DESIGN OF AUTOMOTIVE HOOD USING NX CAD

Drive Link: https://drive.google.com/open?id=15bLdqrTJyGz8emeXbNGYIqJoGARp8izV

AIM:

To design a Hood of a Car based on Engineering inputs, Design considerations & Safety Factors that satisfies industry’s manufacturing requirements and functional requirements using NX CAD.

INTRODUCTION:

The hood which is also known as bonnet is the hinged cover over the engine of a car that allows access to the engine compartment.

The hood release system is common on most vehicles and usually consists of an interior hood latch handle, hood release cable and hood latch assembly. The hood latch handle is usually located below the steering wheel, beside the driver's seat or set into the door frame.

In this project we will be focussing on designing the Inner Panel & Outer Panel of the hood & finally assembling them with the given hinge assembled part and the Latch & Striker Reinforcement.

Since we are initially dealing with sheet metal of infinitely small thickness, the hood is predominantly designed using Surface Modelling tools.

HOOD ASSEMBLY:

The Hood is basically a sheet metal assembly of various parts such as the Outer Panel, Inner Panel, Latch & Striker and the Hinge. Finally, the hemming is done in order to roll the outer panel over the inner panel.

The Inner Panel provides strength while the outer panel functions as the metal cover. They both are held together by mastic seals.

The Latch & Striker is primarily responsible for precise and flush car door closure and also for keeping the door securely closed. Subsequently, the striker, as the decisive counterpart for the latch, is a central element with enormous importance for flawless function of the entire closure system.

The hinge is a movable joint or mechanism on which the hood swings as it opens and closes, while connecting the hood & the engine compartment.

DESIGN INPUTS:

Hood Thickness Information

• Outer Panel Thickness = 0.75mm
• Inner Panel Thickness = 0.75mm
• Reinforcement Thickness = 1.5mm

Master sketch:

INNER PANEL DESIGN:

The inner panel master section was designed based on the master sketch. The inner panel is the critical part of the hood which acts as a force redirecting medium during frontal crash. Hence, it becomes very important to ensure that the inner panel is designed in such a way that the damage caused to the passenger as well as the pedestrian is minimum.

The inner panel consists of different contours known as Emboss & various cutouts made on the emboss which reduces the weight of the hood. It also consists of the mastic sealants extruded from the cutout edges.

The various commands & features that were used to design the inner panel are given as follows:

• Offset Surface : This command is used to create a surface or a body by offsetting a set of faces.
• Offset Curve in Face : Offsets curves along the faces on which they lie.
• Trim Sheet : Trims off a portion of a sheet body.
• Trim and Extend : Trims or extends a set of edges or faces to the intersection with another set of edges or faces.
• Extend Sheet : Extends a sheet body by distance or to the intersection with another body.
• Trim Body : This command is used to trim a portion of a body along a plane or a face.
• Through Curves : Creates a body through multiple sections where the shape changes to pass through each section.
• Law Extension : Creates a law-controlled extension from base sheet, dynamically or based on laws for distance and angle.

• Face Blend : Adds tangent blend faces between selected set of faces based on the radius specified.
• Edge Blend : Rounds sharp edges between faces based on the radius specified.
• Projected Curve : Projects curves, edges, or points onto faces or planes.
• Emboss : Modifies a body with faces made by projecting a section along a vector, with options for end cap location and shape.
• Curve on Surface : Creates a surface spline feature directly on a face.
• Mirror Geometry : Copies geometries and mirrors them across a plane.
• Sew : Combines sheet bodies by sewing common edges together, or combines solid bodies by sewing common faces.
• Thicken : Creates a solid body by adding thickness to a set of faces.

Embosses:

The embosses are projected sections from the inner panel surface that are created in order to redirect or dissipate the forces away from the centre of the hood, towards the hinge to avoid passenger damage.

The embosses with varying depths (5mm, 7mm, 9mm, 13mm) are provided on the hood for reinforcement & to add strength to the inner panel.

The following image shows the path of the force exerted due to collision in such a way the damage to the passengers is minimized.

The arrows denote the impact force during collision and the line of action of force travelling away from the center of the center and towards hinge.

Mastic Points:

The Mastic sealant or the Mastic point is an elastic compound that behaves like a rubber. The mastic points are defined on the hood inner panel where the hood can easily deform.

They help the hood outer panel to come back to its original shape after it gets deformed. The mastic has an influence area of 80mm. They also play a major role in reducing NVH ( Noise, Vibration, Hardness) of the car.

The red circles indicate the mastic positions.

LATCH & STRIKER AND HINGE ASSEMBLY:

This is a critical stage in Hood Design as it plays an important role in the opening & closing of the Hood.

The positioning is done as shown below concerning the Hinge axis so that the circle is passing through the center of the striker with the Hinge axis as its center.Also, The Striker must be perpendicular to the trajectory to achieve the correct position and thus enables it to close with the latch.

HEMMING:

Hemming is the process of joining both the outer and inner panel together. The hemming process roles and wraps the hood outer panel around the inner panel and holds it tightly together. The following image shows the result post hemming process.

Relief rear section:

Front relief:

The final image of the Hood is shown below,

Section modulus:

Section modulus is the geometric property of a given cross-section used in the design of flexural members. Section modulus depends only on the cross section shape of the beam or structure.

Section modulus may be defined as the ratio of total moment resisted by the section to the stress in the extreme fibre which is equal to yield stress.

The material considered for the hood is Stainless Steel AISI 302 cold-rolled (y = 520 MPa)

The Section modulus is calculated at a particular section of the hood by doing a Section Inertia Analysis and obtaining the respective Smax & Smin values of that section.

Section Modulus is given by

S = I/y

Where,

S = Section Modulus

I = Moment of Inertia

y = distance between the neutral axis and the extreme end of the object

Section modulus of Initial Hood Design:

From the above analysis, following minimum and maximum moment of inertia are obtained

Imax = 5.3133 e+06 mm^4

Imin = 2.0181 e+04 mm^4

"y" for section of Hood = 456.5 mm

Smax = Imax / y = 5.3133 e+06 / 456.5 = 11639.21 mm^3

Smin = Imin / y = 2.0181 e+04 / 456.5 = 44.20 mm^3

RESULT:

Thus the hood for a car was designed that satisfies the industrial manufacturing requirements and functional requirements using NX CAD.