Another matter of doubt is how hot an engine bearing may become before the engineer should consider it dangerous. In all two-stroke-cycle engines the bearings will run warmer than on a four-stroke-cycle engine, since the direction of pressure is not reversed; and, as a result, the lubrication is not so good. In case the bearing becomes too hot to touch with the fingers, the load should be thrown off and the engine run slowly while the bearing cools off. It is usually noticeable that on a single flywheel engine the bearing next to the flywheel runs warmer than the other bearings. Many new engines have this bearing slightly higher than the other one, to compensate for the wear. On engines already in service the hot bearing is probably due to in creased wear caused by the extra weight of the flywheel.
Crankshafts.—Investigation of crankshaft failures, in most instances, reveals that the fracture occurred at the junction of the shaft and crank throw, or at the junction of the crank throw, or web, and crank pin. This liability of fracture can be largely reduced by more liberal fillets at these points. The danger of fractures will never be totally eliminated as long as engineers, on starting, allow too much oil to be injected into the cylinder. Not all of this oil burns on the first stroke. Some is trapped in the cylinder and, mixing with the air on the com pression stroke, preignites long before the piston reaches dead center. To better protect the engine against the -operator's carelessness it is good practice to attach a safety valve to the cylinder. A great many engines are so equipped, and all engines have a pet-cock or indicator opening that can be used to connect the safety or relief valve to the cylinder.
There seems to be a deep-rooted belief, held.by many engineers, that a crankshaft must be scrapped at the least sign of grooving or cutting at the bearings. Since a crankshaft represents a considerable per cent. of an engine's total cost, effort should always be exerted to repair the old shaft. If the grooving is slight, the bearing cap should be removed as should also the entire bearing shell. Wooden blocks should be inserted to prevent the shaft from shifting. A file, used while the shaft is turned slowly by hand, will smooth up cuts that are rather deep. When the defect is too far developed to be cured by this method, the shaft should be shipped to a good machine shop and a cut taken off of it at the journals. Where this is done, it becomes necessary to rebabbitt the bearing shells to the now smaller diameter of the shaft.
Flywheels.—Small oil engines up to about 100 h.p. make use of -a, solid rim flywheel, the hub being either split at one side or at both sides, according to the tastes of the builder. In placing a flywheel onto the shaft, iron wedges should be driven into the splits at the hub; a wedge should be used both on the outside and on the frame side of the hub in order to open the split evenly. The shaft should be carefully cleaned with gasolene, and, if any rust is present, this should be removed by emery cloth. The shaft should be well oiled and the flywheel, after
being blocked up level with the shaft, slipped on. The iron wedges usually open up the hub enough to allow the wheel to slip on easily. After the wheel is on far enough to prevent it tilting, rotation of the wheel will cause it to slip along the shaft much more freely. In tightening the hub bolts after the wheel is in place, the same tension should be given each bolt; the nuts should be tightened slowly, working them down uniformly. If one bolt is drawn up before the others have been touched, the wheel may cock a little.
A close observer, in visiting oil engine plants, will notice the practically universal habit of the flywheel's running out of true. In some cases it is because the builder fails to true the wheel up properly. Usually, though, it is caused by the erector cocking the wheel when pulling it on the shaft. While a wheel in this shape is not dangerous, still it detracts from the appearance of an otherwise attractive power plant. Many suggestions have been offered in regard to the best way to correct this. Some en gineers loosen up the hub and insert a thin shim around the shaft on the "out" side. It is seldom that this does any good, and it is not safe since the shims alwys work loose. Others shim up under the key on one side of the hub and attempt to throw the wheel straight. Even if successful, the flywheel is left in an insecure position since now only the key holds it. The most approved way is to peen the rim until the wheel runs true. The engineer should mark the part of the rim that runs out with chalk. This will indicate the particular section of the wheel that must be peened. The engine should be slowed down to about 30 to 50 r.p.m. and a 5-pound hammer used against the rim at the chalked place. The rim should not be struck with much force. Presevarence with the hammer will bring the wheel back into line, and the engine will run without the slightest variation in the rim travel.
Withdrawing a wheel key is probably the hardest undertaking around an engine. This is simply due to the lack of proper facili ties. The average engineer drives a chigel between the key-bead and the wheel hub, mars up the shaft and gets disgusted with the job. A key on an oil engine should be drilled and tapped for a "pull-out" bolt. If this is not already done, the operator should do it, for it is not difficult. Then a piece of iron can be made U-shaped and tapped for the bolt. Placing this U bracket over the shaft, Fig. 286, the bolt can be drawn taut, and a sharp blow on the key will start it. A device like Fig. 287 can be used on the smaller engines with success. A 3z X2 in. flat strip of iron some 16 inches long is bent at the end A. The other end has a square hole which is placed over the head of the key as shown. By striking with a hammer at A the key can be loosened.