| 摘要 |
A rectangular frame set into the concrete surround at the head of a shaft supports one or more slab doors that are hinged to the frame. Adjacent each hinge line brackets fixed to the frame support a fixed spindle upon which a helical spring is secured. The spring has an extending limb at one end that bears against the under surface of the door. The other end of the spring is engaged with a winding bushing rotatable on the fixed spindle, and the bushing has a hollow hub for accepting a detachable winder so that the spring may be pre-torqued to the proper loading to raise the door from its horizontal closed position to a substantially vertical attitude for access to the shaft or other opening. The frame defines a drip trough to collect moisture passing the door edges and when two doors are used a channel extending along the butt line of the doors collects water passing between the adjacent edges of the doors. Safety stops and externally accessible latch handles may be combined with the doors which may have stiffeners fixed to their sides.
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