Hood design-Week 2

HOOD DESIGN

Aim: To design car hood in NX using given master section and provided design parameters.

Hood / Bonnet –

Car hood/bonnet is a hinged cover that rests over the engine in a front-engine car. A latch is provided under the hood in order to prevent accidental opening during driving. Purpose of hood is to provide access to the engine and battery for repair and maintenance. It also protects these parts from direct sunlight and dust.

Design considerations :-

The human comfort factor for automobile design is first consideration. It is a method to safeguard and protect space for the human user and necessary components that make up the vehicle being designed. All safety regulations are ensured during this process. Ergonomical consideration consist of Hood clearance, Hood Opening angle and location of support rod or gas stay.

Hood opening angle is different for different regions or countries. It is decided by the average height of the human in that particular country.

The opening angle is considered important criteria because when the opening angle is less there is a chance of head injury to the human.

By varying the length of the support rod the opening angle can be varied.

Gas Stay is a mini version of the bicycle pump in which the compressed nitrogen is stored in the pump with oil and tightly sealed so that the gas can't escape. The gas allows to store energy which produced by the force acting on the piston and oil damps the movement of the piston and also lubricates it. When force is acted upon the piston, it compresses the gas, and energy is stored and when force is stopped acting on the piston, the stored energy is released moving the piston to its free state position.

Pedestrian safety:

Pedestrian safety is important consideration while designing the hood. According to the wrap around distance and pedestrian head impact zone hood is designed. Safety is the most important criteria in automobile designs which concentrates on both pedestrian and occupant safety. The pedestrians are the most vulnerable road users and are at high risk of traffic accidents. The major reason for the fatality of the pedestrian in traffic accident is due to the head injury resulting from the hard impact of the head against the stiffer hood or the ground. This leads to the necessity of new inner hood panel which is less stiffer and pedestrian friendly. The area at the head impact zone is made softer to reduce the impact force .

Wrap around distance: It is the distance from the road to the point where the head of the pedestrian collides with the hood.

In the below images we can see the wrap-around distance for adults and children.

• Components –

The car hood consists of following components.

1. Hood Outer Panel
2. Hood Inner Panel
3. Hinges
4. Latch and Striker
5. Support Rod
6. Reinforcements

1) Hood Outer Panel

The outer panel works as a metal cover. It is responsible for aesthetics of the vehicle.

2) Hood Inner Panel

The inner panel provides strength and supports the outer panel. It is where hood balancer and hinges are attached.

3) Hinges

Hinges allow the hood to rotate around a fixed axis for opening and closing movement. One part of the hinge is fixed with the engine compartment, while other part is connected to hood.

4) Latch and Striker

Latch and striker mechanism is provided to lock the hood, in order to prevent accidental opening of the hood during driving. The latch is present on the engine compartment while the striker is attached to inner part of hood.

5) Support Rod

It is a device that supports the hood in open position. It is also called as hood balancer rod.

6) Reinforcements

The reinforcements are provided to add material to the inner panel to provide extra strength to the hood to compensate for loss of strength near component installation regions.

Latch Trajectory:

It is the path along which the hood rotates during opening and closing movement. The striker on the hood follows a particular path while opening or closing the hood, which is along circumference of a circle with center at the hinge axis. This path is nothing but latch trajectory. It is a very important aspect in hood design, in order to decide hinge, latch and striker installation points.

Mastic Sealant:

Mastic sealant helps to locally improve strength of the hood. It is generally a hard rubber part, which is introduced as a paste in between inner and outer panel of the hood, which will get hardened inside the furnace. Every mastic sealant point will have a tendency to improve the local strength in around 80 mm diameter. Considering this, the mastic points are positioned such that most of the hood region is covered.

Hood Outer Panel with hemming-

Design Methodology:

The procedure followed for designing the hood and its components is as follows,

• First the inner panel is designed from the given master section using the various commands and features. We create the embossing using the offset surface from the original master section. Similarly, we create the cuts on the emboss by sketching and using trim surface option.
• Then the outer panel is designed from the same master section. Here we are supposed to provide the hemming to outer panel. The parameters of hemming are taken as per given values.

• Now, on the inner panel, the embossing for hinge and striker are produced by considering their respective dimensions and assembly positions. First the emboss for hinge is created and then by fixing the hinge into its position, we draw an axis passing through both side hinge axes to form a common axis. And then we position the striker such that it will move along circumference of a circle whose center is at midpoint of the common axis. Thus, position of striker is decided and we can create emboss for the same.
• Using the hinge emboss we created, we can create our hinge reinforcements by creating new parts in the assembly itself.
• Data Povided –

Outer Panel Thickness – 0.75mm

Inner Panel Thickness – 0.75mm

Reinforcement Thickness – 1.5mm

Important Features used in designing:

• Trim Body – It is used to trim one or more bodies using a face or plane. You can also specify the bodies to retain and the bodies to discard.
• Offset Surface – It allows you to offset one or more existing surfaces.
• Offset in Face – This command will allow you to offset particular edges or curves on selected faces.
• Trim Sheet – It is used to trim off a portion of a sheet body using curves, faces, edges or planes.
• Through Curves – It will allow you to create a body Through multiple sections, where it’s shape changes to pass through each section.
• Law Extension – This command extends a curve or edge to create a sheet body at desired angle length by changing the rules under angle law and length law.
• Trim and Extend – It helps you to trim and extend intersecting surfaces and make corner at their intersection.
• Face Blend – It creates fillets between faces.
• Edge Blend – It creates fillets on edges.
• Extend Sheet – It will allow you to extend an existing surface to required distance.
• Emboss – Emboss command creates an embossing by modifying a body with faces created by projecting a curve/section along a direction. You can modify the emboss using options like endcap location and draft angle.
• Projected Curve – It is used to project an existing curve/curves on a selected plane or surface by selecting a direction for projection.
• Mirror Geometry – As name suggests, it allows you to mirror geometries about selected plane or surface.
• Sew – It combines sheet bodies together by sewing their common edges. There shouldn’t be any gaps larger than specified tolerance.
• Thicken – It is used to convert a surface to solid body by providing thickness.
• Extrude – It helps you to create a feature from a sketch. You are also able to create thin feature using a sketch or curve under this command.
• Wave Geometry Linker – With this you can create, associative or linked copies of any entity of a part into other parts. It may be a surface, edge, curve, body, sketch etc. The changes made to the original entity will be reflected in the copy of that entity.

Sheetmetal Operations in Car Hood Manufacturing Process:

• Deep Drawing –

It 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. The process is considered deep drawing when the depth of the drawn part exceeds its diameter.

• Hemming –

Hemming is a forming operation in which the edges of the sheet are folded over other part or simply folded 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.

TYPES OF HEMMING

• Flattened/closed hemming
• Open hemming
• Tear drop hemming
• Rolled/rope hemming

Fig.12 Hemming process

Fig.13 Roller Hemming process in 3 Stages

• Embossing –

Sheet metal embossing is a stamping process for producing raised or sunken designs or relief in sheet metal. This process is carried out by means of matched male and female roller dies, or by passing the sheet or rolls of metal between rolls of desired pattern.

Fig.14 Simple Embossing with its Section view

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.

IMAGES

Conclusion: I designed the car hood along with its components and assembled the same, using NX CAD, under given parameters.