Challenge

Hood design-Week 2

AIM:To design a hood using nx

Project is about to Design a Hood that satisfies the manufacturing requirements, functional requirements as per Euro NCAP standards.
The Body in white (BIW) Inner panel and Outer panel of the hood must be designed and assembled with the Hinge Assembly, Latch and Striker with their reinforcements.

The Hood is an opening body panel that covers the engine in a front-engine vehicle in some cases like rear engine vehicles it acts as the cover of the boot space.
The hood is also considered as another type of door in a vehicle as it consists of an outer panel and inner panel.
The inner panel provides strength while the outer panel functions as the metal cover.
Hoods also play a major role in the aerodynamics of the car, thereby reducing the drag and increasing the downforce of the car.
Hood also helps to direct impact of the any head-on collisions away from the passenger cabin there-by reducing the damages to the passengers.
These Hoods are designed in such a way that less damages are caused to the pedestrians during accident.

There is a Master Sketch for every region which would be adoptable on different situations which gives us the dimensions of the part to be designed. The inner panel including all its embosses are designed by offsetting the outer panel. The maximum gap between the inner and outer panel in the master sketch is 25mm, under which the Engine compartment is situated. This specification is given taking into consideration pedestrian safety. Process of Deep Drawing is used in manufacturing the inner panel and the design considerations are kept in mind for the same.

Whenever the frontal crash takes place in a car the first part which will get affected will be hood. So, hood must be designed in such a way that it should not come off from the body of the car and must absorb the energy as far as possible and diverting the force towards the side panel of the car so that the amount of force which is sent to the cabin is lesser since the force is lesser the passengers will experience less amount of force so that passenger safety is achieved.
Hood inner panel plays major role in dissipating the force away from the passengers. It is completely depending upon the emboss which is created in the inner panel. A typical emboss is shown below. The force will completely move along the direction shown above minimising the force exerted towards the cabin.

The most important safety requirement to be considered while designing the Bonnet is the Pedestrian Safety from a frontal collision
The European New Car Assessment Program (Euro NCAP) is a European car safety performance assessment program which has developed test specifications and rating systems for assessing pedestrian injuries during impacts with the front of vehicles Such tests include a series of tests using impactors designed to mimic a collision at a speed of 40 km/h. They describe a situation in which a pedestrian is situated laterally and a car first hits one of his legs. Tests are conducted on the leg, upper leg, and most importantly, the head, as head trauma is most commonly responsible for casualties.
Thus to avoid severe injuries to the head, the high impact area for adults and children is subjected to less thickness. Also, no reinforcement should be provided in that area as, during the head collision, the hood will absorb the head impact energy leading to the collapse of the hood and thus protecting the severe head injury.
Another important criterion for the hood design is the protection of the occupants inside the car during the front collision. The design of the hood should be in such a way that during the collision, the force should travel in the upward direction through the embosses and directed towards the body of the car. Thus protecting the occupants inside.

While designing a component a design engineer must make sure that the component can produced by any means of manufacturing methods with least amount of time, material, manpower and power. So that we can reduce the overall cost of car.
When the hood is considered it is manufactured by deep drawing at most of the time Deep drawing is the process where the single sheet of metal is place over a die which has the desired shape of the final output and the sheet is pressed against it with a tool. For the tool to come out from the part usually pressing relief of 5° to 9° will be provided during modelling itself.
In NX draft analysis can be done after modelling so that we can verify whether the product is manufactured by pressing or not. One side of the surface is highlighted by green and other side by Red colour means that the model can be manufactured. Result of the draft analysis is shown in the figure below.

The striker needs to be place on the trajectory of the centre point of the hinge axis to ensure that it fix perfectly on the latch if not locking will not be smooth and affects the comfort and also it will create NVH problems.
The striker is aligned using following steps:

Drawing the line along the axis of the hinge.
Creating the point on the mid-point of the line.
Drawing the circle from the mid-point of the line to the latch where the striker will be mounted.
Drawing the line from centre point to the mid-point of the striker rod.
Aligning the line's end point and the mid-point of the striker to the circle.
Angle need to aligned so that the angle of the striker is perpendicular to the line drawn from mid-point of the axis line to the mid-point of the striker.

Inner panel and outer panel of the hood should be joined rigidly without disturbing the styling surface of the hood which is the outer panel. Here we use special joining methods known as Hemming and Mastics.
Hemming:

Hemming is the process of wrapping the flange of the outer panel to the flange of the inner panel so that they are fitting rigidly. After hemming process, the sealants were applied along the edge of the flange of outer panel so that water tightness is achieved. The below image shows the cross-section of the inner and outer panel joined by hemming process.
Usually, the reliefs were provided at the corners of the hemming flange to avoid the Hemming relief to avoid stress developed while hemming. stress developed during the hemming process.

Mastic points were the gluing points where the paste is applied between the inner and outer panel to join the panels. Each mastic point will stiffness up to 80mm surrounding. So mastics to be added in such a way that it covers entire surface of the hood. If the gap is less than 5mm between the outer and the inner panel without mastic points it will cause vibration resulting in NVH problem. Below image shows the mastic points where the paste is applied, and it will get hardened in further processes resulting in a rigid joint.

Challenges faced during this project :

The tools using where difficult during this project

The constraints at the last mounting of the hood where not visible and the mounting of the striker and the hinges where difficult.

The where overcomed with the help of the mentoring session that where created in the course.

CONCLUSION:

Thus the Hood is designed that satisfies the manufacturing requirements, functional requirements as per Euro NCAP standards.

Ship work like this
in your own course.