AGMA 04FTM10-2004 pdf free.The Failure Investigation and Replacement of a Large Marine Gear by: P. Hopkins, UK MOD Defence Logistics Organisation, B.A. Shaw,University of Newcastle, J. Varo, David Brown Textron, A. Kennedy, UK Ministry of Defence.
It was clear that revised propulsion limitations were required if the risk of further gear tooth failures or deterioration was to be minimised. Therefore, to reduce the loads being experienced by the damaged main wheel teeth, revised single and twin engine torque limits were calculated, based on the loss of tooth contact area and an assessment of the allowable stress to theoretically assure infinite life.
For the undamaged gearbox the torque at which additional engines were brought on-line (change-up criteria) was reduced, to avoid maximum tooth stressing at intermediate ship speeds. Whilst this did increase engine running hours and fuel consumption, it provided a margin of error against failure. The original twin engine maximum limitation was retained, but for ‘urgent’ operational use only and a lower ‘routine’ operational maximum was introduced to reduce the risk further.
A revised procedure was also introduced to ensure that good balance of torque was achieved between the Olympus gas turbines, particularly at high torques. With the torsion meter measuring output torque at the main shaft, it had been possible for one engine to be working harder than the other, effectively overloading the main wheel teeth, but for this to go unnoticed because the total shaft torque was within limits.
With these revised limitations and the close control of transients, the risk of further tooth failure was assessed as small, but the ship was still able to meet her operational requirements. Subsequently a monitoring system was installed, which captured torque readings on a continuous basis, to enable the cause of any transients to be investigated and avoided.
To provide early warning of any further deterioration,
an enhanced inspection regime was devised.
The defects were initially monitored following post repair trials and then by a rigorous three monthly inspection, undertaken using dye penetrant NDE by a specialist team consisting of MoD, David Brown and Design Unit.Other monitoring techniques,including vibration analysis, were considered, but in tests this had not been able to reliably identify and trend deterioration. It was considered that new cracks would not propagate at a rate that would not be identifiable by the imposed regime of dye penetrant NDE.
The task of full scale dye penetrant inspection on a large wheel is time consuming and unpleasant to carry out, but it was felt that this technique gave the best opportunity of providing full coverage of all teeth with good accuracy for identification and monitoring of the crack types known to be forming.Although other methods are available for crack detection (e.g. ultrasonics and eddy current), these were not as easy to apply with confidence in-situ.Being a relatively simple technique to apply, dye penetrant was also made available as a backup during ship’s staff visual inspections.
With the altered running procedures in place, further deterioration did not occur which suggested that the new restrictions had been pitched at about the right level.AGMA 04FTM10 pdf download.