Challenge

Hood design-Week 2

HHussain Akbar27 Nov 2022

Design of hood

Aim

To design the inner and outer panel of the hood from the given class A surface
To consider various design aspects required to design a proper hood
To create the latch trajectory for the striker
To create reinforcements for the hinge and striker assembly
To assemble all the components together to obtain the final assembly

Introduction

The hood or bonnet is the hinged cover sheet metal part placed over the engine of motor vehicles. Hoods can open to allow access to the engine compartment in case of front engine cars, or trunk on rear-engine and some mid-engine vehicles. Hood is generally used to cover the powertrain and electrical components located in the front portion of the car. Hood has many functional and aesthetic requirements which have to be satisfied while designing it.

Components of a hood

Hood outer panel

The outer panel of the hood is the aesthetic structure which is visible to the customer and hence the look of the surface and the air flow (aerodynamics) along the surface of the outer panel are two main factors considered while designing it. The outer panel is attached to the inner panel by the aid of hemming and also mastics which are explained in detail further below.

Hood inner panel

The inner panel is the actual engineering structure which provides strength to the hood assembly. The inner panel is not visible to the customer during closed condition of the hood and hence appearance is not much of a deciding factor for the inner panel. Although, inner panel is the most important part of the complete assembly since it has a direct impact on the overall safety of the structure and also the direction in which forces are transmitted during collision along the hood.

Hinge assembly and reinforcement

Hinge assembly attaches the hood to the engine room with the help of fasteners .The hinge assembly helps the hood to open and close along a pivot axis which passes along the two hinge assemblies. The hinge assembly carries the entire weight of the hood and is a major structural member. For this reason it is provided with reinforcements to further strengthen its purpose.

Striker and reinforcement

The striker is placed over the hood & gets attached to the slot provided in engine compartment. Thus it is used for locking purpose of hood with engine compartment. The striker is provided with reinforcement to further increase its strength and prevent any chances of failure during rough operation of the hood.

Hood stays

Hood stays are generally a rod or a gas stay which connected to the engine bay area and serve the purpose of keeping the hood in open position. They must be capable of lifting the weight of the hood and also few external forces like vibration, wind etc. These are helpful in keeping the hood open during general inspection and also maintenance of the vehicle

Support rod

Supports rods are placed inside the engine bay are operated manually by the operator. One end of the support rod is fixed with the engine bay area while the other end can be hooked in the hood to keep it in open position without Falling

Gas stay

Gas stays are gas filled telescopic cylinders which require an initial action after which it can lift the hood on its own and can keep the hood in open position without any manual movement. It is mostly used in luxurious, heavy vehicles and also in vehicles with a long hood where manual lifting of the hood can be very tough.

Design factors considered for hood

Master section:

The master section provides us with the basic dimensions of the inner panels and its embosses.it also provides the distance of each surface of the inner panel from the outer class a surface of the hood. These dimensions must be strictly followed to ensure there is no interference between components during the final assembly process

Latch trajectory:

The latch trajectory is the path along which the striker moves during the movement of the hood. It provides us the opening and closing angle of the hood and also helps in the placement of the locking mechanism in the engine bay which will engage with the striker. The following steps are followed to create the latch trajectory:

First and foremost the hinge assembly is properly positioned on the hinge emboss created and is then mirrored along the centre plane
Two points are created on the centre of rotation of each hinge assembly
These points are now used to create a line which passes through the points. This line is known as the hinge axis
A circle is drawn from the midpoint of the hinge axis created and it is assigned a radius. On the other hand a horizontal line is created which passes through the centre of the striker hook
The horizontal line is aligned with the circle created to position the striker in its position.
The striker will now follow the created latch trajectory.

Safety

Safety is one of the most important criteria while designing automobile components and hood plays a major role in the safety of the occupants as well as the pedestrians. Keeping this in mind the hood inner panel is designed to ensure maximum possible safety can be achieved

During frontal collisions the hood must be able to absorb most of the impact forces and deform itself while ensuring the least amount of forces are transferred to the cabin of the vehicle. Embosses are provided in the inner panel which help deflect the forces acting on the hood towards the hinge assembly. This ensures that the most of the impact force is transferred toward the structure of the car so the occupants in the cabin do not experience it, thus decreasing the chance of injuries and fatalities

Pedestrian safety during frontal collision is also a very important aspect to be considered during the hood design process.

