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DETERIORATION OF STEEL PROPELLER SHAFTING
By COMMANDER C. H. J. KEPPLER, U. S. Navy
A remarkable instance of pitting of steel shafting after only seven months' service was disclosed recently upon the docking of the U. S. S. New Orleans at Navy Yard, Boston. The New Orleans is one of the older types of protected cruisers, of about 3400 tons, and is sheathed with copper. About the middle of July, 1917, the installation of new propeller shafts was completed at the Navy Yard, Puget Sound, but the urgent war requirements did not permit encasing the steel shafting with the usual composition sleeve. The tail shafting, a hollow steel forging 131" at its greatest diameter, has a section 30' 8" long between the end of stern tube and forward edge of strut bearing, which except for the protective coats of paint was directly exposed to the sea water.
Upon examination in dock on February 16, 1918, the surface of both shafts was found to be covered with innumerable, deep, irregular pits, some areas of from 10 to 16 square inches and from
" to " deep (see Fig. I). Near the struts and stern tubes, under the fair-waters, positions nearest the copper sheathing of the hull, were found the deepest and most numerous holes. The steel fairwaters were honeycombed with large holes and the zinc plates had entirely disappeared (see Fig. 1). A black oxidized metallic substance in the deepest pits was removed with scrapers and had the consistency and appearance of soft putty mixed with lampblack. A chemical analysis of this material gave the following:
(a) Ferrous Oxide, 35.3%.
(d) Undetermined, 18.2%. Ferric Oxide is probably FeO to Fe,O, after exposure to air. FeO is probably hydrated Ferrous Oxide.
After consultation by telephone with the Bureau of Steam Engineering, the following treatment was undertaken:
(e) Shafting wire-brushed and sand-blasted down to bare
(f) Filled all deep pitholes with smooth-on.
(g) Wrapped with layer of Okonite waterproof tapeturns applied opposite to ahead rotation of shaft.
(h) Coated with shellac.
(1) Painted with one coat of anti-fouling paint. The general dimensions of the tail-shafting, together with their areas of greatest deterioration, are shown in Fig. 3.
As a performance of war emergency, the whole operation of cleaning and preservation was accomplished in twenty-five hours of continuous work.
Upon re-examination of the propeller shafting in dry-dock on 28 June, 1918, it was found that the corrosion and pitting had not progressed over the condition existing in February. As a further protective measure, the following additional operations were carried out:
After the coat of anti-corrosive paint, mentioned under (k) above, the shafts were parcelled with canvas sheeting soaked in tallow. They were then served with housline, taking half hitches every four or five inches so as to prevent unravelling in case the housline was cut at any point; finally a coat of molten pitch and tallow, two parts pitch and one part tallow, was applied to complete the protective jacket.
PREFACE "Where do we go from here?" is an expression characteristic of what we of the navy subconsciously are asking ourselves as life unrolls its phase of duties ashore and afloat. Very often we must unceremoniously change from an operating to a designing engineer. Naturally, in such changes we find “nuts to crack." If they are covered by good technical writings we are fortunate; if not, it's a case of dig. That's when we wish some one had blazed the trail with some notes, no matter how meager.
Duty, not long ago, brought me face-to-face with the necessity of determining upon the type of ball bearing to choose for certain high-speed instruments. This was the “nut to crack." From notes made in the investigation of the subject, certain general principles, upon which the theory of ball bearings depends, became apparent, and these are presented in the following paragraphs.
It is understood in a general way that the purpose of the ball bearing is to reduce friction. What else we may expect of it is rather hazy. If the question is asked, "Why are there different types of bearings?”, can we answer intelligently? What would be the answer to these questions with reference to ball bearings for a certain machine?
1. Size of bearing required.