![]() ![]() ![]() Then, dividing by the square-root of 2, we can back into the "correct" length for the straight section, which comes to an awkward 8.838835 inches. So, subtracting the somewhat shorter 2-inch tie and dividing by 2 gives a radius of 12.5 inches. It seems to have been designed with a round number for the overall diameter. O27, which I believe Lionel got from Ives, is the other way around. (I think we're using the same reasoning.) When you double this to get the diameter, and add a tie length of 2.25 inches, the overall diameter comes to 30.534271, which is where we get the modern nominal diameter of O31 (but occasionally O30 for the same track). It's pretty clear that Lionel intended their straight section to be exactly 10 inches, with the result that the radius is 14.142136 inches, the value that Rob and I agree on. With 8 curved sections in a circle, this scheme effectively requires that the radius be the length of a straight section multiplied by the square-root of 2. This assures that a passing siding can be made using only standard sections. The manufacturing tolerances are much tighter than in pipes.I am convinced that the original scheme for O27 and (so-called) O31 tubular track was to coordinate the curved and straight sections so that the joints in a siding alongside a straight main line would match those of the main line. Tubing is measured by the exact outside diameter and the wall thickness (WT). On the other hand, tubes are structural members and are measured with an exact outside diameter. As stated above, they are measured with nominal outside diameters using NPS and Schedule to define the pipe size. ![]() Pipes are used to transport gases or fluids. However, some process piping, utility facilities, and civil piping may require higher schedule numbers depending on the pressures used. The two most common schedule sizes seen in buildings are Schedule 40 and Schedule 80. Stainless Steel pipe schedules append an "S" to the end of the schedule number, for instance Schedule 40S. It was originally intended for these to be phased out, but they occasionally appear. These were original terms used prior to the schedule system that was implemented in 1927. In addition to the schedule numbers, you will often see Standard (STD), Extra Strong (XS), and Double Extra Strong (XXS). The schedule number is roughly calculated as: Schedule = 1000 x (P/S) where P is the internal service pressure of the pipe (psig) and S is the ultimate tensile strength of the pipe material (psi). Wall thickness is important because it determines the amount of internal pressure that the pipe can withstand. A pipe's schedule is the thickness of the pipe wall, which directly affects the interior dimension and weight of the pipe. While the Nominal Pipe Size identifies the outside diameter of the pipe, more information is needed to identify specific pipes. The DN dimensions must conform to ISO Standard 6708. Outside of North America, pipes are identified by Diametre Nominel (DN), which is a dimensionless value that roughly equates to the outside diameter of the pipe in mm. When specific pipes are specified, the NPS, Schedule, and material are required. Nominal Pipe Sizes apply to all materials of pipe. ![]() However, from NPS 14 and above, the NPS value matches the outside diameter of the pipe. Technically, the NPS is non-dimensional and only roughly refers to the diameter of the pipe. Nominal Pipe Size (abbreviated NPS) is a North American standard for identifying pipe sizes.
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