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AUTOMOTIVE HOOD DESIGN INTRODUCTION Hoods are the accessing areas for engine and other major components of the automotive. Some hoods are front flip opening type which is much more ease of access. In this hood design project I have given with the styling outer panel of hood. By using which the BIW (Body in White)…
ABISHEK G
updated on 01 Jun 2021
AUTOMOTIVE HOOD DESIGN
INTRODUCTION
Hoods are the accessing areas for engine and other major components of the automotive. Some hoods are front flip opening type which is much more ease of access. In this hood design project I have given with the styling outer panel of hood. By using which the BIW (Body in White) hood part inner panel is designed using the master sketch available. After developing the inner panel design, the embosses are given to the inner panel which adds stiffness, increases the strength locally and plays an important role in force dispersion. Actually the hood design is influenced by many factors, initially the styling team develops the styling panel/skin panel for the automotive and then we need to assess the technical design and manufacturing feasibility of the product. The inner panel is defined using its opening angle, predefined master sketch, safety standards, wrap around distance, hood side reference angle, etc. substantially. The inner panel consist of features like embosses, contour holes, mastic sealant, reinforcements, land area etc. Hemming is done to fold the outer panel over the inner panel. The hinges were placed on the sides of the hood. The latch and striker arrangement, an opening mechanism is positioned using the hinge axis method.
SOFTWARE USED
SIEMENS NX12
HOOD
Hood is also can be called as Bonnet in some countries. This part is hinged over the car and makes the frontal portion of the car accessible for repair and major maintenance. The hood is opened and fixed using the support rod, sometimes gas stay is used according to the requirement. The hood also associated with some accessories like wiper, hood scope etc. Materials generally used for styling panel is carbon fiber, fiber glass or dry carbon. The inner panel is mostly made up of Aluminum where steel also used. The hood design has main concerns like hood opening angle and pedestrian safety. The opening angle varies upon different country zones. The pedestrian safety is sewed in hood to avoid fatality rate of victim. The hood must be designed in such a way that it should disperse the force generated during a crash along sideways to hinge points. This case also depends on the relative speed of the vehicles and their inertial mass. All the forces can’t be dispersed from the leading edge but a major percentage of impact load can be derailed from travelling towards windshield.
HOOD OPENNING ANGLE
The foremost thing to consider as a hood designer is the opening angle of the hood. This is a very important consideration since this has to be comfort enough to open and view through the parts inside. This opening angle depends on the length of the support rod, either way first the opening angle has to be defined. Opening angle varies according to the country zones. If the opening angles mismatch then it will make the customer to lose ease of access.
HOOD STAY
This is the element that may be a support rod or gas stay type. The primary function of this element is to put the hood in a fixed opening angle. This stay should be rigid enough to support the weight of the hood, if not this will lead to serious injury to the person underneath it. The support rod doesn’t have any specific standard design for it. Therefore the designer has freedom over it, but it is important to make sure the design strength. The loads experienced by the support rod were compressive and bending load. It is also required to calculate the buckling load due to eccentricity of the load impressed. Some consumers prefer using gas stay. Support rod is much more cost effective than the gas stay. Then support rod mounting position has to be figured out. This can be either mounted on the inner panel of hood or at the engine side. The support rod might dangle when in open condition because of load and wind, this can be avoided by adding some distorted design. This little distortions might add strength to the support rod.
The support rod can’t have a curvature which is more than the 35mm from the mounting axis. By following this design standard the weight of hood won’t bend the support rod. If the support rod deviate more which consequently reduces its strength to hold the hood.
Gas stay
Support rod
The support rod is fixed using grommets while the gas stay were fixed using ball studs.
Grommets Ball studs
NVH CONSIDERATIONS
REINFORCEMENTS
The hood possess some added weights like latch and striker, hinge assembly on both sides. Normally when some extra weight is added to the inner panel it losses some strength locally. To compensate this loss the reinforcements were done. The reinforcement thickness might be like 1mm, 1.5mm which depends upon the loss that we get. This feedback has to be inferred from the CAE team. Stiffener pads were also use as a reinforcement.
