Lower Shafts of Fixed Ball Valves: Practical Experience and Advice on the Problem of Structural Deviations

The lower shaft of a fixed ball valve is often an easily overlooked but costly part to get wrong. Whether the ball core rotates smoothly and whether the axis is shifted or not, it often depends on whether the size and centre datum of this lower shaft have been controlled in the machining and assembly stages.

We have dealt with a number of similar problems, a customer with DN500 Class 600 structure, the field opening and closing there is a clear sense of frustration. At first suspected that the actuator or ball eccentricity, dismantled only to find that the lower shaft positioning step runout is too large, bearing seat clearance also exceeded the design value, resulting in the entire ball core running centre displacement.

Lower shaft structural design, it is recommended to use step positioning, and at the same time, it should be clear whether in the end by the shoulder positioning or by the bottom step drop. If this detail is not clearly marked on the drawing, it will be easy to install the wrong one when relying on empirical judgement in the later assembly. When we deal with this side, we will generally take the axis of the ball as the centre to establish the reference line of the upper and lower holes of the bearing, and then adjust the positioning structure according to the upper and lower shaft structure.

Processing technology, rough machining of forgings must be tempered after processing, and then a unified clamping fine-turning of all key surfaces. We often control the step runout within 0.03, coaxiality ≤ 0.015. Some sliding structures will also do cylindrical grinding or nitriding, especially the customer requires low friction or long life operation of the project.

Although this part is not big, once the centre deviation accumulates, it will affect the stability of the whole sealing structure, especially if the ball seat engages insubstantially or the limit structure is offset. We recommend that the mating direction be clearly marked on the drawings, or that the overall structure diagram be provided before assembly, so that we can do an interference simulation in advance to prevent subsequent rework.