Roof challenge

DESIGN AND ASSEMBLY OF ROOF USING NX CAD:

OBJECTIVE:

• The main objective of this project is to design car roof with essential flanges and reinforcements like ditch area, front roof rail, bow roofs, center roof rail and rear roof rail.
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• To find moment of inertia and section modulus for Bow roofs, central roof rail, front roof rail and rear roof rail.
• To perform the draft analysis for the roof reinforcements such as front roof rail, bow roofs, center roof rail and rear roof rail.

INTRODUCTION:

• Design of any product starts from a requirement. The design engineer sketches the product based on the requirement.
• Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD software’s.
• another important thing in the design is that, every product that is designed should be able to manufactured. so the role of the design engineer is to design the product that can be easily manufactured and with cost effective.
• This is the place where the Design for manufacturing [DFM] and Design for assembly [DFA] takes place a role while developing any product.
• The process of development of design carries on until the design satisfies the customer needs.
• Here in this project, we are going to develop a car roof with its required reinforcements.

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.
• In safety point of view, a roof plays a crucial role especially in roll over kind of accidents in which vehicle tips over onto its side of the roof.
• And vehicle roll over crashes causes many fatalities and severe injuries of head, neck, spine trauma around the world.
• Therefore, passenger safety is also very important parameter while designing a roof. For these reason, strength of the roof is very important criteria for passenger safety and safety of vehicle in snow load condition.
• Insurance Institute of Highway Safety performs the Roof crush test which give results of the strength of the roof which provides safety to the passengers when any type of collisions is happened.
• In order to achieve the strength of the roof components, we introduce some reinforcements to the roof such as Front Roof Rail, Bow roofs, Centre Roof Rail and Rear Roof Rail.
• Number of bow roofs is decided on the basis of improvements of the roof strength and the overall length of the roof.

Why reinforcements are needed?

If a roof is in a flat condition, then it is weaker in nature. It doesn’t have that much of strength to withstand the external forces. So, to improve the strength of the sheet metal component, the reinforcements are introduced to withstand against the external forces like snow load and any other kind of impacts. For to improve the strength of the roof component, reinforcements such as Front roof rail, bow roofs, Centre roof rail and rear roof rail are introduced. The reinforcements are joined with the roof outer panel by performing different joining processes like spot welding and with mastic sealants. The thickness of reinforcements of sheet metal should be around 0.75mm to 1.5mm.

Reinforcements of Roof:

• Front Roof Rail.
• Bow Roofs.
• Centre Roof Rail.
• Rear Roof Rail.

Design Considerations of Roof:

• Front passenger Head room clearance.
• Rear passenger Head room clearance.
• Front Visibility criteria.
• Rear Visibility criteria.
• Heat distortion criteria.
• Snow load condition criteria.
• Draft analysis.
• Curvature of the roof.

Design Methodology:

Components of Roof:

FRONT ROOF RAIL:

• Front roof rail is one of the reinforcements provided to improve the strength of the roof which is placed at the front portion of the roof where the windshield joins with the roof.
• The front roof rail and its bottom flange is designed with the considerations of front visibility criteria and head room clearance for the front passengers.
• Front roof rail is joined with the roof outer panel by using spot welds at one end and with mastic sealants by other end for increased strength of the roof.

Front roof rail Master section & Design:

BOW ROOFS:

• Bow roofs are also one of the reinforcements which is provided to increase the strength of the roof at the middle portions of roof.
• The width of the bow roof is around 65 – 75mm and it changes according to the length and size of the roof.
• The bow roofs are joined with the roof outer panel by the help of mastic sealants.
• In this project, we designed two bow roofs named as Bow roof 1 and Bow roof 2.
• Bow roof 1 is placed between the front roof rail and centre roof rail and Bow roof 2 is placed between the centre roof rail and rear roof rail.
• The number of bow roofs requirement differs based on the length of the roof and the required strength of the roof to be needed.

Bow roof Master section & Design:

BOW ROOF 1:

BOW ROOF 2:

CENTRE ROOF RAIL:

• Centre roof rail or reinforcement centre is located at the centre portion of the roof.
• The cross section of the centre roof is more than the bow roofs which supports the left portion and right portion of the roof. Due to the increased cross section of the reinforcement centre, it provides better strength to the roof.
• It is joined with the roof outer panel by the help of mastic sealants and spot welded with the BIW component.

Centre roof rail Master section & Design:

REAR ROOF RAIL:

• Rear roof rail is one of the reinforcements provided to the roof at the rear portion of the roof to increase the strength of the roof component.
• Rear roof rail is joined with both the roof outer panel and the back door of the car.
• Rear roof rail is designed with the consideration of inputs of back door, rear visibility criteria and rear head room clearance.
• Rear roof rail is joined with the roof outer panel by using the mastic sealants at one end and with spot welds at another end.
• In some cases, the rear roof rail is joined by continuous seam welding to avoid gaps and water leak proof.

