Optimization Design of the Structure of the Manual Swing-out Luggage Compartment Door of Passenger Cars

This study studies the design of the manual swing-out luggage compartment door of a medium-sized passenger car, which makes design process more specific and design ideas clearer. Firstly, the tumbler arm is optimized and its weight is reduced by the method of designing a four-bar linkage of luggage compartment door and dynamic and structural strength analysis of the structure of the luggage compartment door is performed. Then, parametric design of the tumbler model is carried out in order to map out different vehicle models quickly. Finally, motion and interferences analysis of the compartment door is made to avoid any potential structure intervention.


INTRODUCTION
The luggage compartment door, one of the most important parts of passenger cars, provides great convenience to pick and place luggage for passengers.With rapid development of national economy, people have an increasing demand for the luggage compartment.In 1980s, the swing-out compartment doors were employed in passenger cars and touring buses in Japan and Europe.In 1990s, this type of compartment doors were also utilized in medium-high grade passenger cars in China.At present, manual swing-out luggage compartment door is widely applied in middle and high grade travel buses due to its characteristics of large opening angle, reliable operation, good sealing property, relatively little room taken when opened and of its convenience to pick and place luggage, etc (Chen, 1999).
Luggage compartment door is a hatch which moves out with free track.The door is supported by the tumbler arm which can drive the door move in parallel motion when it swings.Therefore, the door is also known as the translational door.
This study demonstrates the superiority of the parametric design and aims at, by analyzing mechanics and motion to verify reliability of the structure and analyzing motion interferences, providing guide for production, shortening design cycles and cutting costs.

DESIGN OF THE FOUR-BAR LINKAGE
The four-bar linkage has great application value in passenger cars production.Apart from applications in passenger doors, it is also extensively utilized in the Fig. 1: Assembly model of the compartment door luggage compartment door of medium and high grade passenger cars.The motion of four-bar linkage completes in four parallel planes (to avoid interferences with each other, each bar moves within one plane).This motion analysis can be undergone within one plane, so only trajectories of two bars (i.e., the upper bar and the lower bar respectively) are required and their hems are projected to any plane that is parallel to that of the two bars lie in.Thus, to grasp motion conditions of the whole mechanism, only motion situations of the two projections need to be studied (Sheng, 2007).
Based on existing engineering drawings and practical measurement data, this study builds a 3D solid model of the luggage compartment door for one model passenger car and explains installation procedures and performs interference analysis.The 3D solid model of the assembled compartment door is as shown in Fig. 1.
The luggage compartment door mechanism mainly includes the bus body, compartment door entity, tumbler arm mechanism, left and right balance bars, gas

MECHANICS ANALYSIS
In mechanics analysis of the four-bar linkage of the luggage compartment door, the balance bar can be ignored due to its much light weight, so can be friction force of the hinge point.In order to confirm the lifting force F of the gas spring, the maximum lifting force of the door during its opening process should be determined.This study determines the lifting force through analyzing force conditions of four working conditions of the door.The four working conditions are: close, open to the highest position, the tumbler arm being horizontal and the gas spring supporting tumbler arm to the longest position (Fu et al., 2002;Wen, 2003).
Based on the force analysis and calculation of the four key positions, the following conclusions can be reached (Liu, 2004): • When the door is closed, the torque making the luggage compartment door automatically close is not large enough.Therefore, make the change of the fixed hinge point position of the gas spring so that a clockwise force is produced when the door is closed.
• When the door is opened to the highest position, the required minimum lifting force is 315.54N which can keep the door in balance at the highest position not to fall down.• When the tumbler arm is at the horizontal position, the lifting force required should be larger than 445.24N which can make the door keep moving up.• When the effective arm of force of the gas spring stretches to the longest position, the lifting force required should be larger than 429.04 N so that the door can keep moving up.• When the tumbler arm is at the horizontal position, the lifting force required is the maximum, which is ought to be equal to or larger than 445.24N. • The door can be opened as long as the horizontal force is slightly larger than 73.09N.
As concluded above, the nominal lifting force of the gas spring can be selected as F = 500 N.
During the movement process of the luggage compartment door, the tumbler arm maximum external and internal forces, while the balance bar O 2 O 4 , which just contr trajectories of the door and is relatively light, generally can be ignored in the analysis.This study studies the bearing force analysis of the tumbler arm when the door moves to the top position limitation and touches the stop blot (i.e., the door moves to the highest position).The bearing force analysis of the tumbler arm refers to Fig. 4 and Table 1, where A, B, C, D, E, Z act as six bearing force segment points of the tumble arm.As a result, the shearing force and bending movement of each point can be obtained as described in Table 1.The shearing force diagram and bending movement diagram can be shown as Fig. 5 and 6 Theoretical Mechanics, Harbin Institute of Technology 2002; Sun et al., 2006).

