Hood design-Week 2
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 for maintenance and repair.
The purpose of opening the engine hood is for inspecting engine room, replenishing cooling water and engine oil, changing parts and making repairs etc. Hood are typically made out of the same material as the rest of the bodywork. This may include steel and aluminium. Deep drawing, hemming, blanking and piercing are the common sheet metal operations that are used in manufacturing hoods in automotive industries.
A typical hood assembly consist of hood outer panel, inner panel, latch and striker, hinge, hood stay.
Outer panel: It is the aesthetic part of the hood. In other words, it is the outer skin of the hood which provides a luxurious look to the vehicle.
Inner Panel: The hood inner panel provides strength and rigidity to the hood. It also directs the force of the collision outwards instead of into the engine compartment of the car.
Striker: It is U shaped bend that attaches to the latch in order to lock the hood and prevent any loose movement during the car motion.
Hinge: It is the pivoting point from which the hood opens and closes.
Gas Stay: is a mini version of the bicycle pump in which the compressed nitrogen is stored in the pump with oil and tightly sealed so that the gas can't escape. The gas allows to store energy which produced by the force acting on the piston and oil damps the movement of the piston and also lubricates it. When force is acted upon the piston, it compresses the gas, and energy is stored and when force is stopped acting on the piston, the stored energy is released moving the piston to its free state position.
Gas stay aids in easily lifting the heavy hood and holding the hood in the open position. Hence there is no human effort needed
Support rods: serve the same function as the gas stay but in this case, the hood has to lift with all the loads and it is held in position using the support rods.
Support rods can either mounted on the hood or on the body of the car. The mounting of the support rod is always on the opposite side of the engine compartment.
There is the human effort needed.
Manufacturing Process:
The following are the manufacturing steps done to generate the outer and inner panel of the hood.
Blanking: It is the process where the required sheet metal material is cut from the sheet metal coil for the manufacturing of the hood using a blanking die.
Stamping: The material obtained from blanking is stretched using the stamping die. The stamping die is designed according to the embossed design. This process is called deep drawing.
Trimming and punching: The outer and inner panel is obtained from the forming is placed in the trimming and punching die to carry out trimming and shaping operations to obtain the hood panel with the required dimensions and emboss cut-outs.
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,
- Conventional die hemming
In conventional die hemming, the flange is folded over the entire length with a hemming tool.
- 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 flange. 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.
Design Considerations:
The first important criterion for designing the hood is the opening angle of the hood.
This criterion average height of the humans in the region of consideration. For example, the average height of a human being in India is 5.8 feet whereas in European countries the average height of a human being is 6 feet. Hence the opening angle of the hood greatly depends on the height of the human which can be controlled by the support rod or gas stay.
The opening angle is considered important criteria because when the opening angle is less there is a chance of head injury to the human.
By varying the length of the support rod the opening angle can be varied.
Reinforcements
Whenever there is an addition of material to the component there is a loss of strength locally. In this case, the additional material is the striker and hinge where additional reinforcements had to be attached due to loss of strength locally in the attachment areas.
Reinforcement for Striker and Hinge.
The thickness of the reinforcements should be in the range of 1.5mm. This parameter can be obtained from the CAE team or with a carry-over study.
There is additional reinforcement called a stiffener pad which is attached between the hood inner and outer panel. This is done in order to reduce the noise and vibration during the motion of the car.
Distance between the outer and inner panel should be 5mm where mastic sealants are placed in the mastic sealant points to form a strong bond between them. This also reduces the noise and vibration in the hood in the car.
We are using sealants to join the hood outer and inner panel because the sheet metal used is 0.75mm thick and welding is not possible as it would affect the Class A surface.
Position of the hinge and Striker(Latch trajectory)
The hinges should be positioned on the hinge tentatively such that they lie on the same plane. The position of the hinge can also be decided based on the carry over study.
From the axis of the hinges create a midpoint, from that midpoint draw a circle with a radius from the midpoint to the center of the striker. The circle created is called the trajectory circle. With the trajectory circle, it is to be checked that the striker is tangent to the trajectory circle if not then the position of the hinge should be moved accordingly such that the striker is tangent to the trajectory circle.
This is done because the latch where the striker strikes and locks the hood is positioned with respect to the trajectory circle. If the striker is not positioned accordingly then there will be difficulty in locking the hood.
Emboss
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.