In replacing the stuffing-box packing, square duck packing or braided hemp, well soaked in oil and graphite, is excellent. In drawing up the gland, it is not necessary to exert much pres sure; the 2 or 3 pounds air pressure does not require much gland pressure. If the gland is drawn up tight, a groove will gradually develop on the shaft.
A few engines are equipped with an air-sealing device at the inner face of the main bearing. This is usually in the form of a brass plate placed between the crank web and the main bearing face. It has a close fit with the shaft and, if provided with springs to keep it against the bearing, will prove effective. The objection to this device is that it is hard to replace when made in one piece, as is usually done. This requires the removal of flywheel and bearing; but it should be renewed as soon as the leak ing air causes the oil to blow out of the bearing. It is possible to replace this without the removal of the flywheel by making the ring in two pieces. The Fairbanks-Morse Co. now furnishes a two-piece ring for replacement. Where the one-piece replace ment ring is supplied by any particular manufacturer, the ring can be split. By removing the bearing liners, the two ring halves can be brazed together. Another method of emergency replace ment is the use of a ring of V-shaped leather, Fig. 281. This cir cle can be cut in two to place it on the shaft and the ends secured by fine wire. A coiled steel spring can then be arranged around the shaft, resting in the V. The spring forces the leather against the web and the air-sealing ring, preventing any air leaks. The leather wears rather rapidly but has solved the difficulty in more than one plant where the brass sealing ring began to leak. This sealing ring takes the side thrust of the cranks, and, because of this, the leather cannot be recommended for a permanent re placement. Even with the standard sealing rings the wear, due to the side thrust of the shaft, is rather heavy. In time this causes excessive clearance, which must be corrected by inserting thin brass or steel shims between the sealing ring and the crank web. The ring springs will hold the shims into place. These shims wear rapidly, and the sheet-steel ones last much longer than the brass.
The important thing to remember is that the air seal must be effective, otherwise the oil film will be blown off the shaft. This is more likely to happen on a ring-oiling bearing than on a force feed type and will invariably result in a hot bearing.
Air Seal in Open-frame Engines.—In engines with an open frame, and using the front of the cylinder for the air compressor, the troubles of the inclosed frame are largely eliminated. The points requiring attention are the air valve and the piston rod stuffing box. The latter has a marked habit of wearing, due to the pressure of the rod when the crosshead is not in alignment. In such cases it is practically impossible to keep the packing in condition. A good way to tell when the stuffing-box needs re packing is the presence of air bubbles mixed with the oil on the piston rod. If the leak is bad, the air, jetting out around the rod, will blow the oil entirely off the rod. Metallic packing rings have been used on the rod with some slight success; taken all in all, nothing can eclipse the old diagonal duck and rubber square packing. In inserting the packing, the rings should be cut diagonally and the joints staggered. An oil pipe should be located over the rod immediately in front of the stuffing-box; it is imperative that the rod receive positive lubrication.
Main the low-pressure engine three classes of bearings are commonly employed, as shown in Figs. 282 to 285. Figure 282 is a type best adapted to engines below 50 or 60 h.p. per cylinder. It allows the wear to occur at the center of the bottom shell, which is placed at a 45-degree angle. Conse quently, to take up a slight wear, it is only necessary to remove a shim or two from between the two shells. This, of course, ap plies to the smaller size engines. Much bearing wear causes the piston to increase the cylinder clearance, lowering the compres sion. To partly remedy this, thin shims should be inserted under and back of the lower shell, bringing the shaft center back to its original position. This particular bearing is lubricated by a me chanical oil pump, the oil flowing down into the interior of the engine frame. Figure 283 is the same type of bearing, with the addition of an oil cellar and an oiler chain.