Innovations in naval warfare are not only new weapon systems and sensors. Often overlooked are innovations that affect the less exciting but vitally important area of logistics. In February 1923, Rear Admiral John K. Robison, the Engineer-in-Chief of the U.S. Navy, wrote of the influence sound engineering design could have on strategic problems confronting the line officer—fuel consumption, in particular:
Logistics, speed, and radius of action are inseparable whatever the type of ship. Economy of operation, and economy of design, enable a fleet to take up a strategical position that would otherwise be impossible on account of fuel consumption. . . . A small percentage of saving in the logistic requirements may mean the winning of a strategical position.
Increasing the operating range of the fleet through fuel efficiency became a mania for the interwar Navy. The Navy’s War Plan Orange called for the Pacific Fleet to cross the vast Pacific Ocean accompanied by a train of fleet oilers for replenishment. One of the most important innovations in propulsion efficiency was the Kingsbury-Michell tilting pad thrust bearing.
The introduction of the screw propeller in the early 19th century was a radical innovation in steamship propulsion. The propeller was connected via a propeller shaft directly to the crankshaft of the steam engine. The water displaced by the screw propeller imparts a reactive thrust on the propeller that is transmitted through the shaft to the engine, which, being rigidly connected to the keel, imparts a force into the hull of the ship, moving it forward through the water. To prevent damage to the engine, a collar in the shape of a solid disc is installed on the shaft before its connection to the engine imparts the thrust against the framing of the hull.
As ships grew larger in displacement with ever more powerful engines to match, the simple collar evolved into multiple collars encased inside a thrust box connected to the keel. The thrust box was located at the end of the propeller shaft and just aft of the engine. Each collar on the rotating shaft bore against a fixed vertical bearing surface inside the box. The bearing surfaces were coated with white metal babbitt (any of several alloys of tin, lead, and various other elements) immersed in an oil bath to reduce friction. The bearings were manufactured in halves so they could be removed and replaced as the babbitt wore away. The collars usually ran hot from friction and were often cooled by seawater. They required frequent adjustment and maintenance to keep them from self-destructing under the huge axial loads involved. The RMS Titanic, for example, had a thrust box on each shaft the size of a city bus containing 14 collars for full ahead and 7 for backing.
Friction wastes energy, reduces the power sent to propellers, adds cost in wasted fuel, and shortens operating range. Two civil engineers working hemispheres apart arrived almost simultaneously at a solution to the friction problem. A. G. M. Michell was an Australian engineer looking to make large irrigation pumps operate more efficiently, and Albert Kingsbury was an American seeking an improved bearing to support the massive generators in a hydroelectric powerhouse in Holtwood, Pennsylvania. Their improved bearings were based on the research of Osborne Reynolds, the famous British engineer, who found that when a very small angle was induced in a flat bearing the oil filling the space between the bearing surfaces took on the shape of a wedge, and this wedge-shaped oil film could support a far greater load than a standard journal bearing. Both Michell and Kingsbury hit on the idea of replacing the split bearing collars in a thrust box with a series of sector-shaped pads or shoes, made of bronze and surfaced with babbitt, arranged in a circle around the shaft matching the diameter of the shaft collar. Each pad was supported by a pivot that allowed the pad to tilt a small amount to form a wedge of oil film between the pad and the collar.
Michell found in testing that a single tilting pad thrust bearing could take ten times the load at one-20th the friction compared to the old thrust bearings. Kingsbury found similar results, and when one of his bearings was examined, the wear was so small it was estimated it could last for centuries. The new design could be scaled to meet any load requirement. Michell obtained British and Australian patents in 1907, and Kingsbury was granted a U.S. patent for his design in 1910. Both designs were used initially to support the vertical loads of heavy machinery, but they were soon found to be equally useful for horizontal axial thrust. In a ship’s thrust box, the tilting pads were placed on both sides of the shaft collar to support the axial thrust in the forward and reverse directions, and a single Kingsbury-Michell bearing could easily replace many multiples of the old type.
The Michell bearing was introduced at precisely the time it was needed most. Charles Parsons, the turbine engine maker, was using a gear reduction unit in his designs to improve overall efficiency, as even the lowest-speed turbine caused cavitation on the propeller blades. With gear reduction, the thousands of revolutions per minute (RPM) of a turbine could be matched to the few hundred RPM needed to efficiently turn a propeller. He found, however, that conventional thrust boxes allowed far too much axial play, which created havoc with the close manufacturing tolerances the precision gears in the reduction unit required. Parsons conducted his own tests with the Michell bearing and verified others’ results.
High-speed geared turbines on civilian ferries were among the first to employ the tilting-pad thrust bearing. The first warship to receive one was the British destroyer HMS Leonidas in 1913. All subsequent geared-turbine designs would use a Michell bearing. Each of the four 24-inch diameter shafts of HMS Hood transmitted 36,000 shaft-horsepower (shp) with a single Michell bearing 4 feet, 6 inches, in diameter. The Royal Navy began replacing the old thrust boxes with the new design on both turbine and reciprocating engines where possible, resulting in a reported savings in annual fuel consumption in 1918 of 500,000 pounds sterling. Civilian maritime operators saw the obvious advantages of the new bearings and did likewise.
The U.S. Navy followed suit beginning in 1917, using tilting-pad thrust bearings produced by the Kingsbury Machine Works on everything from submarines to battleships. The bearings provided cost savings in any application requiring a thrust bearing, albeit optionally so in most cases. But they were absolutely necessary wherever a gear-reduction unit was employed. On smaller vessels such as destroyers and submarines, the Kingsbury thrust bearing was built into the housing of the gear-reduction unit, saving space.
Nearly every ship built in the past 100 years has used tilting-pad thrust bearings based on the Kingsbury-Michell design somewhere in its propulsion system. Improvements such as continuous oil feeds and leveling plates under the tilting pads have been introduced, but the same essential design is still being produced today under the Michell and Kingsbury trade names for a large variety of heavy industrial and marine applications.
Bibliography
1. RADM John K. Robison, USN, “The Part of Engineering in Command,” U.S. Naval Institute Proceedings 49, no. 2 (February 1923): 203–18.
2. J. A. Taylor, “The Michell Thrust Bearing,” Proceedings of the Engineering Association of New South Wales 30 (April 1915): 39–50.
3. Journal of The American Society of Naval Engineers, 34 (1922).
4. Bureau of Naval Personnel, Training Division, NAVPERS 10813-B: Engineering, Operation, and Maintenance (Washington, DC: U.S. Navy, 1966), 242–43.
5. Jim McHugh, “Albert Kingsbury–His Life and Times,” Sound and Vibration Magazine (October 2005), 2–9.
6. D. K. Brown, The Grand Fleet (London, UK: Chatham Publishing, 1999), 24.
7. The Kingsbury Bearing at Holtwood (New York: American Society of Mechanical Engineers, June 1987).
