Английский язык: Сборник текстов и упражнений для студентов специальности "Эксплуатация судовых энергетических установок". Писарева Л.П. - 43 стр.

                                           Lesson 17
                                 Location of Troubles (Part III)

      Cylinder liners are a source of work for repair yard. Liner casualties occur either from
wear or cracks. Wear is a normal result of engine use.
      Liners that are originally fitted so that they will drop into place in the jackets with little
or no pressure, sometimes require from 200 to 300 tons hydraulic pressure to remove them.
This is caused by the carbon or scale accumulation that gets into the seating.
      Cracked liners are encountered occasionally. It usually results from piston seizure, and
such seizure usually results from inadequate cooling of the piston. No attempt is ever made to
repair cracked liners, renewal being the only remedy.

                                    Cylinder Covers (Heads)

       The cover is usually the most complicated casting on the engine, and as such suffers most
from heat trouble. To make a successful cover repair, electric welding must be done skilfully
and studding employed.
       Once an emergency repair to a cracked cover was made on shipboard by the crew. Both
bridges, between injection valve and inlet and exhaust valves cracked on bottom face of cover.
The injection valve hole was bored out and threaded and a cast iron bush screwed in, the bush
being then machined to make a seat for valve body. A hole large enough to remove the cracks
was then drilled on each side of the injection valve opening and threaded, after which a copper
plug was screwed tightly into each one. Although this type of repair is not to be recommended,
it did work well in this case and the cover was continued in service.

                                           Crankshafts

      Most repair work on crankshafts 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 re-
new a broken shaft, it is found that the breakage is due to misalignment of torsional vibration.
      An interesting case demonstrating the destructive effects of torsional vibration is that of a
group of ships having crankshafts about 20 in in diameter, turning 87 r.p.m. There was no noti-
ceable evidence of vibration while the engines were running, yet the coupling bolts became
loose or broke, and the coupling faces were deeply scored and worn. In this case it was possi-
ble to correct the trouble by changing the firing order of the cylinders and the relative positions
of the cranks.

                                              Pistons

       The piston seems to be the most troublesome of all 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.
       Either gas or electric welding can be used, but the former is much to be preferred. It can
be used successfully, however, only when the piston can be preheated.
       In connection with the occurrence of heat cracks in the centre of the piston crown, it was
found that such cracks could be prevented by cutting a circular groove in the crowns.

                                                 43