WÄRTSILÄ 31SG, Product Manual

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Crankshaft itself is forged in one piece. Both main bearings and big end bearings temperatures are
continuously monitored.
Counterweights are fitted on every web. High degree of balancing results in an even and thick oil
film for all bearings.
The connecting rods are arranged side-by-side and the diameters of the crank pins and journals
are equal irrespective of the cylinder number.
All crankshafts can be provided with torsional vibration dampers or tuning masses at the free end
of the engine, if necessary. Main features of crankshaft design: clean steel technology minimizes
the amount of slag forming elements and guarantees superior material durability.
The crankshaft alignment is always done on a thoroughly warm engine after the engine is stopped.
4.2.3 Connecting rod
The connecting rod is of forged alloy steel. All connecting rod studs are hydraulically tightened.
The connecting rod is of a three-piece design, which gives a minimum dismantling height and
enables the piston to be dismounted without opening the big end bearing.
4.2.4 Main bearings and big end bearings
The main bearings and the big end bearings are of tri-metal design with steel back, lead-bronze
lining and a soft running layer. The bearings are covered with a Sn-flash for corrosion protection.
Even minor form deviations can become visible on the bearing surface in the running in phase. This
has no negative influence on the bearing function. A wireless system for real-time temperature
monitoring of connecting rod big end bearings, "BEB monitoring system", is as standard.
4.2.5 Cylinder liner
The cylinder liners are centrifugally cast of a special alloyed cast iron. The top collar of the cylinder
liner is provided with a water jacket for distributing cooling water through the cylinder liner cooling
bores. This will give an efficient control of the liner temperature. An oil lubrication system inside the
cylinder liner lubricates the gudgeon pin bearing and also cools piston crown through the oil
channels underside of the piston.
4.2.6 Piston
The piston is of composite type with steel crown and nodular cast iron skirt. A piston skirt lubricating
system, featuring oil bores in a groove on the piston skirt, lubricates the piston skirt/cylinder liner.
The piston top is oil cooled by the same system mentioned above. The piston ring grooves are
hardened for extended lifetime.
4.2.7 Piston rings
The piston ring set are located in the piston crown and consists of two directional compression rings
and one spring-loaded conformable oil scraper ring. Running face of compression rings are
chromium-ceramic-plated.
4.2.8 Cylinder head
The cross-flow cylinder head is made of cast iron. The mechanical load is absorbed by a flame
plate, which together with the upper deck and the side walls form a rigid box section. There are four
hydraulically tightened cylinder head bolts. The exhaust valve seats and the flame deck are
efficiently and direct water-cooled. The valve seat rings are made of alloyed steel, for wear