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

 

Hood Thickness Information

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

 

 

Outer Panel :-

We get the outer panel skin surface from style department with proper aesthetic look, gap etc. The shape of the Outer panel is made aerodynamic in order to minimize the air effect. Shape of Outer panel is reffered as a base to design a inner panel.

 

                                 

 

 

Inner Panel :-

Hood Inner panel is main functional part in hood design. It gives structural support and strength to the outer panel of Hood. The outer and inner panels of the Hood are typically attached together using a hemming operation at the edges.

 

                               

                 

                                

 

                               

Latch and striker :-

This system is the attachment of hood assembly and vehicle body. The hood latch system must be able to catch the striker of the hood upon closing and instantaneously, as well as automatically, lock. Once in the locked state, it must remain locked and only unlock upon the operator’s control. 

 

                                      

 

 

Hinge system  :-

The hinges are used to guide and hold the hood when open & close. Hinge assembly consists of two parts one is connected to the hood inner panel and second is connected to the vehicle body.

 

                             

 

 

Reinforcements :-

Reinforcement are provided mainly in hinge and latch region. When hinge and latch assemblies are attached to inner panel, strength of that area reduces locally so we add some additional material to increase the strength of the area locally. Thickness of reinforcement material is provided according to the design requirements. In this project, 1.5mm thickness has been given to both latch and hinge reinforcement. 

 

                             

                 

                             

 

 

Latch Trajectory :-

Circle is drawn from the hinge axis center so we can finalize the position of latch striker. For proper mating, the tip of the striker should be tangential to the latch trajectory. the position of latch striker follows the path of this circle.

                             

 

 

Emboss definitions :-

Embosses are provided to increase the strength of inner panel. Shape of embosses also determines the direction of collision force distribution. the shape of the embosses should be designed in such a way that, the collision forces should be directed towards outward side of the car (towards hinge side).

                     

                            

 

 

Force distribution :-

 

                             

 

 

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 :-

The manufacturing of deep, cuplike products from thin sheet metal is known as deep drawing. The process involves a punch with a round comer and a die with a large radius. The punch-die clearance is slightly larger than the thickness of the sheet metal to be deep drawn.

As the load applied through the punch, the sheet metal is forced to flow radially and sink into the die cavity to form a cup. The process is best suitable for complex shaped problems.

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 creates a seamless effect on metal, allowing for the creation of a smooth, curved edge. Deep drawn metal parts are created using a single metal blank, not a repeated course of metal blanks as is typically the case with traditional metal stamping.

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.

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).

 

                                        

 

 

 

Piercing :-

Piercing is a shearing process in which raw metal is pierced with a machining tool, resulting in the creation of a circular or other shaped hole.

                                    

 

Hemming :-

Hemming is a sheet metal forming process in which sheets are joined by bending it usually to 180°. Automotive body panels and automotive parts..

This assembly method is widely used in the final stages of production of automobile hoods, doors, or other closures for aesthetic purposes.

It 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.

The typical three-step bending process of the hemming operation comprising of the flanging, pre-hemming, and final hemming process.

 

                  

 

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.

 

               

                         

 

                                 

 

 

Corner relief :-

                         

 

 

Final assembly :-

                         

 

                     

                         

Conclusion

Thus, the hood is designed considering passenger and pedestrian safety as per the master section is successfully complete.