Roof Design
SHEET METAL ROOF DESIGN USING SIEMENS NX CAD
Objective
1.To design a roof of a car with its reinforcements like bow roofs, front roof rail, center roof rail and rear roof rail using the master sections provided
2.To calculate the heat distortion and snow load criteria
3.To check the draft analysis of all the parts
Introduction
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.
When it is seeing from outside of the car, it is just closing the top portion. But in design point of view, it is hard to create the design. Because of you have to create no of support rails for enough strength to withstand the load. Bow roofs are primarily fixed in flatter region to enhance strength.
Front roof and rear roof rails are fixed to the ditch area using spot weld operation. But front, rear bow roof and center roof rails are fixed to roof using mastic sealants. We don’t want so much gap between roof rails and bow roof area. So, we reducing the NVH.
DESIGN CONSIDERATION OF ROOF:
- Visibility criteria
- Head room clearance
- Bow roof position
- Heat distortion and Snow load criteria
- Draft analysis
Parts of Roof
Front roof rail bow roof 1 Center roof rail Bow roof 2 Rear roof rail
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 consideration for the bottom flange. 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 rail
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 strenght and also since this would not affect the styling of the roof.
Center roof rail
Center roof rail is in the center of the roof which is connected to the B pillar and it also helps in strengthning the roof the ends are spot welded to the flanges of ditch area and also joined using mastic seal.
Rear roof rail
Rear roof rail is situated in the rear side of the roof its main function is to give strength to the roof. Also the back door rests on the rear roof rail. Its also joined to the roof using mastic sealants
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]
Judgement Condition : OK< 2.7> 3.1
| Case | Rx | Ry | R | T | I | L | w | Result | ||||
| 1 |
|
|
|
0.75 |
|
2033.705 | 1.7 | ok | ||||
| 2 |
|
|
|
0.75 |
|
2033.705 | 2.1 | ok | ||||
| 3 |
|
|
|
0.75 |
|
2033.705 | 2.1 | ok | ||||
| 4 |
|
|
|
0.75 |
|
2033.705 | 1.6 | ok |
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)
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
250 ≤ s ≤ 380
Moment of inertia of bow roof 1
Moment of inertia of center roof rail
Moment of inertia of bow roof 2
| Case | Y | LY | LX | MY | a | T | L1 | L2 | s | Iy | Rx | Ry | Qr | Result | ||||||||||||
| 1 |
|
2033.705 |
|
|
|
0.75 |
|
|
|
|
|
|
|
ok | ||||||||||||
| 2 |
|
2033.705 |
|
|
|
0.75 |
|
|
|
|
|
|
|
ok | ||||||||||||
| 3 |
|
2033.705 |
|
|
|
0.75 |
|
|
|
|
|
|
|
ok |
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 7 degree is considered for analysis . green colour infer on the parts that all face along the tooling direction has positive draft angle greater than 7 degree and passed in analysis.
Draft analysis of front roof rail
Draft analysis of bow roof 1
Draft analysis of center roof rail
Draft analysis of bow roof 2
Draft analysis of rear roof rail
Pictrures of the roof design in all views
Top View
Bottom view
Side View
Isometric View
Conclusion
We have designed the roof of a car successfuly in siemens Nx Cad by doing the calculations and following the master sections.