Pipe & Tube Manufacturing
This month's feature industry will be an aspect of pipe and tube manufacturing, specifically stainless steel tubing. This is just another type of industry at which I personally have worked on a millwrighting contract. If you have been following this "feature industry" page for a while, you are probably starting to get an idea of the wide variety of jobs and job sites at which construction millwrights can work. Even if you are sent to the same plant more than once, chances are that you are there to perform a completely different assignment from the last time. This is just one of the reasons why construction millwrighting is the best job in the world: it's never the same old thing! And yet, you don't get a feeling of not knowing where to begin a new project, because some things are done the same way whether they happen at a food plant or a steel mill. With your journeyman skills, you know you can handle it!
Once at a tube manufacturer, I was given the assignment of installing safety cable for the in-plant maintenance inspectors' future use. When up in the rafters inspecting the overhead crane, workers must wear a safety harness and tie off their lanyard to a lifeline to prevent falling. Construction millwrights were there first, installing the lifeline to begin with.
For a wire rope to be used as a horizontal lifeline, many factors must be considered. Somebody's life may depend on correct installation. Here are some things which you have to get right.
The safe working load of wire rope is the square of the rope diameter multiplied by 8, in tons. The diameter is measured with calipers from the very outer point of one strand to the outer point of the opposide strand, NOT so that either jaw of the calipers is flush with more than one strand simultaneously. If parts of more than one strand are touching the jaw at once, you are not at the very outer point of a strand, and therefore your measurement will not give you the accurate rope diameter.
The accepted safety factor for a rope which is used to support men is 10:1. Therefore, multiply the maximum load expected by a factor of ten. This allows for shock loads, reduced capacity of the rope due to wear or abuse or variations in quality, inaccuracies in estimating the weight expected, etc.
A feature of wire rope is the "lay". Usually, the individual wires are twisted in one direction to make up the strands, and then the strands are twisted the other direction to make up the rope. However, you can also buy rope in which the wires and the strands are both twisted the same way: this is called "Lang Lay". This rope is more abrasion resistant, resistant to kinking and crushing, and has a smoother outward appearance. However, it cannot be used for hoisting in single line rigging because it will have a tendency to untwist. Lang Lay rope must always be fixed at both ends. Fastened along the crane rails as a lifeline, this requirement is met, and under these conditions its other features may be advantageous. If given a choice of rope, Lang Lay might be a better selection for this purpose; of course, regular lay rope will also do the job.
Another feature of wire rope is your choice of "preformed" or regular construction. Preformed refers to the individual wires which are twisted into shape before being made into a rope. This causes them to fit together tighter, and the resulting rope will be somewhat stronger since there will be less tendency for one wire to take more weight than others. Also the wires, if broken, will not jut out of the rope but rather will continue to lay in place. Normally that would be an advantage, since they won't catch in sheaves or on hands--but it makes it more difficult to see if there are broken wires, and when in use as a lifeline, you want to be able to notice breaks in the rope easily. So for this use, the preferred rope would not be preformed.
Now, when you attach the wire rope to the end fixtures, you must use a 'thimble' to keep the eye from crushing. Once you've passed the "dead" end of the rope around the thimble, it is held in the shape of the eye by the use of cable clips which bolt it back to the "live" end. Remember, the 'saddle' part of the cable clip goes on the live end--"a cowboy doesn't saddle a dead horse". The correct number of clips to use is three times the diameter, plus one. The correct spacing of the clips is to place them each six diameters apart. Keep in mind that the use of cable clips reduces the effective strength of the rope to 80%, since they are more likely to fail than the rope itself is to break.
Quiz Question:
Given the above information, if you wanted to install the lifeline for
maintenance workers, describe the wire rope you would use (size, type, etc),
and exactly how you would install it. The heaviest maintenance worker
at this plant weighs 250 pounds.
Stainless steel tubing is made from strips of stainless steel, usually shipped to the tube manufacturer in the form of huge coils. The strips are uncoiled and precisely, uniformly sized as they begin their transformation into pipe. Next the strips are formed by gradual stages of rolling, increasingly closing the curved edges together in the shape of the tube. When the shape is complete, the longitudinal join in the pipe is welded together in a continuous process. Next the weld is tested, probably by an eddy-current process, to ensure that the pipe is complete. Finally the pipe will be given its finishing processes, which may be anything from simply polishing it to various heat- and electroplating treatments; and of course, cut to final length for shipping.
Here are some links to pipe and tube manufacturers:
Reading Tube in Pennsylvania, although they make copper tube not stainless, have a hugely terrific site with explanations and links to all kinds of useful and relevant information: http://www.readingtube.com
Welded Tubes Inc. has a fantastic site which even includes a plant tour so you can really see how it's done: http://www.weldedtubes.com
Great Plains Stainless discuss how you can customize the alloy of your stainless products: http://www.gpss.com/
Wheatland Tube has a page which explains the history of tube manufacturing from the "tong and bell" method of the 1930's to their own Aetna Standard Mill, a continuous weld mill making "Electrical Metallic Tubing" by electric resistance welding. http://www.wheatland.com/histool.htm
Fischer Canada answers the "how is tubing made?" question in detail, including telling you about their laser welding process, which is so precise it reduces the heat affected zone in the steel. http://www.fischerca.com/fischer3_frameset.html
Arcus Staal BV in the Netherlands discusses heat treatment of their Sosta Products: http://www.arcus.nl/arcus_info/index.html
Rath Manufacturing's site tells you about their own "micro weld technology" and special finishing processes: http://www.rathmfg.com/
Well, you know that if you're given the choice, you'll order non-preformed, Lang Lay wire rope. But if you're not given a choice, these factors are not critical: other types of rope will serve as well, although you might want more frequent thorough inspections performed in that case. The big question is, what size rope must be used?
If the man who might rely on the lifeline weighs 250lbs, a safety factor of 10 means that the rope must be able to support a 2500 lb load. However, you are also planning to attach the ends of the rope with cable clips, which reduce the reliable strength of your system to 80%. To counteract this, base your figures on 2500/0.80, or 3125 lbs. (Eighty percent of 3125 is 2500).
3125 pounds is 1.5625 tons, there being 2000 lbs in a ton. Now, the square of the rope's diameter times 8 has got to come out to at least 1.5625.
1.5625/8 = 0.1953. The square root of 0.1953 is 0.4419. That means the rope's diameter has to be at least 0.44 inches. Therefore, you use half inch wire rope.
You can double check this by calculating the other direction: half inch rope has a safe working load of (0.5 x 0.5) x 8, or 2 tons. That's 4000 pounds. Eighty percent of that is 3200 lbs. Given a safety factor of 10 for men, this rope can be trusted for 320 pounds.
What if you used 3/8" diameter rope? The SWL would only be (0.375 x 0.375) x 8 = 1.125 tons, or 2250 lbs. Eighty percent would leave it at 1800 pounds, which using the correct safety factor would only allow a 180 pound person. The maintenance guys couldwell be heavier than this, so that means 3/8" rope is unsuitable.
Now that you're sure you're using half-inch wire rope, here are the final installation details. Of course you form the end eyes with thimbles. Since the correct number of clips is 3D + 1, and (3 x 0.5) + 1 = 2.5, you use 3 cable clips at each eye. Saddles on the live end! 6D = 6 x 0.5 = 3, so space the three clips 3 inches apart. That means your dead ends will be somewhere around 10 inches long.
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