Main Engine camshaft coupling bolts loosened and dropped

What happened

While staying offshore for port entry, it was discovered that coupling bolts from the stern side gear case situated in the center of the main engine cam shaft had loosened and that nuts had dropped off. Of a total 6 bolts, the corresponding nuts of 4 bolts had dropped off. As for the remaining two bolts, the nuts were loose and were corotating (the bolts which fell off dropped into the gear casing, while the nuts dropped outside of the gear casing.) After discovery, the bolt nuts were immediately picked up, attached and tightened. Later, when the main engine was operated to move the ship from the anchor- age point to the port, the same bolt nuts loosened again. After entering port, to prevent the nuts from loosening, a gas welding rod instead of wire was used to weld all of the nuts. However, during the short time of standing by to leave port, the welding blew off and the bolt nuts loosened again. As emergency repair, the nuts and bolts were welded on-board and the vessel returned to route.
An inspection of the bolts revealed the following points:
1) Major scratching over 2/3 of the bolt periphery near the bolt head.
2) Large-scale abrasion over the entire length of the bolt in the circumferential direction. Bolt diame- ter was reduced by 10/100mm.
3) The area (flange contact surface) at the center of the bolt of about 1mm in width where the said diameter in the circumferential direction was quite large.
Furthermore, for those bolts which exhibited the most significant scratching damage, their correspond- ing bolt holes also exhibited extensive galling. Several groove type scratches were found to other bolt holes in the circumferential direction.

Repair

A certain period would be necessary to completely repair the coupling. Thus, as an emergency meas- ure, the upper section of the cam shaft thrust bearing within the gear case was removed. Using reaming tools, about 11 hours of repair work was carried out. As the diameter of each bolt hole after reaming activity was different for each hole depending on the depth of the galling, the corresponding bolts were cut on land to match these hole dimensions. The bolts were then cooled and then inserted and tightened.

Root Causes

Similar damage had occurred on another licensee engine. Due to the differences in licensee, although the configuration of the coupling bolts were slightly different, both were difficult to tighten down due to the surrounding space. It is surmised that the damage was caused by insufficient coupling bolt tightening during construction of the new vessel.

Preventive actions

Depending on the model, the coupling is entirely covered and its structure makes inspection difficult. Regular inspection for loosening is necessary.