HOOD ASSEMBLY DESIGN

REPORT ON HOOD DESIGN

BY : suraj kendre

INTRODUCTION :

• The Hood or the Bonnet of a car is considered as the frontal BIW enclosure component.
• Bonnets are designed to reduce air resistance as the air flows around the car.
• The design of the bonnet is vital since it affects the aerodynamic flow around the car.

Apart from this, bonnets are also designed to meet several other requirements –

1. They serve a great deal of purpose in protecting the passengers inside the car.
2. The pedestrians in the event of an accident.
3. It also acts as the engine cover.

• The hood material is selected based upon the manufacturers requirement. Some materials like aluminum, fiberglass, carbon fiber and dry carbon. Mostly aluminum and its alloys are often used in automotive industry.
• The hood or the bonnet is commonly manufactured using Stainless steel of various grades.
• For vehicles that weigh less, the bonnet is made up of aluminium.
• The Stainless steel bonnet uses a technique called deep drawing to manufacture the component.
• The Aluminium bonnet employs a procedure called as casting.
• In this project, we are going to design a hood that is manufactured using stainless steel material.

OBJECTIVE :

• The main objective of this project is to come up with a design idea for the bonnet BIW that satisfies-

1.The manufacturing requirements

2.Required Opening angle of hood

3.Positioning of Striker & Hinges

4.Functional requirements

5.Safety of the pedestrians

6.The frontal collision

SURROUNDING DATA FOR HOOD :

• The Surrounding data required for hood design -

1. Engine - compartment

2.Battery - location

3.Two pair of hinges

4.Latch and Striker

5.Front fenders

6.Cowl panel

7.Front bumper

BOM FOR HOOD ASSEMBLY :

The Hood assembly consists of -

1. Hood outer panel 1NOS

2.Hood inner panel 1NOS

3.Latch and Striker 1NOS

4.Striker re-enforcement 1NOS

5.Hinge Assembly 2 NOS

6.Hinge re-enforcements 2NOS

7.Support rod 1NOS

8.Nut and Bolts 8NOS

9.Grommet 2NOS

10.Mastic

11.Stiffener pad

DATA FOR HOOD ASSEMBLY DESIGN :

The Hood assembly inputs consists of -

1.Hood outer panel thickness = 0.75mm

2.Hood inner panel thickness = 0.75mm

3.Striker reinforcement thickness = 1.5mm

4.Hinge reinforcement thickness = 1.5mm

HOOD DESIGN :

• First thing to consider while designing the hood is the opening angle, if it is less there are chances that human head hits the striker.
• The hood design consists primarily of two parts i.e. outer panel and inner panel, Joining of these two panels form the hood.
• The inner panel provides strength while the outer panel functions as the metal cover.
• The outer panel of hood is designed from the styling surface and hemming with corner relief are provided on it.
• The inner panel of hood is designed considering the master section with respect to the styling surface. Embossing and reinforcements are provided on the inner panel so as to improve the strength.

The hood also consists of other components which require in assembly:

1.Hinge mechanism

2.Mastic

3.Latch-striker mechanism

4.Reinforcements.

HOOD OUTER PANEL :

• Hood outer panel is nothing but the styling surface also called as cover of inner panel.
• It is the part which gives luxurious look to the hood.
• The thickness of outer panel varies from 0.65 to 0.75mm.
• Outer panel and inner panel are joined by hemming process not by spot welding as it spoils the outer look of the car.
• Rollers are moved again and again over the flange and the flange is folded so as to avoid springback & half rolling.
• During Hemming process front relief and rare relief are provided at the corners on the hood for proper joining.
• More then one roller are used as it saves time and is more effective while press folding the flange.

HEMMING :

• Hemming is a method used in automotive industry to join inner and outer panel or two panels together where the aesthetic look is important.
• It is the process of bending or folding the flange of the outer panel over the inner panel.
• Following is the consideration for hemming-

HOOD INNER PANEL :

• Hood inner panel is a reinforcement for hood outer panel.
• Hood inner panel provides strength to the hood.
• It is drawn with the help of master section, previous data or benchmarking.
• It has embossing for striker and hinge assembly to be positioned on it.
• It also have embossing on it in such a way that during collision the force gets diverted towards the hinges and the damage to the passengers are reduced.
• It has large cut-outs which reduce the weight as well as the soften the area locally for safe landing of the pedestrian and also for mastic application.

• While designing the inner panel hood that will be manufactured using the deep drawing process, there are some minimum requirements that we need to fulfil:

1.Minimum fillet radius = 3mm+Thickness

2.No sharp corners inside the component

3.No curvatures lesser than 5mm

4.Small embosses should not exceed the maximum depth of 6mm.

5.Mounting areas and welding areas need to be flat

6.Provide appropriate corner relief on the area of curvature.

SUPPORT ROD :

• A support rod is an apparatus of an engine room of a car, which can safely keep an engine hood in an open state even if external force abruptly generated due to vibration, wind and can facilitate an opening or closing operation of the engine hood.
• Whenever a support rod layout is made we should know where the opening and closing angle are, where the rod is going to get mounted and dis mounted. Length of the rod is defined with respect to opening angle.
• Make sure that the rod does not collide anywhere in the engine compartment or else it will create rattling noise.
• Clearance of at least 5mm with non moveable surface and 10mm for moveable surface around the rod should be maintained.

• The length of the rod should not be more than 35mm as there are chances that due to weight of the hood the rod will get bend.
• The supporting rod cannot be made linear it has bends on it to ensure the strength of the support rod.
• Support rod should be on the opposite side of the battery as it will be installed first and should not have clash with battery and easy removal.
• The size of support rod hooking hole should allow minimum clearance of 2mm in hood completely open condition.
• Position of support rod hooking hole in hood side should be in the same x coordinate with body side fixed point and on surface of front bow.

