Hood design-Week 2

DESIGN OF HOOD ASSEMBLY

AIM:

To design the hood inner panel and to fix the striker in path of the trajectory of hinge axis.

INTRODUCTION:

Hood is also referred as bonnet. Hood is the one of the major BIW component of the car. It acts as a styling surface of the car. In designing the hood, the designer should not only focus on the aesthetics of the hood but also should consider the passenger safety in case of collision with other vehicle or the poles, pedestrian safety in case of collision with pedestrians and the safety of engine and other components in the engine compartment in case of a solid body falls on the hood.

Styling engineer will provide the outer surface of the hood to the design engineer, as a design engineer, we need to optimise the hood design such that the components meet all the safety criteria mentioned above without disturbing the styling surface.

A typical Hood or Bonnet usually consists of Outer Panel, Inner Panel, Hinge Reinforcement, Hinges, Striker and Striker reinforcement. Outer panel is the styling surface where any of the operation that involve in deformation of the surface for example, joining methods like spot welds and riveting is not allowed on the styling surface. Disturbing the outer panel also disturbs the aerodynamics of the vehicle.

DESIGN CONSTRAINS TO BE FOLLOWED:

• Passenger safety
• Pedestrian Safety
• Manufacturing Feasibility
• Positioning the striker along the Trajectory of the Centre of Hinge axis
• Joining techniques of inner and outer panel.

PASSENGER SAFETY:

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.

Since the embosses have move curved surfaces it will also absorb the energy transferred during the collision occurs.

PEDESTRIAN SAFETY:

Safety of the pedestrians need to be considered during the design as per the NCAP Euro assessments.

Hood is of two types based on the side reference line. Side reference line is a imaginary line drawn from a ground at angle of 45 degrees. If this line falls on the fender then the hood is inlaid hood and if the line falls on the hood, then hood is wrap around hood.

Child and adult impact area on the hood are defined by the wrap around distance (WAD).

Area between the wrap around distance of 1000 to 1500 mm is the Child head impact zone and from 1500 to 2100 WAD is the adult head impact zone. Now since we know the zones where the pedestrians going to hit, as a design engineer, we must make sure that they are experiencing the least force such that they were not getting any severe injury without compromising the strength of the hood so that passenger safety is also assured. For this reason, the inner panel is kept at 25mm from the outer panel and creating the embosses is to improve the strength of the hood.

Head impact area will be typically looks as shown in the image is defined based on the side reference line and the wrap around distance. These zones will get Red, Yellow or Green ratings based on the Head Injury Criteria calculated using the expression

a= Acceleration of the head during impact (Measured in gravity)

t2-t1= interval between the beginning and end of the recording where the HIC value is maximum (Value can be maximum of 15ms)

HIC<1000 region will be assigned green rating if HIC value is between 1000 to 1350 then the region will get yellow colour and HIC value more than 1350 then region becomes red.

MANUFACTURING FEASABLITY:

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^o to 9^o 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.

POSITIONING THE STRIKER:

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

1. Drawing the line along the axis of the hinge.
2. Creating the point on the mid-point of the line.
3. Drawing the circle from the mid-point of the line to the latch where the striker will be mounted.
4. Drawing the line from centre point to the mid-point of the striker rod.
5. Aligning the line’s end point and the mid-point of the striker to the circle.
6. 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.

7. Now the striker will move along the trajectory of the circle drawn.

   JOINING THE INNER AND OUTER PANEL OF THE HOOD:

   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.

   

   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 stress developed during the hemming process.

MASTIC POINTS:

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.

Image shows the mastic points where the paste is applied, and it will get hardened in further processes resulting in a rigid joint.

CONCLUSION:

Thus the hood is designed considering passenger and pedestrian safety as per the master section and striker also place in the path of trajectory of the hinge axis.