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Tomislav Skračić, MA Undergraduate English Course for MARINE ENGINEERS 3rd Semester Essential reading: SPINČIĆ, A., PRITCHARD, B., An English Textbook For Marine Engineers 2, Pomorski fakultet, Rijeka 2009. LUZER, J., SPINČIĆ, A., Gramatička vježbenica engleskog jezika za pomorce, Pomorski fakultet, Rijeka 2003. Lesson 2 Crankshaft, Main Bearings and Shaft Alignment KEY WORDS crankshaft (n.) main bearing shaft alignment reciprocating motion at a top center bending stress connecting rod thrust crankpin (n.) adjacent bearing torsional forces semi-built design crank web (n.) forge (v.) solid-forged cast (v.) - koljenasto vratilo / osovina, radilica - glavni / temeljni ležaj - centriranje osovine - stapno gibanje - u gornjoj mrtvoj točki - naprezanje zbog savijanja - potisak ojnice - oslonac koljena, osovina letećeg ležaja - pokrajnji ležaj, "odrivni" ležaj, "thrust" bearing - torzijske / zaokretne sile - izrađen od više dijelova - 'pršuti' za uravnoteženje - kovati - izrađen u jednom dijelu - lijevati Lesson 2 Crankshaft, Main Bearings and Shaft Alignment KEY WORDS shrink on to (v.) - navući u toplom stanju crank journal - oslonac temeljnog ležaja journal (n.) - blazinica, ležajna čaša (materijal koji se troši) journal bearing - leteći ležaj carbon steel - ugljični čelik (FeC) alloy steel - legura čelika nickel - nikal chromium - krom molybdenum - molibden (Mo) crank (n.) - koljeno vratila multi-throw shaft - osovina s više koljena, višecilindarska koljenasta osovina firing order - redoslijed paljenja torque (n.) - zakretni moment torque wrench - moment-ključ load (n.) - opterećenje exhaust arrangement - ispušni sustav / raspored ispuha Lesson 2 Crankshaft, Main Bearings and Shaft Alignment KEY WORDS misalignment (n.) - loše centriranje, nepoklapanje osi bearing shell - blaznica temeljnog ležaja / košuljica ležaja lining (n.) - obloga, sloj copper-lead - bakar-olovo aluminium-tin alloy - legura aluminija i kositra flash (n.) - sloj obloge lead (n.) - olovo indium - indij seating (n.) - sjedište, postolje transverse sadles - poprečni nosači thrust bolts - vijci temeljnog ležaja preopterečeni na tlak shim (n.) - podložna pločica bearing play - zračnost ležaja pressure lubricated - tlačno podmazivane tie rod - kotveni vijak tensioned - stegnut Crankshaft, Main Bearings and Shaft Alignment DEFINITION OF A CRANKSHAFT The crankshaft converts reciprocating motion in the cylinder into rotary motion of the propeller shaft. PARTS The crankshaft is made up of throws and journals. A throw consists a pin (secured or attached to big / bottom end bearings) and two webs or cranks. Journals rest or lie in the main bearings. STRESSES (fluctuating) Bending (when the piston is at TDC); Sheer stress (in operation); Torsion (due to speed change) Crankshaft MANUFACTURING Solid forged built in a single piece (small-slow speed engines) Semi-built design (large medium-speed engines). Crankpins & webs are forged or cast in one piece and shrunk on to the journals. Fully-built (cast in single piece – webs are shrunk on to the crankpins and journals). Crankshaft MATERIALS • Carbon steel • Alloy of nickel, chromium & molibdenum • Specialy alloyed grey cast steel Exercise 1 - Read the following text: The crankshaft, which converts the reciprocating motion of the piston to rotating motion, must resist the bending stresses caused by the connecting rod thrust when the piston is at the top dead centre. Then the maximum gas pressure acts straight down on the crankpin and tends to bend the shaft between the adjacent bearings. The crankshaft must also withstand the torsional forces produced by the change of speed. Exercise 1 – Complete the sentences: The crankshaft converts ... ... to .... .... It must resist the bending stresses caused by the connecting rod thrust when ... ... . Then the maximum gas pressure acts straight down on the crankpin and tends to ... .... The crankshaft must also withstand the ... .... Exercise 2 – Read the text: Medium speed engines have crankshaft usually solid forged, i.e. made from a single piece, while slow speed engine crankshafts are mostly of semi-built design with crankpins and webs forged or cast in one piece and shrunk on to the journals. The type of steel used, which is carbon or alloy steel containing nickel, chromium and molybdenum, is chosen for its strength, resistance to fatigue and hardness of bearing surface. Exercise 2 – Complete the sentences: Medium speed engines have crankshaft usually solid forged, i.e. made ... ... Slow speed engine crankshafts are mostly of semi-built design with ... ... They are ... or ... in one piece and ... on to the journals. The type of steel used for crakshafts is ... ... It contains nickel, ... and must be resistant to ... Exercise 3 – Read the text: The cranks of a multi-throw shaft are set at appropriate angles giving a “firing order” for the engine. The firing order is chosen primarily to obtain a smooth torque and the best mechanical balance. However, main bearings loads, exhaust arrangements suitable for turbocharging and torsional vibration may also be taken into account. Although the crankshaft appears to be robust, they rely on the main bearings to develop their full strength. Exercise 3 – Complete the sentences: cranks of a multi-throw shaft are set at appropriate angles giving ... ... The firing order is chosen primarily to obtain ... ... and ... ... However, we must also take ito account: ... loads, exhaust arrangements suitable for ... and ... ... Crankshaft rely on the ... ... to develop their full strength. Crankshaft, Main Bearings and Shaft Alignment Questions & Discussion 1. 2. 3. 4. 5. 6. 7. State the function of the crankshaft. What forces is a crankshaft subjected to? What kind of crankshafts are used in: a) Medium speed diesel Slow speed diesel 4. What does the choice of steel type for crankshaft depend on? What is the “firing order”? What else is taken into consideration in designing a crankshaft?