The bearing is now cast with the bore an inch smaller in diam eter than the shaft. The halves, still clamped together, are now removed from the plate and cleaned of fire-clay. They should be placed on a lathe and bored out to exact shaft diameter. After being separated, a bevel should be cut along the parting edges so that these edges do not tend to scrape off the oil as the shaft revolves. It is of no benefit to peen the surface of the bab bitt. This practice is likely to loosen the bond between the babbitt and cast-iron shell. After boring the shell, the two halves should be lightly coated with Prussian blue and placed one at a time on the shaft. By rotating the half of the liner, the engineer can note the high spots and scrape the bearing to a perfect con tact. This scraping is a tedious affair, but the resultant long life of the bearing justifies the labor. Oil grooves of ample length should be cut in the babbitt. In cutting the oil grooves none of them should be carried to the edges of the shells. If this is not observed, the open ends of the grooves offer too free a passage to the oil; consequently but little oil flows over the babbitt surface.
Frequently the housings are not bored exactly true, and the bearings are a trifle cramped, touching the shaft on only a part of their length. To guard against this, a new liner should be examined after the engine has been run an hour or so on light load. If the shaft bears only partially, scraping the high end will bring all the surface to a contact. This is better than shimming up the low end since, if the latter is done, there is a liability of the cast-iron shell fracturing due to the poor support it receives. In bolting down the bearing cap the nuts should be drawn up snugly.
In no case is it desirable to loosen up on the bolts from the lower shell by means of shims or separators. These must be of the proper thickness to allow running clearance between cap and shaft. This clearance can be .001 inch per inch of shaft diameter. By placing lead wire or fuse wire between the cap and shaft, the cap can be drawn up as tight as possible by the bolts. Removing and measuring the thickness of the wire enables the engineer to determine the proper amount of shimming that is necessary be tween the two bearing halves to obtain this running clearance.
Hot hot bearing is a trouble that every engineer some day will experience. Generally this occurs when the load is heavy and when a shut-down is an impossibility. The engi neer must maintain his presence of mind even though the bearing It is an all too common practice for the operator to excitedly douche the bearing with a bucket of water. This is of no avail. Water is at best a poor lubricant, and it will merely make matters worse by washing off the little oil that does cling to the shaft. The water strikes the shell and the shaft; this results in the contraction of the shell, thereby loosening the babbitt. Another bad practice is the use of an air hose in a vain endeavor to cool the bearing. The only correct procedure is to run an oil pipe or hose to the bearing and feed a heavy stream of cool oil through the inspection hole in the cap directly on to the shaft. If the bearing 'is provided with an oil cellar, the drain cock to this should be opened and the oil allowed to flow out after passing over the shaft. It is best to run the engine light until the bearing cools off. If the engine is arranged to allow a cylinder to be cut out, the two cylinders adjacent to the hot bearing should be operated idle, with the exhaust valves blocked open. This relieves the damaged bearing of part of the pressure due to the cylinder explosion. After shutting down,
the bearing liners should be examined and any necessary repairs made.
Clearance Between Bearing and engines develop considerable side-play in the crankshaft after being in service a few years. This is attributable to excessive clearance between the end of the main bearings and the crank-cheeks or throws. Engines of different makes vary as to the allowable clearance, but .007 inch is a representative value. If the clear ance is too great, the remedy is to tin the ends of the bearing and run a ridge of babbitt around these ends, Fig. 39. Using a file and scraper, all surplus metal can be removed and this ring of babbitt reduced to the desired thickness. This will prevent the side-play and should last for at least a year before requiring renewal. Still another method is the employment of a sheet-steel ring. This may be made along the lines of Fig. 40. The wings at the side provide space for the fasteners.
Scored are occasions when an engine crank shaft becomes scored to a serious extent or worn unevenly at the journals. The average engineer is prone to think that a shaft in either of these conditions is worthless and should be removed at once. When the damage is severe, of course this is necessary. More often a little work will allow the engine to still pull its load. In cases of scored shafts an emery stone will serve to smooth up the shafts, finishing with a scraper and a final polish by lapping. The scores do not cause trouble pro viding the edges of the cuts are smoothed off, preventing the babbitt from being picked up. When the wear is uneven, the bearing shell should be scraped to a fit, even though the diameters at the two ends vary to a marked degree.
Fractured Crankshafts.—The bogie of the Diesels when first introduced in this country was broken crankshafts. Undoubt edly the one thing that an obstacle to the introduction of the oil engine was this question. There were a few isolated cases of fractured shafts. These without reservation were due to one of two things. The early Diesel operator knew but little about the engine and believed in letting well enough alone. No attempt was made toward the adjusting of the various parts. The fuel valve frequently got out of order and opened too early, causing preignition. These excessive pressures had to be relieved by some means. Often the head gave way, but at times the head proved stronger than the shaft, and so a fractured shaft was the result. The second cause was the failure to take up the wear in the main bearings. Frequently the inside bearings became worn, allowing the shaft to be supported by the two outside bearings only. This produced a deflection in the shaft which was repeated and reversed each revolution. Ultimately the shaft gave way. As a rule the break occurred between the pin and the web or throw. More liberal fillets at this point along with more knowledge acquired by the engineer has elimi nated this danger. Several score of Diesel plants, to whose records access is had, report no trouble with fractured shafts. Hot Bearings—Two-cycle Engines.—The horizontal two stroke-cycle Diesels are as free from bearing trouble as are the four-stroke cycle engines. At the reversal of the stroke the bearing pressure is somewhat relieved, thereby allowing the oil to form a film. With vertical two-cycle engines the constant downward pressure results in faulty lubrication. This is the cause of the many instances of hot main bearings. The operator must maintain vigilance, seeing that the oil supply is ample, and at the first sign of a hot bearing the engine should be shut down.