Roof challenge
ROOF
OBJECTIVE:
To develop a roof panel from the given roof skin by using the given mastersection in NX CAD.
To develop the roof rails from the given skin surface.
Calculate the Section modulus, Heat distortion and Snow load criteria for the created Roof.
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.
Because the earliest automobiles were designed in an era of horse-drawn carriages, early automobile roofs used similar materials and designs.
The solid models that are created from the given skin are
- Roof Outer Panel
- Front Roof Rail
- Bow Roof
- Centre Roof Rail with Reinforcement
- Rear Roof Rail
DESIGN METHODOLOGY
ROOF OUTER PANEL:
Extract the working surface from the given skin.
Create flanges on the front , rear and the side edges.
Blend the flanges together with required radius.
Mirror the part and sew it.
Thicken it to the required thickness.
FRONT ROOF RAIL:
Create the front roof rail by offsetting the given skin surface to the required distance.
Create flanges as per the mastersection.
Blend the surfaces as per the given radius.
Create emboss on the surface.
Mirror and sew the surface.Thicken it.
CENTRE ROOF RAIL:
Create the centre roof rail as per the given mastersection.
The centre roof rail is made with the reinforcement.
The reinforcement will increase the strength of the roof rail.
Create mastic points on the reinforcement.
The mastic points are created to reduce the NVH created in the car during travel and also to join the panels.
Thicken the surface to about 0.75mm.
BOW ROOF:
Similarly create the bow roof by offseting the working surface.
It has to be created as per the mastersection.
The flanges are connected with the use of Studio surface.
The distace between the roof and the bow roof rail has to be 5mm.
The 5 mm must be given because of the mastic points.
The mastic points are effective around a area of about 80 mm .
Mirror the surfae and sew it, thicken it to about 0.75mm.
REAR ROOF:
Create the rear roof by offseting the surface to the required distance.
The flanges are created as per the master section.
Blend the surfaces as per the given radius.
Create emboss on the rail to improve the strength.
Mirror the surface and sew it.
Thicken the surface.
DRAFT ANALYSIS:
Draft Analysis Overview Designers of plastic parts and mold tooling manufacturers can use the Draft Analysis tool to check the correct application of draft to the faces of their part.
With draft analysis, you can verify draft angles, examine angle changes within a face, as well as locate parting lines, injection, and ejection surfaces in parts.
Here we have analysed the draft angle of the created parts by specifying the ejection direction.
Roof outer Panel
Front Roof Rail
Bow Roof Rail 01
Centre Roof Rail
Bow Roof Rail 02
Rear Roof Rail
HEAT DISTORTION CRITERIA:
Heat Distortion Temperature. The heat distortion (or heat deflection) temperature (HDT) is an important property of polymers.
It gives an indication at what temperature materials start to "soften" when exposed to a fixed load at elevated temperatures.
The deflection temperature is also sometimes called the deflection temperature under load .
This temperature gives an estimate of the service temperature a given product can withstand.
However this is only an estimate, and cannot predict how the final part will perform.
Other settings like speed of the temperature increase or even part design will significantly influence the final thermal performance of a given part.
Here we have to calculate this criteria with the use of a 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]
The Calculations are tabulated
The given model passed the heat distortion criteria.
SNOW LOAD CRITERIA:
It is the strength of the roof to withstand the snow load and retain its original position when the load is removed.
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)
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
The centre to bow 02 does not pass the snow load criteria.
Now we have to decrease the curvature of Ry to below 8000 range by modifying the arc radius.
SECTION MODULUS :
Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members.
Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness.
Front Roof Rail:
Bow Roof Rail 01:
Centre Roof Rail
Bow roof rail 02
Rear Roof
CONCLUSION:
Thus the roof panels are developed from the given skin and the analysis are done.
The roof assembly passed the Heat distortion criteria but in snow load criteria it fails at Bow roof 02, so we have to change the curvature with the help of the styling team.