Roof Design

ROOF DESIGN USING NX CAD

 

OBJECTIVE:

          1.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 front, bow roof rear, center roof rail and Reinforcement are creating to build the strong roof.

          2.Do curvature study on Roof and performance calculations for Heat distortion and Snow load criteria to determine the position of the Bow roofs.

          3.Finding the moment of inertia and section modulus for Bow-roofs and center roof rail.

          4.And, also finding the Draft Analysis of each part.

SOFTWARE USED: NX 12.0

• To design the Sheetmetal ROOF in NX by using some Solid/Surface features.
• NX Software is a flexible and powerful integrated solution that helps you deliver the products faster and efficiently.
• NX Cad software is widely used automotive, aerospace, thermal power station. Because of it is having modelling, drafting and manufacturing platform.

 

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 issues.

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 are,

• 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.

CURB WEIGHT MEASUREMENT:

          Curb 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. 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 attached rocker panel support system, is placed on      these beams. The vehicle is adjusted so that:

1. The longitudinal centerline of 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.

DESIGN SPECIFICATION FOR THE AUTOMOTIVE ROOF COMPONENT:

            Master section data for front roof rail, rear roof rail, bow roof front, bow roof rear, Center roof rail and reinforcement. (*Master sections are only use in reference purpose only).

STYLING SURFACE FOR ROOF:

FRONT DITCH AREA:

          Front ditch area is creating with help of the dump solid of class A surface developed by considering the parameters to design the front ditch area,

We have to create the extension sheet with minimum 15mm, and give the positive draft angle 3 degree considering the following drawing operation.

REAR DITCH AREA:

          It’s same to the front roof ditch area but we have to fix the rear door in this area, when considering the parameters to design the rear roof ditch area.

FRONT ROOF RAIL:

          Front roof rail is used for supporting the roof for externally applied forces occurring at accidents. When we increasing the front roof rail is good for strength but also it is not for drivers because of it hide visibility of road.

          When considering the parameters of roof rail, give them 60-80mm length, proper draft angle and also using the embosses for increasing the strength. Front section of front roof rails spot welded to the front ditch area and other end is fixed with help of mastic sealants.

Sheetmetal thickness t = 0.75mm

MASTER SECTION FOR FRONT ROOF RAIL:

REAR ROOF RAIL:

          Rear roof rail is creating to length 80-100mm with proper draft angle. Give embosses on the sheet to ensure the enough strength for holding the roof structure. When considering the all parameters like length, width, sealants, rubber materials, wind shield to create the design properly.

MASTER SECTION FOR REAR ROOF RAIL:

BOW ROOF 1:

          The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure.

          Positioning of the bow roofs are decided by the curvature study. The Heat distortion and Snow load criteria are checked to analyze the bow roof position by various condition.

          We also give the Mastic area for fixing the Bow roof to the roof outer. Each mastic sealants are having 80mm circular effective area of surroundings. Thickness of the Bow roof is 0.75mm.

BOW ROOF 1 MASTER SECTION:

BOW ROOF 2:

The bow roofs are given to the improve the torsional stiffness and load bearing capacity of the roof structure. When number of bow roofs are given by the roof dimension. Longer the roof area we have to give the more bow roof to the structure.

BOW ROOF 2MASTER SECTION:

CENTER ROOF RAIL:

           Center roof rail is effective support to the flat area structure roof. Because of center area of roof is more susceptible to failure under action of load. Usually, Reinforcement center roof rails are fixed at the center of the roof for connecting to the B – Pillar support structure, it will used for roll-over test.

 Thickness of the center roof rail is 0.75mm.

MASTER SECTION FOR CENTER ROOF RAIL:

MASTIC SEALANTS OF ROOF:

          The Mastic sealant or the Mastic point is an elastic compound that behaves like a rubber. The mastic sealants are applied at the many places like roof outer to front roof rail, rear roof rail, bow roof and reinforcement center roofs. They help the roof outer panel to come back to its original shape after it gets deformed. The mastic has an influence area of 80mm.

          It will give the three methods in one operation, it fills the gap between the two or more substrates, it forms a barrier through the physical properties of the sealant itself, and by adhesion to substrates.

