Stern Tube Machining: Two Case Studies and a Time-Lapse Video

Within the marine industry, in-situ machining of stern tube bearing pockets or of bearings themselves belong to a group of line-boring assignments that we carry out very frequently. In this post we would like to introduce two recent cases, one performed in Las Palmas, Canary Islands, and the other in Singapore.

Case 1: Machining of stern tube forward and after bearing pockets in Singapore

Damage to the stern tube bearings is found during the routine dry docking of a vessel in a shipyard in Singapore. Upon this discovery, the ship owner turns to us for advice. As this is evidently an unplanned and serious case, our specialists mobilize very quickly and carry out an initial laser check of the pockets’ alignments and geometries. The check reveals that the bearing pockets are misaligned and that the ovality that is measured is excessive.

The customer concurs that line boring presents itself as the best remedy. Again, our machining specialists mobilize quickly and rectify the poor alingment and ovality. Both the forward and after bearing pockets are machined. Working around this clock, this is acomplished in just five days.

We arrange two new bearings to be made in Spain on an urgent basis. Once they are delivered, we supervise their installation at the shipyard in Singapore. A final laser alignment check confirms that the alignment is correct now. We also carry out a load test of the entire shaft line and attend the sea trial, which goes smoothly.

Case 2: Machining of stern tube after bearing pocket in Las Palmas, Canary Island

A Norwegian ship owner decides to upgrade the stern tube bearings and seal assembly on a 20-year old ship to a newer, improved design. The upgrade means that the after bearing pockets has to be machined to accomodate the new bearing bush and seals.

Our work scope is as follows:

  • Measure stern tube diameter using a micrometer
  • Find the existing center line through laser measurement
  • Set up the CLB80 line boring machine inside the stern tube and align it with the help of lasers
  • Machine the stern tube according to drawings
  • Final measurement of the stern tube using a micrometer and laser measurement equipment

As is usually the case with stern tube bearing bushes, three different inner diameters (Ø525, Ø524 and Ø523 mm) have to be machined. The  total lenght of the of the stepped area is 935 mm. Two QuantiServ specialists from Gothenburg, Sweden, complete the work in two weeks.

They use our new CLB80 line boring machine that we designed and built ourselves. This machine is capable to very accurately bore holes with Ø140 – 600 mm that are up to 10,000 mm long. Its flange facing capability ranges from Ø90 – 700 mm.

Here is a time-lapse video of the line boring work performed in Las Palmas. The laser alignment and measurement works taking place before and after the line boring are omitted from the video to keep it short.


Two-stroke Bedplate Line Boring in Mexico

When a six year old bulk carrier suffered main bearing failures on its Japanese-made main engine, QuantiServ was called in for an initial inspection and for discussions on how to arrange the repair in the fastest and most economical way. The inspection in Veracruz, Mexico, showed that main bearings # 7 and 8 failed and that the crankshaft as well as the main bearing pockets were damaged.

The crankshaft was beyond repair and had to be replaced by a new one. The bed plate, on the other hand, could be recovered by line boring. With the engine frame lifted up, QuantiServ’s in-situ specialists carried out

  • a thorough inspection of the bedplate, including NDT crack detection and hardness measurements
  • laser alignment checks before line boring
  • line boring of main bearing pockets # 7 and 8
  • laser alignment checks after line boring
  • blueing checks

The work was carried out successfully while the vessel was alongside in the shipyard in Mexico.

In-situ machining of lateral surfaces on 20V32 engine block in Bangladesh

Lateral surfaces before and after in-situ machining

Lateral surfaces before and after in-situ machining

In-situ machining of lateral surfaces on a 20V32 engine block in a power plant in Bangladesh

In October 2016, QuantiServ received an urgent request to carry out in-situ machining on a 20-cylinder 32-bore engine block in a power plant in Bangladesh. During the replacement of the crankshaft it was noticed that both lateral surfaces of main bearing cap number 5 showed signs of severe fretting and were in need of machining.

Immediately, in-situ machining equipment was prepared at QuantiServ’s Dubai workshop and was sent to site. Once the equipment had arrived at site, QuantiServ’s engineers from Dubai performed in-situ machining on the engine block to achieve a clean surface that was free from damage. The in-situ machining process was constantly monitored by laser to ensure perfect alignment and adherence to very tight machining tolerances.

The main bearing cap was sent to a local workshop in Bangladesh for machining and installation of compensation plates. This process was supervised by QuantiServ’s engineers. Once the machining was completed, all mating surfaces for the main bearing cap were checked with marker blue to ensure a perfect fit.

Once the work was completed, a final check by laser on the assembled bearing cap showed that both the bore alignment and diameter fully conformed to the engine maker’s specification.