Roof Design
- TABLE OF CONTENT:
- OBJECTIVES & INTRODUCTION
- DESIGN CONSIDERATIONS
- DFMEA
- MANUFACTURING PROCESS
- DESIGNING USING THE SIEMENS NX TOOL
- CALCULATIONS AND RESULTS
- CONCLUSION
- OBJECTIVE:
The objective of the project is to create the Car Roof, which includes creating engineering features like Front roof rail, rear roof rail, bow roof front, bow roof rear, center roof rail, and Reinforcement which are built to strengthen the Roof Outer panel.
Finding the section modulus and moment of inertia for Bow-roofs and center roof rail.
Do a curvature study on the Roof and performance calculations for Heat distortion and Snow load criteria to determine the position of the Bow roofs.
And finally, perform Draft Analysis of each part.
The Tool Used for the task is Siemens NX CAD, which is the best Software for Creating Sheetmetal Components. NX Software is a flexible and powerful integrated solution that helps to deliver products faster and more efficiently. NX Cad software is widely used in automotive, aerospace, thermal power station, etc...
INTRODUCTION:
An Automobile roof or Car top is the portion that sits above the passenger compartment, protecting the person's inside from sun, wind, rain, and other external elements.
When seen from the outside, it is just closing the top portion. But from the design point of view, various engineering analysis are needed to be performed to give a good meaning to its presence.
For example, the roof outer panel is not curved in all sections and is not flat everywhere, as we know the curved parts are having strength to withstand top loads, but in the flatter regions the strength will not be up a to desired level, hence the reinforcements railings are to be provided to strengthen.
DESIGN CONSIDERATION OF ROOF:
- Visibility criteria
- Head room clearance
- Bow roof position
- Heat distortion and Snow load criteria
- Draft analysis
ROOF CRUSH TEST:
OVERVIEW OF THE TEST:
Roof strength evaluations consist of a quasi-static test conducted on a vehicle’s roof in a manner similar to tests used to judge compliance with Federal Motor Vehicle Safety Standard 216 (Office of the Federal Register, 2009). The main differences 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 is,
- Specifies testing one side of a vehicle’s roof
- Does not include a headroom criterion
- Specifies testing to a given displacement instead of a given force level
- Specifies setting the vehicle’s pitch angle 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 127 mm of plate displacement.
KERB WEIGHT MEASUREMENT:
Kerb weight values used for calculating the SWR are based on IIHS measurements of a vehicle, not the manufacturer’s specified curb weight.
Vehicle curb weight is measured with full fluid levels using scales manufactured by Longacre Racing Products (Computer scales DX series 72634).
TEST VEHICLE SELECTION:
For information on how we select vehicles for our crash test programs, including how we define typically equipped vehicles, see Test Vehicle Selection (IIHS, 2021). Whenever possible, the vehicle acquired for testing is the same vehicle used for the curb weight measurement.
At times, it is necessary to test a vehicle that does not meet the IIHS definition of a typically equipped vehicle, and the curb weight measurement is applied from another vehicle that does meet the definition. In these cases, the test vehicle does not differ from our typically equipped vehicle definition in any way that might influence the roof strength (e.g., a vehicle with a sunroof would not be tested if the typically equipped vehicle lacks this option).
Once the test vehicle has been acquired, either the driver or passenger side of the vehicle is selected for testing. For most vehicles, the test side is chosen at random. However, for roof designs that appear asymmetrical, engineering judgment is used to select the side of the roof that may result in a lower peak force.
TEST VEHICLE PREPARATION:
With the vehicle on a level surface, the on-road pitch angle at the front door sill is measured on both sides of the vehicle. Unless the vehicle manufacturer requests otherwise, roof racks and other nonstructural items that may be contacted during the test are left as installed from the factory.
Any trim or other components are removed if they interfere with supporting the vehicle along its rocker panels. For vehicles with vertical pinch weld flanges on the bottom of the rocker panels, the vehicle support system consists of one I-beam (HR A-36 W4X13) for each rocker panel.
Each I-beam has a clamping system incorporated on the top that is tightened against the pinch weld flange to clamp the system in place (Figures 1 and 2). When the pinch weld flange has a bend that prevents supporting the sill with one I-beam, more than one I-beam can be used on each side of the vehicle.
For vehicles with no pinch weld flange, or with a nonvertical flange angle that precludes clamping, mounting by another method may be necessary, such as that described in the National Highway Traffic Safety Administration’s Laboratory Test Procedure (2006).
Prior to testing, the front-row seat back on the side being tested is reclined to prevent interaction with the crushing roof. The rear seats are latched in their upright position. All windows are closed, and doors are locked.
TESTING OF ROOF:
Roof strength evaluations are conducted on a quasi-static test system manufactured by MGA Research Corporation (Figure 3).
The system consists of an upright assembly and an attached loading head that can be fixed at varying heights from the ground as well as at pitch angles ranging from −5 to +5 degrees to accommodate testing on the driver or passenger side.
The roll angle is permanently fixed at 25 degrees. Four hydraulic actuators control the movement of the platen along two linear guides. The entire system is mounted on a T-slot bed plate anchored to the floor of the test facility.