Rear roof rail Master section & Design:

ROOF DITCH AREA:

• Roof ditch area is the extended portion of the roof at the front portion and also at rear portion of the roof.
• Front portion of the front roof rail is joined with the front ditch area of the roof and windshield glass is also fixed with the front ditch area of the roof.
• Roof ditch area is joined with other components with the help of spot welds and with the mastic sealants based on the requirement.
• Rear ditch area is joined with the rear portion of the rear roof rail by spot welds to provide the better strength.
• A dynamic rubber sealant is fixed with the rear ditch area flange which also supports the tailgate door.

FRONT DITCH AREA OF ROOF:

REAR DITCH AREA OF ROOF:

EMBOSSES ON ROOF:

• Embosses provided on the roof is also like an reinforcement which provides the strength to the overall area of the roof.

STRENGTH OF THE ROOF:

• After designing the roof and its reinforcements, the strength of the roof is calculated by some tests to know whether the strength of the roof is enough or need to improve.
• To check the strength of the roof, Roof crush test is performed by Insurance Institute of Highway Safety.

ROOF CRUSH TEST:

• After analysing so many accidents, IIHS has designed some rules and regulation to perform the roof crush test.
• IIHS gives ratings based on the results of tests. For this, they consider two things while giving the rating. They are, how fast the roof can reform to its original shape after the roll over and how deep is the protrusion after the roll over.

IMPORTANCE OF ROOF CRUSH TEST:

• To minimize the protrusion into the occupant compartment, so that the seat belts and air bags can do their job in protecting the occupant.
• There are two conditions to be followed for the roof crush test. They are,
• Initial Condition – 1) Close and lock all the doors. 2) Close all the doors.
• Fixing the test sample – Fix the body sill or chassis frame on a hard, level surface horizontally.

Method of Force applying:

• The load is applied in such a way that 5° 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 25° 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.

RESULTS:

• If the displacements go 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.

HEAT DISTORTION STUDY:

• This study is done to check how the sheet metal components perform in the extreme heat conditions. Sheet metals have tendency to change its shape in high temperatures.
• To avoid any failure of sheet metal components this study is done to determine its capacity.

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 ≤ W < 3.5.

L = Roof Length in X – direction [mm] (Roof dimension with 0Y).

R = Roof curvature.

R = 2(Rx x Ry) / (Rx + Ry).

Rx = X curvature.

Ry = Y curvature.

t = Roof plate thickness [mm].

l = Bow roof span [mm].

Results:

From the above table it can be concluded that all values of w<2.7 and also W is not >3.1. So thus, infers that current positioning of bow row are good in state as per design and satisfies all the conditions for all components.

SNOW LOAD CRITERIA:

• The snow load test is performed to know how the roof is behaving when snow occurs on the roof in cold regions.

Roof design must pass these calculations to ensure that it has sufficient stiffness to withstand the load due to accumulation of snow over the roof.

Prediction formula: Qr = [ly x t²]/[αxSx[(Rx+Ry)/2] ²x10^-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 vehicle 0Y [mm].

→Length of Roof panel with the centre point between Roof rail FR/RR as the reference point of the front/rear.

Lx = Distance between the left and right end of the roof on the roof bow [mm].

→Width of Roof panel exposed on the surface.

Y = Distance from FR rail centre 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 FR/VIEW.

Ry = Longitudinal direction curvature radius of roof panel X cross section on roof Bow [mm].

→Roof panel X curvature radius of the length S in SIDE/VIEW.

The above are values of each individual roof Bow.

Results:

As per standards the Qr value supposed to be greater than the 3.1. In the Above Design, we have found the Qr value is greater than 3.1 in all Components. Hence the Snow load criteria is satisfying.

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 colour ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values.
• Minimum draft angle of 7° is considered for analysis. green colour infers on the parts that all face along the tooling direction has positive draft angle greater than 7° and passed in analysis.
• The limit angle should be given as 7° for each component.
• When the colour is green it means it has a positive draft it means area which are more than 7 degree is shown as green. If it is shown in yellow that means it has less than 7 degrees and if in blue or red then it means it has negative draft.

Front roof rail:

Bow roof 1:

Bow roof 2:

Centre roof rail:

Rear roof rail:

Section inertia analysis:

Bow roof 1:

Bow roof 2:

Centre roof rail:

ROOF IMAGES:

TOP VIEW:

SIDE VIEW:

ISOMETRIC VIEW:

CONCLUSION:

We were successfully able to design the roof with all the roof rails for better support and strength. We designed it so as to satisfy the norms and regulations by calculating the heat distortion criteria and the snow load criteria. While doing this we also understood about the methodology behind designing the roof of the car and what all we take into consideration while designing so.