Speaking of the valve stem in the pump valve fitting, it is actually a shaft type part, and its length itself will be larger than the diameter. Generally it will also be composed of outer cylindrical surface, external threads, external conical surface and its outer four aspects. According to the different structure of the end, the valve stem in the pump valve fittings will be divided into two forms with a sealing cone and without a sealing cone.
The stem structure in the pump valve fitting has an outer cylindrical surface with a very high roughness requirement. The outer cylindrical surface of the valve stem is in close contact with the packing. In order to prevent the valve stem from scratching the packing during rotation or axial movement, it is necessary to ensure that the seal between the packing and the packing is performed afterwards. Roughness requirements are high, generally Ra3.2 ~ 0.8um.
The stem structure in the pump valve fittings with external threads also requires attention. Except for valve stems of individual valves such as ball valves, most stems have external threads. The external thread is used to convert the rotational motion of the power unit or the transmission into the axial movement of the valve stem. In this respect, the opening and closing of the valve can also be achieved and the valve can be subjected to a large axial load. In general, rods will also use single-headed standard trapezoidal threads.
The main technical parameters of the stem structure in the pump valve fittings are surface accuracy and tolerance. For the diameter accuracy, it is generally at level 11, and the surface roughness should also be noted at Ra3.2~0.8um. The precision of the thread is 7~9, and the surface roughness should be Ra6.3~3.2um. The surface roughness of the sealing cone is Ra3.2~0.4um.
The valve stem structure tolerances in the pump valve fittings shall not exceed 1/2 of the outer diameter tolerance for the full length of the shaft line of the valve stem. The taper of the outer cylinder is also to be noted that it must be no more than 0.015 mm per 100 mm length. The concentricity of the stem and seal cone facing the outer cylinder of the valve stem structure in this pump valve fitting generally does not exceed 1/2 of the outer cylinder diameter tolerance, and the force conditions are more complicated, requiring special attention.http://www.dmgcastings.com/