The rotating wedge: A method of obtaining strain-free locations in a centrifugal field

Abstract

In strain measurement on rotating members using electrical resistance strain gauges and slip rings, difficulties associated with slip ring ‘noise’ may be overcome by several means, one of which is to adopt the full rotating bridge. As well as the active, the three inactive gauges of the bridge are mounted on the rotating side, the compensator preferably at a strain-free location. Investigators have adopted several methods of achieving this. A new method has been devised whereby the inactive gauges are mounted axially on the segmented circumference of a thin disc of the same material as the test member, rotating with it on the same axis. Axial slots, running in depth radially into the disc, divide the circumference into a large number of small segments or ‘teeth’, each of which is virtually the wide end of a narrow rotating wedge. Experimental evidence has been examined, demonstrating that the only significant stress in the narrow rotating wedge is radial stress. The theoretical solution for the stresses is therefore of the form of Boussinesq's two-dimensional elasticity solution for the stationary wedge symmetrically loaded at the apex. The solution for the narrow rotating wedge has been indicated on this basis. Subsequent to tests to determine the strain equivalent to slip ring noise in a new commercially produced slip ring unit, an investigation using strain gauges was carried out to verify that the segment locations of a 10 in overall diameter rotating aluminium segmented disc were strain-free to a speed of 2400 rev/min. The geometry of the segmented disc has been considered in respect of notch depth, nominal circumferential stresses in the body of the rotating disc and stress concentration at the notch roots. Other means of producing the necessary strain-free ‘platforms’ have been suggested for circumstances in which material is unavailable for manufacture of a segmented disc. An arrangement of test member, segmented disc and strain gauge circuit has been adopted for a steady-state centrifugal strain investigation on a water turbine.