the feature industry this month is....
Aluminum Extrusion Manufacturing
Aluminum is a useful metal. Being non-ferrous ('doesn't contain iron'), it is not prone to rust like steel, and yet it is still very strong and has a huge list of potential uses in all kinds of industry and construction. One common way to make a useful component out of aluminum is known as extrusion. By the process of extrusion, aluminum can be shaped into things like the structural supports for machines or work platforms, or into conduit and tube-like shapes, among many others. It is a wonderful system, and it is also just another one of the many industries which need millwrights to install and maintain their machinery.
One time when I was at an aluminum extruder's factory, the job I was assigned was the maintenance of exhaust fans. Therefore this millwrighting lesson will be about fans and fan belts. A fan is essentially the same as an axial air compressor, just that the air is not pressurized into piping but instead moved freely in the area of the fan, often for the purpose of exhausting waste gases, or air-cooling components (or a room!). Most fans are propellor type, although on rare occasions you might encounter a centrifugal fan. In any case a fan is a machine used to move large volumes of air under low pressure, rather than the high pressure low volume of a compressor.
There are 3 general types of fans: induction draft, forced draft, or balanced draft. In the first type, air is moved by the exhausting of 'old' air, causing 'new' air to move into the low pressure area to take its place. In forced draft, air is moved by pressurizing the air coming IN to an area: in other words, the fan creates a high pressure area, causing the air movement away from this area. In balanced draft, a combination of fans is used in an area with one creating the low pressure area and the other creating the high pressure area at the same time.
With small fans such as you might have in your window at home, the motor may be attached to the fan directly. With large fans it becomes more common to see the motor attached indirectly, usually by a belt or belts running between sheaves on the motor and sheaves on the fan. V-belts are designed to run on their sides, and should never be installed touching the bottom of the groove on the sheave. This will cause the sheave to wear out of shape and you will need to replace not just the belts but the sheaves as well--much more expensive! If a V-belt sits down in the bottom of its groove, it is the wrong size belt for that sheave, and you need a larger belt.
There are several types of belts, but standard V-belts come in letter series, A to E being smallest to largest cross section. The vee is approximately a 42 degree angle. The number on this type of V-belt will look something like A60, for example. This means a belt of "A" cross section (these have about a half-inch top width), 60 inches long along its inside length. To calculate the correct length of belt for your application, a simple rule of thumb is to add the "pitch circle diameters" at both sheaves, and multiply by 1.5 (this is just easier than using pi/2, and close enough for a belt system with reasonable adjustment amounts). Add twice the center distance between the sheaves, then go up to the next size belt (to make up for not using pi). The pitch circle diameter is measured at halfway on the belt between its ID and OD.
You have a fan and a motor 36 inches apart center-to-center. The sheave on the motor has an 8 inch diameter. The sheave on the fan has a 12 inch diameter. The sheaves will fit a "B" series belt, and let's say that it so happens that half this belt height lands right at the sheave OD. What size belt do you need to order?
To create the many useful structural shapes made of aluminum, an extrusion factory starts with "billet", or basically a bulk block of aluminum (although billet comes in solid or hollow for different uses). The billet must be heated up to a temperature which will soften it so that it can be extruded, but it is important to note that it is softened, not melted. At the correct softness, it is forced through a steel die with an opening shaped according to the desired finished extrusion shape. This "forcing" is done by a hydraulic press which can generate the huge force necessary to squish the aluminum through the die. (I have heard the process compared to squeezing icing through cake decorating tips, or dough through a pasta maker.) While the billet is being squeezed, nitrogen is often passed over the aluminum to act as a sort of flux to prevent the intake of impurities into the extrusion.
Coming out the other side of the die, the shaped extrusion is still warm and may be a little twisted from the deformation of its molecules under the high pressure. Therefore the next steps at the factory will be to cool or quench the extrusion, and then straighten it if needed in a straightening machine. Depending on the alloy used and the desired finished product, it may go through a different oven to "age harden" the metal. Finally the extrusions are cut to length, finished as required (eg buffed or polished, or painted, etc) and then all that's left is to pack them up and ship them to where they are going to be used!
Millwrights will see aluminum extrusions not just if they work at the extrusion factory, but on other jobs as well. It is becoming quite common to see conveyor systems in which the frames are made of aluminum extrusions, especially in food industries where the possibility of rust falling into their product from the conveyer is just not acceptable. Construction millwrights might receive prefabricated conveyor sections made of aluminum extrusion, or might on another job receive the extrusions uncut and build the frames from there. In any case, aluminum extrusions are a common sight in many industries and a very useful item for many purposes in a factory.
Here are a few links to sites related to aluminum extruding:
There is actually an Aluminum Extruders' Council, whose web site lists many applications for extrustions, has an "FAQ" section, and includes technical information and great diagrams to describe the process. http://www.aec.org/
Indalex, a large aluminum extrusion manufacturer, has a web site which shows standard and custom extrusion shapes and describes anodizing and other finishing processes. http://www.indalex.com/website/home.nsf/Home?OpenForm
The BonL Aluminum company has a site which shows parts of a typical extrusion die, and describes factors affecting extrusion. http://www.bonlalum.com
Northern States Metals at http://www.extrusions.com/ has a "Design Ideas" page showing many extrusion shapes and their various uses.
American Aluminum Extrusion's web site shows the capabilities of the presses, and describes some of their specific equipment and processes. http://www.americanaluminum.com/
At the Global Sources website, you can look up extrusion manufacturers around the world. http://www.globalsources.com/manufactureres/Aluminum_Extrusion.html
Expressed algebraically, the rule of thumb is:
L = [ (PCD1 + PCD2) x 1.5 ] + (2 x CD)
where L is the belt length, PCD1 and PCD2 are the pitch circle diameters of the two sheaves, and CD is the distance between sheave centers.
Therefore in this example,
L = [ (8 + 12) x 1.5 ] + (2 x 36)
or 102 inches.
Remember that the rule of thumb suggests you go up to the next size belt, so you should order a B103 belt in this case.
See you on the next update of the Construction Millwright Feature Page!
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