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bolt”, if tensioned rapidly, and if “thin” so only a few threads engage, substantial bending stresses
can occur on the shaft or “bolt”, and this is exacerbated if the nut is “thin”, as was the case here.

5. CONCLUSIONS

An experimental stress analysis of a Rotork actuator incorporating a disc brake assembly has
been undertaken. Use was made of a double rosette strain gauge system and lead through wires to
the shaft core with simple, yet elegant twisting wire connections to the monitoring equipment, which
obviated the need for telemetry or slip rings and balancing.
On site tests of the actuator arrest under trip, stall and disc brake conditions was undertaken
with the strain gauge bridge configured to monitor sequentially either bending, tension or torsional
stresses.
High bending offset stresses (187f 12 MPa), initially attributed to the disc brake,were in fact due
to poor setting up and loose tolerance of the thin lock nut-Belvel washer combination, which was
canting over and applying significant bending stress, via the actuator housing, to the shaft. Such
stresses were easily high enough to have caused the rapid fatigue cracking.
Redesign of the lock nut system effectively incorporating a longer nut system which could not
cant over on the shaft, together with improved processing of the shaft, effectively solved the problem.

REFERENCES

1. Burrell, I. J., Rotork Engineering report ER181, June 1991.
2. Blackbeard, P. J., ESKOM ReportG91121011,12 September 1991.
3. Schemer, O., Scientific Investigation Bureau, Report 920546, Rotork Actuaror Shafts, 25 May 1992.
4. Tait, R. B., Rotork 30 NABT Actuator Wormshaft-April Report, 21 April 1992.
5. Shigley, J. E., and Mischke, C. R., Mechanical Engineering Design, 5th edn, McGraw-Hill, 1989.
6. Ward, K. A., Nuclear Electric Report, No TD/SIP/mem/1190091, Section 3.5 and Appendix C. 1991.