Crosshead Bearing Unscheduled Failure

As a recently appointed chief engineer you are requested to survey the crosshead of a main engine following an unscheduled repair due to bearing failure.

Out line the information you would request prior to the survey

Describe the survey procedure you would adopt stating, with reasons, the areas which should receive particularly close attention.

State with reasons, what information you would request and the operation s you would require observe after reassembly of the crosshead

Ans)

Information requested prior to survey:

Previous survey reports: Engine hours when survey was carried out, details of recorded clearances, details of any of any possible marking on pin, bearing condition

Manufacturers service letters appertaining to crossheads, which may give a clue as to reason for failure.

Recent LO analysis; Any deterioration in the lubrication quality due to contamination by cylinder oil or fuel may give a guide to why the bearing failed.

Sample of any detritus found in Lo filters, plus any records of anything found in the days leading up to the failure. If white metal is or has been found, it may indicate that the bearing material was failing over a period of time.

Engine room log for the weeks leading up to the failure. And since the repair, giving details of oil pressures and temperatures, fuel rack readings, exhaust temperatures etc. plus power card readings before the failure. This will give an indication whether overloading has taken place. Reports of engineers made at time of repair. As to action taken, conditions found etc together with any photographs they may have taken.

The old pin(if replaced ) and bearing shells.

(b)

Assuming that the crosshead pin and piston rod assembly is resting on the support brackets bolted to the guides and that the conrod is swung down to crankcase door level inspect bearing half in con rod(the loaded half) for evidence of scoring, squeezing, wiping, cracking of bearing metal, scoring will indicate hard particles in LO or damage to crosshead pin. Squeezing and cracking points toward overloading, whilst wiping will indicate poor lubrication and/or overloading. Some manufacturers have an overlay of a lead/tin alloy on the tin aluminum bearing metal, and the state that the bearing should be replaced if this overlay has worn away. Other manufacturers have wedges machined in the bearing material and recommend replacement when the wedges are reduced to half original width.

If it is considered necessary to inspect the top half of the bearing, then the con rod must be reconnected the engine turned to BDC and the top bearing cover lifted.

Inspect closely the underside of the pin. this is the side which is continuously loaded. The finish should be mirror like with no evidence of scoring or roughness. If any is found then the pin must be replaced or polished. Because the crosshead bearing is not hydrodynamic ally lubricated, and relies on boundary lubrication at the beginning and end of the swing then any damage to the pin will damage the beading material.

Examine the guide shoes and guide strips for evidence of squeezed out metal, also check the sump pan for white metal. If there is any doubt as to the condition of the guide shoes, then the con rod must be removed and the pin must be lowered to the crankshaft so that the guide shoes can be examined.

On reassembly the crosshead pin and the guide shoe clearances should be taken and recorded. Incorrect clearances will not allow a LO film to be built up when the rubbing speed is at a maximum. The engine should be turned through several revolutions and the turning ,motor ammeter observed for any change in load. This will ensure that there is no binding between moveing parts The oil pumps should be switched on and the flow through the bearing and guide shoes observed. This will ensure that there is an adequate flow for cooling and lubrication purposes. The oil pressure should be recorded.

After boxing up and preparing the engine, the engine should be run for 30 seconds,2 minutes, 10minutes and 30 minutes, on low load, opening up after each run and checking the bearing for signs of overheating or squeezing out of white metal. This should help prevent damage to the pin if the bearing does, wipe. Load on the engine should be slowly built up over a period of time and then a set of power cards taken to ensure that the engine is not overloading or unbalanced.