SlightlyLessthanabsolutevalue
When the tumbler arm is at the horizontal position, the lifting force required is the maximum, which is ought to be equal to or larger than 445.24N. The door can be opened as long as the horizontal force is slightly larger than 73.09N.
bove, the nominal lifting force of the gas spring can be selected as F = 500 N.
During the movement process of the luggage compartment door, the tumbler arm O 1 O 3 bears the maximum external and internal forces, while the , which just controls movement trajectories of the door and is relatively light, generally can be ignored in the analysis.This study studies the bearing force analysis of the tumbler arm when the door moves to the top position limitation and touches the e door moves to the highest position).The bearing force analysis of the tumbler arm O 1 O 3 refers to Fig. 4 and Table 1, where A, B, C, D, E, Z act as six bearing force segment points of the tumble arm.As a result, the shearing force and bending movement of each point can be obtained as described in Table 1 movement at the movable position E where the gas spring moves about which is 19.73 Nm.So the door is easy to be sheared or bent on the two positions and frequently encounters failure problems near there in practical applications.Therefore, without regard to specifications, economies and other factors, when selecting the rated lifting force of the gas spring it only requires the lifting force is larger than the necessary maximum lifting force when the tumbler arm is at the horizontal state (Chen, 2004).
It is learned that the tumble arm bears the maximum internal force at the fixed bearing position from the shearing force and bending movement analysis of the tumble arm, as a result, it is actually easy to be sheared or bended.So strength checking is necessary.
A type of 45# steel pipe is chosen as the material of tumble arms, of which diameter D is 32 mm, thickness t is 2 mm and internal diameter d is 28 mm.And its tensile strength is about 600MPa, shearing strength is 370 MPa.So it is available that: Bending moment of the tumbler arm at the movable position E where the gas spring moves about which is 19.73 Nm.So the door is easy to be sheared or bent on the two positions and frequently encounters failure problems near there in practical applications.Therefore, without regard to ifications, economies and other factors, when selecting the rated lifting force of the gas spring it only requires the lifting force is larger than the necessary maximum lifting force when the tumbler arm is at the ned that the tumble arm bears the maximum internal force at the fixed bearing position from the shearing force and bending movement analysis of the tumble arm, as a result, it is actually easy to be sheared or bended.So strength checking is necessary.
type of 45# steel pipe is chosen as the material of tumble arms, of which diameter D is 32 mm, thickness t is 2 mm and internal diameter d is 28 mm.And its tensile strength is about 600MPa, shearing strength is Section modulus of bending resistance: The maximum shearing force of the tumbler arm: F It is thus clear that if the strength of the tumble arm is sufficient the diameter of the steel pipe can be reduced as applicable to achieve light weight and cut costs.Besides, according to the checking calculation, the gas spring whose rated force is quite large is also very qualified.

GAS SPRING DESIGN
In practical applications, after primary selecting according to specifications which a bus company provides, the rated lifting force of gas springs is confirmed based on the lifting force the compartment door actually requires, finally the specification of the gas spring is determined as YQ10/22-180-500 (O-O) 500.Its parameters are as follows: diameter of the piston rod is 10mm; external diameter of the cylinder tube is 22 mm, stroke is 180mm, extend length is 500 mm, nominal force is 500 N.
Installation of the gas spring is simple and a preferable method is the ball joint connection.In the installing process, the position of the piston rod of the compressed gas spring must be downward so as to lessen friction and ensure the best damping mass and buffering property (JB/T 8064.1, 1996).
The position of the hinge point of the gas spring must be determined before selecting the lifting force of the gas spring.Within allowable design space, the movable hinge point O 5 should be as far as possible from O 3 so as to reduce the necessary lifting force of the gas spring.Through mechanical calculation, it is learned that when the tumbler arm O 1 O 3 is at the horizontal state, the lifting force comes to the maximum value.In this case, when the tumbler arm of the compartment door is at the horizontal position, the distance from point O 5 to point O 1 is 87.1 mm and O 5 to O 3 is 562.6 mm.
On the basis of structure and workmanship requirements and engineering experience, also considering the longest position of arm of force and the horizontal position of the tumbler arm, the moveable hinge point of the gas spring is finally confirmed as shown in Fig. 7.
The fixed point O 3 can be determined based on the maximum extend length of arm of force as shown in Fig. 8. Combining this geometry condition and the gas In addition, length of the gas spring changes with the opening angle of the compartment door which can be demonstrated as: the longer →the shortest →the longest and torque generated by the gas spring as well as is changed: negative moment →0→positive moment; While the stretching force of the gas spring always changes little in the whole process.

PARAMETRIC DESIGN AND MOTION INTERFERENCE ANALYSIS OF THE COMPARTMENT DOOR
This study uses 3D parametric modeling method to build a model of a tumbler arm of the compartment door.
Because tumbler arms are used for different vehicle types or various compartment doors of the same vehicle type, their structure sizes will vary.In order to quickly work out 3D and 2D drawings of different types of buses or different tumbler arms of compartment doors, parametric modeling technology should be applied.
It is easy to know that by changing x, y coordinate values of point O 3 which are relative to O 1 , track of the centerline of the tumbler arm can be changed accordingly.Then rescan the track to obtain a manual solid model which can be used for different vehicle types or different compartment doors.Provided the coordinate value of point O 3 that is relative to O 1 is P13 = 73.026mm and the coordinate value of O 2 is relative to O 1 is P12 = 514.384mm; P13 and P12 can be changed easily in "formula dialog box".If P12 is changed to 600 mm, a new trajectory can be obtained.Besides, a relational expression between P12 and P13 can be established.
Different compartment doors of the same type bus have different door widths, so just change their widths when designing different doors.Because the solid model of the tumbler arm is created by zygomorphic design in the model building, its size can be easily modified within its plane of symmetry (Leo and Jens, 1995).

CONCLUSION
The primary design of the compartment door is finished in a 2D plane, which can determine a rational and economical lifting force of the gas spring.Besides, the force analysis, motion interference analysis and parametric design on the manual swing-out luggage compartment door mechanism are undergone in this study to ensure structure strength of the compartment door and achieve configuration optimization and weight reduction of tumbler arm mechanism.

Fig. 4 :
Fig. 4: Bearing force conditions of six segment points of the tumbler arm Bearing force conditions of six segment points of theBearing force values of six segment points of the tumbler -

Table 1 :
Bearing force values of six segment points of the tumbler arm