Figure 284 is a bearing quite generally used on both vertical and horizontal engines. In tte horizontal engine the direction of pressure is against the front side of the bearing. Both the upper and the lower halves wear oblong; consequently only a small amount of wear can be compensated for by means of shims. If the engineer desires to eliminate all play in the bearings, it will be necessary to replace these bearing shells quite often. Due to this, the bearing is suitable for the smaller sizes of horizontal engines only.
For vertical engines this is the most acceptable bearing. The pressure is always downward, and the wear occurs uniformly Qll the lower shell. It is then an easy matter to shim this shell back to its original position. On a single-cylinder vertical engine, to shim up the shell it is necessary to jack-up the shaft from the outside. The jacks should be placed under the flywheels, or, if it is impossible to do this, wood blocks with a V cut into their up per side should be placed under the shaft where it extends beyond the wheels. By setting the jacks under the blocks, the shaft may be raised with ease. Before jacking-up the shaft, if the en gine is a belted unit, the belt should be removed so that there will be no outside force acting on the engine. In a three- or four cylinder engine, a jack should be placed under the center crank pin box to assist in raising the shaft.
The quarter-box bearing, Fig. 285, when used on horizontal engines, is very attractive from the operator's viewpoint. It is customary to have the wedge on the front side only. This takes care of all adjustments since the direction of pressure is invariably toward the front side of the bearing. This being true, it is not essential to have the rear quarter-box adjustable. Ordinarily, the front and bottom of the bearing are made in one piece. The weight of the flywheel and shaft wear the bottom only a slight amount, and usually this wear will not require attention for some years. It is very easy to keep the bearing snug since all that need be done is to take up on the front wedge bolts. This should never be done when the engine is running.
Some engine builders reverse the position of the wedge, placing it at the rear side. This does not allow adjustment for wear, but merely enables the engineer to reduce the shaft play by bringing this side box up against the shaft.
Various attempts have been made to water-cool the main bearings, Fig. 285. Since there is no reversal of pressure, the bearings will run warmer than in case of a steam engine. The water-cooling involves complicated pipe lines and valves, and it is difficult to take care of the piping. If a bearing requires
water-cooling, it is proof that the bearing surface is not liberal enough and the pressure per sq. inch is too high.
Figure 272 shows a bearing that was the favorite in the earlier days of the oil engine and is still used by a few builders. This bearing housing is in the form of a flange which is bolted to the engine frame. The bearing is a cylindrical one-piece bushing without means of adjustment. In case of any considerable wear, the sole remedy is replacement of the entire bushing. The bearing is lubricated by means of a mechanical oil pump, assisted by a ring oiler which dips into a cellar below the bushing. Since the bearing wear must be of some extent before the engineer feels that replacement is justified, air leaking along the shaft is a trouble often encountered. The air-seal ring should prevent the air leaks, but the " jump " of the shaft, due to the worn bearing, will wear the ring until it is valueless.
Adjustments.—In making bearing adjustments the operator should keep in mind the fact that the shaft must be leveled up true in respect to the housing. To line them up with the cylinder, a good plan is to place the engine on both dead-centers and see if the connecting-rod is in the center of the crank throw. In case of general repairs center lines should be established.
The question is frequently asked as to what amount of play should be allowed between the shaft and upper bearing cap. If the bearing be one where the cap does not carry any of the pressure, a good plan is to loosen the cap, or tighten it, until the shaft can be "jumped" about one or two hundredths of an inch. Side play between bearing face and crank web can be from Ho 0 to 364 inch without danger, though the first value is one that conforms more closely to good engineering.
The real difficulty in main bearings is the question of lubrica tion. The suggestions outlined in the care of Diesel bearings apply with equal force to the low-pressure engine. Even with an oil-cellar design stream lubrication is highly desirable. The practice many engineers have of using the same oil for months without refiltering is open to severe criticism. No engine bearing will stand up long if dirty oil be used. For a small plant, where first cost of accessories is of serious moment, a 5-gallon can with a false bottom and with the upper part filled with waste makes a good filter. A high-grade filter, purchased from a filter manu facturer, will pay for itself in a short time, in oil saving alone.