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nber and a deci- a dimension of ace accuracy. ic. Moving the micrometers. a meter - an a metrically mensions are fix mm does 10.05 -02- Own as is tol- ths of point parts ntion s and mbol ndi- the di- us e. 20 ft 24. Where the upper and lower limits of tolerance are shown as plus or minus measurements, the tolerance is said to be 25. Tolerances on elevator layouts are usually designated in 26. On escalator layouts, tolerances are more apt to be shown in form. 27. To distinguish between the metric and U.S. systems, a zero is placed ahead of the decimal point if the measurement is in the system. THREADS The most common fastener used in elevator installa- tion is the threaded fastener. From large bolts securing the guide rails to tiny screws inside hall fixtures, you will find threaded fasteners securing all parts of the elevator. In addition to holding parts in place, screw threads are used on adjustable parts. Certain electrical contacts are adjustable by means of threaded parts. An example of a large part adjustable by means of threads is the elevator braking mechanism. Another function served by screw threads is the trans- mission of power. The worm gear in an elevator driving machine is a prime example. Simple mechanical devices used by prehistoric man include the lever, wheel, and inclined plane. Early builders undoubtedly found they could raise a heavy object by rolling it up an inclined plane - a ramp. During building of the great pyramids, ramps were constructed circling around each pyramid. Rolling the heavy stones around the ramps resulted in raising their elevation. This was probably the first spiral inclined plane. The screw thread is a form of spiral inclined plane, so it is not a great leap from the pyramid builders to the screw thread. A spiral inclined plane is called a helix, and the angle it makes with horizontal is called the helix angle. In screw threads, helix angle is the angle between the thread and a plane perpendicular to the axis of the screw. As with the wheel, history of the invention of the screw thread is lost. What is well known however, is the history of attempts at standardization. Prior to about 1840, each machine shop designed threads for their own convenience. Imagine having to buy bolts and nuts from only one source because there was no interchangeability. In 1841, Sir Joseph Whitworth pro- posed a standard thread having a 55° angle between sides of the thread. This became the British Standard Whit- worth thread used for general engineering. In the United States, William Sellers in 1864 proposed a standard thread having a 60° angle between sides. This was later accepted as the U.S. standard. Toward the end of World War II, the 60° Unified Standard thread was rec- ognized by the United Kingdom, Canada, and the United States. This thread form was accepted by the International international standard. Organization for Standardization (ISO), and became the In 1958, publication of standards began for thread sys- tems in inches and in metric units. Except in a few special conform to standards. cases, inch threads and metric threads are now made to Unified Standard. Figure 13 compares the Whitworth thread with the the next is called the pitch. The distance from the crest of one thread to the crest of 13 CREST CREST P 55° WHITWORTH 60° ROOT 60% ROOT UNIFIED STANDARD FIGURE 13 Unified Standard and Whitworth Threads United States Standards The two classes of threads used in the United States are Unified National Coarse (NC), and Unified National Fine (NF). National Fine has a greater number of threads per inch than National Coarse, but thread profiles are identi- cal-conforming to the Unified Standard. Because the United States Standard system of mea- surement uses the inch, threads are specified in number of threads per inch. In the metric system, threads are speci- fied by pitch. Upon close examination, a good reason is seen.