configuration the frame 1 defines a rectangular aperture one side of which is partially closed by a face plate 2 having a slot opening 3 which when the valve is open provides a passageway between the two regions separated by the valve the top of the frame 1 is closed by a mounting plate 4 through which mechanical movement is imparted to a gate 5 which is adapted to close the opening presented by the slot the surface of the gate which faces the slot has a circumferential groove into which a suitable seal such as an o ring 101 fig7 9 or gasket is inserted when the gate is pressed against the faceplate the seal provides a vacuum tight junction effectively closing the aperture of the slot the gate is supported by two rods 6 not visible in fig1 and 2 one is shown in fig4 7 9 the rods 6 pass through apertures in the mounting plate 4 and through bellows 7 omitted in fig6 see fig9 11 which are provided around the moveable rods 6 to give a vacuum tight seal for the apertures through which rods 6 move a hinge block 8 is mounted on the mounting plate 4 in the following manner the hinge block 8 itself includes axle shafts 9 fig4 and 6 which project beyond the ends of the hinge block 8 each axle shaft 9 is rotatably supported in a bearing member 10 fig4 within a support 11 which in turn is affixed to the mounting plate 4 the dimensions of the hinge block 8 axle shafts 9 and.