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The New Zealand Railways Magazine, Volume 1, Issue 6 (October 24, 1926)

Uniform System of Screw Threads

page 27

Uniform System of Screw Threads

Screw-threads are those of bolts and serews used in bolting up steam engines, and other machinery. The variety of threads adopted by different manufacturers, causes not only great inconvenience, but adds to the cost of repairs in the railway workshops.

The difficulty of ascertaining the exact pitch of a particular thread occasions much embarrassment, especially when it is not a sub-multiple of the common inch measure. This difficulty would be completely obviated by uniformity, the thread becoming constant for a given diameter. The adoption of uniform screw threads in our workshops would enable us to dispense with the costly screwing apparatus now in use, and, further, it would remove beyond doubt the confusion and delays occasioned thereby. It would also prevent the waste of bolts and nuts which is now unavoidable.

The impulse given to the manufacture of locomotives and general mechanical apparatus during recent years, has tended to increase this waste. The adoption of a uniform system in the railway workshops would mean a considerable saving. Take for example the fitting shops of the railways. Here the variety of apparatus rendered necessary by the want of uniformity will correspond with the number of distinct manufacturers by whom the engines are supplied, whereas if the same system of screw threads were common to the different engines and other mechanical rolling stock a single set of screwing tackle would suffice. Were a uniform system adopted for locomotives there is no doubt that it would be extended to other mechanincal appliances and machinery. Particular threads will be required of course for particular purposes, but uniformity should be the paramount factor in serews for general use in fitting up machinery.

Petrol Rail Tank Car (5,000 gals. capacity). Built at Petone Railway Workshops

Petrol Rail Tank Car (5,000 gals. capacity). Built at Petone Railway Workshops

The use of the screw bolt is to unite certain parts of machinery together in firm contact. It is adapted for this purpose by the compact form in which it possesses the necessary strength. The ease and firmness with which distinct parts of machinery can be united and held together, and the facility with which they may be separated, are conditions of the utmost importance. No other known contrivance fulfils these conditions so well as the serew bolt. There are three essential characters belonging to the screw thread. These are pitch, depth, and form. Each of these may be indefinitely modified and any change will affect more or less the several conditions of power, strength, and durability.

The mechanical power of the screw depends upon the pitch, which, for a given diameter determines the angle of the inclined plane, and on the form of thread which regulates the direction in which the force applied will act. The strength of the screw in the thread varies with each of the three characters.

In the selection of the thread considerable latitude of choice will be found to prevail in reference to all the characters. No definite rule for determining any one of them can be given. It may be manifest that particular threads are too coarse or too fine, too deep or too shallow, but there are intermediate degrees within which the choice of thread, like that of diameter, is arbitrary, and must be guided rather by diseretion than by calculation.