Hood design-Week 2
H00D DESIGN
Introduction of hood
A car hood also referred to as a bonnet in some other countries is the hinged cover that rests over the engine of a front-engine vehicle. Its purpose is to provide access to the engine for repair and maintenance. Car hoods are typically constructed from steel and sometimes from aluminum.
Hood design considaration points
- opening angle
- hood balancing system
- NVH
- pedestrain safety rating
- Euro NCAP regulations
- latch and striker position
- hinge axis
Hood opening angle
This is one of the main criteria in designing of hood. This opening angle varies from person to person and country to country. In India, the average height of a person is 5.8 feet/ 177 cm (Current data) where as in the European countries the average height is 6 feet, these points refers variation in opening angle of hood.Opening angle is considered as an important criteria,when the opening angle is less then required there are possibilities of human head injury.
Hood balancing system
1.stay rod
These support rods are widely used in cars to support the hood in open Position. These support rods are cost effective compare to gas stay. In this type of hood support the hood opens manually. The support rod is fixed on the hood or the engine compartment, to hold hood (in Open Position) the support rod needs to positioned(manually) on the supporting rod point which is opposite to the support rod position. Generally these support rods are used only in the lighter weight hood Vehicles.
2.Gas stay
These Gas stay are type of support rods which is used for heavy weight hoods. In these gas stay rods, pressurized gas is filled which helps to hold the hood in open position. Gas stay just needs the initial force to open the hood. Gas stays are convient to use but these are expensive when compared to support rods.
Gas stay hold hood open position
Purpose
An automotive engine hood is arranged to be opened for the purposes of inspecting the engineroom,replenising cooling water and engine oil.changing parts,making repair etc.
NVH-(Noise,vibration,hardness)
Noise, Vibration and Harshness (NVH) is the study and modification of the noise and vibration characteristics of vehicle, particularly cars and trucks. To improve NVH, mastic sealants were provided on the inner panel to attach with outer panel. Here mastic points on the inner panel plays a major role to avoid Noise and Vibration into the cabin. NVH is measured by the instruments, namely microphones, accelerometers and force gauges, or load cells. Component and subassembly NVH tests are most commonly performed using a vibration shanker while entire vehicles usually on the test track or road stimulator test beds.
A SHORT STUDY ABOUT THE WAD:
During most pedestrian-vehicle crashes the car front impacts the pedestrian and the whole body wraps around the front shape of the car. This influences the head impact on the vehicle. Meanwhile, the windscreen is a major impact point and tested in NCAP conditions. The severity of injuries is influenced by car impact speed; type of vehicle; stiffness and shape of the vehicle; nature of the front (such as the bumper height, bonnet height, and length, windscreen frame); age and body height of the pedestrian; and standing position of the pedestrian relative to the vehicle front. The so-called Wrap Around Distance WAD is one of the important measurements for the assessment of the protection of pedestrians and of bicyclists as well because the kinematic of bicyclists is similar to that of pedestrians.
For this study accidents of GIDAS were used to identify the importance of WAD for the resulting head injury severity of pedestrians and bicyclists. GIDAS (German In-Depth-Accident-Study) collects accidents as a representative sample of the German accident situation based on in-depth investigation. A total number of n=548 pedestrian and n=436 bicyclists suffered a head impact with the car collected within n=27,666 accidents with all kinds of traffic participants and injured persons from the years 1999 to 2013 which are used for the study
Results: A significant influence respectively an increase of WAD by the driving speed of the bicycle cannot be found. The cumulative frequencies show, that WAD 1500 only covers about 8% of bicyclists but 18% of pedestrians. WAD 2100 covers about 51% of bicyclists and 74% of pedestrians. The WAD for pedestrians as a measure of distance from the ground to the impact point of the head is dependent on the impact speed of the car. For lower speeds, the front hood area is more often contacted, and for higher speeds the windscreen. For Vans and off-road cars, front hood impacts can only be seen in lower-speed collisions. The WAD of bicyclists, in general, is higher than for pedestrians. The cumulative frequencies of body heights show that bicyclists are larger than pedestrians. The injury severity of the head (AIS head) is increasing with the age of the pedestrian or bicyclist. Only 6.9% of pedestrians and 3.3% of bicyclists in the age group of 16 to 25 years suffer head injuries of AIS 3+, in the age group over 55 years there are 19.7% of pedestrians and 16.1% of bicyclists.
Master sketch:
The master section is created based on the requirements received from the marketability team and the manufacturing requirements that we saw before. Below are the images of the master section used for the design.
Hood Thickness Information
- Outer Panel Thickness = 0.75mm
- Inner Panel Thickness = 0.75mm
- Reinforcement Thickness = 1.5mm
1.Hood outerpanel
Outer panels are the aesthetic elements of a car. It is the styling team who provides the skin for the 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 initial 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 requirement. According to the design requirement here it is given 1.5mm thickness to both hinge and latch reinforcements.
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.
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 appearance. 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.
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.
Final Assembly
