Machinery's Handbook, 31st Edition
Historical and Miscellaneous Threads 2121 Harvey Grip Thread.— The characteristic feature of this thread is that one side inclines 44 degrees from a line at right angles to the axis, whereas the other side has an inclina - tion of only 1 degree. This form of thread is sometimes used when there is considerable resistance or pressure in an axial direction and when it is desirable to reduce the radial or bursting pressure on the nut as much as possible. See BUTTRESS THREADS . Lloyd & Lloyd Thread.— The Lloyd & Lloyd screw thread is the same as the regular Whit- worth screw thread in which the sides of the thread form an angle of 55 degrees with one another. The top and bottom of the thread are rounded. Lock-Nut Pipe Thread.— The lock-nut pipe thread is a straight thread of the largest diameter which can be cut on a pipe. Its form is the same as the American or Briggs standard taper pipe thread. In general, “Go” gages only are required. These consist of a straight-threaded plug representing the minimum female lock-nut thread, and a straight-threaded ring represent - ing the maximum male lock-nut thread. This thread is used only to hold parts together, or to retain a collar on the pipe. It is never used where a tight threaded joint is required. Philadelphia Carriage Bolt Thread.— This is a screw thread for carriage bolts which is somewhat similar to a square thread, but having rounded corners at the top and bottom. The sides of the thread are inclined to an inclusive angle of 3 1 ∕ 2 degrees. The width of the thread at the top is 0.53 times the pitch. SAE Standard Screw Thread.— The screw thread standard of the Society of Automotive Engineers (SAE) is intended for use in the automotive industries of the United States. The SAE Standard includes a Coarse series, a Fine series, an 8-thread series, a 12-thread series, a 16-thread series, an Extra-fine series, and a Special-pitch series. The Coarse and Fine se - ries, and also the 8-, 12- and 16-thread series, are identical to the corresponding American Standard series. The Extra-fine and Special-pitch series are SAE Standards only. The American Standard thread form (or the form previously known as the US Standard) is applied to all SAE Standard screw threads. The Extra-fine series has a total of six pitches ranging from 32 down to 16 threads per inch. The 16 threads per inch in the Extra-fine series applies to all diameters from 1 3 ∕ 4 up to 6 inches. This Extra-fine series is intended for use on relatively light sections; on parts requiring fine adjustment; where jar and vibration are important factors; when the thickness of a threaded section is relatively small as in tubing, and where assembly is made without the use of wrenches. The SAE Special pitches include some which are finer than any in the Extra-fine series. The special pitches apply to a range of diameters extending from No. 10 (0.1900 inch) up to 6 inches. Each diameter has a range of pitches varying from five to eight. For example, a 1 ∕ 4 - inch diameter has six pitches ranging from 24 to 56 threads per inch, whereas a 6-inch diameter has eight pitches ranging from 4 to 16 threads per inch. These various SAE Stan dard series are intended to provide adequate screw thread specifications for all uses in the automotive industries. Sellers Screw Thread.— The Sellers screw thread, later known as the ‘United States stan dard thread,” and now as the “American Standard,” is the most commonly used screw thread in the United States. It was originated by William Sellers, of Philadelphia, and first proposed by him in a paper read before the Franklin Institute, in April, 1864. In 1868, it was adopted by the United States Navy and has since become the generally accepted standard screw thread in the United States. Worm Threads.— The included angle of worm threads range from 29 ° to 60 ° ; for sin - gle-threaded worms 29 ° is common; multiple-threaded type must have larger helix and thread angles to avoid excessive under-cutting in hobbing the worm-wheel teeth. AGMA recommends 40 ° included thread angle for triple- and quadruple-thread worms, but many speed reducers and transmissions have 60 ° thread angles. The 29 ° angle is the same as the Acme thread, but worm thread depth is greater and widths of the flats at the top and bottom are less. If lead angle is larger than 20 ° , an increase in included thread angle is desirable. Worm gearing reaches maximum efficiency when lead angle is 45 ° , thus explaining the 60 ° thread angle. Thread parts of a 29 ° worm thread are: p = pitch; d = depth of thread = 0.6866 p ; t = width, top of thread = 0.335 p ; b = width, bottom of thread = 0.310 p .
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