Many inductive sensors currently used for steam valve position feedback for power generation have a 3/16 inch [4.75 mm] diameter high permeability core with a 4-40 UNC female thread (smaller and finer than an M3 x 0.5 thread) attached to a 3/16 inch [4.75 mm] diameter stainless steel core extension rod. The 4-40 UNC male thread on the core extension rod is typically 3/16 inches [4.75 mm] long, with a minor diameter about 0.086 inches [2.2 mm]. These two threaded parts may be silver-brazed together, but more often they are merely crimped. Over a period of time, turbine vibrations work harden this small threaded joint, so the core breaks away from its extension rod and the sensor then fails mechanically.
In ASG's PG and LA-27 series LVDT sensors for the power gen industry, as shown in the illustration below, the LVDT core is enclosed in a 3/8 inch [9.5 mm] diameter thick-wall stainless steel tube with a threaded plug in its end that is installed with high temperature thread locker and staked in place by rolling over the end of the tube below the thread diameter so the plug can never come out. This larger diameter core rod assembly rides in a bronze bushing in the LVDT's housing that is sealed from any contaminant ingress by two double-contact, high temperature elastomer shaft seals. The LVDT's bore clearance for the LVDT core rod assembly is small, so any lateral motion due to vibration is miniscule.
Mechanical connection to PG/LA LVDT core rods is a 5/16-24 UNF-2A male thread (on special order: M8 x 1.25). ASG offers rigid female coupling nuts in a few lengths, or a ball-joint coupling that permits small misalignments of the core extension rod screwed into the coupling, to connect to the steam valve actuator shaft. Typically this extension rod is 5/16 or 3/8 inches [8 or 9.5 mm] in diameter, so the entire ASG LVDT sensor assembly attached to the turbine is much sturdier and more robust than an LVDT or inductive half-bridge sensor with the much smaller diameter shaft hardware originally put into service.
