to check out the night sky while staying inside the car.
Structural components of the roof:
- Front roof rail reinforcement
- Bow roof rail reinforcement 1
- Center roof rail reinforcement
- Bow roof rail reinforcement 2
- Rear roof rail reinforcement
Front roof rail:
Bow roof 1:
Center roof rail:
Bow roof 2;
Rear roof rail:
Complete roof design:
Draft analysis
Draft analysis is the method of checking the draft angle on various faces of the part with a particular direction.It is performed based on colour ranging identifying zones on the analysed element where the deviation from the draft direction at any point,corresponds to specified values.
Calculation for determing the bow roof position:
Heat distortion criteria:
Heat distortion temperature is the temperature at which the metal softens and deforms under a specific load.This study is done to check the ability of the roof to withstand high temperature with respect to the position of the reinforcements which contribute to increased strength.
Create sketch for calculating heat distortion
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
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]
| HEAT DISTORTION CRITERIA | ||||||||||||
| S.NO | BOW | Rx | Ry | R | I | L | t | [1.73*10^(-3)*L] | [1.85*10^(-8)*(R^2)/t] | [1.10*10^(-3)*I] | W | OK/NG |
| 1 | FR to BR1 | 5573 | 2468 | 3421 | 402.4 | 2011.5 | 0.75 | 3.47 | 0.28 | 0.44 | 1.51 | ok |
| 2 | BR1 to CTR | 6438 | 4936 | 5587.8 | 374.4 | 2011.5 | 0.75 | 3.47 | 0.77 | 0.41 | 1.97 | ok |
| 3 | CTR to BR2 | 10286 | 3952 | 5710.11 | 456.8 | 2011.5 | 0.75 | 3.47 | 0.8 | 0.5 | 2.09 | ok |
| 4 | BR2 to RR | 13655 | 3266 | 5271.22 | 352.6 | 2011.5 | 0.75 | 3.47 | 0.68 | 0.38 | 1.85 | ok |
From the above result,the desired designed roof statisfies the heat distortion criteria.
Snow load criteria:
This study is used to check the ability of the roof to withstand snowfall and bounce back to its original shape without any permanent deformation once the load is removed.
Sketch for calculating snow load criteria
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
Moment of Inertia(Iy) :
Bow roof 1:
Center reinforcement roof:
Bow roof 2:
| SNOW LOAD CRITERIA | ||||||||||||
| S.NO | BOW | Iy | α | Lx | Ly | Y | S | Rx | Ry | t | Qr | OK/NG |
| 1 | FR to BR1 | 78793 | 0.79 | 1038.5 | 2011.5 | 479.8 | 445.35 | 2918 | 2740 | 0.75 | 4.47 | ok |
| 2 | BR1 to CTR | 124074 | 0.86 | 1147.9 | 2011.5 | 411.5 | 452.55 | 6053 | 3166 | 0.75 | 2.16 | NG |
| 3 | CTR to BR2 | 83838 | 0.93 | 1115.5 | 2011.5 | 493.6 | 476.55 | 10304 | 4114 | 0.75 | 0.46 | NG |
| 4 | BR2 to RR | - | 0.85 | 1094 | 2011.5 | 459.5 | - | 14270 | 4262 | 0.75 | ||
As a result,our roof design does not get statisfying in the snow load prediction,so we can do some correction ie to make changes in the position of bows and add an extra bow to make strengthening the bow and also change the radius of curvature of the part.
Final roof design:
