Hood design-Week 2

Aim: Design Hood outer panel, inner panel and the necessary reinforcements by following the Master section.

 

Description:

 

Given data:

Hood Thickness Information.

• Outer Panel Thickness = 0.75mm
• Inner Panel Thickness = 0.75mm
• Reinforcement Thickness = 1.5mm

Master section

 

HOOD:

A car hood, also referred to as a bonnet in some other countries, is the hinged cover that rests over the engine of a front-engine vehicle. Its purpose is to provide access to the engine for repair and maintenance. A concealed latch is typically used to hold down the hood. Hoods sometimes also contain a hood scoop, wiper jets, power bulge, and/or hood ornament. Car hoods are typically constructed from steel and sometimes from aluminium.

A car hood is meant to protect the performance parts of your vehicle, including the engine, while also providing easy access for repairs and maintenance. It is possible for the hood to incur damage as well. One of the most common types of damage is to the hinge, which is the device that connects the hood to the body of the vehicle. Over time, the hinge can become loose or may fall off completely. Scratches and dents can also lead to exposure to the elements that can result in rust and corrosion. This can be a serious problem that can ultimately expose the engine in your car to snow, sunlight, and rain. When this happens, it can impact the performance of your car's engine.

Car hood is divided into Outer and Inner panel:

Hood outer panel:

Hood outer panel is an ornamental feature that covers the inner panel with different kind of designs. It provides an aesthetic look for the car. The thickness of the panel varies from 0.65 to 0.75mm

Hood Outer panel

 

Hood inner panel:

Hood inner panel is designed in such a way that it not only strengthens the hood panel but also reduces the injuries of the pedestrians in case of any accident. The thickness of the panel varies from 0.65 to 0.75mm.

Hood Inner panel

  

Emboss:

Embossing is a forming process in which a high pressure is applied to a workpiece to change its surface. Embossing is done for providing the local strength and dissipating the force.

Embossing is a sheet metal forming process for producing raised or sunken designs or relief in sheet material by passing sheet between rolls of the desired pattern or by using matched male and female roller dies.

Embossing has two main functions:

• It provides the local strength.
• Helps to dissipate the forces.

Yellow colour lines indicate the force dissipation on impact.

 

Hinge & striker:

Hinge:

Hinges are placed on the flat emboss and helps in opening and closing of the hood. The design for the hinge changes from company to company.

Hinge on car hood

  

Striker:

Striker is fixed on the hood inner panel which get locked with the latch when the hood is closed.

Striker on car hood

 

 

Design standards:

The Hood outer and inner panel are designed in such a that it can satisfy the NCAP safety standards.

NCAP is abbreviation of NEW CAR ASSESSMENT PROGRAMME.

The safety standards are as follows:

 

Wrap Around Distance:

WAD (Wrap-Around Distance) means the distance from the ground to the point on the bonnet along the vehicle front structure.

Wrap-Around Distance

 
 When it comes to safety both the driver and the pedestrian safety is important. Driver has got safety within car (Airbags) but when the car hits a pedestrian the whole-body wraps around the front shape of the car & the head impacts on the hood to reduce the head injuries the hood should be designed according to the norms given by the NCAP. They set a range (shown in below figure) where the adult or child head may receive minimal injuries after an heat impact on the hood.

Wrap around of Human body	Head impact region for Adult and Children

 

 

NVH

The acronym is short for Noise, Vibration and Harshness. NVH applies both outside and inside the car.

For example, if you hear some squeaks in the cabin while driving or your rear-view mirror vibrates excessively, that’s poor NVH control.

It could be caused by something like bad fitment of interior panels.

If you feel serenely cocooned in a car, with nary a sound, then that car’s NVH is excellent.

Similarly, if a car drives by without a lot of engine roar, that cars got great NVH suppression.

There are many sources of NVH, such as:

• Air-conditioning (car’s HVAC system)
• Drivetrain
• Transmissions
• Tyres
• Brakes
• Electric motors on an electric vehicle (EV)

These sources of noise can propagate through the chassis and into the cabin.

  

Deep drawing:

Deep drawing Process

The deep draw forming process begins with metal blanks. At each step in the deep drawing process, the metal blank is shaped through pressure applied by a metal die.

Though deep drawing is similar to metal stamping, the terms are not interchangeable. Stamping does not leave a single machine until the metal has completely formed. In general, deep drawing is used to fabricate parts and products that are deeper than metal stamping.

The formability limitations of conventional deep drawing are a barrier for some industrial uses. Radial drawing stress and tangential compressive stress are a common concern that can result in wrinkling, fracturing or cracking in some applications. Numerous unconventional deep drawing techniques have recently been implemented that have helped increase the industrial uses of deep drawing. These processes include hydroforming, hydro-mechanical deep drawing, aqua drawing, hydraulic deep drawing, the Guerin process and the Marform process.

 

Hemming:

Hemming is kind of forming where the edge of the sheet metal is rolled over itself. Hemming is done for perfect.

