SCREW 



5270 



SCREW 



Park. Near the city in the surrounding hills 

 are various lake resorts. Prominent features 

 of Scranton are the Federal building, the court- 

 house, standing in the center of a parked 

 square, a fine city hall, Albright Memorial Li- 

 brary, the administration buildings of the Inter- 

 national Correspondence Schools, Y. M. C. A. 

 and Y. W. C. A. buildings, Masonic Temple, 

 several fine hotels and the new Delaware, 

 Lackawanna & Western Passenger Station. 

 Benevolent institutions include the Moses Tay- 

 lor and state hospitals, the Oral School for the 

 Deaf and Dumb and the Home of the Friend- 

 less. In addition to several private educational 

 institutions the city has an Historical Society 

 and a Society of Natural Science. 



Scranton was founded in 1840 by Joseph H. 

 and George W. Scranton. In 1854 it was incor- 

 porated as a borough and in 1866 and 1901, re- 

 spectively, was incorporated as a third- and a 

 second-class city. 



SCREW, a mechanical device consisting of a 

 cylinder with a spiral projection or ridge on its 

 surface. The projection is called the thread 

 and the cylinder the body of the screw. Cut 

 from paper a right-angled triangle with an alti- 

 tude at least 

 three times the 

 length of the 

 base. Draw a 

 heavy line along 

 the edge of the 

 oblique side of 

 the triangle (its 

 hypotenuse), and 

 then roll the pa- 

 per around a pen- 

 cil, beginning at 

 the wide end (see 

 1, in Fig. 1). The 

 dark line around 

 the pencil cor- 

 responds to the 

 thread of the 

 screw. In 2 of Fig. 1 is shown a section of the 

 projecting spiral. The vertical distance (s) be- 

 tween two whorls of the thread is known as 

 the pitch. The screw works in a nut having a 

 spiral groove into which the thread of the 

 cylinder fits, and the power is usually applied 

 by a lever or a wheel which moves through 

 the circumference of a circle. The power, it 

 should be understood, is applied at the end of 

 the lever. 



The mechanical advantage of the screw is 

 stated thus: 



FIG. i 



(1) The screw is a spiral 

 inclined plane. (2) The pitch 

 of the screw is shown at s. 



With the screw a given power will support a 

 weight as many times as great as itself as the 

 circumference described by the power is times as 

 great as the pitch of the screw. 



A complete revolution of the screw raises 

 the weight a distance equal to the pitch. A 

 screw having five threads to the inch has a pitch 

 of one-fifth inch. 



Application. To ascertain the weight that 

 can be raised by a screw through the force of 

 a given power, multiply the power by the cir- 



FIG. 2 



(1) Vise; (2) jackscrew ; (3) micrometer 

 screw (see page 3791) ; (4) worm wheel. 



cumference of the circle which it describes, and 

 multiply this product by the denominator of 

 the fraction representing the pitch. (The cir- 

 cumference of a circle^ 2Xthe radius X 3. 1416.) 

 What weight can be raised by a power of 20 

 pounds, acting upon a lever 5 feet long and 

 attached to a screw having a pitch of % of an 

 inch? The power moves through a circle whose 

 circumference=3.1416X2X5 feet or 31.416 feet. 

 31.416X12=376.992, number of inches. 376.992 

 X20=7539.84. 7539.84X5 (denominator of 

 fraction representing the pitch) =37,699.2. 

 Therefore a weight of 37,699.2 pounds can be 

 raised by the screw. (This is the theoretical 



FIG. 3 

 Two forms of endless screws. 



result. Practically, the result is somewhat less 

 because of the power used in overcoming fric- 

 tion.) 



Uses. The screw is applied to so many uses 

 that to name them all would be impracticable. 

 We are all familiar with the wood screw used 



