Connecting-rods.—Figures 265 to 269 show the types of con necting-rods generally used on the low-pressure engine. Figure 265, Bessemer Oil Engine, has a marine-type crank end and a round non-adjustable piston pin end. As the latter has no means of tak ing up wear, the engineer should not allow this solid end bearing to become too worn; prompt replacement by a new bushing will help maintain a smooth-running engine. Figure 266, Buckeye Oil Engine, shows a rod along somewhat similar lines. The principal difference is in the split wrist-pin bearing. In taking up the wear, this split bearing will not maintain a perfect bore. It will tend to go out of round. When making adjustment, the brass should be slipped over the pin, which is removed from the engine and coated with Prussian blue. Rotating the bushing reveals where it bears hard on the pin. The operator can bring the bushing true by judicious scraping. Too many engineers feel that such attention to details is unnecessary; nevertheless, the life of any engine can be doubled by the exercise of due vigilance by the operator. Figure 267, Primm Oil Engine, shows a rod having a wedge adjustment at both ends. Figure 268, Muncie Oil Engine, shows a rod used on some engines with both ends of the marine type. Figure 261, De La Vergne D.H. Engine, shows a rod that closely follows Diesel practice, with the addi tional feature of oiling the piston pin by a passage in the rod itself.
The connecting-rod used in the Fairbanks-Morse Vertical Engine is shown in Fig. 269. The crank bearing has a renewable liner, and any slight wear can be taken up by removal of a shim. The piston-pin bearing is a bronze bushing, split on one side. This bushing fits into the rod and is held by the adjusting, or take-up, block set-screw. When the bearing has worn, it is necessary to remove the rod and piston pin from the piston. The brass shim or strip, which• fills the space between the two edges of the bushing, is taken out and filed until the bushing will fit the pin. This must be done gradually in order not to reduce the shim too much. The bushing and shim are placed in the rod and the pin inserted; the take-up block is then forced down to a solid contact by the screw. Nothing should be taken for granted. The pin should be rotated, being coated with Prussin blue, and any high spots in the bushing scraped away. The engineer must remember that the bushing must come into con tact with the shim. If the high spots are not scraped, the bushing will not bear true since its design is such that it becomes elliptical when adjusted.
Adjustments.—In operation, the engine should never be allowed to run with any play in the _brasses. -Wear is easily detected by the thumping sound as the piston reverses at the end of the explosion stroke. At the moment of explosion the impetus given the crank and flywheel is sufficient to cause it to run ahead of the piston toward the end of the stroke. The pin, then, might be said to be out of contact with the pressure side of the brass. Upon reversal of the crank, the connecting
rod brass comes into contact with the piston pin, producing a blow or thump. If the brasses are snug, the pin is never out of contact. If the wrist end has a wedge adjustment, it is an easy matter to take up the lost motion without pulling the piston.
The same applies to the crank end, regardless of the particular type of bearing. However, if the wrist end be a marine-type or screw-adjusting box, it will be necessary to pull the piston in order to make the proper adjustment. The brass should be drawn up tight, and then the bolts should be backed off a sixth of a turn. Some engineers attempt to adjust without using shims between the two halves of the brass. It is advisable to insert shims so that the pressure of the bolts is on the shims and not on the pin itself. In tightening the brass, the pin is with drawn from the piston. When the correct fit is secured, the pin is driven out of the rod, and the piston, connecting-rod and pin are reassembled. No connecting rod will run without heating if there is no provision made for a small amount of clearance be tween pin and brass. The fit should be snug, yet not tight. A good way to determine whether the adjustment is correct is to "jump" the piston. If a 5-foot pinch bar will just cause the piston to move, it is safe to assume that the brasses are snug enough.
Connecting-rod Brasses.—The piston-pin bearing, or box, is usually made of phosphor bronze—a babbitt bearing will not stand up very well on account of the heat conditions within the piston. If is well to remember that it is not necessary to have the bearing completely surround the pin. The pressure actually falls on a very small portion of the bearing, and the lubrication is better with part of the brass cut away, as shown in Fig. 270. Frequently, the engine manufacturer does not do this, and the engineer will make no mistake if he removes the surplus portion before using the bearing.
The crank-pin bearing used on this type of engirie up to 35 h.p. is usually a split babbitt bushing enclosed in the bear ing housing or box. Ordinarily, the manufacturer die-casts these bearings and reams or broaches them to exact size. If the die-casting is not so finished, the surface is not perfect, and the metal has a tendency to "drag." With the die-cast bearing, in case of renewal the cheapest way is to buy a new one from the engine builder, since the plants using these small engines are not equipped, as a rule, to make a bushing. In case of urgency, where it is impossible to wait for a new bushing from the factory, it is easy to have a local machine shop cast a babbitt bushing, around a mandrel placed in a piece of pipe. The engineer should see that the machinist who fits the bushing to the engine pin uses extreme care in bringing it to a perfect contact with the pin.