1. Continuous operation, or almost continuous operation of machinery at designed full power for any length of time is almost certain to disclose defects or weak parts of that machinery. Naval vessels, and many others as well, were subjected to these conditions during the recent war. While the percentage of vessels whose machinery was not equal to the continued strain was comparatively small, a number of vessels did require repairs of a more or less urgent nature. Some of these repairs were beyond the capacity of the ship's force, and it became necessary for these ships to go into dry-docks at navy yards. The repairs in dry-dock consisted principally of shaft repairs, and by far the greatest trouble with shafting, aside from routine re-wooding of bushings, etc., was experienced with outboard couplings. If these constitute a weak point, as they evidently do, it may be possible to improve them.
2. Since the adoption of marine machinery and propellers, the engine and the propeller have fought continually—the engine to turn the shaft, and the propeller to keep it from turning. This conflict of forces is nothing less than a great battle, with science and invention endeavoring to aid first one and then the other belligerent. In the meantime the shaft—as you may imagine—is "torn between two emotions." At one end the engine turns it, at the other the propeller resists the turning, pushing against it, and—driving the ship. The shaft, made up of sections bolted together with couplings, and supported in bearings throughout its length, is acted upon by an almost infinite variety of forces—forces tending to turn it, forces tending to compress it, forces tending to bend it, and (when the ship backs), forces tending to stretch it.
3. For this very reason shafts are constructed with ample allowance for strength. With slight variation for different types of installation, there are the following sections: main engine shaft, thrust shaft, one or more sections of line shafting, the stern tube shaft, and the propeller shaft. The couplings securing the end of these sections to each other, are of the same type for the engine, thrust, and line shaft sections, and consist of a flange shrunk or forged on each end of each section so as to be practically integral with it. These flanges are bolted together with through-bolts so that the shaft is continuous, with very little additional weight, and if anything, with greater strength at the couplings. The stern tube section is secured to the after end of the line shafting by a special sleeve fitting over the forward end of the stern tube shaft and bolted to the flanged end of the line shaft. This sleeve is kept from turning on the shaft by longitudinal keys, fitted between the shaft and the sleeve. It is prevented from pulling off, or moving lengthwise along the shaft, by a locking ring, or by segments of a ring, which fit into a grooved recess in the shaft. This construction is used so that the end of the shaft will not be enlarged, and so that the shaft can be drawn outward through the stern tube if desired. In some ship yards (Newport News, for instance), it is customary to force this sleeve on the shaft while heated. Any one can imagine the enthusiasm with which the task of removing one of these coupling sleeves is approached in a navy yard. One of the inboard couplings of the U.S.S. Michigan resisted all efforts for about a week, although a force of approximately 60 tons was applied, and the coupling finally had to be cut off, and a new sleeve forged. This coupling had been forced on cold, over a .003 inch, to .004 inch difference in diameter. These couplings do not, as a rule, cause much trouble in operation. The chief trouble develops when it becomes necessary to remove the coupling in order to withdraw the stern tube section of the shaft. The other special coupling, outside the hull, causes more trouble.
4. The coupling between the stern tube shaft and the propeller shaft is the heaviest, and probably the strongest part of the whole shaft. Owing to its inaccessible location, outside the hull and under water, special precautions have been taken in its design and construction. There are two general types in use. The one most in favor with naval men consists of a hollow cylindrical sleeve, tapered inside to fit a special taper on the ends of the shafts. Two opposite longitudinal keys between the shaft and sleeve, keep each end from turning in the sleeve. Cross keys through each shaft, and the sleeve, draw the ends into the taper and keep them from withdrawing. This type gives general satisfaction, the only troubles experienced being due to keys working loose, fractures caused by too tight a fit at the sides of cross keys, etc. In the other type the sleeve is made in two halves, bolted together along the sides. The shafts are prevented from withdrawing by a single locking ring in halves fitting into annular recesses in both shafts and into a recess on the inside of the sleeve. Longitudinal keys keep the shafts from turning in the sleeves. A great deal of trouble has been experienced with this type, arising from crystallization of the bolts, in spite of the fact that the strongest materials, even class "A" nickel steel forgings, have been used. The importance of these couplings and their inaccessible location, make it essential that their design and construction should embody the best that mechanical science and experience afford.
5. An impartial "estimate of the situation" makes it clear—strange as it may seem—that the greatest trouble in shafting occurs at the heaviest and strongest part. It is possible that an investigation of the forces of vibration in the shaft will disclose the reason why this particular part should cause so much trouble.
6. Vibration may be divided into three general classes: transverse, longitudinal, and torsional. Different parts of the shaft are subjected to stresses in various directions. Each crank-pin of the reciprocating engine tends to force the shaft in a direction dependent on the angle of the cranks to each other. The shaft is restrained by bearings between each crank. The fact that these bearings are equally spaced along the shaft increases the tendency of the shaft to spring, the amplitude being equal to the distance between bearings, the nodes being at the bearings, and the frequency depending upon the speed of revolution. This tendency, started at the engine, travels through the length of shaft until it reaches the heavy outboard coupling, and this coupling is of such heavy construction that the vibrations are damped—absorbed. The end of the shaft tries in vain to shake itself free in the coupling. If the bearings are not in exact alignment, or if some are down more than others, there is a tendency for the shaft to sag or to be bent at these points. As the shaft revolves, it tends to bend successively to every part of its circumference. This causes the same transverse vibration of the end of the shaft in the coupling sleeve. Moreover, aside from its weight, there is another factor tending to cause the vibrations to be absorbed in this coupling. The propeller, turning in water affected by the wake current of the ship, by currents diverted by the rudder, and by the greater pressure of water at the bottom than at the top, has a tendency to spring the after end of the shaft. The supporting strut bearing is not tight; it permits slight play of the shaft. This transverse vibration, being dependent upon so many uncertain features, can hardly coincide with the vibrations in the remainder of the shaft, and these vary with the speed of rotation. Is it not natural to suppose then, with the propeller shaft trying to shake itself loose in one end of the coupling sleeve, and the stern tube shaft endeavoring to shake itself loose in the other, that great and unknown stresses are set up at this point?
