Hood design-Week 2

                                                                               Design of a Car Hood

Aim:

To design a car hood inner panel and the outer panel and to assemble the hinges and the striker to the inner hood as per the trajectory.

Abstract:

A styling surface has been provided in the challenge with which the outer hood panel and the inner hood panel are to be designed. Both the hood panels are to be designed in such a way considering the manufacturability, openings for assembling the components and the safety standards. 

Introduction:

The car hood system is an access panel to the engine compartment to enable maintenance of the power train, drive belts, battery, fluid levels and lamp units. It is fundamentally a reinforced skin panel with many safety and quality requirements. The car hood is manufactured such that it is strong and less in weight so it protects the engine components and the passengers in the vehicle in case of an accident.

                                                           Fig 1: Car hood in real life.

 A car hood is made up of the following components:

(i) Car hood Outer panel.

A car outer panel is a sheet metal that is shaped into the required styling surface. Usually, the thickness of the outer hood is approximately 0.75mm. The outer hood is shaped using the hydraulic press and stretch forming process and the edges are bent to hold the inner hood panel with the help of hemming process.  

                                                                         Fig 2 : hood outer panel

(ii) Car hood inner panel:

A car hood inner panel is usually manufactured in such a way it has a patterned holes structure for the following reasons shown below.

(i) To reduce some weight of the hood.

(ii) A patterned hole structure provides stiffness to the hood panel.

(iii) The pattern is designed in such a way that the force acting on the hood panel in case of an accident will be distributed or directed towards the hinges attached so that to reduce the impact on the passengers inside the cabin.

(iv) For the easy escape of water particles from the hood.

The inner hood panel is manufactured as done for the outer hood panel and the patterned hole structures are made using a deep drawing or blanking process.

                                                                       Fig 3: Hood inner panel

(iii) Hinges:

The hood hinge is a pivot and bracket mechanism that makes it possible to open and close the hood. 

(iv) Striker:

Striker in the car hood is hook structured component which helps in looking the hood panel to the bumper structure of the car with the help of latch mechanism.

                                    Fig 4: Striker                                                                    Fig 5: Latch

(v) Hood Support rods or gas stays:

Hood support rods are the parts used for holding the hood panel when opened while accessing the engine. These support rods are structured in such a way that has enough stiffness and strength for holding the hood in position. 

Few cars with longer or heavier hoods will be having gas stays so that a user can easily lift the hood as half of the load is already beared by a gas stay.

                        Fig 6: Support rod.                                                                                              Fig 7: Gas stay

Manufacturing Process:

A sheet metal undergoes the following manufacturing process to form a car hood component.

(i) Blanking:

Blanking is a steel manufacturing process in which a flat, geometric shape is created by feeding a sheet metal into a press and die. In this process, the blank is punched out from large sheet metal.

                                                             Fig 8: sheet metal blanking.

(ii) Stretch Forming:

Stretch forming is a metal forming process in which a piece of sheet metal is stretched and bent simultaneously over a die to form large contoured parts. Stretch forming is performed on a stretch press, in which a piece of sheet metal is securely gripped along its edges by gripping jaws.

                                                                Fig 9: Stretch forming process.

(iii) Punching:

The punching process of sheet metal is to remove the unnecessary or excess sheet metal out from the required sheet metal. The punched out or trimmed metal known as slug is then removed in scrap.

(iv) Deburring:

The deburring process is a smooth finishing process of sheet metal that removes the rough finishing of the sheet metal known as burrs The burrs on a sheet metal are dangerous causing injury and may reduce the life of the component.

                                                            Fig 10: deburring process.

(v) Hemming:

Hemming is a process used in industry to join inner and outer closure panels together (hoods, doors, etc.). It is the process of bending or folding the flange of the outer panel over the inner one. The accuracy of the operation affects significantly the appearance of the car’s outer surfaces and is therefore a critical factor in the final quality of a finished vehicle.