MASTIC SEALANTS
The main purpose of using mastic sealant is to adhere the outer panel with the inner panel. This will reduce the NVH value majorly and simultaneously increases the strength locally. The mastic seal points have a vicinity of about 80mm. after passing the dry chamber the sealants get harder like a rubber, which is good in absorbing the vibrations.
HOOD REQUIREMENTS
MATERIAL SELECTION METHOD
Material selection foe bonnet can be done based on prioritization matrix. The functional requirements, the design options and the system considerations have now been formalized. Each element of the design may now be assessed for material selection using a simple prioritization matrix.
HOOD DESIGN PROCESS
The design involves the following
STYLING PANEL
This is the styling panel of the automotive. This panel just have the skin data for the inner panel. The thickness of this panel is 0.75mm. By using the master sketch of the panels the engineering feasibility of the given styling panel is interpreted. There were Inlaid hood and Wrap around hood. In Inlaid hood the end point of car lies on fender were as in wrap around hood the end point of car lies in hood. This makes some sensible difference in the side reference angle. The styling panel influences the following factors
BONNET SIDE REFERENCE LINE (SRL)
The Bonnet Side Reference Line is defined as the geometric trace of the highest points of contact between a straight edge 700mm long and the side of a bonnet, when the straight edge, held parallel to the lateral vertical plane of the car and inclined inwards by 45° is traversed down the side of the bonnet top and A-pillar, while remaining in contact with the surface of the body shell, any contact with door mirrors is ignored. Where multiple or continuous contacts occur the most outboard contact shall form the bonnet side reference line.
WRAP AROUND DISTANCE
Wrap around Distance lines created by wrapping a tape measure from the ground onto the vehicle in successive X-Z planes.
EURO – NCAP PEDESTRIAN SAFETY
For adult and child headform impacts, the EEVC WG17 test protocol proposes a maximum HIC of 1000 during an EEVC WG17 head impact zones 40-km/h impact. The head impact zones are defined by boundaries in the vehicle's longitudinal (X) and transverse (Y) directions.
Head injury criteria (HIC) - The head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact.
Impact tests
Head and legform impact
According to EURON-CAP pedestrian safety the impact zone has to be higher for having a better safety rating. The tests carried by EURO-NCAP were head impact test, upper legform to bonnet leading edge test and legform to bumper. The third most contact surface for head injuries were hood of vehicles.
Head injuries contacts surfaces
The deciding factor of higher impact zone in hood is the skin panel configurations. If side reference line is on fender then wrap around distance is more, consequently lesser impact zone oh hood. If the SRL is on hood then WAD is less which increases the impact zone. The zones were child and adult impact zones. The area (mm^2) of these zones were plotted using skin panel data.
Impact zone grid points Identification of headform grid points
Identification of upper legform and legform grid points
Therefore the grid points were defined using skin panel data.
INNER PANEL
The inner panel is the BIW part of the hood. This part is designed by interpreting the master sketch given for hood. The thickness of the inner panel is 0.75mm.
Master sketch
Master sketch interpretation – The distance between the inner and outer panel were 0.2 mm on both sides. There were three flanges to be designed. Flange 1 is 0.2 mm offset from the parent surface (skin panel). Flange 2 is 0.4 mm offset from skin panel. Flange 3 is 25mm from skin panel, this dimension depends upon pedestrian safety considerations. For hemming purposes inner panel offsets 15mm from the peripheral edges of skin panel. Flange 3 is having a width of 40mm, this dimension depends on width of the car. Engine is mounted directly underneath the flange 3. It is also important to make sure the drawability of the design. Therefore always the opening angle to be given more than 45 degrees. Draft analysis tool helps to check this condition. The blend radius has to be greater than the thickness of panels.
The 25 mm offset distance depends on under bonnet clearance.
