Hood design-Week 2

SHEETMETAL DESIGNING OF HOOD OUTER AND INNER PANEL

AIM

In this project a Hood Assembly will be designed which would consist of hood outer panel, hood inner panel. The latch for the hood will also be placed at appropriate position considering that the hood should be opened and closed easily using its mechanism. The hinges will also be placed at appropriate position for the rotation of the hood inner panel.

 

INTRODUCTION

The hood or bonnet is the hinged cover over the engine of the vehicle. Hoods can open to allow access to the engine compartment, or trunk on rear-engine and some mid-engine vehicles) for maintenance and repair.

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. The hood is also prone to receiving scratches and dents. Although this might appear to be a minor problem, dents and scratches can lead to depreciation in the value of your vehicle. 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.

The first important criteria for designing the hood is to set an opening angle till which the hood panel can be opened. This opening angle is different for different countries which largely depends on the average height of a person in that particular country.

The movement of the hood can be controlled by either one of them:

1. Gas stay

2. Support rod

1. Gas Stay

A gas stay is a type of spring that, unlike a typical mechanical spring that relies on elastic deformation, uses compressed gas contained within an enclosed cylinder sealed by a sliding piston to pneumatically store potential energy and withstand external force applied parallel to the direction of the piston shaft.

 2. Support Rod

This is the most commonly used rod for opening and closing the hood at a certain angel. One end of the rod is hinged to the engine compartment and the other end is set free so that it can be locked to the hood inner panel when the hood is opened. This is usually used when the weight of the bonnet is small.

It is very important to consider that the support rod should not collide inside the engine compartment.

Padestrian safety NCAP (New car assessment program) standards:

Globally, various NCAPs are present that carry on the pedestrian safety tests on new vehicles and rate them accordingly.

1. Head Impact

This test is carried out for a vehicle speed of 40 km/h. To have an estimate of the potential head injury in the fateful event of a vehicle striking an adult or a child, a head form impactor is used in a series of impact tests and the impact sites are then assessed for and rated accordingly.
This test will give essential information about the deformation clearance, energy-absorbing structures and deployable protection systems such as pop-up bonnets and external airbags in a new vehicle.

2. Upper Leg Impact

The facility carries out the test at the vehicle speed of 40 km/h. An adult upper leg form impactor is used in a series of impact tests and then the impact sites are assessed. The protection is then rated as good, adequate, marginal, weak or poor. The test procedure allows to check the energy-absorbing capacity of the structures of a vehicle and also check whether the geometry of the vehicle allows mitigation of injuries.

3. Lower Leg  impact

Euro NCAP carries the test at the same speed standard as the above ones. It uses an adult leg impactor and a series of tests are carried out. Impact sites are then assessed and the protection rating is offered.

4. AEB Pedestrian

For this test, the NCAP uses a specially-designed pedestrian form which has articulated limbs to replicate the walking motion of a human. Cars are assessed on the basis of how forgiving the front-end design is in case of impact. AEB pedestrian tech may not be able to completely avoid a collision in some cases. The tests are done in the day as well as night to ensure vulnerable road users are protected all the time.

5. AEB Cyclist

For this test, Euro NCAP uses a specially designed cyclist form which will replicate the actual cyclist on the road. Euro NCAP offers the highest rating if the collision is completely avoided. The vehicle is also awarded if the speed is reduced as any reduction in speed of impact can significantly reduce the injury to the vulnerable user.

In order to measure the potential head and leg injuries in the front portion of the car illustration of crash mitigation tests is designed below:

A research was performed which shows the percentage of pedestrian fatalities by the age of vehicle that struck the pedestrian fron 2008 to 2018 is shown below:

Approximately 1.2 million road traffic fatalities occur each year (The World Bank, 2002). The number of pedestrians surviving vehicle impacts with injuries far exceeds the number of fatalities. 

