Abstract

A flush valve is provided for opening and closing a drain connecting a toilet bowl to a waste holding tank maintained at a partial vacuum. The flush valve is located external to the drain and comprises a pinch or nip mechanism surrounding a flexible section of the drain and an electro-mechanically operated actuator for moving the pinch mechanism to close the drain by pinching it shut or to open the drain by pulling it apart. The actuator includes a motor for moving a piston in opposite directions, and a cylinder having one end connected to the pinch mechanism and another end containing an enclosure. The piston is retained and reciprocable a limited distance within the enclosure, and a spring is interposed within the enclosure to provide biasing means between the piston and the cylinder. The spring has sufficient stiffness relative to the force needed to shut the flexible drain section, in the absence of an obstruction, so that the flexible drain section may be fully closed without compression of the spring by the piston. However, when the flexible drain section is nipped shut, or if an obstruction is encountered, the spring will be compressed within the enclosure by the piston, causing operation of a switch to deactivate the motor and prevent motor stalling and damage to the valve. A manually operable shut-off flush valve also is provided, comprising a plate slidable through a coupling in the drain line to bring an opening in the plate in or out of registration with the drain line to open or close it, respectively.