Rigging ​

Hinge rig ​

Download scene: hinge_rig.ma

1. Create left and right flaps, center hinge
2. Parent left and right flaps to center hinge
3. Move pivot for left and right flaps to sit near (or on) the hinge
4. Create locators, put them far left and far right of the flaps
5. Select left locator, left flap, aim constraint, aim axis is -1 0 0, limit to Y axis only
6. Select right locator, right flap, aim constraint, aim axis is 1 0 0, limit to y axis only
7. Move the parent hinge, the flaps should try and keep aiming at their respective locators.

Hinge on rails ​

Download scene: hinge_rail_rig.ma

Added a set driven key that drives the locator x position by the hinge z position.