In its latest edition of the Safety Digest, the UK MAIB investigated an incident occurring during a ship engine’s overhauling, where the grinding disc exploded and caused injury in one engineer, highlighting the importance of not exceeding the safe operating speed of a grinding wheel.
The incident
The engineers on a large container vessel were overhauling a slow speed two-stroke main engine cylinder. They had removed the cylinder head and piston and were getting ready to clean and calibrate the liner. The wear ridge formed at the top end of the cylinder liner had to be ground off before the new piston could be inserted. However, the grinding disc normally used for this was worn out. The second engineer found another one in the store, but the hole in the centre of the disc was larger in diameter than the original and did not fit the spindle of the hand-held pneumatic grinder.
The second engineer promptly fabricated a spacer redesigned to make the disc fit. The grinder had a maximum rated speed of 22000rpm. The second engineer put on a protective face mask and entered the liner with a ladder. However, as soon as he started grinding, the grinding disc exploded into several pieces. One piece smashed through his mask, hitting him just above his left eye, leaving a deep gash. He lost consciousness and had to be lifted out of the liner. He was hospitalised immediately and made a full recovery. The crew could not determine the speed rating of the exploded grinding
disc, as there were no more similar discs on board. It was most likely to have been rated well below 22000rpm, as most general purpose grinding discs are rated between 5000 and 15000rpm.
Lessons learned Exceeding the safe operating speed of a grinding wheel can cause it to crack or disintegrate. When the wheel speed is doubled, the stresses it experiences are quadrupled. Always ensure that the tool’s rotational speed does not exceed the maximum allowed speed of the grinding disc. Face protection masks are not designed to withstand the impact of high momentum objects striking them. It was extremely fortunate that the second engineer did not lose an eye or suffer fatal injuries.