Two HR A-36 — W10x88 I-beams are mounted on the bed plate perpendicular to the longitudinal axis of the platen. The vehicle, with an attached rocker panel support system, is placed on these beams. The vehicle is adjusted so that:
1. The longitudinal centerline of the platen is within 10 mm of the initial roof contact point.
2. The yaw angle of the vehicle relative to the longitudinal axis of the platen is 0 ± 0.5 degrees.
3. The midpoint of the platen’s forward edge is 254 ± 10 mm forward of the most forward point of the roof (including windshield trim if it overlaps the roof) lying on the vehicle’s longitudinal centerline.
4. The pitch angle of the vehicle matches the on-road angle, while also accounting for any difference between the platen’s pitch angle and the nominal −5 degrees. The maximum combined difference of the vehicle and platen pitch angles from their targets is ± 0.5 degrees. (For example, if the onroad vehicle pitch angle is −0.2 degrees and the platen pitch angle is −5.2 degrees, the target sill angle for the test is −0.4 ± 0.5 degrees.) If necessary to achieve this angle, shims are inserted between the rocker panel supports and the W10x88 I-beams attached to the bed plate.
Once the vehicle is positioned correctly, the rocker panel supports are clamped to the two perpendicular I beams, and the beams are marked to allow confirmation that the vehicle position is maintained during the test.
For body-on-frame vehicles, the frame is supported to prevent the weight of the chassis from stressing the body at the body mounts. The roof is crushed to a minimum displacement of 127 mm at a nominal rate of 5 mm/second.
Some tests are conducted to a greater displacement to collect additional strength data for research purposes. Force data are recorded from five load cells (Interface Inc., model 1220) attached to the loading platen.
Displacement data are recorded from four linear variable displacement transducers (LVDTs) (MTS Temposonics model GH) integrated into the hydraulic actuators. Figure 4 shows the locations of the load cells and LVDTs on the loading platen.
Force and displacement data are collected with a National Instruments USB-6210 data acquisition system and reported at 100 Hz. These data are based on a sampling rate of 2,000 Hz, with every 20 points being averaged to produce the output data at 100 Hz.
The displacement-time histories from the LVDTs are compared to verify that the platen’s roll angle and pitch angle (relative to the vehicle’s on-road pitch) were maintained at 25 ± 0.5 degrees and −5 ± 0.5 degrees, respectively.
The pre-crash and post-crash conditions of each test vehicle are documented with still photographs. The position of the vehicle in the test fixture also is recorded. Motion picture photography is made of the test with real-time video cameras.
CALCULATING THE STRENGTH-TO-WEIGHT RATIO (SWR) RATING:
Force and displacement data are recorded for 5 seconds prior to each test, while the test system holds the loading platen at initial roof contact.
The data recorded from 1 to 4 seconds of this hold time are averaged for each channel to produce a measurement offset that is subtracted from the data recorded during the crushing of the roof.
After removing the offset for each channel, the force-displacement curve is plotted using the summed output from the five load cells and the average displacement from the four LVDTs. The maximum force prior to 127 mm of platen displacement is divided by the measured curb weight to obtain the SWR.
Both the force and curb weight are rounded to the nearest pound prior to performing the calculation, and the resulting SWR is rounded to the nearest one-hundredth of a unit.
Displacement is rounded to the nearest 0.1 mm. (The maximum displacement where the load can be used for the vehicle rating is 126.9 mm.) The vehicle’s rating is assigned based on the boundaries listed in.
All trim levels sharing the tested vehicle’s body type and roof structure are assigned the same rating as the typically equipped trim level, provided their curb weights do not exceed 110% of the selected vehicle’s weight.
Based on published curb weights from multiple sources, any trim levels that may exceed this weight are identified and weighed. If the weight does exceed 110% of the weight of the selected vehicle, a unique SWR is calculated for that trim level.
If this SWR results in a lower rating, both ratings are reported for the model, with a split according to trim level. If a trim level weighs more than 110% of the typically equipped model, but the lower SWR does not fall in a lower rating band, only the original SWR and rating are reported for the model.
Let us see the available Master section and analyze them to implement in our design.
The front roof and rear roof rails are fixed to the ditch area using spot weld operation. But the front, rear bow roof, and center roof rails are fixed to the roof using mastic sealants. We don’t want so much gap between the roof rails and the bow roof area. So, we reducing the NVH issues.
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]
Judgment Condition
OK< 2>
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)
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 from 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 lengths in Side View
Judgment condition
Qr ≥ 3.1
250 ≤ s ≤ 380
ROOF MANUFACTURING PROCESS
The manufacturing of the fender majorly involves these operations.
- Embossing
- Bending
- Cutting and Blanking
- Embossing
Embossing is a metal forming process for producing raised or sunken designs or relief in sheet material using matched male and female roller dies, theoretically with no change in metal thickness, or by passing a sheet or a strip of metal between rolls of the desired pattern.
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 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 an elevated temperature. In order to avoid bending or damage on the roof, based on the heat distortion temperature, this study will predict the bow roof position which helps to strengthen the roof.
This criteria is not checked for the roof with sunroof feature,
SNOW LOAD:
The snow load criteria study is used to check the durability of the roof to withstand the snow fall the roof and reflex is an original shape without any permanent deformation after the load is removed.
If the criteria is not OK or Failed as per standards,
1. Increase thickness,
2. Add extra Bow roofs,
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.
A minimum draft angle of 7deg is considered for analysis. green color infers on the parts that all face along the tooling direction have a positive draft angle greater than 7deg and passed in analysis.