TYPES AND ADJUSTMENTS The Diesel connecting-rod shares with the main bearings the questionable honor of giving the operator hours of worry. To the engineer who is versed in steam engine practice it remains a source of wonderment why even the smallest amount of connecting-rod brass wear can cause such heavy pounding. The cylinder pressure of the Diesel at the moment of initial com bustion mounts into the hundreds of pounds: many times, on starting, the pressure runs as high as 750 lbs. per sq. inch in those engines not equipped with relief valves. The pressure is in the nature of a hammer blow, even where the fuel valve adjustment is correct. It is to be expected that, because of this constant hammering, the wear on the brasses will be far more than that which occurs in the steam engine where the steam enters the cylinder in a less violent manner. Furthermore, a condition of brass clearance that would be perfectly acceptable with the steam unit cannot be tolerated with the Diesel. This explains the continual brass adjustment that, to the uninitiated, seems to indicate that the engine builder had failed to properly manu facture these parts.
American Diesel Engine Co.'s Connecting-rod.—The first Diesel manufactured in America was fitted with a connecting-rod similar to Fig. 41. This gave place to the type illustrated in Fig. 42. The wedge design was quite prevalent in gas engine work, and the Diesel builders evidently obtained it from that source.
In adjusting the piston-pin bearing, it is necessary to remove the rod from the piston. To reduce the clearance resulting from the wearing of the brass, the cap is unbolted and the re quired amount of shims removed. The wedge must then be brought up snug against the pin. Some operators, under a mistaken idea, use no shims or separators, depending solely upon the wedge. It must be conceded that many engines have operated fairly successfully under this condition. However, the shims serve to keep the two bearing halves rigid, and, in the event wear occurs, the play of the two halves will not hammer the wedge. The objection to this form of wedge is based on the frequency of fracture of the wedge bolt. The wedge has a steep angle, and the end thrust against the bolt is of considerable proportion. The break usually occurs in the thread, allowing the wedge to shift. A remedy for this can be obtained by reduc
ing the wedge angle. This, of course, lessens the total adjust ment obtainable, but shims can be interposed between the wedge and brass, giving a further lift.
The crank or big end follows conventional Diesel lines in being of the marine design. The housing is cast steel and has the babbitt run directly on its inner surface.
Allis-Chalmers Diesel Connecting-rod.--An improved form of the wedge-type rod is found on the Allis-Chalmers Diesel. This rod has a marine crank end and a wedge-adjustable piston pin bearing, Fig. 43. The improvement consists, mainly, in the employment of adjusting or separating set-screws in conjunction with the wedge. In taking up the bearing wear the set-screws are slackened off and the wedge brought up hard against the brass. The wedge bolt is then backed off an eighth of a turn and the set-screws tightened. These set-screws serve to separate the two bearing shells enough to provide running clearance between the pin and the bearing. This design enables the oper ator to correct any piston-pin bearing wear without removing the rod from the engine. After the clearance assumes propor tions beyond the capacity of the wedge, shims can be inserted between the shell and the wedge block. The big-end bearing follows the usual marine design; the babbitt is cast directly onto the housing halves. Separators of %- inch thickness are placed between the housings when the bearing is bolted together for boring. The adjustment is secured by the addition or removal of separators.
Snow Oil Engine Connecting-rod.--This rod, Fig. 44, is quite dissimilar to the rods used on other American engines. The piston-pin end is of the marine type while the big end is provided with a wedge-adjustable open-end bearing. The engine has a crosshead design piston. Consequently it is not necessary to remove the pin for the purpose of taking up the wear in the bearing. The bearing bolts can be slackened off and the proper amount of shims or separators removed. Since the crosshead pin is large, but little adjustment need be made on this end. Adjustment at the piston-pin end reduces the connecting-rod length, and thereby increases the compression clearance in the cylinder.