                                                                         Fig 11: Roller Hemming process

Design Considerations as per the safety standards:

The car hood panels are designed to be safer not only for the passengers inside the car but also considering the safety of the pedestrians in case of an accident. A vehicle-to-pedestrian crash generally starts with the bumper-to-leg contact, followed by the impact between the hood leading edge and the upper thigh or pelvis. Depending on the relative heights of the pedestrian and the vehicle front, the hood leading edge can hit the abdomen or chest as well. During the impact, the pedestrian often wraps around the front of the vehicle until the head and upper torso are struck by the top surface of the hood and/or windshield. The final impact in a pedestrian crash is almost always caused by the ground, but most pedestrian injuries occur due to the interaction with the vehicle. Despite what seems to be a well-defined sequence of events, the precise trajectory and injuries of a pedestrian depend on many factors, including pedestrian 4 factors (age, height, etc.), crash factors (impact speed, impact angle, etc.), and vehicle factors (vehicle type, stiffness, curvature, etc.). As per the EuroNCAP safety standards, the pictures shown below gives some of the considerations about the frontal crash of the car with a pedestrian.

                Fig 12: Wrap-around distance                                                      Fig 13: Impact during a pedestrian accident.

Procedure of designing the car hood using CATIA software:

1. Considering the given master sketch for the hood design, create the inner hood from the given styling surface.

                                                              Fig 14: Given styling surface.

2. Create the geometry and the embosses in such a way that the sheet metal should direct the forces applied towards the hinges when an impact occurs in case of an accident. The embosses are created for strengthening the hood panel. Another importance of an emboss is for providing relief for the sheet metal while manufacturing process.

                                                                      Fig 15: design of the inner hood.

3. Provide clearance for the reinforcements of hinges and latches which are to be created with reference to the trajectory lines.

4. Now as per the master sketch create flanges for the outer hood panel such that it holds the inner door panel by hemming process. While creating flanges, provide clearance on the edge curves of the hood up to 3mm so that for a good finish of the sheet metal while hemming process. Maintain the clearance of the flange as shown below.

                                                     Fig 16 & 17: Providing clearance while hemming process.

5. Now create the reinforcements for the hinges. The reinforcements on the sheet metal should be flat irrespective of the inner hood panel, for easy rotations of the hinges as shown below.

                                                    Fig 18: Providing reinforcements for hinge attachment.

6. Using the assemble workbench import the hinges given for the challenge and offset constraint the reinforcements and hinges as shown below.

                                                                     Fig 19: hinge attachment.

7. Now the striker is to be installed to the hood as per the trajectory curve. The procedure for creating a trajectory curve is as shown below:

a. Create a reference line connecting the centres of the hinge connections as shown below.

                                                         Fig 20: created reference line for trajectory curve.

b. Create a midpoint on the created reference line such that it becomes the centre of the trajectory curve.

                                                                         Fig 21: midpoint creation.

c. Now create a reference line on the tip of the striker such that it’s parallel with reference to the striker rod connecter.

                                                                   Fig 22: striker reference line

d.  Now create a trajectory curve by creating a circle from the created midpoint of the line to the reference line of the striker.

                      Fig 23: Created trajectory curve                                    Fig 24: Adjusted the sticker as per the reference line and trajectory curve.

8. Now adjust the striker’s height and angle such that the reference line of the striker and the trajectory curve are touching each other parallel as shown below.
9. Now fix the striker to the inner hood with the help of reinforcements as shown below.

Mastic Data:

Mastic Data on the hood inner panel are the references used for attaching the outer panel to the inner panel with the help of mastic sealants. This process is done so that both the surface panels can be attached together without damaging the smooth finish of the sheet metal.

                                                              Fig 25: Hood inner panel mastic data.

There are totally 50 mastic data provided for the inner panel.

Result:

Therefore designing the car inner and outer hood panels, assembling the hinges and striker to the panel as per the trajectory and pricing the mastic data for the inner panel is completed as per the safety standards.

                                                                    Fig 26 & 27: Design for the car hood.