Maintenance and Repairs

Replacing a Damaged Cam on a B&W Slow Speed Engine

How a slipped cam was removed and replaced on the camshaft of a large 2 stroke marine diesel engine
 

Thanks to Willie Chree for contributing this story of a slipped cam on a B&W 7K90GF engine, how the engine was prepared for emergency running and how the repair was effected.

About seven years ago we were on our way back from the US Gulf to the Humber on board a large bulker of 100 000 dwt. The engine was a  B&W 7 cylinder K90GF (900mm bore, 1350 stroke). We were in the Western Approaches and we were running UMS (Unmanned Machinery Space) at the time as per night orders from the Chief Engineer. I was Second Engineer and I was on the squeak (duty engineer) that night when my phone rang about 0200. I recollect it was the second mate advising me that the engine seem to slow down and there was a fair number of sparks coming from the funnel. Then off went my alarm so I had an idea something was seriously wrong.

On entering the Engine Control Room I noticed that the camshaft lub oil pressure was low and all the exhaust temperatures were abnormally high apart from cylinder number 3 which was low. I pressed the panic alarm (Note: - For those of you unfamiliar with this term, this is the engineers call alarm which is there to summon assistance; marine engineers with their inverted sense of dry sarcastic humour refer to it as the panic alarm. However it doesn't mean the engineer is panicking - not that he'll ever admit it anyway! - WM) and got lub oil pressure restored to the camshaft system. By this time the Chief Engineer and the other engine room staff were down below. We could not stop as we were approaching the separation zone so we slowed down as much as possible whilst still maintaining steerage on the vessel.

On further investigation it was obvious what the problem was;
"Notice anything?" asked the Chief
"Yes" I replied
"What?"
"Number 3 and number 4 exhaust valves are opening and closing together" I replied.

We shut the fuel off Number 3 and  headed for the Channel Islands where we stopped to prepare the engine to run on 6 of the 7 cylinders

On investigation of number 3 exhaust valve cam and operating  gear we found that number three exhaust cam had slipped on the shaft. This had been caused by the failure of one of the thrust retention collars (distance rings) on the duplex roller race inside the cam follower roller. The roller failed to turn as it caught between the guide and the cambox top. The follower then must have freed up and the exhaust valve started to operate again at the wrong time.

Roller and guide from a K90 engine (although not the damaged one in the story)

 

The C/E called the Engineer Superintendent at home, who then gave him a B&W phone number to call to arrange parts and service.

When the chief called Copenhagen and told the tale of woe he was greeted with a very laconic reply:

 

"Oh ja Chief you haff a problem indeed!"


We followed his instructions for emergency running:

  • We removed No 3 exhaust valve cam roller,  guide and actuator.

  • No 3 exhaust valve was now being held closed by the return springs

  • To doubly ensure the fuel could not be delivered to the unit we removed the fuel pump cover and lifted the plunger up to the top position  with the extraction tool.

  • Cylinder lubrication was not adjusted on no 3.


To stop the turbochargers from surging the following was done.

  • The access cover was removed from the charge air manifold.

  • The air side of the forward turbo was restricted by fitting canvas gags to the air filter to give better balance as number 4 unit is common to both turbos.

It was trial and error but it worked; The engine speed we managed was about 60 rpm and gave us a speed of 9 knots.

When we got to the Humber we carried out a full repair with the assistance of the B&W service engineer.

 

The cambox bottom was dropped. Reference marks on the camshaft were verified. The section with the damaged cam was uncoupled and removed.

 Using hydraulic jacks to remove the coupling

 

 

 

After removing the coupling using hydraulic jacks and removing the sealing rings the old cam was then removed. This should have been done using heat alone, but we found that the heat was not enough from the tiger torches so it was ground off with a little acetylene torch assistance This was a joint decision with B&W, the superintendent and the C/E. There was no noticeable damage to the camshaft; no tears and no sign of bluing.

 

In order to fit the new cam the cam must be heated either in a oven or an oil bath. Because of the logistics and the time frame it was decided to go with the latter. A large portable wash tank was filled with 500l of cyl oil. It's the cheapest and has a high flash point. A geny pyrometer was used, and the cam was slung in the bath from one of the E.R.Cranes. The operation involved checking the cam was  evenly heating as when we took the damaged one off the lobe was absorbing the heat and then radiating it as the engine room was cool (April in the UK).

 

The service engineer from B&W was very precise with the measurements and ensured the cam bore had expanded evenly; this was done using the vessels instruments which had been properly calibrated the previous voyage. The cam was positioned on to the camshaft and allowed to shrink. The coupling was replaced in a similar manner, and the section of camshaft replaced in the engine. The timing was then checked and the engine returned to its normal running condition.

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