Rods and boxes

Engine manufacturers have addressed the problems encountered with the stuffing box/piston rod interface on new long-stroke two-stroke engines.

Concerns over performance of the stuffing box/piston rod combination in two-stroke engines can be addressed through a number of upgrade and reconditioning options, say engine builders and component suppliers.

Problems arose with the new generation of long-stroke engines with higher stroke-to- bore ratios and increased scavenge air pressure. This combination places higher demands on the stuffing box/piston rod interface, which must achieve a better seal if it is to function adequately and prevent excessive system oil contamination.

Wartsila NSD, who has suffered with stuffing box problems for some time, introduced a new design with the RTA60 (The Motor Ship, August 1999). It has been field tested for more than 10,000 hrs on a 9RTA58, a 7RTA84M and an 8RTA84C and forms the basis of a retrofit package for older RTA engines in operation. More than 100 cylinders have been, or are being, retrofitted to date. The new housing is easily fitted during a regular overhaul, and without pulling the pistons on RTA engines with oil cooled pistons. The modification also cuts the excessive oil loss through the neutral space which has been experienced by many users.

The engine designer strongly recommends using hardened piston rods to minimise scraper ring and rod wear and achieve the best performance – though the new stuffing box design can be used with existing soft rods if they are inside wear limits. Another option is to grind the rods to 2mm or 4mm undersize and fit undersized scraper rings. Hardening the undersized rod would also improve performance in this case. Wartsila NSD will only supply hardened rods from now on, and has stopped using metal spraying to recondition old ones. Price levels for new hardened rods have dropped and are only slightly higher than reconditioning, it says.

Hardened piston rods were fitted as standard on MAN B&W’s MC engine family from the beginning, but some licensees did not adopt the company’s lamella scraper ring design. These can easily be retrofitted in the same ring spaces – drain is adjusted to handle the higher volumes of oil scraped from the rod.

DMI’s Hamburg subsidiary, DMI (Drechsler), has developed a chrome alloy metal spray process for reconditioning piston rods which increases surface hardness to 520 vickers from the standard 250 vickers. This is in excess of the hardness achieved with induction hardening says the company. Exchange rods can often be supplied to save time.