Hood design-Week 2

TITLE:HOOD DESIGN

Objective

Design hood outer panel, inner panel and reinforcements with the given outer hood skin, master section and design parameters.

Introduction

The hood or bonnet is the hinged cover over the engine of motor vehicles. Hoods can open to allow access to the engine compartment (or trunk on rear-engine and some mid-engine vehicles) for maintenance and repair. In design perspective hood should meet some specific conditions like pedestrian safety, hood head zone for the lock striker, force distribution when subjected to frontal collision (occupant safety), manufacturability etc.

Design parameters

Hood Thickness Information

Outer Panel Thickness = 0.75mm

Inner Panel Thickness = 0.75mm

Reinforcement Thickness = 1.5mm

Master Section

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 definitions

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

Hood commonly possess large surface area and hemming can only be provided through its boundary. We cannot use other joining techniques like welding, riveting etc because they leave marks on the outer panel which spoil its aesthetic appearance. So, mastic sealants are used to stick the inner surfaces of outer panel and inner panel. Mastic sealants are elastic compound that behave like rubber. It is applied between the inner and outer panels at specified spots. Each spot of mastic sealants has its effect around a circular region of 80mm diameter. Thus, the mastic points are arranged in such a way that it covers the areas away from the hemming which are more vulnerable to vibrate.

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,

1. Conventional die hemming
2. Roll hemming

Final Assembly