HINGE ASSEMBLY :

• There are different hinge assemblies used for hood mechanism depending upon the opening angle of the hood.
• It requires two pair of hinges to operate the hood mechanism.
• The hinges are bolted to the inner panel of hood with embossing to increase the strength locally also providing reinforcement brackets.

STRIKER :

• It provides lock and unlock mechanism for the access of the engine parts and other components. It is the point which will get attached to the hood for locking purpose.
• Whenever the hood is open there should be a clearance between the tip point of the hood and the striker.
• To lock and unlock a striker a primary latch mechanism is used which is placed under the drivers seat.
• In primary situation the lever used below driver seat will release the hood striker and remain at a particular height.
• After this we get access or gap which is used for secondary liver so as to open the hood completely.
• Whenever the primary latch is released we should have a gap of 10 to 15mm so that we can access the secondary lever easily.

STRIKER POSITIONING :

• After positioning the hinge assembly and the latch striker mechanism. The position of the striker will be obtained from the master design or the parent surface where the striker to be rested.
• The trajectory is derived from the hinge axis center distance from the striker. It is denoted as circle shown.
• The hood closes properly only if the trajectory is perpendicular to the striker. Otherwise misalignment may happen.

REINFORCEMENTS :

• Whenever we add weight to a component we loose the strength of the area locally. Example Latch and Striker, Hinge assembly to the inner panel of hood.
• When we add striker bracket on the inner panel the weight is added to the hood hence the local area near the striker loose strength so as to compensate this we will add additional material which we call it as reinforcement bracket so as to increase the strength.
• Hence reinforcement are one which are additionally given to compensate the lose that happen on area locally and intentionally given to improve strength of that area locally.

MASTIC :

• Mastics are like paste when heated get hardened and join the components.
• It is used between inner and outer panel. When it will get into the hot bath inside the furnace it will get harden and join the outer panel to the inner panel.
• Gap between inner panel and outer panel should be 5mm and it will be field by mastic sealants.
• Mastic sealants are used to join two panels where the spot welding is avoided such as the styling surface and its reinforcement so as to keep aesthetic look of the car.

Mastic points for Mastic sealants are added to improve the NVH of a vehicle .

Every mastic sealant have the tendency of improving the strength locally around 80mm diameter.

STIFFENER PAD :

• It is spongy material little hard placed between inner and outer panel to increase the strength locally.
• It is mostly used to avoid the vibration between the outer and inner panel during vehicle in motion.
• Stiffener pad in most of the cases is used during the initial level of the project and later it is removed as cost reduction item.

EMBOSSMENTS :

• Whenever there is collision happening the force goes straight towards the passenger compartment and harm the passenger.
• So as to dissipate the force outside the car towards the hinge assembly as early as possible embossing is provided on the inner panel of the hood.
• Embossing direction changes the force direction outside towards the hinge. Hence during collision the force is reduced and the passenger inside the cabin are not affected.
• The embossing done on the inner panel of the hood should maintain along the Y or V direction so that forces dissipate equally outside the car.

SECTION MODULUS :

• Section modulus is a geometrical property of a given cross section used in the design of a component.
• It gives us an idea about how much of load the given structure can take.
• If the section modulus increases the moment of resistance increases and hence its load bearing capacity increases.
• Section modulus is calculated by using the formula –

S = I/Y

S is section modulus

I is moment of inertia

Y is distance between the neutral axis and the extreme end of the object.

Unit for section modulus is mm^3

I = 3.925x10^-7 m^4 Y = 478.127/2 mm

I = 3.925x10^5 mm^4 Y = 239.063 mm

• S = I/Y
• S = 3.925x10^5 mm^4 / 239.063 mm
• S = 1.6418x10^3 mm^3

DRAFT ANALYSIS :

• Draft analysis is carried out to see that the panel can be drawn or not.
• It also helps to find the negative draft present in the component which can cause difficulty in manufacturability.

SAFETY CONSIDERATIONS :

• While designing a hood safety considerations such as EURO NCAP regulations, Pedestrian ratings, deformable brackets, etc are considered.
• EURO NCAP is a team who analyses all the tests which are carried over to understand how much the car is safer for a pedestrian.
• The European New Car Assessment Program (Euro NCAP) is a European car safety performance assessment program which has developed test specifications and rating systems for assessing pedestrian injuries during impacts with the front of vehicles.
• Whenever there is collision with the pedestrian, tends to hit inside the bonnet and this hitting zone is defined in two categories-

1)Child head impact zone- 1000 WAD to 1500 WAD

2)Adult head impact zone- 1500 WAD to 2100 WAD

• WAD is wrap around distance which is the distance from ground level to hoods end.
• There are mankind used for pedestrian collision during the test. Most of the incidents are recorded and average value is taken.

• They describe a situation in which a pedestrian is situated laterally and a car first hits one of his legs.
• Tests are conducted on the leg, upper leg and, most importantly, the head, as head trauma is most commonly responsible for casualties.

Child head impact criteria-

1.Impactor diameter- 130mm

2.Impactor velocity- 45 km/hr

3.Impactor angle- 50-52 deg

Adult head impact criteria-

1.Impactor diameter- 165mm

2.Impactor velocity- 40 km/hr

3.Impactor angle- 65-67 deg

• Thus to avoid severe injuries to the head, the high impact area for adults and children are subjected to less thickness.
• Also no reinforcement should be provided in that area as during the head collision, the hood will absorb the head impact energy leading to collapse of the hood and thus protecting severe head injury.

HOOD ASSEMBLY :