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## Re: [TowerTalk] Chrome-moly VS: steel

 To: "Mike Bragassa" , Re: [TowerTalk] Chrome-moly VS: steel "D. Scott MacKenzie" kb0fhp@comcast.net Fri, 14 Jul 2006 00:44:21 -0400
 ```There are really two questions there combined into one question. Schedule 80 is a pipe wall thickness designation....It does not specify alloy - there are approximately 30+ alloy designations for pipe: http://www.key-to-steel.com/Articles/Art24.htm But making it simple, it it is assumed that the material is standard black steel pipe to ASTM 53, typical strength has a 52,000 psi yield. That would be about 70,000 psi ultimate strength, and a hardness of about 79 Rockwell B. Normalized 4130 pipe, has a hardness of about Rockwell C 30, with an ultimate tensile strength of about 130,000, and a yield strength approximately of 110,000 psi. Just based on strength, the 4130 is better. But our failure criterion is bending, or plastic deformation. Now we have to compare the wall thicknesses and the strength, so see which works better. This is where the metallurgist, being the jack-of-all-trades, instantly transforms into a stress engineer. The maximum stress in a beam is given by s=Mc/I, where s is the stress, M is the moment, c is the distance of the outer fiber from the neutral axis, and I is the moment of inertia. I for a thin walled tube (good enough in this illustration) is: I =pi*t8r^3 Assuming a 2" diameter, the I for the two cases are: 4130 (2" dia. x 0.18" wall): 0.565 A53 (2" dia x 0.25" wall): 0.785 c is the fiber distance from the neutral axis, and is c = r + t/2differs for each one because of the wall thickness: 4130 (2" dia. x 0.18" wall): 1.09 A53 (2" dia x 0.25" wall): 1.13 M is unknown, and is a function of the length of the mast, and the windloading. For the purposes of this, the masts are the same length, and the windloading is the same. Failure is yielding of the material, or when the stress equals the yield strength (less any safety factor - we will ignore that for the time being). For the 4130 pipe, yield is 110,000 psi = s = Mc/I = 1.93M For the A53 pipe, Yield is 50,000 psi = s = Mc/I = 1.44M The max moment at failure can be calculated by rearranging: for 4130 M = 110,000/1.93 = 56,994 ft-lbs for A53 M = 50,000/1.44 = 34,722 ft-lbs In other words, the 4130 can take about 1.64 times the moment than the A53 pipe, even with a thicker wall. Since the lengths of the mast are assumed to be the same, then the 4130 can take 1.64 times the load that A53 can before bending can occur, for this specific example, and specific material conditions. The next question is, is this strength necessary for the application? It depends on the windload of the antennas, and the length of the mast. From that calculation, and a suitable safety factor, the maximum antenna loading for a specific mast can be determined. I hope that this clarifies some things - finnaly I have been able to answer someone's question and contribute, instead of just asking questions..... :) Scott MacKenzie, PhD Metallurgist KB0FHP -----Original Message----- From: towertalk-bounces@contesting.com [mailto:towertalk-bounces@contesting.com]On Behalf Of Mike Bragassa Sent: Thursday, July 13, 2006 11:39 PM To: towertalk@contesting.com Subject: [TowerTalk] Chrome-moly VS: steel Importance: High For you metallurgists in the group: Re: 20 ft mast pipe How does a 0.18 inch chromemoly pipe compare to a 0.25 inch schedule 80 pipe? 73, Mike, K5UO _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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