These components are computer calibrated – a stepper motor is used to push the plug in until the required flow is achieved. Twenty-seven highly accurate flow ranges (plus a virtually limitless number of specials) can be made from one standard set of parts, greatly reducing inventory. The flow selector shaft has a mushroom head into which a number of 2mm diameter radial holes are drilled, which meet with corresponding 1.6mm axial drilled holes in the underside of the head. The positional and dimensional accuracy of the radial and axial holes; minimal burr where the two holes break through into each other; and the repeatability with which the flow selector shaft and the setting plugs are manufactured are crucial to the assembly and performance of the design.
Brass is readily and easily machined and drilled at high speeds, (giving minimal drill breakage) produces little or no machining burr and cutting tool wear is minimal giving good dimensional repeatability. The tight limits on the required interference of the plugs into the holes is therefore maintained because of the machining properties of brass. In addition brass is compatible with medical gases (oxygen, anaesthetics etc.) without the need for plating making it an ideal choice of material for this application. |
