COMMON FASTENING AGENTS 257 



tween the screw and nut is not parallel to the axis of the 

 screw, there is a tendency to burst the nut. Therefore this 

 form of thread is unsuitable for transmitting power. 



The Whitworth's screw (Fig. 118b) is slightly rounded 

 at the top and bottom. Compared with the American threads, 

 the difference is in the angle between the sides, which is 55. 

 The French have a standard screw with the thread at an 

 angle of 60, with a flat top and bottom. Its pitch and dia- 

 meter are given in millimeters. An international standard 

 for metric screw threads was adopted at Zurich in October, 

 1898. This thread is based on the United States standard, 

 which is an equilateral triangle truncated (cut) one-eighth 

 of its height at top and bottom. 



The United States standard thread (Fig. 118c), often 

 called Seller's thread (from the man who first manufac- 

 tured it), is also made with its sides at an angle of 60 to 

 each other, but its top is cut off to the extent of one-eighth 

 its pitch, and the same quantity is filled in at its bottom. 

 The advantages claimed for this thread are that it is not 

 easily injured, that the taps and dies retain their size 

 longer, and that bolts and screws made with this thread 

 are stronger and have a better appearance. As this thread 

 has been recommended by the Franklin Institute of 

 Philadelphia, it is sometimes called the " Franklin Institute 

 Standard." 



Since this thread is flattened or cut off at the point and root 

 an amount equal to one-eighth of the pitch, it is only three- 

 fourths as deep as a V thread of the same pitch. For example, 

 a 1-in. bolt threaded with a United States standard thread 

 will have a root circle .837 in. in diameter, while a V thread 

 of the same pitch cut on a 1-in. bolt will have a root circle 

 .784 in. in diameter. This shows that the V thread cuts into 

 17 