WELD SPOTS AREA:

CURVATURE STUDY ON THE ROOF:

• Heat Distortion
• Snow Load

• HEAT DISTORTION:

            The heat distortion study the plays an important role in Sheetmetal application. Heat distortion is temperature limit above which material cannot be used for structural application. This study is useful to fin the where the material starts to soften when exposed to a fixed load at elevated temperature. In order to avoid the bending, damage to the roof based on the heat distortion temperature. This study will provide the which position of the bow roof give enough strengthen to roof.

  Bow Roof Prediction Formula:

  HEAT DISTORTION CRITERIA:

  W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/2] + [1.10 x 10^(-3) x I]-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(R_X*R_Y)/(R_X+R_Y)

            R_X = X Curvature

            R_Y = Y Curvature

            t = Roof plate thickness [mm]

            I = Bow roof span [mm]

  Thickness for the Roof(t):

            Roof Outer Panel, t= 0.75mm

            Front Roof Rail, t = 0.75mm

            Rear Roof Rail, t = 0.75mm

            Bow Roof Rail Front, t = 0.75mm

            Bow Roof Rail Rear, t = 0.75mm

            Reinforcement roof rail, t = 0.75mm

R_X & R_Y (X & Y Curvature):

• From 0-Y axis, at -3Y about distance 300mm is drawn parallel on the perpendicular plane. After that 100mm lines are offset to either side of -3Y line. Now center lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch. Make all geometry lines are construction lines except points.
• Project the intersection points on sketch to the Roof surface.
• Join the intersection points in roof surface using the arc curve an measure the radius using the minimum radius comment.

R_X Value:

R_Y Value:

FINAL RESULTS OF HEAT DISTORTION:

          We have the necessary values for finding the heat distortion value(W):

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/2] + [1.10 x 10^(-3) x I]-2.68 

Apply the all the values,

SI no	BOWs	Rx	Ry	R	I	L	T	W	Results
1	Fr & 1st	2360.88	5746.05	3346.70	342.42	2033.97	0.75	1.49	OK
2	1st & CTR	5955.46	5046.18	5463.24	426.39	2033.97	0.75	2.04	OK
3	CTR & 2nd	9873.98	4145.18	5839.06	407.64	2033.97	0.75	2.12	OK
4	2nd & RR	13856.83	3394.94	5453.71	402.07	2033.97	0.75	2.01	OK

From the above table we, conclude that all the values are of W< 2.7 and also W> 3.1. So, that all bow roofs areas are placed at the right place.

SNOW LOAD:

          Snow load criteria study is used to check the durability of the roof to withstand the snow fall on the roof and reflex it is original shape without any permanent deformation after load being removed. 

SNOW LOAD CRITERIA:

Q_r = [I_Y x t²] / [α x s x [(R_X + R_Y)/2]² x 10^-8]

Where,

α = M_y x L_X² x10^-12, M_{y =} Y(L_y–Y)

Judgement condition = Q_r > 3.1

                    250 < s <  380

 t = Roof plate thickness[mm]

L_Y = 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 rails front/ rear as the front and rear reference point.

L_X= Distance between the left and right end of the roof on the roof bow[mm],

          With of the roof panel exposed on the surface.

Y = Distance between Front roof rails to roof bow [mm],

s = Distance for which roof bow bears divided load [mm],

          s = L₁/2 + L₂/2

I_y = Geometrical moment of inertia of roof bow (Y cross – section) [mm4],

R_X = Lateral direction curvature radius of roof panel Y cross section on roof bow [mm4],           

          Roof panel curvature radius of the length L_X in front view.

R_Y = Longitudinal direction curvature radius of the roof panel X cross section on roof bow[mm4],

          Roof panel X curvature radius of length s in side view. 

R_X & R_Y (X & Y Curvature):

• At 0-Y axis on perpendicular plane to roof. Draw 100mm lines offset to either side of the of 0Y axis line. Now center lines are drawn between each adjacent roof rail respective. Intersection points are marked on the sketch. Make all the geometry in construction lines except points.
• Project the intersection points in the sketch are projected to roof surface.
• Join the project points using arc curve and measure the radius value using minimum radius comment.

