Abstract
M.Phil.
Pulleys are critical items in belt conveyors. Their primary role is to drive large mining conveyor
systems, facilitating the transportation of ore over extensive distances, both in South Africa
and abroad.
The effect of the manufacturing process (with specific emphasis on the induced residual
stresses) on the fatigue performance of conveyor pulleys is herein investigated and reported.
A pre-selected pulley was chosen based on size, suitable for experimental work as well as
practical specifications. The static and fatigue performance of the pulley were investigated
both with the current design criteria as well as Finite Element Analysis, with comparisons
drawn.
The material data for the Finite Element Models was obtained experimentally with tensile
tests of the SANS 1431 350 WA plate. The magnitude of the residual stresses were obtained
experimentally by using the incremental hole-drilling technique for non-uniform residual
stresses. The method was verified by comparison with the Finite Element Analysis results for
the non-linear material analysis of the roll-bending of the shell.
The fatigue analysis revealed that the stress ranges of interest for the pulley were below the
non-propagating stress range, and hence theoretically infinite fatigue life would be possible
under constant amplitude conditions. The operational fatigue life required for the pulley would
be possible, when considering the latest S-N curve for "very high cycle fatigue". The stress
intensity factors for the weld details were also below the threshold value and hence crack
growth should not occur, upon crack initiation.
A new design criteria was proposed for the fatigue analysis considering either fatigue
assessment standards or fracture mechanics for the assessment of the butt-welds.
This investigation showed that the manufacturing-induced residual stresses may play a
significant role in the fatigue life of a pulley. The fatigue strength of a machined stressrelieved
joint is higher if the stress range is partly compressive. The fatigue strength of a
machined as-welded joint is higher than estimated by the fatigue classifications. This is due to
residual stress relaxation that occurs at the weld toe because of yielding and hence a
subsequent reduction and redistribution of the residual stresses. This reduction in the mean
stress level, with a stress range that is partly compressive, would mean an increase in the
fatigue strength of the joint.
This would in conclusion result in similar fatigue strengths for a stress-relieved and an aswelded
joint. This would additionally depend on the extent of the reduction of the residual
stress in the as-welded joint.
Recommendations were suggested for further experimental and numerical work for both the
T-bottom and Turbine-type pulleys.