Development of strain gauge techniques for the measurement of propeller shaft torque in distant water trawlers

• Published in: Journal of Strain Analysis
• Main Authors: Bennett, R, Hatfield, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: SAGE Publications 1966
• Subjects: General Medicine; Arctic; North Atlantic
• Online Access: http://dx.doi.org/10.1243/03093247v012102; http://journals.sagepub.com/doi/pdf/10.1243/03093247V012102

In the course of research and development work on deep sea fishing vessels, a continuing requirement is for a system to measure and record propeller shaft torque that will remain reliable and accurate on commercial fishing voyages of up to seven weeks duration, in the extreme environment of Arctic and North Atlantic waters, in all weathers, and at all seasons of the year. The system should be suitable for use in conjunction with dial indicators and recorders on voyages manned by development engineers and also suitable for use in automatic data logging systems. A system has been developed and tried consisting of d.c. energized foil strain gauges and simple slip rings consisting of silver strip wrapped on to the shaft, with unamplified readout on to galvanometers of either direct reading light spot type or those in trace recorders. The system can be used for the measurement of both steady and oscillatory torques. Almost 100 per cent reliability has been achieved on a number of installations. Both calibrated and uncalibrated installations have been used. The calibrated systems have an accuracy of better than 1 per cent. From experience gained to date the conclusion is drawn that an uncalibrated system can be expected to have a mean accuracy of about ±31/2 per cent, using the maker's gauge factors and a mean value of 11.9 ×10 6 for the torsional modulus for mild steel. In several vessels special short lengths of intermediate shafting have been provided in duplicate, in order to facilitate frequent re-calibration. Scatter in a given set of readings is low, the standard deviation being about 1 per cent at a surface stress of 3000——4000 lb/in 2 (steaming power range) and up to about 21/2 per cent at stresses in the range 1200——2500 lb/in 2 (trawling power). Determining scatter due to the variability of the instrument in the face of variations due to weather, ship displacement and other causes was one of the problems encountered during early trials on commercial voyages. It was anticipated that zero drift could be a very severe problem but this has not been the case. The maximum drift over several weeks has been 21/2 per cent of maximum torque and it has frequently been negligible. The system is now being applied to the measurement of thrust, using hollow shafts. Both torque and thrust signals are being fed into data loggers on certain trawlers. The problem of automatic logging of mean thrust and torque in the face of oscillations with periods of up to 14 seconds, induced by ship motion, without imposing heavy strains on analytical and computing facilities, has been solved.