Fatigue of Counterweight Sheave Trunnions - Heavy Movable ...

Fatigue Life Extension of Main Counterweight Sheave Trunnions
The report does not give any specific load or stress analysis data but by extrapolating the data contained
in the report, the nominal bending stress in the trunnion, at the transition fillet, appears to have been
between 17.5 and 19.3 ksi, not including any increase in stress due to stress concentrations. We
calculated the stress concentration factorto be 1.96. This yields a factored stress range between 68.6 and
75.7 ksi.
Valleyfield Bridne - Quebec
One of the counterweight sheave trunnions on the Valleyfield Bridge in Valleyfield, Quebec, which is
owned and operated by the St. Lawrence Seaway Authority (SLSA), fractured in the mid 1980's after an
estimated 800,000 plus cycles. The bridge was constructed in 1956.
The allowable nominal bending stress used for the design of the sheave trunnions was 15.0 ksi. The
SLSA issued their own design specification in 1956 for the bridges built on the Seaway. This
specification was probably based on the Canadian Engineering Standards Association Specification A20
published in 1927. The actual nominal bending stress is calculated at 15.6 ksi, not including any stress
increase due to stress concentrations. The calculated stress concentration factor is 1.8. The factored
stress range is 56.16 ksi.
Lubrication grooves in the trunnions for Valleyfield are stress raisers. As part of a sheave-strengthening
contract in 1962, the grooves were filled with silver solder and the lubrication grooves were moved to
new bearing bushings. The situation was made worse by the drilling of holes in the trunnion lubrication
grooves to help anchor the silver solder in the grooves. Reportedly, cracking initiated at one of these
anchor holes near the transition fillet in the shaft; leading to the eventual fracture.
Calumet River Bridne - Illinois
The investigation and repair of sheave trunnions in a railroad lift bridge over the Calumet River in Illinois
took place in 1953 and the history is somewhat spotty.
Various repairs had been made to the built-up sheaves, by welding, to tighten up the fit of the web plates
to the cast hubs and the cast hubs to the trunnions. The welds that connected the hub to the trunnion,
broke, allowi~lg the sheave to tilt relative to the trunnion. As corrective work was being carried out, a
crack was visually detected in the trunnion. The crack was 318" deep and covered 1 10' of the
circumference. The crack was in the journal at the root of the transition fillet adjacent to the web of the
sheave. The repair welding between the hub and trunnion apparently caused pre-existing but obscure
cracks in the trunnion to open up.
Eventually, cracks were found in 7 of 16 trunnion journals. The cracks varied in depth 118" to 112" and
covered an average of 120' circumferentially. We have no records concerning the eventual corrective
work.
The nominal bending stress is 11.6 ksi. The stress concentration factor at the transition fillet is 2.3. The
factored stress range is 53.36 ksi. The number of stress cycles between 191 6 and 1953 is not known.
Fatigue Analysis
Counterweight sheave trunnions are typically stepped cylindrical shafts with fillet radii at all section
transitions. The transition fillet produces a stress concentration that magnifies the nominal (bending and
torsional) stress. The radius size and trunnion proportions govern the magnitude of the stress
HEAVY MOVABLE STRUCTURES, INC.
gth Biennial Movable Bridge Symposium