The door consists of two equal wings 8 and 9. Wing 8 is connected by turning pair B to slider 7, which reciprocates along fixed guides b. Wing 9 turns about fixed axis O and is connected by turning pair A to wing 8. Rigidly secured to wingk 9 is lever 6, which is connected by turning pair C to link 5. Link 5 is connected by turning pair D to rod a of piston 3. The left end of cylinder 1 is constantly connected to air tank 2 so that piston 3 is held by compressed air in its extreme right-hand position in which the door is fully open. At this, three-way valve 4 connects the right end of cylinder 1 to the atmosphere. When the handle of valve 4 is turned, the right end of cylinder 1 is connected to air tank 2. The difference in effective areas on the sides of piston 3 moves it to the left, closing the bus door and holding it in the closed position.
$4302$CHP,FD$