R_X Value:

R_Y Value:

MOMENT OF INERTIA:

BOW ROOF 1:

Therefore, we take the Minimum moment of inertia value is I_Y1 = 1818.10mm⁴.

BOW ROOF 2:

  We take the Minimum moment of inertia value is I_Y2 = 1513.83mm⁴.

CENTER RAIL:

Minimum moment of inertia for center reinforcement rail is I_Y3 = 2100.56mm⁴.

FINAL RESULTS OF SNOW LOAD CRITERIA:

          We have the necessary values for finding the Snow load value(W):

BOWs	My	Lx	Ly	L1	L2	T	S	Qr	results
Bow roof 1	1818.10	1137.76	2030.99	501.75	520.8	0.75	511.24	16.50	OK
CTR	2100.56	1084.39	2030.99	520.74	503.75	0.75	510.65	10.59	OK
Bow roof 2	1513.83	1057.07	2030.99	500.57	441.08	0.75	470.82	6.57	OK

If roof structure pass to the snow load condition. They, must have Q_r values from above 3.1.

          Q_r   > 3.1 

So, we done the curvature study on the roof. Heat distortion and snow load condition my roof is pass the two criteria, so position of the bow roofs and center reinforcement roofs placed at right area.

SECTION MODULUS:

          The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object, which is collectively termed as flexural rigidity (EI). Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.

Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.

There are two types of section modulus we have,

• Elastic section modulus
• Plastic section modulus

Elastic modulus is defined as,

          S = I/Y

     S – Section modulus,

     I – Second moment of area

    Y – distance from the neutral axis to outer most fiber of the object.

BOW ROOF 1:

Moment of inertia (Max), Imax = 3.819 x 10⁴ mm⁴

Moment of inertia (Min), Imin = 181.8 x 10³ mm⁴

          Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

          S = I/Y

          Y = 30.07 mm

For maximum MOI,

          S = 3.819 x 10⁴/30.07

          S_MAX = 13.20 mm³

For minimum MOI,

          S = 181.8 x 10⁴/30.07

          S_MIN = 628.77 mm³

^{BOW ROOF 2:}

          

Moment of inertia (Max), Imax = 2.698x 10⁴ mm⁴

Moment of inertia (Min), Imin = 151.3 x 10³ mm⁴

          Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

          S = I/Y

          Y = 30 mm

For maximum MOI,

          S = 2.698x 10⁴/30

          S_MAX = 9.35 mm³

For minimum MOI,

          S = 151.3  x 10⁴/30

          S_MIN = 524.50 mm³

^{CENTER ROOF RAIL:}

Moment of inertia (Max), Imax = 5.694 x 10⁵ mm⁴

Moment of inertia (Min), Imin = 210.05x 10³ mm⁴

          Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

          S = I/Y

          Y = 45 mm

For maximum MOI,

          S = 5.694x 10⁵/45

          S_MAX = 13.286mm³

For minimum MOI,

          S = 210.05x 10³/45

          S_MIN = 480.78mm³

^{FRONT ROOF RAIL:}

Moment of inertia (Max), Imax = 1.793 x 10⁵ mm⁴

Moment of inertia (Min), Imin = 917.08 x 10³ mm⁴

          Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

          S = I/Y

          Y = 60.28 mm

For maximum MOI,

          S = 1.793 x 10⁵/60.28

          S_MAX = 3.123mm³

For minimum MOI,

          S = 917.08 x 10³/60.28

          S_MIN = 1567mm³

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 is 7 degree is considered for analysis. Green colour show positive draft angle of the part more than 7 degree.

ROOF OUTER PANEL:

FRONT ROOF RAIL:

BOW ROOF 1FRONT RAIL:

CENTER ROOF RAIL:

BOW ROOF 2:

REAR ROOF RAIL:

ROOF VIEWS:

FRONT VIEW:

TOP VIEW:

BOTTOM VIEW:

CONCLUSION:

          I did the My roof design and check heat distortion and snow load condition for bow roofs are placed at right place or not. So, after finishing my design I did the heat distortion and snow load condition.

          After, finishing the curvature study. Then go to check the section modulus. And last, we check the draft analysis. Because, whether part is manufacturable or not. That, why we doing this analysis. That’s all my design pass the all requirements for placing the roof on the car.