Answer MEP Question 59

Q. What examinations must be carried out when a crosshead bearing of a large, slow-speed engine is opened up for survey? 

General Inspections:

- Inspect bearing half in con rod (the loaded half) for evidence of scoring, squeezing, wiping, cracking of bearing metal. Scoring is usually due to LO failures and introduction of foreign particles or abrasives, Squeezing and cracking shows that the bearing is overloaded. Wiping indicates both LO failure and overload. 

- The top half of the bearing is to be inspected, for this the connecting rod must be reconnected the engine turned to BDC and the top bearing cover lifted. 

Crosshead pin: 

- Bottom portion of the pin should be inspected to check the appearance which shall show glossy appearance without any physical scoring or scratches, if found the pin shall be replaced or polished by approved means. Pins with scoring or roughness will further damage the bearing as lubrication technic used here is boundary lubrication and not hydrodynamic type. - The pin is also subjected to thorough inspection for cracks by NDT (Non Destructive Testing) methods, especially in way of any radii. 

Guides:

- Guide shoes and guide strips to be closely inspected for squeezed metal or wear articles, if any seen then the connecting rod must be removed and the pin is lowered for further closer examination. 

Further checks:

- The engine Is turned through several revolutions and the turning gear motor ammeter observed for any change in load. This will ensure that there is no binding between moving parts. - The lithe oil pumps started and the flow through the hearing and guide shoes observed to ensure that there Is an adequate flow for cooling and lubrication. 

After installation

Clearances:

Prior to reassembly the crosshead pin and the guide shoe clearances should be taken and recorded. Incorrect clearances will impair lubrication.

Clearance between pin & shoe:

The radial clearance between the crosshead 'pin' and 'shoe' is very difficult to measure when the pin is fitted in the engine. It can only be taken by measuring the pin outside diameter and the shoe inner diameter by a micro-meter. 

Clearance between Shoe & Rail:

- It is measured with a long feeler gauge inserted at the top and bottom of each guide shoe. 

Clearance between Plate a Rail:

- The complete crosshead must be pressed axially to one side with suitable hard wood edges or similar aids. This side pressure should be exerted onto the shoe and not the pin. Clearances taken With a feeler gauge. 

- After installation the engine shall be put on run-in condition following the manufacturer recommendation on time and load. Between each run, the crankcase is opened for checking the bearing for signs of overheating or squeezing out of white metal. 

- Set of power cards taken to ensure that the engine is not unbalanced or overloaded. 

Information Required for Cross Head Bearing Survey:

- Check with class about the correct unit which is due for inspection. 

- History of the unit shall be drawn from planned maintenance record to ascertain all previous inspections/maintenance carried out on crosshead bearing under survey inspection. 

- Prior to and after the job acquire set of indicator cards and log readings taken from the engine to inspect for excess cylinder pressures, and the latest lube oil analysis to inspect for lube oil condition and impurities. 

- Ascertain no recommendation or Conditions of Class is present which could directly or indirectly associated to the cross head.