Roof challenge

AIM:To design roof of car with roof rails & perform heat distortion, snow load analysis, section modulus inertia analysis, draft analysis.

                                                FIG_1: ROOF OF A CAR.

OBJECTIVE : 

1. To design ditch area for outer panel of roof on given styling surface.
2. To design Front & rear roof rail for roof.
3. To design Bow roof rails & reinforcements.
4. To examine heat distortion & snow load criteria.
5. To Perform draft analysis on individual parts.
6. To perform section modulus inertia calculation.

INTRODUCTION :

• The design process starts from the need of the customers.
• According to the demand of the customer & rules of the automotive society we have to sketch or set the parameters.
• Afterwards we have to consider design for manufacturing & assembly (DFMA) parameters.
• Heat distortion analysis, snow load analysis, section modulus inertia analysis, draft analysis are considered for given design & do corrections to meet the requirements.
• 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.
• On the front side of the roof we have to place wind shield glass & on the rear side we have to place backdoor or rear hood.
• We have to perform calculations for heat treatment & snow load by plotting curves on the outer panel. 
• Visibility criteria and head clearance will be considered according to different regions & rules.

REINFORCEMENTS :

• The reinforcements like roof rails are provided for making the roof stronger & should withstand with various loads such as snow, luggage, etc.
• In these reinforcements the front & rear roof rails, Bow roofs are involved to provide basic structure for the positioning of roof, wind shield & backdoor.
• Different joining or welding processes are involved in this positioning of the assembly.

PARTS INVOLVED IN ROOF ASSEMBLY:

1. ROOF OUTER PANEL WITH DITCH AREA
2. FONT ROOF RAIL
3. REAR ROOF RAIL
4. BOW ROOF RAIL L1 & R1
5. CENTRE REINFORCEMENT
6. CENTRE BOW ROOF RAIL

                                 FIG_2: REAR DITCH AREA OF OUTER ROOF PANEL

                                 FIG_3: FRONT DITCH AREA OF OUTER ROOF PANEL

                                           FIG_4: OUTER ROOF PANEL TOP VIEW

 

                             Fig_5: Components of Roof Assembly

FRONT ROOF RAIL:

• In the front roof rail design the wind shield positioning & visibility of driver as well as head space of the driver is considered.
• The front side flange is joined with spot welding & rear flange is joined with roof using mastics.
• Front Roof Rail is designed using the given master sketch & tool opening angle is only 3 degrees.

                                           FIG_6: MASTER SKETCH OF FRONT ROOF RAIL

                                                            FIG_7: FRONT ROOF RAIL

REAR ROOF RAIL:

• Rear roof rail is designed for positioning the backdoor on the rear flange.
• The rear roof rail front flange is joined by using mastics to the outer panel.
• The rear flange is joined using spot weld with outer panel.
• The rear roof panel is designed using the given master sketch of rear roof panel.

                                       FIG_8: MASTER SKETCH OF REAR ROOF RAIL

                                                       FIG_9: REAR ROOF RAIL

CENTRAL ROOF RAIL & REINFORCEMENT:

• Centre roof rail is positioned at the centre of the roof.
• It is wider than other bow roof rails because it bears the most of the load of roof.
• To strengthen the centre bow roof rail the reinforcement is provided.
• The reinforcement is joined with mastics to the outer panel
• The reinforcement & centre roof rail is spot welded.

                            FIG_10: REINFORCEMENT POSITION ON CENTRE BOW ROOF RAIL

                                                        FIG_11: CENTRE BOW ROOF RAIL

BOW ROOFS:

• Bow roofs are given to improve torsional stiffness and load bearing capacity of the roof structure.
• We can add number of bow roofs as per the length of the roof.
• we have used two bow roofs on left of the central bow roof & right of the central bow roof.

