Roof Design
AIM:--
For the Given Roof styling, create an ASSEMBLY by developing the essential flanges for the outer roof and creating the reinforcements.
Provide appropriate dimensions and check for Draft analysis.
Do a curvature study on the roof and perform calculations to determine the position of the Bow-roofs.
Suggest and implement remedies in the design if the criterion is not full-filling
Start with the creation of a roof ditch area with a tool opening angle of 7 degrees.
Develop the reinforcements as separate parts
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< 2.7
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
Here are some Reference images for the Master sections - These images are just to give you a brief idea about the design.
Front Roof Rail Master section
Master section of the Rear Roof Rail
Master section of Bow Roof
Master section of Center Reinforcement Rail
ANSWER:--
WHAT IS ROOF IN AUTOMOBILES?
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.
ROOF CRUSH TEST :
Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing the many accidents IIHS has then fixed some regulations , that how to perform the crush test .
IIHS gives the rating based up on this test . For this they consider two points while giving the rating
- How fast the car can reforms 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 there job in protecting the occupant
CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :
There are two conditions 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 50 is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width . The load is applied at the position of 250 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
RESULT :--
If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength . the stronger the roof the better it can protect the occupants in the roll over crashes.
Now lets see what are all the parts are there in roof:--
- OUTER ROOF
- FRONT ROOF RAIL
- BOW ROOF 1
- CENTER ROOF
- BOW ROOF 2
- REAR ROOF RAIL
- OUTER 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.
- FRONT ROOF RAIL:--
Front roof rail is the one which joins the wind shield glass , body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given
Front Roof Rail Master section
Rear Roof rail: It helps in joining the rear door. The design of the rear roof rail differs by the opening method of the rear door.
* Center roof: It helps in providing support to the flat area of the roof. As the name goes, it is placed at the center of the roof between the front and rear roof rail.
* Bow roof: These additional roofs called bow roof are introduced to improve the torsional stiffness and load bearing capacity of the roof structure. The number of bow roof used varies by many factors, one to mention is the roof length.
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.
Roof:
Two HR A-36 — W10x88 I-beams are mounted on the bed plate perpendicular to the longitudinal axis of the platen. The vehicle, with attached rocker panel support system, is placed on these beams. The vehicle is adjusted so that:
- The longitudinal centerline of platen is within 10 mm of the initial roof contact point.
- The yaw angle of the vehicle relative to the longitudinal axis of the platen is 0 ± 0.5 degrees.
- 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.
- 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 is also recorded. Motion picture photography is made of the test with real-time video cameras.
If the displacements go above the requirements level, then we have to increase the bow roof thickness or add more mastic points to increase the strength. The stronger the roof, the better it can protect the occupants in the roll-over crashes.
CALCULATIONS:
For our study, we are considering the following conditions for testing the roof.
* Heat Distortion Criteria:
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
For the above, we can find can record and calculate as shown below:
From the above table, we can conclude that the positions at which the bow roofs are placed satisfy the heat distortion condition and is good to go for manufacturing.
* Snow load Criteria:
This test is done to know how is the roof behaving when there 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 requirements for snow load criteria.
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)
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 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 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]
Section analysis:
* Bow roof 1:
* Bow roof 2:
* Center roof:
For the above, we can find can record and calculate as shown below:
From the above table, we can conclude that the positions at which the bow roofs are placed satisfy the snow criteria, except the bow roof 2. Thus, to improve, we can add embosses to the roof to increase the strength of the roof.
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 70 is considered for analysis. Green color infer on the parts that all face along the tooling direction has positive draft angle greater than 70 and passed in analysis
Front roof rail:
Rear roof rail:
Center roof:
From above calculations the value of 'Q' obtained is always greater than 3.1
ROOF ASSEMBLY
CONCLUSION
Design of roof was succsessfully completed using SIEMENS NX as per the given master sections.The position of bow roof rails are as per the given standards.Draft analysis & section analysis ensure the manufacturability of every components.