Page:The New International Encyclopædia 1st ed. v. 17.djvu/787

* SCREW. 713 SCREW PROPELLER. Beparating and hniidliiifr vM-h lilank respectively as the nature of (lie nperation denunuls, and pro- ducing finished screws with wonderful rapidity, regularity, and perfection. 'V" Thread. Square Thread. Whi+worth Thread. Powell's Thread. STANDARD SCREW THREADS. Formerly all wood screws were cut screws, that is, the metal of the body of the blank was cut away in grooves, leaving the thread projecting. In recent years, however, a process of rolling and press working has been employed by which the threads are raised without loss of the metal be- tween them. In nearly all ca.ses the threads of wood screws and machine screws are triangular in shape. The extensive use of screws has led to standard shapes and dimensions being adopted for screw threads. In England this standard is the Whitworth thread, designed by Sir Joseph Whitworth : in the United States it is the United States standard or Sellers thread, devised by William Sellers of Philadelphia. The.se standards relate particularly to the threads of machine screws, bolts, and nuts, etc. For tables of dimensions of screw threads and various other data regarding the use and efliciency of screws, see Kent's Mechanical Enfihicers' Pocket-Boole (New York, 100(1) ; also Rowland, on "Screws," in the Knci/rIi>iKr dia. Britannica. SCREW DOCK. See Caisson. SCREW-PINE (Pandanus). A genus of plants of the natural order Pandanace*, natives of tropical Asia and of the South Sea Islands. Many of them are remarkable for their prop roots. Their spiny-edged, sword-shaped leaves, 3 to 4 feet long, are spirally arranged in three rows. In general appearance, when unbranched they resemble gigantic pineapple plants, whence their popular name. Pandanufi odoratissimtis is a widely distributed spreading and branching tree, 25 feet high, much used in India for hedges. It grows readily in a poor soil and is one of the first plants to appear on newly formed islands in the Pacific. The unexpanded flowers are fre- quently boiled with meat. Oil impregnated with the odor of the flowers and the distilled water of them are highly esteemed East Indian per- fumes. The terminal biuls, the soft, white bases of the leaves, and the fleshy part of the drupes, which grow together in large heads, are eaten ■in time of scarcity. The spongy and juicy branches are used as cattle food. The leaves are uscil in (hati-hing. and in making a kind of um- brella conunim in India, and their tough longi- I I ' ^ ,v,' A., BCBBW-piNE (Pandanus utilia). tudinal fibres for making hats and cordage. Their spindle-shaped fibrous roots are split for basket- making. Alore valuable as a fibre plant is an allied species, Pandanus utilis, the vacoa of Mau- ritius, which grows to a height of about 30 feet, but from continual cropping of its leaves usu- ally grows to U or 10 feet. The fibres of its leaves are used for making bags, which rival in cheapness and usefulness the gunny bags of India. In temperate and northern climates these plants are commonly cultivated in greenhouses for ornament. SCREW PROPELLER. A contrivance for l)ropelling vessels which acts in the water like a screw bolt in a nut. It consists of a hub of cylindrical or spherical shape to which are at- tached the Idades that form the screw. Screw- propellers are cast in one piece or built up. the blades being attached to the hub with bolts. The latter plan is now common, though small screws arc usually cast in one piece. Propellers are made of cast iron, cast steel, or bronze. The best are made of bronze of fine quality, because, though not equal to stbel in strength, it cor- rodes very slowly — a very important point, as the corrosion not only diminishes the strength, but makes the blades rough and ragged at the edges, thereby reducing their elficicncy. The blades of a screw propeller may be con- sidered as parts of separate threads winding around the hub and shaft, cut off by planes perpendicular to the shaft and at a distance apart about equal to the length of the hub. simple true screw would be made of such form, but experiment has shown that some variations from the simple form are desirable. In the first ])lace. the edges of the blades nuist be sharj) and the thickness near the hub sufiicient to stand the strain of propulsion. The outer ends are pointed or have the corners cut off to rediice the vibration, and in many screws the driving or leading edge is thrown to the rear from the normal radial line for the same reason. The nundicr of blades varies from two to four. Two- bladed screws are at least as economical in smooth water as screws with more blades, but in