On larger engines the removable bushing is seldom used. The crank-pin bearing housing is usually made of steel or cast iron, though a few housings have been made of bronze; the cast-steel housing is by far the best. The box is lined with babbitt, and the best babbitt is always the cheapest in the long run.
Quite often one sees a crank box that will not retain the babbitt. The fault generally proves to be a lack of sufficient anchorage in the bearing. If the box is merely drilled in a number of places for babbitt anchors, it will be well to put it on a planer, or shaper, and cut dovetail slots in the surface of the housing. These slots will hold the babbitt under unusually severe conditions.
Babbitting a Crank rebabbitting a box, it is never advisable to run it with the box cold. If this is done, the babbitt will seldom unite with the box. Where the engine is small, as good a method as any is to secure a mandrel the same diameter as the crank pin and, after placing this in the housing (which has been heated), run the beatring around the mandrel. Any bad spots in the babbitt can be smoothed up, and the entire bearing scraped into a good fit to the pin. On large engines the same method as is used on a Diesel engine should be_f °Rowed. In running a new bearing, a liberal amount of shims should be used between the two housing halves in order that the babbitt can wear considerable and still leave room between the two hous ing parts for "take-up." Extreme care should be exercised to see that the babbitt is not overheated in melting. Regardless of the question of economy, it does not pay to use the old scrap babbitt in the ladle. It carries too much abrasives; it is best re served for ordinary work, such as line-shaft boxes.
clearance for the crank pin, which should not exceed .02 inch, is indicated by a very slight "jump." As regards the value of side play between the big-end bearing and the crank web, this of course differs in various make engines. Some demand more liberal clearance than others if the brass does not bind after warming up. A fair value is inch, or just enough to be detected by using a small pinch bar.
When an engine is first started or after a new bearing is in stalled, the engine should be run without load for a couple of hours. If a bearing runs hot in service, it never pays to shut down, for the babbitt will surely grip the pin. The load should be thrown off and the engine run at a slow speed, with the lubri cation increased, until the bearing cools off.
and the oiling of the wrist-pin box has caused more trouble and worry to the en gineer than the oiling of all the rest of the engine. Some builders depend on picking up enough oil from the cylinder walls to lubricate the pin, the pin being drilled lengthwise and provided with a scoop at one end, Fig. 271. The great objection to such a method of oiling is the likelihood of the scoop picking up carbon particles and thereby clogging the oil passage, causing a trouble some pin. A second arrangement is to have a pipe passing through the cylinder walls, allowing the oil issuing from the end of the pipe to be picked up by a groove on the piston and then conveyed to the pin, Fig. 272. Still another plan is to supply the pin by means of a drilled hole extending through the connecting rod from the crank end, Fig. 261. Probably the best way to oil the pin .of a horizontal engine is to use a device such as a wick or wiper oiler in connection with a mechanical oil pump, Fig. 251.
A contrivance that has been used on an inclosed-frame engine is outlined in Fig. 273. A 1N-inch brass pipe is slotted on one side and the ends capped. This pipe is fastened to the inner wall of the piston by two clamping bands and extends out 16 inches be yond the end of the piston. A 34-inch connection is run to the pin boss connecting with the oil passage already in the pin. The 1N-inch pipe receives the oil through a tube running from the oil pump, which is mounted on the engine frame back of the cylinder.
Since the slot is as long as the engine stroke, the oil supply to the pin is positive. The inertia of the oil as the piston moves assists in insuring a steady oil stream. No matter what system be used, it must be given attention. The piston pin is in a high tem perature zone and needs a constant oil supply to keep' in normal condition.
The usual way of oiling the crank pin is by a centrifugal oil ring, Fig. 272, which is by far the simplest and most reliable de vice. Only one precaution need be exercised. The big-end bear ing should have liberal oil grooves cut in the babbitt in order to distribute the oil as it emerges from the drilled passage in the pin. Whenever the bearing gets warm, the babbitt may be forced into the oil passage, closing it. The oil passage, then, should always be cleaned out after the hot box has been cooled.