Roof Design
Objective
The main objective of the project is to create the roof Sheetmetal design in NX cad, and also designing Front roof rail, Rear roof rail, bow roof 1&2 than centre roof rail.
Do curvature study on roof and performance calculation for Heat distortion and Snow load criteria to determine the position of bow roofs.
Then finding the Moment of inertia and section modulus of all the bow roofs, front and rear roof and also centre roof rail.
Also finding the Draft Analysis of each part.
Software
To designing the Sheetmetal roof in NX by using some solid/surface features.
Introduction
An automobile roof of a cartop 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.
Design consideration of roof
- Visibility criteria
- Head Room Clearance
- Bow Roof position
- Heat distortion and snow load criteria
- Draft analysis
Roof crush test
Over view of the test
Roof strength evaluation consists of a quasi-static test conducted on a vehicles roof in a manner similar to tests used to judges compliance with federal motor vehicles safety standard 216.The main difference between the procedure specified by the insurance institute for highway safety (IIHS) and that specified by the U.S. federal government are that the IIHS procedure are,
Specifies testing on side of a vehicles roof
Does not include a head room criterion
Specifies testing to a given displacement instead od a given force level
Specifies setting the vehicles pitch angles during testing based on the measured-on road pitch angle.
An overall rating is assigned based on the peak strength to weight ratio (SWR) measured within 127mm of plate displacement.
Roof Crush Test
Roof crush test is the design given by the IIHS. After analysing the many accidents IIHS has then fixed some regulation, that how to perform the crush test.
IIHS gives the rating based upon this test. For this they consider two points while giving the rating.
How fast the car can reform to its original shape after the roll over and how deep is the protrusion after the roll over.
Goal Of Roof Crush Test
To minimize the protrusion into the occupant compartment, so that the seat belts and air bags can do their job in protecting the occupant.
Condition to follow for Roof Crush Test
There are two condition 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 5 degrees is tilted from the front end to the rear end the load will be in 1800mm in length and 750mm in width. The load is applied at the position of 25 degrees from the flatness of the roof. The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that.
CURB WEIGHT MEASUREMENT
Curb weight values used for calculating the SWR are based on IIHS measurements of a vehicles, not the manufacture’s specified curb weight.
Vehicles curb weight is measured with full fluid levels using scales manufactured by long care racing products.
Design Procedure
In these challenges we are creating
Front Roof Rail
Bow Roofs (Front and Rear)
Centre Roof Rail
Rear Roof Rail
Ditch area
Open the assembly navigator in NX-CAD software
Adding outer roof part file in NX-CAD assembly and create roof .
Roof Outer Panel
First, we will be extracting the top haft of the portion of the roof surface. Since the roof surface is symmetrical in shape, we can easily work only on half of the portion and reflect it at the end to create the whole roof.
Front Roof rail
Front roof rail is designed using master section of the front roof rail shown below considering the dimensions windshield which is accommodated in front of the roof, adjacent to windshield. Dimensions shown 32mm and 80mm in the image is based on the factors such as head room clearance and visibility criteria.
Bow Roofs (1&2)
The bow roofs are used to improve the torsional stiffness and load bearing capacity of the roof structure. Bow roof are placed on the flat region of the roof which provides strength in bigger curvature region and in between front roof rail, centre roof rail and rear roof rail. The number of bow roof present is depending on the overall size of the roof. Presently in this project two bow roofs are added. Bow roofs are also designed based on the design consideration similar to front roof rail, referring the master section attached below.
Bow roof 1
Bow roof 2
Centre Roof Rail
Centre roof rail helps in providing efficiently support to the flat area of the roof as it is more susceptible to failure under the action of load. Generally centre roof rail is placed at the centre of the roof which is connected to the B-pillar support structure. This centre roof rail helps in adding the strength to the roof during the roof- over test.
Rear Roof Rail
Rear roof rail is the member used to give strength of the roof in back end. Rear roof rails main usage is to join back door and body side outer.
Ditch Area
Ditch area is the extended portion of the roof styling surface which is used to mount the reinforcement and also ensure sealing between parts of the roof. Opening angle of ditch area is 7 degrees.
Heat Distortion Criteria
The heat radiating from the sun causes the roof of the car to change in the metallic region. 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 he heat distortion temperature at where the material starts to soften when exposed to affixed 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.
W = [1.73x10(-3) x L]+[1.85x10(-8) x (R2)/t] + 1.10x10(-3) x I]-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)
I = Bow roof span (mm)
Snow Load Criteria
This test is done to know how is the roof behaving when threr is a snow over. 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 removal the roof should go it its original position. This is the basic requirement for snow load criteria.
Qr = [Iy x t2] / [α x s x [(Rx+Ry)/2]2x10-8]
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 rail on the vehicle along with 0Y(mm)
Length of roof panel with the centre 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 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 movement 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)
In order to find snow load criteria apply all the necessary values from the roof to the given formula:
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 addition of flanges and embosses increase the section inertia of the part. Here the master section of the roof and roof rails and their section analysis are given
Front Roof Rail
Bow Roof 1
Bow Roof 2
Centre Roof Rail
Rear Roof Rail
Draft Analysis
Front Roof Rail
Bow roof 1&2
Centre Roof Rail
Rear Roof Rail
Conclusion
Hence the roof, front roof rail, bow roof rail, centre roof rail and rear roof rail are designed as per the given master section than section moment of inertia and draft analysis also check feasibility of manufacturing.