Hood design-Week 2

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 maintenances and repair.
• In design perspective hood should meet some specific conditions like pedestrain 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 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 hoods 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 oprtaions that are used in manufacturing hoods in automotive industries.

DESIGN PARAMETRES:

• Hood Thickness information:

Outer Panel Thickness =0.75 mm

Inner Panel Thickness =0.75 mm

Reinforcement Thickness =1.5 mm

COMPONENTS:

1. Hood Outer panel:

Outer panels are the aesthetic elements of a car.It is the styling team who provides the skin for 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 intial 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 requirements here it is given 1.5mm thickness to both hinge and latch reinforcements.

4. 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.

5. Emboss defintions:

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.

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 because they leave marks on the outer panel which spoil its aesthetic apperance. 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.

7.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 tranformation process with material retention.The processor 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 stresses ( 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.

8.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 appearances 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 apperances 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:

In conventional die hemming, the flange is folded over the entire length with a hemming tool.

2.Roll hemming:

It is the roll hemming process that is used for automotive sheet metals.Roll hemming is carried out incrementally with a hemming roller. An industrial robot guides the hemming roller and forms the flanges.Roll hemming operation can also be divided into several pre-hemming and final hemming process steps.Roll hemming is very flexible to use and tool costs are significantly lower as compared to those of conventional die hemming.However, the cycle times are much higher since the hemming is realized using a hemming roller which follows a defined path.

FINAL ASSEMBLY: