2015-10-22
457
Notes:
The principal source of trouble from this cause is the large bearings of the main shaft, cranks and crossheads. In general the bearing pressures are higher in the Diesel than in the steam engine and more careful and accurate fitting is required, so that the maximum bearing surface will be obtained. All bearings must be relieved at the sides enough to prevent gripping and to assist the spread of the lubricating oil around the bearing but not enough to reduce the bearing surface too much when they expand with the heat. All of the bearings, large and small, about the engine are subject to wear and looseness may develop in any of them in service but the large bearings just mentioned have the most severe duty to perform and require adjustment more often than the others.
Mechanical defects
Mechanical defects resulting in failure of parts in service may be due to errors in design, errors in machining or defects in the material itself. Errors in design have been too numerous to catalog but in the case of most well established builders such errors have been eliminated by experience in operation. Errors in machining are not the rule in any engine, if the manufacturer has an efficient inspection department, but they do occur and sometimes cause a great deal of trouble. Such errors are usually difficult to discover because they are not expected and the troubles resulting are assigned to other causes.
Sometimes careful investigation and analysis will reveal that a breakage that was apparently caused by a mechanical defect was due to bad work or neglect on the part of the engineers.
Exercise 1. Answer the following questions.
1) What is the principal source of trouble from wrong fits and clearances?
2) Why may looseness develop in any of the bearings in service?
3) What are the sources of mechanical defects?
4) What errors are usually difficult to discover?
5) What will sometimes careful investigation and analysis reveal?
Exercise 2. Give the equivalents of the following.
In Russian: To perform severe duty, accurate fitting, require adjustment, efficient inspection department, careful investigation, to prevent gripping, errors in machining, main shaft, cranks and crossheads, to assist the spread of the lubricating oil, expand with the heat, parts in service, cause a great deal of trouble.
In English: главный вал, кривошипы, крейцкопф, подвержены износу, требуют пригонки, иметь в виду, вследствие небрежной эксплуатации, конструктивный дефект, аккуратная подгонка, предотвратить заедание, дефекты устранены, погрешности при механической обработке деталей, тщательное расследование, сократить поверхность подшипника.
Exercise 4. Translate into English.
1) Большие подшипники главного вала, кривошипов, крейцкопфов являются основными источниками аварий.
2) Все подшипники подвержены износу в процессе эксплуатации двигателя и поэтому требуют периодической пригонки
3) Механические дефекты возникают вследствие конструктивных ошибок, погрешности при механической обработке деталей, дефектов в самом материале.
4) Это следует иметь в виду во время работы двигателя.
5) Такие погрешности трудно обнаружить, потому что на них не рассчитывают, и возникшие неисправности приписывают другим причинам.
6) Иногда тщательное расследование и анализ аварии показывает, что она произошла вследствие небрежной эксплуатации, хотя на первый взгляд кажется, что авария была вызвана конструктивным дефектом.
Exercise 5. Make up a dialogue of your own based on the one given below.
THE ENGINE
A.: If all the stored air is lost how can the engines be started?
В.: In most cases a small emergency compressor is provided, driven by a very small oil engine that can be started by hand After a small amount of air is pumped up an auxiliary engine can be started and air pumped up to start the main engine.
A.: How much starting air capacity should the tanks have?
В.: The usual requirement is enough to start the engine 12 times. A better rule is to provide at least 35 times the volume swept through by one piston for each main engine.
A.: Describe the preparations for starting the engine.
В.: Jack the engine to see that it is clear for running, start lubricating oil pump and see that oil reaches all bearings, open by-pass valves in fuel oil lines to injection valves and pump oil through by hand to clear out all air and fill the pipes with oil, open stop valves in starting air and injection air lines, give cylinder lubricators a number of turns by hand to get some oil into the cylinders, start jacket cooling water pump and see that the propellers are clear. If means for heating the cylinders are available and the weather is cold, warming up should be started in time to have all the cylinders hot before starting.
A.: Why should the bearings of a new engine be gone over after 800 to 900 hours of running?
В.: То take up looseness due to the initial wearing in.
A.: Why will a piston sometimes run without trouble at normal load and drag or stick if the load is increased?
В.: Because the clearance in the cylinder may be sufficient at normal load but when overload causes all parts to become hotter the increased temperature of the piston causes it to expand more and reduce the clearance.
A.: What is the effect of too much or too little clearance between a cam and its roller?
В.: Too much clearance causes late valve opening, early closure, decreased lift and noisy operation. Too little clearance causes early opening, late closure, increased valve lift and may prevent complete closure.
A.: What are some of the common troubles experienced with fuel measuring pumps?
В.: Leaky suction and discharge valves, due to grit in the oil or to particles of dirt or lint becoming caught under the valves; pump becoming air bound; incorrect clearance between suction valve stems and tappets; leaky plungers, and change in timing, due to looseness in the operating gear.
A.: Why is it desirable to have thermometers so located that separate temperature readings of the cooling water discharge from the cylinders can be taken?
В.: If merely the temperature of the common discharge from all cylinders is taken, a single cylinder jacket may become air bound and the flow of cooling water through it stopped altogether without giving any indication at the common outlet.
A.: What are the effects of too many or incorrectly placed oil grooves in a bearing?
В.: The bearing surface is reduced and the oil escapes from the bearing without spreading over its surface.
A.: What is a common trouble with the cylinder lubricating system?
В.: Excessive feed of lubricating oil into the system, causing collection of gum and carbon in the ring slots and leakage of black oil into the crank pits.
Exercise 6. Speak on the following.
1) Derangements likely to occur in connection with wrong fits and clearances.
2) Derangements likely to occur in connection with mechanical defects.
