Hood design-Week 2
Aim :- To design a Hood outer panel, inner panel and the necessary reinforcements by the given Master section and design parameters.
Theory :-
Hood is a main component of front portion of a car. It is an opening body panel that covers the engine in front engine vehicle. It consist of an outer panel and inner panel. The inner panel provides the strength while the outer panel functions as the metal cover. It is an access panel to the engine compartment to enable maintenance of power train, drive belts, battery, fluid levels and lamp units.
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
. 
There are two types of hood stay, support rod and Gas stay which holds the position of hood at desired angle.
1) Support rod :-
It is a simple metalic rod. It is not usually made linear to increase rigidity of rod but the length from line connecting support rod retaining point with retaing point on body side to maximum bending point should be less than 35mm to ensure the strength of support rod.
It should have sufficient clearance with other engine components to avoid rattling noise.
Gourmets are provided to keep the support rod in rest condition.
Advantages of support rod are as follows
- They are economical
- Light in weight.
Disadvantages of support rod are as follows
- When striking with the automotive body panel to leave scars which are very likely to cause rust.
- The metal rod is often too hot to be gripped by hands immidiately after the vehicle has been stopped.
2) Gas stay :-
It is a device used for supporting the engine hood of a vehicle in its open postion by means of an actuator which is activated by gas.
In case of gas stay only intial force is required to open the hood then it automatically opens at desired angle with the extension of pneumatic piston in cylinder of gas stay assembly.
Advantages of Gas stay are as follows
- No human effort is required
- Easy to operate
Disadvantages of Gas stay are as follows
- They are heavy in weight compared to support rod.
- Cost is more.
Safety requirements :-
The most important safety requirement is regarding the frontal collision.
It should withstand in high collision forces as well as it should divert the forces in outward direction so intensity of collision forces would be reduced to cabin side of car.
Pedestrian safety :-
Design should be such way that it should reduce the injuries to the pedestrians in the event of a pedestrian vehicle collision. The objective of these measures is to reduce the number of road accident fatalities and the severity of injuries sustained by pedestrians involved in a collision with a vehicle in urban traffic.
The height, weight and age of the pedestrian all play a role in the kinematics of the event and in his ability to survive. Most fatalities for the younger population are related to brain damage caused by head impact on the bonnet. For older people additional risks include rupture of arteries in the lower limbs and pelvis from bumper and bonnet leading edges.Impact frequency and seriousness of injury has been studied for many years, resulting in rating systems and improved design.
Evaluation of pedestrian safety for a bonnet must be carried out in the context of its surrounding elements like Vehicle styling, size (wrap around distance “WAD”) and under bonnet clearance to other elements (considered as hard points). Local bonnet stiffness is influenced by mounting point stiffness such as hinges, bump stops and latches. This is better understood by superimposing the kinematics of a dummy onto the test conditions.
Wrap Around Distance (WAD) :- It is the geometrically traced distance from the contact point with the Ground Reference Plane, vertically below the front face of the bumper, to any point on the vehicle front structure.
Side Reference line :- The geometric trace of the highest points of contact between a straight edge, held parallel to the lateral vertical plane of the vehicle and inclined 45 deg. is traversed down the side of the front structure, and the side of the front structure.
Child Head Impact Zone :- 1000 WAD to 1500 WAD
Adult Head Impact Zone :- 1500 WAD to 2100 WAD
Besides functioning as an engine compartment cover, the hood of modern vehicles can also help manage the impact energy of a pedestrian’s head in a vehicle-pedestrian impact. However, a hood’s ability to absorb impact energy may be impeded by the proximity of the hood to components packaged inside the engine compartment, i.e., by its underhood clearance.
For example, for a given hood design, the hood’s ability to absorb impact energy through deformation can be significantly reduced when the hood and engine block are in close proximity.
Therefore, a large underhood clearance would be preferred for pedestrian protection.
The predicted level of protection offered by the vehicle is verifiedby EURO-NCAP.
Design
Master section :-
Master section is a predefined sketch which will be adaptable for different situations.
Four main parts of Hood assembly are as follows
- Outer Panel
- Inner Panel
- latch and striker system
- Hinge system
Design parameters
Hood Thickness Information
Outer Panel Thickness = 0.75mm
Inner Panel Thickness = 0.75mm
Reinforcement Thickness = 1.5mm
Components
1.Hood outer panel
Outer panels are the aesthetic elements of a car. It is the styling team who provides the skin for the outer panel. The shape, position and size of the hood should satisfy the safety norms. So while designing the hood we should give more importance to pedestrian safety. The outer panel is attached to the inner panel by hemming. So, it is necessary to provide required hemming flange and reliefs for the hemming operation. We also provide mastic sealants between outer panel and inner panel to reduce vibration.
2.Hood inner panel
Inner panel is the part that gives strength and rigidity to the outer panel. The master section represents the initial emboss shape and size. Then we provide the emboss for the latch and hinge elements after fixing the location.
3.Reinforcement
Reinforcements are provided according to the design requirements. Mainly it is provided at the hinge and latch regions to improve the strength locally. Usually thickness of reinforcements will be higher than that of the inner panels and the value varies according to cost and requirement. According to the design requirement here it is given 1.5mm thickness to both hinge and latch reinforcements.
Latch Trajectory
Latch trajectory is given by the circle drawn from the hinge axis point. Hinge axis point is the centre point of the hinge axis. First, we can fix either the latch or the hinge. If we have fixed the hinge position then we should plot the Trajectory circle for the latch. For proper mating condition the striker axis through the tip of the striker should be tangential to the latch trajectory.
Emboss
Throughout the inner panel we provide different embosses. We can increase the stiffness of the inner panel through these embosses. The shape and size of the embosses also determines the direction of the force distribution resulted from frontal collision. The force should be always distributed away from the passengers. Here in the figure the embosses are designed in such a way that the force is directed to either sides of the car.
Mastic data :-
Mastic sealants used between the outer and inner panel. mastic sealants are the rubber like elasic compound which connects the outer and inner panels of car hood at specified points. Mastic sealants are used in the region between the inner and outer panel because other joining process like welding or riveting are not good for car asthetic design. It also helps in improving NVH (Noise, vibration, and harshness) characteristics.
Mastic salant points are effective around a circular region of 80mm diameter.
Manufacturing Aspects
The most important design consideration of any component is manufacturing aspect. In sheet metal, the component is manufactured by press tool dies. The Press tool is stamping which includes cutting operations like shearing, piercing etc. and forming operations like bending, Deep drawing, hemming etc. The main focus of manufacturing aspect is to manufacture hood assembly with minimum operation sequence and process time.
Deep Drawing
Deep drawing is one of the most widely used processes in sheet metal forming. Apart from its use in many other sectors, it is applied in the automotive industry for the manufacturing of car body parts. The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions.
Sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing the part through a series of dies. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles. Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.
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.
The accuracy of the hemming operation is very important since it affects the appearance of the surface and surface quality. Material deformations, which occur during the hemming process, can lead to dimensional variations and other defects in parts. Typical hemming defects are splits or wrinkles in the flange, material overlaps in the corner areas or material roll-in. This is why it is important to provide required reliefs at the corners.
There are two types of hemming operations,
- Conventional die hemming
In conventional die hemming, the flange is folded over the entire length with a hemming tool.
- Roll hemming
It is the roll hemming process that is used for automotive sheet metals. 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.
Corner relief:
Final Assembly
