Hood design-Week 2

Objective :

To Design Hood Outer Panel, Inner Panel and the necessary reinforcements by following the Master Section mentioned with the Design parameters and submit an ASSEMBLY of Hood Inner Panel, Outer Panel, Latch, Hinge and the Reinforcements.

Also to prepare a detailed report about the Hood Design which includes a detailed explanation about hood and its components, Latch Trajectory, Mastic Data, Emboss Definitions, Manufacturing process like Deep-Drawing and Hemming, and all the parameters followed in the design.

Data to be used to design the Hood assembly are as follows:

Hood Thickness Information

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

Introduction to Hood

Hood also called as Bonnet is a hinged thin sheet which covers and protects the main parts of car like engine.

Few of its main functions are as follows :

• Direct access to the engine, battery and other crucial parts of the cars for repair and maintenance and to protect these parts from direct sunlight and dust.
• Directs impact away from the passengers during crash.
• Reduces damage to the pedestrian (low impact) during frontal collision.
• Helps the car aerodynamically stable.

Design considerations for hood assembly are pedestrian safety, hood head zone for the lock striker, force distribution when subjected to frontal collision, manufacturability etc.

A typical hood assembly consist of hood outer panel, inner panel, latch and striker, hinge, hood stay. Hood stay is an apparatus for supporting the engine hood of a moto vehicle 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 are typically made out of the same material as the rest of the bodywork. This may include steel and aluminum. Deep drawing, hemming, blanking and piercing are the common sheet metal operations that are used in manufacturing hoods in automotive industries.

The components and manufacturing data of the hood design are as follows :

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 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.

Hemming in Sheet metals

Hemming is a sheet metal forming process in which sheets are joined by bending it usually to 180°. Automotive body panels and automotive parts such as deck-lids, trunk-lids, doors, hoods, and tailgates are formed by hemming processes.

Conventional die hemming is suitable for mass production. In die hemming, the flange is folded over the entire length with a hemming tool. Normally the actual hemming is a result of a forming operation in which the flange is formed with a hemming tool after the drawing and trimming operations have been completed
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. If weld or bolts were used to join the hood panels then the design and the aerodynamic features would get damaged so hemming is more significant to use.

The above figure shows Hemming between Hood outer panel and Inner panel.

There is Relief given in the areas specified during Hemming to reduce redzone area.

2)Hood inner panel

The hood inner panel is required to have a high initial reaction force against the impact exerted on the hood to
enhance the energy absorption capacity.
Main objective is to design a hood inner panel capable of reduction of reaction forces of the hood inner panel even if a level of deformation of a hood of a vehicle increases, and ensuring a sufficient energy absorption capacity. Safety of pedestrians and as well as the driver can be improved as the inner panel design is improved. Reinforcements, hinge assembly, striker are all assembled to the inner panel.

Deep Drawing

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. The deep drawing process is a forming process which occurs under a combination of tensile and compressive conditions.
Sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered "deep' drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing 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 stresseg,hoop stresses) result in flange wrinkles. Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.

Mastic points

Hood commonly possess large surface are 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.

The above figure shows the mastic points.

Latch placement in the inner panel

While opening or closing the hood, latch needs to be perfectly aligned to the stricker point and in order to achieve that condition the latch trajectory must be perpendicular to the hinge axis.

3)Hinge, Latch and Reinforcement

Latch

A hood latch is a very simple component. It's basically the fastener that keeps your hood closed shut.
This latch ensures that your hood is securely connected to your vehicle and that it doesn't pop open unexpectedly When tension is applied to the cable from the hood release handle, it pivots the secondary latch, which pulls away from a retaining seat located under the hood. By design, the secondary latch then releases the hood a few inches before a safety latch takes over.

Hinge

Hinge is a component that attaches the hood to the fenders of the car. The latch much be fixed perpendicular to the hinge axis so that the hood opens/closes properly.

Reinforcements

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.

Conclusion

The Hood design is completed and its assembly looks as follows