Under bonnet clearance – This is the clearance in which the whole hood setup is moved 90mm towards the hard points of components. The impact deformation space is built under the styling panel. After offsetting the bonnet this space is being interferenced with the inner components of hood. These points are called hard points and they were critical in design. These points has to be relocated.
To reduce the number of red zones usually the designers will take off back some materials from inner panel of hood to get extra under bonnet clearance. Another technique to reduce the red zones were using the deformable mounting points. This system is collapsible during crash, hence this reduces the risk of hard points of interference.
Collapsible sections
After interpreting all the dimensions and features from the master sketch the inner panel is designed.
CREATING THE FLANGES AS PER THE MASTER SKETCH
Top view
Front view
CREATING THE EMBOSSES AS FORCE DISPERSION FEATURES
Front view
This clearly illustrates the force vector flow of the inner panel and majority of the forces have been deflected towards the hinges of the hood.
Top view
HEMMING THE OUTER AND INNER PANEL
Required hemming length = 0.75+0.2+0.75+0.2+0.75mm
Hemming the styling panel with inner panel Rear relief given after hemming
STRIKER
HINGE ASSEMBLY
HOOD ASSEMBLY
Front view
Back view
Top view
Side view
Isometric view
REINFORCEMENTS
Generally reinforcements are enabled for compensating the loss of material or the structure strength locally. The reinforcements can be given in different forms. Even the inner panel is a type of reinforcement for the outer panel. The thickness of reinforcements were about 1 to 1.5mm. Here 1.5mm thickness reinforcements used. This value of reinforcements have to pick up from CAE team as a loss compensation feedback.
The high lightened green areas were the reinforcements given. Reinforcement material can be rubber, steel, etc. stiffener pads were also used. These reinforcements were significantly helpful in reducing the NVH score.
POSITIONING STRIKER AND HOOD TRAJECTORY
The latch and striker has to be positioned tentatively, with the help of hinge axis taken. Whenever e swinging part is designed its trajectory and positioning of components have to be checked. The hinge is positioned on the emboss surface using assembly constraints. By using mirror assembly tool the other side of hinge is placed exactly same the before. Thereby sketch a hinge axis which is co-axial for both the hinges. Mark the middle point of the hinge axis. This hinge axis point is zero weigh point. Now the striker is already positioned in the pre-determined area, this idea of placing can be capitalized from a previous car. This method is going to validate whether the position is correct or wrong. Now the radius is taken as the distance between the hinge axis point and the striker midpoint. After drawing a circle two things can be inferred from the circle. The circle is the trajectory of the hood opening and closing. By drawing a line tangent to the point normal o the midpoint of striker and inferring whether the striker is locally normal to the tangent. If it possess local tangency then the latch and striker arrangement is validated to be correct.
MASTIC SEALANT
The main purpose of using mastic sealant is to adhere two surfaces, the outer panel and inner panel. Although a hemming joining process is performed the mastic sealant makes the two panels to adhere rigidly. This sealant makes the inner compartment of panel as air tight towards corrosive fluids. The sealants were applied with no dripping on the specified location. The applied vicinity of the drop is about 80mm. Therefore this sealing covers almost all the area between inner and outer panel. The sealant becomes rigid or like rubbery after coming out of the dry chamber for ED coating.
DRAFT ANALYSIS
This command helps in analyzing the tooling feasibility of the product. This draft analysis is generally done for molding, plastic and stamping parts. Our hood product involves more stamping process. Therefore it is mandatory to analyze whether all the draft angle are within the tolerance zone. The negative drafts can never be manufactured, which leads to rejection of design. The draft analysis command has four zones of criticality. In which the product design has to be in the green zone for manufacturability. The other zones indicate the negative draft present in the design. The design to be free from negative drafts, abnormal blend radius, etc.
Before drafting
After drafting
CONCLUSION
Thus the Hood design is designed using the class A surface and MAster sketch given.
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