Pedestrian are most vulnerable road users and the chances of fatalities due to head injury from collision with vehicles are very high. The pedestrian safety aspect has been given a lot of importance during the past decade and hence it becomes very important for manufactures to design the components in a way which can reduce the level of severity of injuries to pedestrians during the collisions while keeping the overall structure of the car very strong.

In our design we have provided cut-outs all over the hood inner panel. These cut-outs help reduce the impact during collisions with pedestrians and hence increase their safety

Wrap around distance

When a pedestrian collides with a car, the whole human body wraps around the front of the car, and the head impacts on the bonnet or the windscreen region in most of the cases. The distance at which the head impacts on the car from the ground is mentioned as Wrap Around Distance (WAD).

During the crash analysis, based on the Wrap Around Distance(WAD), two test areas will be created namely the Child head impact zone and the Adult head impact zone. The child head impact zone is between 1000 to 1700 mm WAD and the adult head impact zone ranges between 1700 to 2100 mm WAD

Design for manufacture:

Draft angle

Positive draft angle is provided over all the surfaces of the inner panel to facilitate easy separation of the part and tool and reduce any chances of damage to the part during ejection. In most cases a minimum draft angle of 7 degrees is required for sheet metal components to ensure no problems occur during the processing of the sheet metal.

Mastics

Mastic sealants are basically gluing agents placed between the inner and outer panels of the hood. They are initially in liquid state when applied and are solidified once they are heated in the in the hot furnace. Mastics help increase the stiffness of the area in which it is placed and it also helps in reducing the NVH.A single mastic sealant covers a diameter of 80 mm and hence based on this the positions for mastics are decided and placed along the emboss in the hood inner panel as shown below

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. In car part production, hemming is used in assembly as a secondary operation after deep drawing, trimming and flanging operations to join two sheet metal parts (outer and inner) together. The edges of the outer panel are folded and wrapped around the inner panel to join them together.

Hemming operation has a great influence on the surface and dimensional accuracy of the sheet metal and hence precise hemming operation is necessary to obtain the best possible finish. Hem reliefs are provided along the curved areas of the hood ensuring the hem is tightly secured along the edges without any damage due to stress caused near the curved areas.

Ergonomics

Ergonomics is one of the most important factors considered while designing automobile components. In case of a hood the opening and closing angle as well as the maximum height of the top portion of the hood from the ground are the major considerations taken before starting hood design. It is easy very important to make sure that during the open condition the hood or the striker should not hit the head of the person who is operating it and hence it must be considered. These considerations are based on standards which defer from country to country and they must be adhered by the manufacturers while designing the hood

Material

Material selection is one of the toughest and most important criteria for any product since it has a direct impact on the overall cost, safety, manufacturability, recyclability and the weight of the final product. Car hoods are typically constructed from steel and sometimes from aluminium. Aftermarket car hoods may be constructed from various other materials like CFRP (Carbon Fibre Reinforced Plastic) which is very light in weight and a vastly used in material in the field of motorsports.

Aerodynamics

Hood has to be made with a very aerodynamic shape since it interacts with a large volume of air during car movement. This has a direct impact on the overall efficiency of the car and hence it is a very important aspect to be considered during the hood design

Styling

Aesthetic look of the car is enhanced by the hood and hence a lot of importance is given to it while designing to make sure all functional aspects are satisfied without disturbing the aesthetic looks. The aesthetic look of the hood is decided by the styling team and it may require some changes based on the feedback provided by the engineering team.

Commonly used sheet metal operations

Shearing

Shearing is performed by slicing through a piece of sheet metal with a blade that’s most often affixed to a tool or machine. The location of the cut is controlled by a squaring arm that allows a sheet of metal to be placed in a precise location.

Once the sheet metal is placed with the squaring arm, the top blade is lowered and compresses the metal against a lower blade, creating an exact cut.

Blanking

Blanking is a metal production process, during which a metal work piece is removed from the primary metal strip or sheet when it is punched. The material that is removed is the new metal work piece or blank. The metal strip is now the scrap.

Punching

Sheet metal punching is a cutting process in which material is removed from a piece of sheet metal by applying a great enough shearing force. CNC Punching can be used to produce holes and cutouts of various shapes and sizes

Piercing

Piercing is a shearing process in which raw metal is pierced with a machining tool, resulting in the creation of a circular or other shaped hole. As the raw metal is pierced, the metal from the newly created hole is considered scrap

Deep drawing

Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It can also be called as a shape transformation process with material retention. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter.

Embossing

Metal embossing is used to impart a design of different profiles upon metal sheets. The metal is pushed with an embossing tool or a punch in order to create a raised effect on the opposite side