a device to control the flow of fluid uses a cam and ratchet wheel . a valve is operated by the cam portion of the wheel , which bears against the valve stem causing it to open . fluid flow causes a pawl to drive the ratchet portion of the wheel , rotating the wheel until the valve stem bears against a low point in the cam , permitting the valve to close . a part of the ratchet portion does not contain ratchet teeth , in which area the pawl cannot drive the wheel . in this area , the cam has an increasing radius from a radius corresponding to a closed valve , to a radius corresponding to a fully open valve . this permits manual control of fluid flow without disengagement of the device .