Crank shafts

Breakage of crank shafts in years gone by was accepted as one of the hazards inherent in Diesel-engine operation, but at the present time such breakage is very unusual. Most repair work on crank shafts consists of refinishing pins and bearings that have become scored or worn, or correcting alignment of the shaft in its bearings. Whenever it is necessary to renew a broken shaft, it is found that breakage is due to misalignment of torsional vibra­tion. Wear of main bearings is more rapid in marine engines than in land engines. The more flexible nature of the structure on which the engine rests, as compared to the mass of concrete under the land engine, makes it more difficult to maintain a continuous­ly unbroken oil film between shaft and bearing box.

Torsional vibration is sufficiently well understood that its occurrence can be predicted with accuracy in new engines and steps taken to eliminate it. It is a much less common source of trouble than in former years, but it is found occasionally.

Connecting rods and crossheads

Connecting-rod repairs are so infrequently required that they may be dismissed without discussion. Crosshead trouble is normal­ly confined to slipper wear.

Piston rods

Piston-rod troubles may be said to be non-existent in single-acting engines that have such rods, but in double-acting designs piston-rod breakage has been rather common.

Pistons

From the repairman's point of view, the piston seems to be the most troublesome of all the major parts of the Diesel engine. The trouble encountered is usually cracking, or wear of ring slots. There are two ways in which cracking occurs: approximately ra­dial cracks develop in the crown or circumferential cracks in the side or skirt. The first is the most common kind, and the appear­ance of the cracks indicates that it is associated with the phenom­enon of metal growth by repeated heating. The second type of fracture, cracks in the sides, is almost invariably due to piston seizure in the liner and is usually associated with cooling-system failure. Welding procedure has been developed to the point where most cracked pistons can be salvaged at a cost substantially less than the price of a new one.

Valves and valve gear

Under this heading there is comparatively little to be record­ed. The exhaust valve is the one most frequently in need of re­pair, due to the fact that a leak usually results in a burned-out valve or seat.

Exercise 1. Answer the following questions.

1) What is the most complicated casting on the engine?

2) What is the usual head repair?

3) Why was breakage of crank shafts accepted as one of the hazards inherent in Diesel engine operation?

4) What does most repair work on crank shafts consist of?

5) Why is torsional vibration a much less common source of trou­ble now than in former years?

6) What is crosshead trouble normally confined to?

7) In what engines may piston-rod troubles be said to be non­existent?

8) From whose point of view does the piston seem to be the most troublesome of all the major parts of the Diesel engine?

9) What repair does the exhaust valve frequently require?

Exercise 2. Give the Russian equivalents of the following.

1) Torsional vibration.

2) Piston seizure

3) Cooling system failure.

4) A burned-out valve.

5) Expansion and contraction stresses.

Exercise 3. Use the correct words from the text.

1) The piston seems to be the most... of all the major parts of the Diesel.

2) The trouble encountered …

3) Wear of main …

4) Connecting-rod repairs …

5) The second type of fracture …

Exercise 4. Translate into English.

1) Предпочтение отдается газовой сварке, если расположение трещины позволяет применить ее.

2) Прежде поломка коленчатого вала в эксплуатации дизеля считалась естественным явлением. Сейчас это редкое явление.

3) Крутильные колебания достаточно хорошо изучены. Их появ­ление в новых двигателях может быть точно предсказано и для их устранения принимаются соответствующие меры.

4) Шатун так редко нуждается в ремонте, что о нем можно и не говорить.

5) В двигателях простого действия, можно сказать, не бывает аварий штоков поршня.

6) С точки зрения ремонтников, поршень доставляет больше всего хлопот.

7) Сейчас методы сварки достигли такого совершенства, что большинство поршней можно восстановить, причем затра­ты оказываются значительно меньше стоимости нового поршня.

Exercise 5. Make up a dialogue of your own based on the one given below.