The most common method of defining the injury severity in road accidents is Abbreviated Injury Scale (AIS) with a scale from 1 to 6 where 1 represents a minor injury, 2 moderate, 3 serious, 4 severe, 5 critical and 6 fatal injuries. Additionally, AIS 9 is used to indicate unknown status of injury severity. Distribution of AIS 2+ injuries from the Pedestrian Crash Data Study (PDCS) database in the U.S. shows that the frequency of injuries to the lower extremities and head dominates

Because injuries to the head often lead to death so head injuries are considered as the most serious injuries. Lower limb injuries are also important because they could lead to long-term impairments and result in high social costs.

The vehicle is marked by following the marking guidelines given by EuroNCAP (Euro NCAP Pedestrian Testing Protocol Version 5.2.1). The various marking lines used are bumper reference lines, bumper corners, hood leading edge reference line, hood
side reference lines and hood top WAD lines. 1,000-1,500 WAD lines mark as child head impact zone and 1,500-2,100 WAD lines mark as adult head impact zone. Head 45 impact speed of 40 km/h, child headform impact angle of 50o and adult headform impact angle of 65o is used.

The above table shows the EuroNCAP Injury Criteria

The above image shows the zoning of pedestrian head impact protection for NCAP

According to the requirements of the Australian New Car Assessment Program (ANCAP) for pedestrian protection, it needs to define zones of car hood for analysis, as shown in Figure 4. It can be noted that when the collision projection point locates between (Wrap Around Distance) WAD 1000 and WAD 1500, the head type will use the children head type. The adult head type will be used while the collision projection point locates between WAD 1700 and WAD 2100.

In 2012, EuroNCAP head injury assessment has been revised to be reported as overall performance score value (Euro NCAP Pedestrian Testing Protocol Version 6.0). Based on this new assessment criterion, Manufacturer must provide the head impact test data for all grid points tested and should be presented based on color scheme as:
Green HIC < 650 1.00 point
Yellow 650 ≤ HIC < 1000 0.75 point
Orange 1000 ≤ HIC < 1350 0.50 point
Brown 1350 ≤ HIC < 1700 0.25 point
Red 1700 ≤ HIC 0.00 point
A maximum of 24 points is available for head impact test zone. Total score of all grid points is calculated as percentage of maximum achievable score and then multiplied by 24. For e.g. if 190 grid points are tested which resulted into total points of 96. Then percentage achievable score is 96/190 = 50.5%, which multiplied by 24 gives 12.12 points.

From the above information we can conclude that pedestrian safety has become of the major factors while designing the hood and overall body of the car inorder to avoid accidents and deaths that happen on the roads.

NVH (Noice, Vibration and Harness)

Noise, Vibration and Harshness, and is basically a measure of how much unpleasant aural and tactile feedback the car delivers as you drive.

What does NVH actually mean?

The noise is what you can hear, the vibration is what you can feel and the harshness is how much of an effect thumps, bumps, noise and vibration have on the cabin and its occupants. Removing and isolating passengers from NVH is a field that's seen monumental advances over the past few decades. Originally the preserve of the most luxurious marques - and an acronym that was only even discussed by premium brands - NVH reduction is a global push, spurred on by consumers who want increasing insulation from the environment they're driving through. 

What causes NVH? 

Noise, vibration and harshness are caused by the car's mechanical and electrical systems, as well as the car's interaction with road surfaces and its passage through the air.

The main sources of NVH in a car are its engine, drivetrain and tyres, as well as the sound of airflow along its body.

Tyres are a huge source of noise, due to vibrations created by their rotation, which then travel along suspension mounts and into the cabin.

Low-profile tyres, found on sportier cars, tend to transmit more vibration through the suspension because the tighter, stiffer rubber can't absorb as much. Wide tyres are more susceptible due to a larger contact patch, but the soft, wide tread patterns of off-road tyres are the worst offenders.

Airflow over the car's body causes quite a bit of noise, especially at higher speeds and in large cars with the aerodynamic properties of a shipping container. Any interruptions the air faces in its journey across the car – such as aerials, roof racks and door mirrors – are rewarded with howling, whipping or buzzing noises in the cabin.

