Hood design-Week 2
Objective:
To 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 vehicle. Hood is the entrance of 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.
A typical hood assembly consist of hood outer panel, inner panel, latch and striker, hinge, hood stay. Hood stay is a part for supporting the engine hood of a vehicle in staying in its open position. The purpose of opening the engine hood is for inspecting engine room, replenishing cooling water and engine oil, changing parts and making repairs etc. Common types of hood stays are Support Rod and Gas Stay. Support rods are manually operated and they are cost effective and simple to design whereas gas stays require no man power but they are costly. Hood is made of the same material as the rest of the body. This may include steel and aluminium. Deep drawing, Hemming, Blanking and Piercing are some common sheet metal operations that are used in Hood manufacturing process in automotive industries.
Design Parameters:
Hood Thickness Information
Outer Panel Thickness = 0.75mm
Inner Panel Thickness = 0.75mm
Reinforcement Thickness = 1.5mm
Figure 1 – Design Data
Master Section:
Figure 2 - Hood Master Section
Components:
1.Hood Outer Panel:
Hood Outer panels are the aesthetic elements of a car 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 process. There should require Hemming flanges and Hemming reliefs for the hemming process. There are mastic sealants are using between outer panel and inner panel to join them and reduce vibration and prevent aesthetic look of outer panel.
Figure 3 – Hood Outer Panel
2.Hood Inner Panel:
Hood Inner panel is the part that gives strength and rigidity to the outer panel. The master section represents the initial emboss shape and size. Inner panel consist the different patterns of emboss which are designed to reduce the stress concentration on the Hood during the impact. The latch, striker and hinge elements are fixed on the inner panel.
Figure 4 – Hood Inner Panel
3.Reinforcement:
Reinforcements are provided according to the design requirements. Mainly it is provided at the latch and hinge regions to improve the strength at mountings. Usually thickness of reinforcements will be higher than that of the inner panels and it varies according to design cost and requirement. According to the design requirement here it is given 1.5mm thickness to both hinge and latch reinforcements.
Figure 5 – Striker and Hinges
4.Latch Trajectory:
Latch trajectory is the path defined to the latch and striker through which it travels and get locked. It is drawn with the help of the hinge axis. First, we draw the hinge axis and then draw a circle using mid-point of the hinge axis. After that we draw a horizontal line from the centre of circle and a line from centre of circle to parameter of circle at 82 degrees from horizontal line. This circular path is called the Latch Trajectory.
Figure 6 – Latch Trajectory
5.Emboss:
Throughout the inner panel Design we provide different shape and size of embosses. We can increase the stiffness and strength of the inner panel through these embosses and also can reduce stress concentration on inner panel. 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 side of the car
Figure 7 – Emboss on Inner Panel
6.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 on Panel joining because they leave marks on the outer panel which spoil its aesthetic appearance. So, there are mastic sealants used to stick the outer panel and inner panel. Mastic sealants are elastic compound that behave like rubber. These are 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.
Operations using in Hood making:
Deep Drawing process:
It is one of the most widely used process in sheet metal forming. This process is applied in the automotive industry for the manufacturing different car body parts. The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions.
Figure 8 – Deep Drawing process
Blanking process:
Sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape changing process with material retention. There is slight difference between blanking and Deep Drawing process. It is considered as Deep Drawing if the depth of the drawn part exceeds its diameter. This is achieved by repeating the process on 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 stress results in flange wrinkles which can be prevented by a blank holder, which is used to facilitate controlled flow of material into the die radius.
Hemming process:
Hemming process 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 with the help of different type of rollers. Normally hemming operations are used to connect parts together, to improve the appearance of a part and to reinforce part edges.
In car part manufacturing, Hemming is used in assembly as a secondary operation after deep drawing, trimming and flanging operations to join Outer and Inner panel together. Typically, this process is used in assembly of parts like hood, door, trunk lids, fender etc.
The Hemming operation is to be very precise because it affects the surface quality and its appearance. During the hemming process, due to material deformation it can lead to dimensional variation and other defects in parts.
Figure 9 - type of Hems Figure 10 - Roll Hemming
Some of Hemming defects are
· Splits or wrinkles in the flange,
· Material roll-in or overlaps at corners
So, the reliefs are provided at the sharp corners to prevent these types of defects.
Figure 11 – Reliefs on sharp corners
There are two types of hemming operations,
1. Conventional die hemming, in which the flanges are folded over the whole length with the help of Hemming tool.
2. Roll hemming, which is done with the help of Hemming rollers guided by industrial robot. It is very flexible to use and its tool cost is also low as compared to the die Hemming.
Final Assembly:
Figure 12 - Final Assembly