7. As the propeller turns over, the force required to turn it and the thrust which it delivers to the shaft vary with the currents in the water in which it turns. This variation is for the most part periodical, and dependent upon the resistance encountered by each blade as it passes the points of unequal resistance. The thrust then becomes a sort of pounding, varying a little above and a little below the mean thrust delivered to the shaft. But the shaft is held from longitudinal movement, however slight, by the thrust bearing. Once the thrust collars are tight up against the thrust shoes, no further longitudinal movement is possible. The shaft vibrates in length, and this vibration is also absorbed in the outboard coupling. The stress exerted in the coupling sleeve may be likened to the action of a wedge subjected to vibrations. The tremendous force which may be exerted by a wedge is not dependent entirely upon overcoming skin resistance between the wedge and the faces it is forcing apart. The wedge itself contracts in length and expands in width on being struck by a sledge. It is evident that the pounding effect of unequal thrust in the shaft exerts a very similar stress upon the coupling sleeve. And in much the same way the inequalities in water resistance affect the uniform rotation of the propeller.
8. Thus, torsional vibration of the propeller shaft is due to unequal delivery of torque by the propeller, even though the torque delivered to the shaft by the engine is uniform, as in turbine and electric installations. What place in the shaft is most likely to receive the full benefit of vibrations caused by momentary differences between the torque applied and the torque delivered? We conclude that the outboard coupling, because of the great weight concentrated on this one part of the shaft, absorbs almost all the vibration of the shaft. There must be some means of diverting these vibratory effects from this inaccessible location.
9. Is it possible to distribute the shaft vibrations throughout the length of the shaft? Would this decrease our shaft troubles?
10. To distribute the vibrations throughout the total length of shaft requires that the shaft be made practically uniform in weight and strength. The solid couplings used on line shafting give practically no trouble and have the advantage of parallel opposing faces to assist in checking alignment of the shaft. If these couplings could be used in place of the special outboard and inboard couplings now used, the shaft would be uniform in weight and strength throughout its length. The deterrent factor is the necessity for withdrawing the stern tube and propeller sections of shaft through the stern tube and strut bearings. Can the stern tube and strut be modified to permit the removal of these sections if fitted with flanges? They would have to be enlarged about 8 inches, in diameter. Clearly it is impracticable with ships with built-in stern tubes. But new struts might be fitted. This would eliminate the necessity for the outboard coupling. It is believed that the bushings which fit into the stern tube and strut to form the bearing for the shaft, could be fitted into a sort of distance bushing, about 4 inches thick, fitted to the inside of the stern tube and strut. These bushings would not be removed except when necessary to withdraw the shaft. It is more than probable that this is entirely feasible in future construction, and that the advantages offered to ships at present in commission may warrant the installation of new struts incorporating this feature, and eliminating the present type of outboard coupling.
11. This would mean, then, that for new construction, the stern tube would be about 8 inches larger in diameter, and that in each end would be fitted a distance bushing of brass, 4 inches thick, cored and ribbed for lightness and strength. The familiar wooded bushings would fit inside the distance bushings in the same way in which they now fit in the stern tube. The struts would be changed in the same way, except of course, that only one distance bushing would be required for each strut, while two would be required for each stern tube. This feature of construction is suggested as a means of distributing the vibration throughout the full length of the shaft.
12. It is, of course, possible that more thorough investigation and experiment may disclose some better method. But before we attempt to estimate this possibility, let us consider the remaining factors.
13. First, reduction of vibrations caused by the propeller would seem to be improbable. Many types of propellers have been used in the navy. The present type is the result of endless calculation, experiment, and trial. The vibrations which originate at the propeller are not caused by any fault of design or construction. They are the result of unequal areas of pressure of water against the blades, arid it is extremely doubtful if any means will ever be provided to improve this condition.
14. Secondly, while the vibration caused by the engine has been reduced to what approaches a minimum, there still remains the shaft vibration, which no engine design can eliminate. Yet the more modern the engine, the more imperatively does its increase of power demand the elimination of vibration. Great strides have been made toward eliminating vibratory stresses caused by the engine. The newer installations, turbine direct connected, turbine with reducing gears, and electric drive, all provide a uniform torque and practically no transverse pressure on the shaft. These advances in main engine installation, however, bring increased power as well, and while the general result has been a great improvement in shaft vibration, the newer installations require even less vibration that at present, for more satisfactory operation. It is readily understood that an old reciprocating engine will "mote" even if the shaft is shaking around a good deal, while a high speed turbine or an electric motor would rack itself into pieces under the same conditions. This only serves to illustrate the difficulties attendant on the "war problem" confronting engineers at each step of progress in marine engineering. To revert to the conflict of forces, amounting to a continual battle between the propeller and the engine, we may say that the engines now have the advantage. There is no improvement in sight for the propeller. The only way in which progress can be facilitated appears to be by improvement in the shaft.
15. The causes of shaft vibration have been treated very briefly. It has been shown that torsional and longitudinal vibration do not disappear with uniform application of torque. It is to be expected that vibration will be present in future installations, though it will undoubtedly decrease as installations become more mechanically perfect. If this vibration can be distributed throughout the total length of shafting, whether by the method of eliminating concentration points at heavy couplings, as suggested in this article, or by some other practicable method, it is certain that the many difficulties experienced with present shaft installations will be greatly minimized, and some at least, will be completely eliminated.