 

                                                           FIG_12: BOW ROOF RAIL L1

                                                           FIG_13: BOW ROOF RAIL R1

HEAT DISTORTION ANALYSIS:

• Heat distortion temperature study is really important in the domain of sheet metal analysis.
• At the elevated temperatures above the heat distortion temperature the sheet metals can't be used for structural applications.
• This heat distortion calculation shows at what temperature on fixed load the sheet metal will start soften & deformations will be experienced.
• We will get the result on roof rails postioning after the heat distortion calculations.
• We can strengthen the roof by rightly positioned the roof rails by getting optimum values from calculations.
• 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            

• 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(Rx*Ry)/(Rx+Ry)

• Rx = X curvature
• Ry = Y curvature
• t  = Roof plate thickness [mm]
• l = Bow Roof Span [mm]

 

                       FIG_14: NOTATIONS OF PARAMETERS FOR HEAT DISTORTION CALCULATION

Procedure for calculation of heat distortion criteria:

• we have created 0-Y line offset it through -3Y & then created 100 mm right & left side of -3Y.
• The vertical lines are plotted between the roof rails at midpoints.
• The vertical lines are creted by offsetting 100 mm right or left.
• Then we marked the intersection points & projected on the outer roof panel.
• The three point arcs are used to create curves on outer panel.
• The Local radius command is used for Rx & Ry calculation.
• The all values are measured through projected sketch & used in excel sheet.
• The all required values & formulae are feeded in excel sheet for given calculations.

• We can conclude that positioning of roof rail is OK for heat distortion analysis because all values lies between less than 2.7 & not greater than 3.1.

SNOW LOAD ANALYSIS:

•  This analysis is conducted to observe the behaviour of our roof desiogn under snow load.
• Initially deformation is observed due to the snow load in the roof.
• But it should be elastic deformation not plastic deformation is the important thing in this analysis.
• The dent should regain its original position after removal of snow
• We have to make offset lines at 0-Y 100 mm offset on both sides instead of -3Y.
• We have to follow the same steps as per heat distortion in this analysis too.

                           FIG_14: NOTATIONS OF PARAMETERS FOR SNOW LOAD CALCULATION 

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)
• 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 front and rear reference point.

• 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]

        Roof panel X curvature radius of length s in Side view

• From the above calculations we can observe Qr > 3.1 so the positioning is OK for snow load.

SECTION MODULUS:

• It is the ratio of second moment of area and distance from neutral axis to the extreme fiber.
• It shows the stiffness of the given member.
• Section modulus is a geometric property of cross-section used in the design of flexural members or beams.
• The stress in the outermost section of the beam is calculated by section modulli method.
• There are two types of section moduli, the elastic section modulus and the plastic section modulus.
• The elastic section modulus is defined as,

S = I / y

where I = Second moment of inertia 

         y = Distance from the neutral axis to extreme fibre

• The unit of sectional modulus is cubic millimeter.
• The bodies having more shear modulus are tougher & can withstand more load.

Section Modulus Calculation of ROOF RAILS:

METHODOLOGY:

1. Create a datum plane at the centre of the Roof Rails.
2. Use the Intersection Curve option.
3. Project the 2D sketch From given 3D Sketch
4. Use trim option & studio spline option to get required closed geometry.
5. Apply section inertia modulus command to get section modulus for solid section.

• Section modulus inertia of front roof rail:

• Section modulus inertia of rear roof rail:

• Section modulus inertia of Bow roof rail L1:

• Section modulus inertia of Bow roof rail R1:

• Section modulus inertia of Central Bow roof & Reinforcement:

• Tabular calculation of section modulus inertia:

DRAFT ANALYSIS:

• Draft analysis helps to judge the product we designed will be easily removable from dies.
• The results of the draft analysis is shown through colors green, blue, yellow, red.
• In the given analysis toooling opening angle is mentioned 3 degree or more.
• The green color will predict the all tooling direction having positive draft angle more than 3 degrees.

                           FIG_15: Draft analysis for roof outer panel

 

                             FIG_16: Draft analysis for Front Roof Rail

                               FIG_17: Draft analysis for Rear Roof Rail

                              FIG_18: Draft analysis for Bow Roof Rail L1

                               FIG_19: Draft analysis for Bow Roof Rail R1

                         FIG_20: Draft analysis for Reinforcement

                           FIG_21: Draft analysis for Centre Bow Roof Rail 

ROOF OVERVIEW:

Conclusion:

• As above we have designed all essential parts of the roof assembly.
• We have examined all criteria are fulfilled by the above design.
• We have good results for Heat distortion & snow load criterias.
• The Roof Rails positioning is also better too bear all loading conditions.
• We have provided embosses on roof outer panel too improve its strenth.
• The all embosses & material removal is executed too improve stiffness & make it light weight.
• All parts having good results for draft analysis as well as sectional inertia analysis.
• In this way the roof design project is completed successfully.