Hood design-Week 2

Car Hood Panel Design

Introduction:

The Hood or Bonnet system is an access panel to the engine compartment to enable maintenance of power train, drive belts, battery, fluid levels and lamp units. It is fundamentally a reinforced skin panel with many safety and quality.

Hood and Boot lid form a sub-segment of vehicle closures,which also contains doors. Hood and boot lid usually do not directly open onto the passenger cell which greatly reduces the importance of certain of the functional requirements normally associated with closures such as passenger safety, air tightness, low cycle fatigue strength etc.

On front-engined cars, the hood may be hinged at either the front, or rear edge or in the earlier models, it may be split into two sections, one each side, each hinged along the center line. A futher variant combains the hood and the wheel arches into one section and allows the entire front body work to tilt forwards around a pivot near the front of the vehicle.

Hoods are typically made out of the same material as the rest of the body work. This may includes steel, aluminum, fiberglass or corbon fiber.

Components of Hood:

• Outer Panel(Skin)
• Inner panel
• Striker
• Hinges
• Reinforcements
• Gas Stay or Support Rod

The inner panel and the other components like striker, hinges, reinforcements are not visible from the outside, those are integrated with the outer panel of the hood called skin surface which gives good impact strength inside and good aesthetic look from outside.

Factors that influence hood design:

• Pedestrian Safety
• Vehicle Quality
• Occpant Safety
• Styling
• Manufacturing

Pedestrain Safety:

In May 2013 the World Health Organization(WHO) reported that more than 270,000 pedestrians lose their lives on the roads each year accounting for 22% of the total 1.24 million road traffic deaths. Despite the magnitude of the problem, most attempts at reducing pedestrian deaths had historically focused solely on education and traffic regualtion. Since the 1970s, crash engineers have begun to use design principles that have proved successuful in protecting car occupants to develop vehicle design concepts that reduce the likeihood of injuries to pedestrians in the event of car-pedestrian crash. These involve redesigning the bumper, hood and the windshield and the pillar to be energy absorbing without compromising the structural intefrity of the car.

EuroNCAP and other vehicle assessment bodies have been instrumental in increasing public awareness of the effectiveness of design for pedestrian safety.

The impact frequency and seriouness of injury in car-pedestrian collision is high in hood zone than any other zone in the car body.

Wrap Around Distance:

Wrap Around Distance(WAD) is the geometrically traced distance from the contact point with the ground reference plane vertically below the front face of the bumper, to any point on the vehicle front structure.

Side Reference Line:

The geometric trace of the highest point of contact between a straight edge held parallel to the lateral vertical plane of the vehicle and inclined 45 deg is traversed down the side of the front structure and the side of the front structure.

Hood Design Parameters & Methodology:

Outer Panel Thickness: 0.75mm

Inner Panel Thickness: 0.75mm

Reinforcement Thickness: 1.5mm

With the help of the given Styling surface, we got the boundary line and the shape for the inner and outer panel to develop further. The Master sketch provides the basic parameters that have to be taken into account to go with detailed design process.

The flow of designing the inner panel by offsetting the styling surface and creating embosses with positive drafts, edge fillets to avoid sharp corners and cutouts. Make some area on the top embosses for the mastic points.

Inner Panel

Emboss:

Embossing is a metal forming process for producing raised or suken design or relief in the sheetmetal by means of male and female dies, with no changes in material thickness. Embosses are created to provide additional strength and to direct the force while collision.

Mastic Sealant:

Mastic sealant is in paste form which is used to join two surfaces, it acts like elastic rubber once get soldified. The main purpose of using mastic sealants in joining the inner and outer panel is to not to affect the aesthetics of the outer surface, since the process like reverting and welding affects the aesthetics of the car. It also has the tendency to increase the strength of the panel, where a point influences upto 80mm surrounding and aslo damp the vibrations.

Striker, Hinge & Trajectory curve:

Striker and Hinge are sub parts of the hood assembly, these will get mounted on the inner panel of the hood. For the making this sub assembly, the inner panel should have the proper oriented area to mount the striker and hinge.

The other side of the hinge will mounted on the engine comaprtment and helps to open and close the hood.

The striker will get locked with the latch pressented in the engine compartment, for the proper locking and opening of hood, the striker and the latch have to be in perpendicular. For this allignment, a trajectory curve is drawn with the help of center point in hinge axis and the center of u-clamp in striker, which gives the opening and closing path.

Reinforcement:

While adding striker and hinges with the inner panel, the inner panel will lose some of its strength at those loactions, so we need to add some reinforcement in other side of panel to regain the strength and to support the striker and hinges.

Hemming:

Hemming is a forming operation in which the edges of the sheet are folded or folded over another part in order to achieve a tight fit. Normally hemming operations are used to connect parts together, to improve the appearance of the part and also to reinforce the part edge.

The accuracy of the hemming is very important, since it affects the appearance of the surface and surface quality.

Types of hemming process:

• Conventional Die Hemming
• Roll Hemming

Here the outer panel and inner panel edges are joined by the hemming operation. And relief is provided in the corners to remove the stress.

Rear Relief

Front Relief

Outer Panel

Deep Drawing:

Deep drawing is a sheet metal forming process in which a sheet metal blank is drawn into a forming die by the action of punch where it goes with both tensile and compression.

In pure deep drawing there is no reduction of sheet metal thickness, forming is achieved in stretch forming purely as a result of decrease in sheet metal thickness.

When drawing complex parts like car body parts, there is usually a combination of stretch and deep drawing prosess will involve.

Cross Section

Draft Analysis:

Draft analysis is a process of analysing the part that it having a positive or negative draft. Where with the positive draft, the part is good to go for drawing process, but where with negative draft, the manufacturing of the part will not be easy.

Conclusion:

Thus as per the given parameters and as per the EuroNCAP safety standard the Car Hood Design is completed and draft analysis is taken to find the part is drawnable in manufacturing using NX CAD.