ON THE SMALL SCREW (UUGE. 



447 



while not extravagantly outside those of the screws to which the data are 

 to^ be applied, provide quantities convenient for measurement with a 

 micrometer gauge, and for the testing-machine. 



The following results were obtained :- 



A. — Torque required to turn a pair of the screws in two nuts, each 

 •226 inch thick (3-6 threads) drawn apart with a strain of 1 ton. 



Angle of thread. 

 40° 



50° 

 UO" 



Torque required. 

 11 foot pound.s 

 l"j )> II 



18 .. 



11 X cos 20'" = 10-34 

 i;!xcos25° = ll-7 

 18xcos30° = 15-r, 



It will be observed that frictional resistance to unscrewing increases 

 with the angle of the thread much more rapidly than in proportion to the 



Fig. 6. 



increased surface pressures due to the oblique thrust. The above figures, 

 from specimens black from the hardening process, are higher than one 

 obtanied from a screw with a bright surface in a preliminary experiment 

 by about 90 per cent. ' 



P~ I^ull on the screw required to shear the thread out of a brass nut 

 •226 inch thick (3-6 threads) cut from flat drawn strip. 



Angle of thread. 



40° 

 50° 

 00° 



Force required to 



shear thread. 



5,500 lb. 



(5,220 lb. 



fi,590 lb. 



Area of thread 

 sheared. 

 •2275 sq. in. 

 •250 „ 

 •270 „ 



Shearing force per 

 square inch. 

 24,1(50 lb. 

 24,880 lb. 

 25,200 lb. 



In this table the area of the thread sheared is obtained from a measure- 

 ment of the space left for the thread of the nut between the successive 

 threads ot the screw, so that this area io less as the screw thread has a 

 wider top. 



