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The metallurgical examination of the bolts showed that the failure was not associated with any
material or manufacturing defects. The bolts conformed to the specification requirements in all
respects.

8. CONCLUSIONS
(1) The catastrophic failure of the raise boring machine is associated with the fracture of the 32
drive head bolts. Thirty of the bolts have failed as a result of corrosion-induced fatigue.
(2) The bolts have failed due to a combination of high cyclic stressing induced by the operation of
the equipment at 13% above maximum thrust and corrosion from the water in the flushing
system.
(3) Chemical analysis, microscopic examination, and hardness testing have established that the
bolts conform to the required SAE J429 Specification.

9. RECOMMENDATIONS

(1) To prevent corrosion of the bolts the following measures are recommended:
(a) An oil-based red lead primer should be used to create a barrier at the cover-body connection.
(b) Mains water should be used at all times for flushing.
(c) Equipment should not be stored underground for any length of time.
(2) Excessive thrust pressures during operation should be avoided, Le. the equipment should be
used within the limits for which it was designed.
(3) All components should be torqued to the correct figure to prevent excessive movement in the
drive head.

10. FINAL NOTE
Since the investigation, a strict quality control system has been introduced at the mine for the
control of bolt sets used on raise boring machines. In addition, all the report recommendations have
been implemented, and the torque settings on the drive head bolts have been increased with the
approval of the machine manufacturer. Following subsequent finite element modelling, the thickness
of the cover and the length of the drive head bolts have been increased for greater stiffness. The
equipment has now operated without problems for several years.

REFERENCES

1. Hammond, I., Austrafiun Mining, 1992, 84(5), 14-18.
2. Cook, N. G. W. and Lancaster, H. F., in Tunnelling in Rock (a course of lectures held at CSIR, Pretoria, 22-26 October
1973), ed. Z. T. Bieniawski. Pretoria, 1973.