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
Fig. 2: Model of the tumbler arm mechanism
Fig. 3: Coordinate values of each hinge point when the door is closed
springs and some other significant components. The tumbler arm mechanism is as shown in Fig. 2.
For convenience of calculation analysis, a simplified mathematical model can be set up as shown in Fig. 3. The compartment door can be simplified into a four-bar linkage which is an approximate parallelogram, for example, the tumbler arm mechanism is simplified into O1O3 bar, the balance bar is simplified into O1O3 bar, the compartment door entity is simplified into O1O2 l bar and the bus body is simplified into O3O4 rack.
Take O4 as the origin of a coordinate and establish a Cartesian coordinate as shown in Fig. 3. On the basis of on-site measurements and drawing information, coordinate values of each hinge point when the door is closed can be obtained as: O1 (68.4, -181.7), O2 (73.7, -
512.9), O3 (-4.7, 332.6), O3 (0, 0); Length of O1 O3 bar:
19.5 mm, O3 O3 bar: 521.1 mm, O3 O3 bar: 331.2 mm,
O3 O3 rack: 332.7 mm, as shown in Fig. 3.
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 315.54 N 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
445.24 N 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.