Hood Design
Design of hood or bonnet:
A hood or bonnet of a car/vehicle is one of the most important parts in designing BIW. It is designed as a closure to the engine compartment but has many other jobs. A hood provides protection and easy access to the engine compartment components such as radiator, engine, battery, alternator and other electrical systems etc. from dust, stones, water, snow, sand, etc. It is crucial design component in terms of pedestrian collision and safety. It is also responsible for dissipating maximum energy in front impacts.
A hood generally consists of two sheets of metal joined together. An outer panel or skin sheet which is for aesthetics importance is made to look appealing with lines and design features to flow along the whole vehicle. The inner panel however is purely structural component which is hidden below the outer panel. Its design purpose it to provide strength and rigidity to the outer panel.
For the given parameters and master section sketch a Hood was designed.
Hemming :
The outer panel as mentioned in the aesthetic part of the hood and needs to have clean surface hence any kind of welding is not an option. Hemming is used here to join the inner and the outer panel together. It involves folding or bending an overlap of the outer panel around the inner panel with predetermined clearances.
Hemming is done using press and moulds or hemming rollers with help of robots. To prevent intersection of outer panel along corners a relief is provided which is basically a shorter overlap.
As per the given parameters, a rear relief of 29mm. and a front relief of 200mm. Was provided.
Inner panel Design:
As stated earlier inner panel being the structural component of the hood is crucial to design. It usually has symmetric features for better manufacturability, and intricate parts such as embosses, cavities, slits, etc.
As per the hemming parameter clearance between the panels is maintained at the outer boundary. Generally a 2mm clearance is maintained between the outer panel and the uppermost face on the inner panel design.
Inner panel reinforcement:
Generally the thickness of the inner and outer panels vary as per the vehicle it is designed for in the range of 0.75mm – 2mm. Local reinforcements are provided wherever needed instead of a thicker panel to keep the weight and cost low. Liable areas such as hinge fixture area, latch fixture area, gas stay or rod fixture area, where the structure is weak due to holes and other fastenings, a thicker than usual panel is attached to the inner panel in the particular area to improve rigidity and strength.
In the given case the thickness of both the panels is 0.75 mm each. A 1.5mm thick reinforcement is introduced at the latch and hinge attachment area.
Positioning of Hinge and Striker:
Hinges help with the opening and closing of the hood as it constrains all other motions and allow the hood to only rotate in the hinge axis. This makes the positioning of hinges an important aspect in hood design.
Striker is the part of latch arrangement. Striker is the part which actually locks the hood in closed position.
The positioning of the latch is usually defined using the data from the parent car or by the design calculations. The important point here is the relationship between striker point and hood trajectory. It is essential that the striker point is perpendicular to the hood trajectory for better fit and finish. The trajectory of the hood is traced by drawing a circle with radius equal to the distance between striker point and hinge axis centre. The hinge axis centre is adjusted by tentatively placing the hinge assembly to meet the condition.
Mastic Data:
Mastic sealant is used as sealant and adhesive to join the two panels together on the inner region. It is a rubber like material which hardens during the baking process. It seals and joins the panels and also helps keeping the NVH low. The effective area of the sealant is within 80mm. diameter circle and are to be placed statistically. Usually it is applied where the clearance in constant between the panels.
In the given assignment to get a constant seal the mastic seal is provided on the edges of the cavities on the uppermost face of the inner panel. A constant gap of 2mm. is maintained between the panels to achieve constant seal.
Emboss Data:
Embossing is drawing out parts of sheet with the help of male and female dies or moulds and press. A flat sheet of metal is weaker than an embossed sheet as it improves the stiffness. Due to the added stiffness the panel is less prone to vibrations and is rigid than a flat sheet panel. A well designed embossed panel with pathways can help divert forces in case of front impact towards the sides and hinges of the hood which results in a safer design. For better manufacturability, a suitable draft angle is provided to the embossed portion. A well designed emboss pattern could improve the vehicle in terms of safety but may increase the cost due to poor manufacturability hence a balance between the two is to be found in designing phase.
In the given assignment emboss was provided to get the pattern such that the mastic seal points can be maintained at a constant gap of 2mm. from the outer panel. Cavities and V-shaped pathways are formed to divert the forces towards hinges and sides of the panel using emboss operation. The pattern was made symmetric with a draft angle of 5 degrees for better manufacturability.