How to reduce NVH?

Cancelling out NVH comes down to three basic principles – reducing it at the source, isolating it from the car's main structure and absorbing as much as possible before it enters the cabin.

The first – if not the easiest – way to reduce NVH is to curb the number and volume of the sounds, vibrations and shocks produced in the first place.

The second and perhaps equally complex way to reduce NVH is to isolate the car's main structure from the source. Complex and expensive suspension tech such as independent, multi-link and hydro-pneumatic systems isolate the chassis and steering column from road surfaces, removing vibration and shocks.

To further isolate the sources of noise, engine, suspension and exhaust components are mounted on rubber and polyurethane bushes rather than solid metal; the softer compounds work against the transmission of vibrations.

The last resort in the war against NVH is also the most conventional: absorbing as much as possible. Mechanically speaking, this tends to refer to steering dampers, which absorb the worst of the shocks introduced by poor road surfaces.

Designing of the Hood using NX software

Input Data

1. Outer hood panel thickness = 0.75 mm

2. Inner hood panel thickness = 0.75 mm

3. Reinforcement thickness = 1.5 mm 

Master sketch that should be used to design the outer and inner hood panel is shown below:

COMPLETE VIEW OF THE HOOD CAN BE VIEWED THROUGH THE BELOW VIDEO:

Outer Hood Panel

For designing the outer hood panel the class A surface was provided as input and then outer edges were extend to form a flange which was designed in a manner which represents the hemming being done to hold the inner panel with the outer panel from the outside

At the corners reliefs was also designed so that the hemming operation can be performed easily.

Detailed view of the hemming cross section at the edges is shown below:

Hood Inner Panel

The inner panel of the hood acts as a reinforcement for the hood outer panel which provides strength to the outer panel. The thickness of the hood inner panel is 0.75 mm. In order to reduce cost and increase strength of the inner panel embosses are made on the sheetmetal along with the cutouts.

Emboss

Sheet metal embossing is a stamping process for producing raised or sunken designs or relief in sheet metal. This process can be made by means of matched male and female roller dies, or by passing sheet or a strip of metal between rolls of the desired pattern. It is often combined with foil stamping to create a shiny, 3D effect.

The metal sheet embossing operation is commonly accomplished with a combination of heat and pressure on the sheet metal, depending on what type of embossing is required. Theoretically, with any of these procedures, the metal thickness is changed in its composition.

Metal sheet is drawn through the male and female roller dies, producing a pattern or design on the metal sheet. Depending on the roller dies used, different patterns can be produced on the metal sheet. The pressure and a combination of heat actually "irons" while raising the level of the image higher than the substrate to make it smooth. The term "impressing" refers to an image lowered into the surface of a material, in distinction to an image raised out of the surface of a material.

In most of the pressure embossing operation machines, the upper roll blocks are stationary, while the bottom roll blocks are movable. The pressure with which the bottom roll is raised is referred to as the tonnage capacity.

The above image shows the embosses that were designed on the hood inner panel.

The sheet was trimmed in the middle portions of the embosses to improve the strength of the hood inner panel.

To attach the hood inner panel with the hood outer panel mastic sealer will be used, in the automotive industry the most important criteria while making any product is that the aesthetic part of the vehicle which is visible to the naked eye should not have any sort of irregularities of damages, considering this important factor mastic sealer will be used to attach both the panels.

The mastic sealant is droped through a machine on the mastic points and then the sealant gets spread around 80 mm in diameter when pressed. In this way the outer hood panel is attached with the inner hood panel.

The above video shows how the mastic sealer is dropped on the hood panel.

The assembly of the hood inner and outer panel is shown below:

 Both of them are attached together with the help of hemming operation which is performed on the edges of the hood outer panel.

The cross sectional view of the hood outer and hood inner panel is shown above along with a detailed view of the hemming region.

Procedure to place latch at proper position

The latch should be placed at appropriate position so that it does not become a obstacle while the hood is opened and closed. For this purpose it should be placed perpendicular to the hook.

