Abstract: A strain transducer is adapted to measure strain at a chosen depth below the surface of a workpiece, such as at the neutral axis of a draft bar. The transducer has a hollow cylindrical plug (6) projecting from a co-axial shoulder (7) of slightly larger diameter, the distal end of the plug being force fitted with a disc-like diaphragm (9) on which strain gauges (10) are mounted. These are on the inside face, and connections (11) extend up inside the plug to encapsulated electronic circuitry at the proximal end. The workpiece is bored (3) to an extent greater than the chosen depth and then counterbored (4) to match the plug and shoulder, and the transducer is inserted with an interference fit.

Abstract: A strain transducer of the kind in which a plug is inset into a workpiece surface requires an accurate interface fit with a drilled hole. One method proposes an oversize hole (12) which first receives an annular insert (14) in whose aperture (15) the transducer plug is a precision fit. The outer cylindrical surface of the insert has axially parallel serrations (16) which bite into the sides of the hole. Another method also has an oversize hole (22) whose mouth is constricted by a cylindrical punch (25) around a precision mandrel (24) which is then replaced by the transducer plug (28). Serrations (35) may alternatively be provided directly on the transducer plug (32), which can then be driven into a drilled hole (36) of only approximate accuracy.