Hood design-Week 2
HOOD
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 maintenance and repair. Hoods are usually made from the same material as the rest of the body. This can be steel, aluminium, fiberglass, or carbon fibre. However, some aftermarket companies produce fiberglass or carbon fibre steel hood replacements to make the vehicle lighter. The hood release system is common on most vehicles and typically consists of an inner hood latch/lock handle, a hood release cable, and a hood lock assembly. The hood lock handle is usually located under the steering wheel, next to the driver’s seat, or in the door frame.
COMPONENTS OF HOOD.
The major components of car hood, are as follows: -
1. Hood Outer Panel.
2. Hood Inner Panel.
3. Hinges for opening and closing of Hood.
4. Striker for the locking mechanism of hood during closed condition.
5. Hood Support System.
1) Hood Outer Panel.
This is the outer skin of the hood and covers the engine, radiator, Battery & Transmission system. Outer Panel is a single piece metal sheet component. The outer panel is the first part of the hood, followed by the internal structure comprising of inner panel. The outer panel is the body of hood and inner panel forms the skeletal system.
2) Hood inner Panel.
The Hood Inner Panel is extracted from the Outer Panel and forms the major part supporting the outer panel, in terms of strength of the hood. The inner panel plays an important role in terms of design, as it helps to protect the under part, with minimum damage during an crash/accident met by an obstacle.
3) Hinges for opening and closing of hood.
Hinge assembly is an assembly similar to door hinge in house. The main purpose of the hinge is to rotate the part in a given angle to serve the suitable purpose. In the Hood design, the placement of hinge assembly plays an important role, for making the hood to actuate at certain angle. The Center axis line of the two sub-parts of hinge assembly should lie in a single line, for making the hinge to rotate at a common axis. A line is drawn from the centre of the axis of a hinge assembly at one end of the inner panel to the similar hinge assembly at the opposite end of the inner panel, in order to make the centre axis of both the hinge assembly to lie on same line.
4) Striker for locking mechanism of hood during closing condition.
Striker is the part present in the inner panel of the hood assembly at the top end, in the middle of two hinge assemblies facing opposite to the hood outer panel. The important function of the Striker is to perform the locking mechanism, during closing of the hood/bonnet. The Striker comes with a U-Shaped lock attached to a plate, which engages with latchet at the closing section of the bonnet. The placement of the striker in the hood inner panel, is done by drawing a circle at a plane present in the middle of the hood inner panel, facing perpendicular to the center line axis of the hinge assembly. After drawing the circle, a tangent is drawn perpendicular to the direction of striker placement, in order to obtain the exact alignment. Based on the line drawn, the striker is placed and fixed to the inner panel.
5) Hood Support system.
Hood Support system is nothing but the Hood Stay being an internal part present at the side of the engine compartment of a car, which is used to support the hood during the opened position.
There are two type of Hood Stay as follows :-
I) Support rod System.
Support Rod System is a simple rod, provided with a certain bending in accordance to its resting position to the side of the engine compartment. The one end of the Support rod system acts as a hinge, and the other end has a hook, which engages to the lock, present in the hood inner panel. The Support rod system supports the entire hood, by taking the load during the open condition of hood.
The support rod used is slightly bent intentionally, as shown in figure above, so that it adds strength & rigidity to the rod which can hold the hood effectively. Also there are restrictions to the design of the hood stay, The maximum distance a rod can be bent is less than 35mm, as shown in Figure below. And also the hood support rod should be fixed at the opposite end of the battery compartment enabling the ease of operation near the battery area.
Gromets are used to fix the support rod.
II) Gas Stay Hood Type.
Gas Stay System is a advanced type of supporting system, which has a linear actuating device, used to control the motion of the hood, while opening and closing. These type of Hood Support does not require much of Man power and is very useful when the weight of the hood is more and difficult to lift. This is usually seen in bigger cars and also it is expensive. These gas stays are provided in the back door to all car segements these days.
It has two ends where one end is fixed to engine compartment and the other to the hood itself. The length C & D is kept constant and only the (B-A) length is varied. We could say that if the length of the Hood increases , (B-A) length also increses.
Stud ball is used to fix the gas stay support.
Materials used in Hood.
1) Steel
2) Aluminium
3) CFRPs - carbon - fiber - reinforced plastics
Amid the raging debate on the suitability of aluminum as a replacement for steel in BiW, a major transformation has occurred in the form of the entire BiW being made up of carbon-fiber-reinforced plastics (CFRPs) in BMW’s i3 model, a fully electric car.
Design Considerations for Hood.
Hood Head Zone for Lock Striker
The hood is designed to open at an angle based on the average height of the people in that region / country.
Euro - NCap ( The European New Car Assessment Programme )
They are the team who analyse and give ratings on how safer is our car for the pedestrians based on the crash tests performed by them. If the car gets high Euro NCap ratings then the car is very much safer for the pedestrians and also becomes a very good selling point in the market. To improve Euro N-Cap Ratings we can start removing materials inside the hood to make it more softer so that, the person when collides with the car will hit the empty area which will have less impact on him.
