With the pressure system it is essential that there be a good seal between the shell and the housing. If not, the oil will escape at the ends, and the shaft will fail to receive proper lubri cation. Ordinarily, if the oil passage to one bearing becomes clogged, the gage on the oil-pressure line will show an increased pressure. If another bearing is worn, thus allowing a freer passage than usual, it is possible for one line to be completely clogged without any pressure change being evidenced.
McEwen Diesel Main 33 shows the two piece bearing of the McEwen Horizontal Engine. Since these engines are small-powered per cylinder, the two-piece construc tion offers no objection. It should be understood by the operator that the shaft pressure will wear the bearing oblong and that no adjustment is practical beyond the removal of the shims. After the wear becomes pronounced, the only remedy is replacement of the worn part. The oiling is effected by two rings which dip into the oil cellar below the bearing.
De La Vergne Diesel Main De La Vergne Engines use a two-piece bearing, as appears in Fig. 34. The bottom and top halves are set at a 45-degree angle which allows the pressure to bear on the center of the bottom shell. By removing, shims from between the two shells the necessary adjustment can be made.
Aligning of Bearings.—In lining up shaft bearings the first step is to establish the shaft center line, using a fine piano wire. Next, the bottom shells are placed in the housings. If the shaft is 9 inches in diameter, the shell is adjusted until the radii from all points on the shell surface to the center line are 4;2 inches. Where the bearing is of the bottom-wedge type, it is easy to make the adjustment. If the engine is a horizontal one, or a vertical engine with a wedgeless bearing, the only way to raise the shell is by the insertion of shims between the housing and the shell. Practically all manufacturers bore the housings true to a test bar. Figure 35 is self-explanatory as to the method of truing the housing. It should never be necessary to shim up the bear ings of a new engine. When an engine is installed, among other records the engineer should make note of the thickness of the bearings, including the cast-iron shell and the babbitt lining. Then, in case of wear, the engineer knows exactly how much he should shim up to bring the distance back to the original value. The variation in the bearing levels should not exceed .003 inch. If more than this, the shaft will likely spring in operation. On engines with the flywheel mounted on an extension shaft the wear is more rapid on the main bearing adjacent to the flywheel. The thoroughly versed erector always sets this bearing from .003 to .004 inch higher than the other bearings. This allows a wear
of .006 to .008 inch before the" bearing becomes too low.
Because of the initial misalignment this particular main bearing always runs warmer than the other bearings. Many_ operators experience a great deal of worry over the temperature condition of this bearing when, in fact, the higher temperature actually indicates that the bearing is in proper shape. When the tem perature becomes lower than with the other bearings, it can be taken as evidence that this bearing has worn low and must be raised.
Babbitting a Bearing.—There are many instances where a bearing becomes hot, causing the babbitt to drag and cut; yet in the majority of cases it is not necessary to rebabbitt this bear ing. The proper procedure is to use scrapers made of old files along the lines of Fig. 36. The babbitt should be smoothed down and all loose and damaged metal cut out. This may cause part of the liner to be so low that it fails to contact with the shaft. This is not objectionable since the ordinary shell has an excess of bearing surface. Where the damage is considerable, a new babbitt liner is the only solution. Under these circumstances it is first necessary to melt out all the old metal by placing the shell in an iron pot over a fire until all the babbitt becomes soft enough to flow off of the cast iron.
A good many engineers think that tinning the cast-iron shell is uncalled-for labor, but it does guarantee that the babbitt will hold. To cast the new liner it is advisable to secure a cast-iron or sheet=steel plate. The two halves are then clamped together, with a paper separator where the two halves meet, heated and placed in an upright position on this plate, inserting an asbestos sheet between the shells and plate to prevent the babbitt from adhering to the plate, Fig. 38. The mandril may be either of cold rolled steel or a piece of pipe about an inch smaller in diam eter than is the engine shaft. The shell base is dammed with fire-clay to avoid any leakage of the molten babbitt. The bab bitt, which should be of new metal, is now heated in a ladle or cast-iron pot until it will char a pine splinter. The dross that floats on the surface of the hot metal is best skimmed off before the bearing is poured. The pot should contain enough metal to fill the entire cavity. It is impossible to use two pourings on a bearing for the two will not unite. Most bearings have a solid babbitt liner for the top of the bearing. Since there is practically no upward thrust, all that is required is sufficient surface to pre vent the shaft from lifting. To save babbitt, it is a good plan to fill in the top shell with fire-clay with the exception of the ends, where space must be left for about 2 inches of bearing metal, similar to Fig. 37.