A quarter-turn valve includes a valve body having an inlet, an outlet, and a diaphragm adjacent arranged in said valve body for movement between a closed and opened position for restricting and controlling fluid flow. The valve further includes an input shaft for receiving a torsional input load in a closing direction and a torsional input load in an opposite opening direction. A cam is mounted on the input shaft. The cam transforms input loads to linear output loads that flex the diaphragm between its closed and opened positions.