In some case small deformable brackets are given inside fender assembly to improve green zones shown in below figure and eventually to achieve better ratings.
HIC Criteria :
= or < 1000 (Rating : Green)
1000 ~ 1350 (Rating : Orange)
= or > 1350 (Rating : Red )
More the Green zones, Better will be the Ratings for the car.
Wrap Around Distance.
WAD (Wrap-Around Distance) is the geometrically traced distance from the contact point with the Ground Reference Plane, vertically below the front face of the bumper, to any point on the vehicle front structure.
NVH - Noise vibrations and Harshness.
Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks. While noise and
vibration can be readily measured, harshness is a subjective quality, and is measured either via jury evaluations, or with analytical tools that can provide results reflecting human subjective impressions. Stiffner Pad are introduced between outer panel and inner panel to improve strength locally and to avoid the vibrations during motion.
Mastic Seal.
Mastic is commonly used as an adhesive and sealant, to join two surfaces. In designing the Hood Inner Panel, there are small points are created near the cut-outs of inner panel structure, where the mastic sealant is applied. In joining of the hood outer panel and hood inner panel surface, joining process like – spot welding is not suitable. Hence, Mastic sealant are applied to joint the two surfaces. The Mastic Data is created by drawing a circle of 80mm Diameter at the centre of the semi-circle points near the cut-outs in hood inner panel.
Embossing in hood inner panel.
Sheet metal embossing is a stamping process for producing raised or sunken designs or relief in sheet metal. This process can be made by means of matched male and female roller dies, or by-passing sheet or a
strip of metal between rolls of the desired pattern. The metal sheet embossing operation is commonly accomplished with a combination of heat and pressure on the sheet metal, depending on what type of embossing is required. Theoretically, with any of these procedures, the metal thickness is changed in its composition. Metal sheet is drawn through the male and female roller dies, producing a pattern or design on the metal sheet. Depending on the roller dies used, different patterns can be produced on the metal sheet. The pressure and a combination of heat actually "irons" while raising the level of the image higher than the substrate to make it smooth. The term "impressing" refers to an image lowered into the surface of a material, in distinction to an image raised out of the surface of a material. In most of the pressure embossing operation machines, the upper roll blocks are stationary, while the bottom roll blocks are movable. The pressure with which the bottom roll is raised is referred to as the tonnage capacity. The Embossing in the Hood inner panel is done for Hinges and the Striker, for its placement during Hood assembly process.
Reinforcements.
Reinforcements are added in the region of striker and hinge assembly placements in the Hood Inner Panel. When the Striker and hinge assembly are assembled in their respective sections of hood inner panel, then the area of that place, occupied by the striker and hinge assembly experiences a loss in strength. In order to compensate the loss of strength, respective reinforcements are added to nullify the loss.
Reinforcements are additionally given to compensate the loss, which happens on that area locally, intentionally given to improve the strength on that particular area locally.
Deep Drawing.
Deep drawing is a sheet metal forming process in which a 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 following list includes the five stages of development for deep drawing process in sheet metal: -
▪ Punching the Metal Sheet
Deep drawing processes begin when the punch hits the metal sheet for the first time. Previous to the first hit, you need to place the sheet on the machine handling the task. You also need to make sure that the punch is in the right place, and the right pattern for fabrication has been programmed. Make sure that the equipment does not vibrate with a hit. That could harm
the finished product.
▪ Bending the Sheet
There should be an initial application of force on the sheet. This first hit should bend the contour of the die cavity. As the bending process goes on, the strength applied on the sheet should never exceed the amount of pressure applied to the metal being pressed. That way, we can avoid wrinkles on the metal sheet.
▪ Straighten the Work
The next step is to increase the force applied to bend the sheet on the blank. By doing this, we will straighten out the metal without breaking it. During this procedure, we will sink the annular punch-die clearance. When we straighten out a metal sheet, we get shorter vertical walls. This is also a process that allows us to see how much we can actually extend metal sheets to get what we need.
▪ Friction and Compression
On this stage, the blank will flow following a radius pattern as friction works to do the rest. These frictional forces will act on the lower surface of the piece as well as the upper part of the die. Frictional forces will slowly decrease as the metal sheet moves along the surface of the die. After the friction passes, the blank will show compression, and it will shrink to a smaller size than the die.
▪ Releasing Tension
In the last stage of this sheet metal manufacturing technique, all of the stress of the piece is released. The blank has to fit in the die cavity creating a vertical wall. The remaining blank takes the shape of a small
flange, and the new piece will be treated with uniaxial tension to be ejected.
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 Vehicle 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.
There are various types of hemming operations:
• Conventional die Hemming.
• Roll Hemming.
Assignment (challenge) cross section of hood.