Hemming is a metalworking process that involves rolling the edge of sheet metal over and onto itself, essentially creating an area of two layers. During this process, the edge of sheet metal is folded over. The two newly formed layers then lie flush with each other.

Sheet metal is available in a variety of gauges. The gauge, of course, refers to the thickness of sheet metal. High-gauge sheet metal is thicker than low-gauge sheet metal, which can make it difficult to manipulate using processes like hemming. With the right tools, however, manufacturing companies can hem sheet metal by bending and rolling the edge over and onto itself.

There are various types of hemming operations:

Conventional die hemming

Convectional Hemming

In conventional die hemming, the flange is folded over the entire length with a hemming tool. Conventional die hemming is suitable for mass production.  

Roll hemming

Roll Hemming

In roll hemming, the hemming roller is guided by an industrial robot to form the flange. Roll hemming is carried out incrementally with a hemming roller.

 

Procedure for Design of inner panel:

1 Given class-A surface is split into half and the after creating the features all can be mirrored.

Given Class A surface

 

2. The surface is off set for 1.7mm (to avoid intersection) and the boundary of the sheet is offset to a distance of 15mm. Below this the hemming is done. Further the sheet is offset to 2.75, 4.75 & 25.75mm accordingly and blends are applied in the sharp edges to reduce the stress concentration

 

3. Then the offset of the boundary is done for about 2mm using offset face in curve, so that the sharp edges can be blended. By which the stress concentration is reduced.

 

4. A profile for emboss is created and projected on the surface and using the emboss option the embossing is done. Blends are applied on the sharp edges to reduce the stress concentration.

 

5. Profile is drawn for the mastic sealants and they are then projected onto the inner panel surface and trimmed using the ‘Trim sheet’ option.

 

Hinge & striker placement:

a. Hinge placement:

Procedure:

1. A flat emboss is created on the corner of the hood inner panel for placing the hinge.

 

2. After creating the flat emboss the hinge is given an assembly constraint, ‘Touch align’ with the emboss on the inner panel.

 
3. Then, using ‘mirror geometry’ option entire hinge with the constraints applied is mirrored.

 

4. Mastic sealant:

A mastic sealant is a type of liquid sealant that cures in an elastic state, thus making it flexible while holding the bond of the surfaces that has attached together. It adheres to just about any material making an all-purpose type of sealant. Wood, aluminium, glass, marble, and even duct board are all standard applications for mastic sealant. This helps the inner and outer hood panel to join together.

The adhesive effect of each mastic is around 80mm diameter, So the position for the mastic should be done in such a way that it should exhibit more strength.

A profile for sealant is drawn and projected on the top emboss and trimmed using the 'Trim sheet' option.

The circle drawn shows the effective area of the sealant.

 

 

5. The hinge axis should be parallel to X-axis, as the hinges are placed on the flat emboss and a point is created at the centre of the hinge shafts. Then a line drawn connecting both the points.

Hinge Axis (Blue line)

 

6. The entire surface is sewed and thicken of 0.75mm is applied for inner panel.

Thickned hood inner panel.

 

b. Striker trajectory:

Procedure:

1. The position of the striker can be decided by using the hinge axis. A circle is created with the hinge point as centre. This circle will be the trajectory for the striker. After drawing the circle, two lines are drawn,

• one at an angle: This is angle of hood opening.
• Second, tangent to the circle: This one will be in contact with the striker & makes the striker move according to the line which is place at some angle.

 

 

2. A line is drawn at the centre of the striker and is made coincide with the line which is tangent to the line tangent to circle. 

Striker	Strike line to be coincided

 

 

3. Finally, emboss for the striker is created.    

Emboss for Striker	Striker alligned with the emboss

 

Procedure for Design of outer panel:

1 Given class-A surface is split into half and the after creating the features all can be mirrored.

Given Class A surface

 

 2. The surface is offset for 1.7mm (to avoid intersection) and the boundary of the sheet is offset to a distance of 6mm using ‘offset face in curve’ option and then required part is kept and the remaining is trimmed off using ‘Trim sheet option’.

 

3. Then the offset of the sheet boundary is done for about 0.5mm and then a surface is created using ‘Law extension’.

 

 4. Using the above three surface a face blend is done with 3-face option and finally a proper blend is obtained.

 

5. Reliefs are provided over the hemmed portions to avoid part deformation and tearing.

Corner relief	Front relief

 

6. The entire surface is mirrored and sewed and thicken of 0.75mm is applied.

Outer panel thickned.

 

Reinforcement:

Reinforcements are added to reduce the impact and provide local strength.

Hinge reinforcement plate	Reinforcement palced on inner panel.

 

Finally, assembly of the hood panel with other features is done by providing the appropriate assembly constraints.

Final Assembly	Exploded view of hood panel

 

Conclusion:

Hood inner and outer panel are developed according to the master section.

Reinforcement plates are fixed.

Hinge axis and striker trajectory are defined.