To achieve the above condition a line will be made using the axis of the hinges on the opposite ends. About the mid point of this line a datum plane will be made and then a sketch will be on this datum plane.

Inside the sketch a circle will be made which will indicate the trajectory of the opening and closing of the hood and then a line will be made which will exactly position the latch at appropriate position.

A tangent line will be made on the circle which will touch with the previous line.

Another straight line will be sketched on the latch which would be perpendicular to the latch.

Now both the straight lines will be coincided with each other using the align feature in the assembly workbench. By doing this the latch will get arrested perpendicularly and will follow the the circular patch so that the hood can be easily opened and closed.

The above image shows the procedures performed to place the latch at appropriate position.

DIFFERENT VIEWS OF THE COMPLETE ASSEMBLY

1. Front view

2. Side view

3. Top view

4. Back view

5. Isometric view

Reinforcements for latch and hinges

The material that strengthen the existing material when adding to it are called reinforced material. As the latch and the hinges will be placed on the embosses so that strength at these position will be less and that portion can get easily damaged due to movements.

In order to provide strength at these places a 1.5 mm thickness reinforcements will be placed. Reinforcement will be a metal part spot welded into the emboss of the hinge and the striker. It can't be seen from the outside as it will be welded in the back portion of the hood inner panel.

The above image shows the latch and the hinge reinforcements that were used in the above project.

Manufacturing process involved in making different features of the sheetmetal

1. Deep Drawing

The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions. A flat sheet metal blank is formed into a hollow body open on one side or a hollow body is formed into a hollow body with a smaller cross-section.

Deep drawing processes are divided into three types:

• Deep drawing with tools
• Deep drawing with active means
• Deep drawing with active energy

In the automotive industry, deep drawing is usually carried out using rigid tools.

The deep drawing operations is shown above

The rigid tools consist of a punch, die and binder. In deep drawing, the plate holder closes after the metal sheet blank has been inserted.

Next the sheet is clamped between the die and the binder. This process slows down the flow of the sheet while it is being drawn and thereby prevents wrinkles from forming under the binder. The punch stretches the sheet over the die radius and forms it in the die. The amount of punch force necessary for forming is thereby continually increased up to the lower dead centre of the punch.

Whereas in pure deep drawing there is no reduction of sheet metal thickness, forming is achieved in stretch forming purely as a result of a decrease in sheet metal thickness. Stretch forming is extensively used for the forming of only slightly curved parts with low depth of draw (e.g. roofs, doors).

When drawing complex car body parts in practice, there is usually a combination of stretch and deep drawing involved. It is necessary that the sheet metal is stretched as well as possible without reaching the material’s limits (e.g. splits, wrinkles).

2. 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 a part and to reinforce part edges.

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. Typical parts for this type of assembly are hoods, doors, trunk lids and fenders.

There are various types of hemming operations:

• Conventional die hemming
• Roll hemming

Conventional die hemming

Conventional die hemming is suitable for mass production. In die hemming, the flange is folded over the entire length with a hemming tool. Normally the actual hemming is a result of a forming operation in which the flange is formed with a hemming tool after the drawing and trimming operations have been completed. The formed flange is then hemmed in several process steps. These steps include, for example, the pre-hemming and final hemming depending on the respective opening angle of the flange. Production plants for conventional die hemming are very expensive, but the cycle times are very low.

Roll hemming

Roll hemming is carried out incrementally with a hemming roller. An industrial robot guides the hemming roller and forms the flange. Roll hemming operation can also be divided into several pre-hemming and final hemming process steps. Roll hemming is very flexible to use and tool costs are significantly lower as compared to those of conventional die hemming. However, the cycle times are much higher since the hemming is realized using a hemming roller which follows a defined path.

CONCLUSION

The complete assembly of the hood outer and hood inner panel was designed as per requirement.

The latch and the hinges were also placed at appropriate position by following a latch trajectory strategy.