Design of backdoor

DESIGN OF BACKDOOR

AIM:

The aim of this challenge is to create a design of backdoor assembly with all the components from the given information and the skin data of the outer panel using Siemens NX-CAD.

THEORY:

The backdoor of a car/ automotive is the rearmost BIW component that provides space for the passenger luggage. A backdoor is also known as a tailgate in the case of SUV’s, station wagons and pickup trucks. Backdoors are mainly classified into three types based on the positions they are hinged to the car body namely top-mounted, side-mounted and bottom-mounted backdoors. Backdoors can also be classified into two types based on the mechanisms used to open the backdoor like in some cases gas stays are used on either side of the backdoor to assist the user while lifting the door and in some cases, a powered spindle is used. The material used in making the backdoor of the car is mainly steel but nowadays Aluminium is also used in making the panels. The main advantage of Aluminium over steel is that the density of Aluminium is very less as compared to steel so it reduces the overall weight of the car. But Aluminium is costly as compared to steel.

COMPONENTS:

The main components that a usual backdoor consists of are:

1. Backdoor outer panel

It is the outer part of the rear door and is mostly responsible for the aesthetics of the vehicle. Outer and inner panels are connected by mastic sealants in the centre and by hemming process at edges. The outer door handles and in some cases, the number plate may also be installed on the outer panel of the back door. It is further divided into the backdoor upper and lower outer panels. The outer panel is manufactured by stamping sheet metal. The thickness for the outer panel is O.75 mm.

2. Backdoor inner panel

The inner panel, as the name suggests, is the inner part of the door. It is where most of the reinforcements and components are mounted. It is manufactured either by casting (aluminium) or sheet metal stamping (steel). The thickness of the inner panel is 0.75 mm.

3. Reinforcements.

Reinforcements are provided to provide support and add extra material to the sheet metal part to increase its strength near component installation regions and minimize the impact of collisions. We have considered the following reinforcements during our design. The reinforcements may be manufactured by stamping. The thickness of all reinforcements is 1.5 mm.

Hinge reinforcements – Hinge reinforcements are provided where emboss for the hinge assembly is positioned based on the design.

Gas stays reinforcements – Gas stay reinforcements are provided to accommodate the gas stay assembly and the thickness is the same as that of the hinge reinforcements.

4. Latch and striker assembly

Latch and striker assembly is used to lock the backdoor and the latch trajectory must be tangent to the circle/ arc drawn with the midpoint of the axis as a centre and the distance between the midpoint and the latch mounting area as the radius. The thickness is the same as that of the Hinge reinforcements.

DESIGN CONSIDERATION:

• REAR VISIBILITY CRITERIA

The design has to be made focusing on improving rear visibility criteria without affecting other parts of the backdoor.

• LOAD CASES

There are several types of tests that are performed on the backdoor while analysing the design of the backdoor such as:

1. Bend Test – The two upper hinge positions and similar positions on the lower side are constrained and two equal loads in the same direction are applied approximately on the centreline of the backdoor to test how much the part is able to restore its original position upon unloading.
2. Torsion Test – Two hinge positions and the latch positions are constrained to be fixed and two equal and opposite loads are applied on the centreline of the lower backdoor to test how much the part is able to restore its original position upon unloading.
3. Abuse Test – This test is done to know how the part behaves to over slamming and overhanging condition (when people lay their body weight on the backdoor when it is opened) and the right lower position is constrained to be fixed for this test.
4. Dent Test – For this test a dent is made on the backdoor and how much the part is able to restore its original position.
5. Assembly Load cases – When the backdoor outer panel is assembled with other components a certain amount of force is applied. This test is done to ensure that the components are able to withstand this force or not.
6. Endurance Test – As a door the backdoor will be opened and closed many times. So, to know the endurance limit this test is done and the number of cycles is noted.

• DEEP DRAWING

Inner panel and outer panel are manufactured by a sheet metal operation known as Deep drawing. In this operation the sheet metal is placed between dies and a punch is used to draw the sheet metal in the tooling direction. So, Draft analysis is used to check whether the inner and outer panels have any negative draft angles.

• LANCING

The flange in the inner panel that is used to join the inner and outer panels through mastic sealants is manufactured using sheet metal operation known as Lancing. Lancing is a piercing operation in which the workpiece is sheared and bent with one strike of the die. A key part of this process is that there is no reduction of material, only a modification in its geometry. So, the area of the portion that we trim off the inner panel must be greater than or equal to the area of the flange and also the distance between the outer panel and this flange must be 5mm for the mastic sealant.

• HEMMING

Hemming is a sheet metal forming process used to join sheet bodies by bending them to 180 degrees. It usually happens in three different stages firstly, bending it to 90 degrees and in the second stage bending it to 135 degrees and finally to 180 degrees. There are usually four different types of hemming processes:

1. Open hem
2. Closed hem
3. Teardrop hem
4. Rolled hem

Open type hem is used to join the inner and outer panels of the backdoor. So, a 2mm distance and 0.2mm tolerance values must be maintained between the inner and outer panels.

• LATCH TRAJECTORY

As seen in the hood design the same principle has to be followed in order to ensure proper locking of the backdoor.

DESIGN METHODOLOGY AND FINAL DESIGN:

It is necessary to make sure that all the surfaces are properly joint to their adjacent surface and there are no disjoints at their intersection edges to avoid further errors. we try to fix this issue by hiding the current surface and creating a new surface and after getting a good outer panel surface we separate both upper and lower surfaces of outer panel.

OUTER PANEL:

INNER PANEL:

EMBOSS:

TOOLING DIRECTION:

REINFORCEMENTS:

HEMMING:

CORNER RELIEF :

DRAFT ANALYSIS:

ASSEMBLY: