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the feature industry this month is....
Glass Making
This is a very interesting industry, and a frequent user of construction millwrights' services. A lot of regular maintenance is required on the various machines where glass is made. "Glass" might bring different images to different people, but the mass produced kinds like windows and bottles are the sort we get work in the factories of the most. This month I will recall a flat-glass factory which recently closed in my area.
A while back we spoke of pneumatic gas laws and did a quiz on a simple system. Let's 'expand' on that. ;)
The most common type of pneumatic (or hydraulic) actuators is a cylinder. Single acting cylinders would extend by pneumatic or hydraulic pressure on the ram, and retract by spring force or gravity. Double acting cylinders operate by fluid pressure in both directions: in other words, there is a pressure connection on both sides of the ram to force it one way or the other within the cylinder, depending on the valve setting. These actuators are therefore capable of a power stroke in both directions. Sometimes the piston is even fixed and the cylinder moves back and forth around it.
In the world of double acting cylinders, you can find single rod types and double rod types. Single rod cylinders have a rod leading off the piston on one side, and are smooth on the other. The difference in area (and therefore action) between the rod end and the smooth end of a single rod cylinder's ram will of course cause a slight difference in action each direction, but this is usually small and therefore usually ignored. (Remember the law, Force=Pressure x Area)? Double rod types have a rod coming off both ends of the piston. This may be perhaps so that the piston can do work in both directions, or it may just be to equalize the areas on both sides so that the force and speed of the actuator will be perfectly the same in both directions.
Sometimes, however, the system may require that the force and speed are NOT equal both directions. In this case you would need to install a 'differential' piston. The uneven cylinder areas will create a slow powerful work stroke in one direction, and a fast retract stroke back.
Quiz Question:
Picture a double acting, differential hydraulic cylinder with a single rod. Let's say that hydraulic pressure in the system is 1000 psi, and the flow rate is 300 cubic inches per minute. The surface area of the piston face is 6 square inches, and the area of the large single rod is 2.5 square inches. Calculate the working force for each direction of action, and the extend and retract speeds.
Glass making is at once a simple process which has been around for centuries, and a marvel of modern technology. Glass is basically made of sand, limestone, and sodium carbonate. These ingredients, crushed, are melted together in a very hot furnace until they are a smooth homogeneous fluid with no air bubbles. The liquid glass may be formed into molds, rolled into tubes, or, as was actually the case in the glass factory I mentioned earlier, it entered into their special "float" process. In this, the molten glass flows continuously by gravity onto a bath of molten tin. The liquid glass floats on the denser tin, and becomes a smooth glass ribbon with flat surfaces. Then it goes through a cooling and annealing step to reduce internal stresses, most often by using air blown simultaneously onto both sides. After this it is cut into workable sizes for finishing (it may require additional heat-treating or tempering for certain safety applications). Cutting can be with a glass cutter tool similar to the ones you buy in the hardware store, or by heating the cut line with a torch then cooling the line with a jet of water, causing it to snap there. Any broken product (called "cullet") may be returned to the melting furnace for recycling, and in fact this aids the melting process.
Here are a few links to sites related to glass making:
The Corning Museum of Glass lists articles varying from The Discovery of Glassmaking to Glass and The Space Orbiter. They even have pages explaining how students can make a type of glass in the classroom!: http://www.cmog.org/page.cfm?page=77
A technical Glass Facts site created by a family of floatglass companies and engineers: http://www.glassfacts.com/glasfaqs.html
The Primary Glass Manufacturer's Council site contains an online film on glass creation, their Energy Report, and more FAQ's: http://www.primaryglass.org/index.htm
The Ottawa Museum of Civilization has a page on the history of glassmaking, with pictures of some items in their collection: http://www.civilization.ca/membrs/canhist/verre/veint00e.html
The NZ Glass Encyclopedia site has a long list of links (mostly collectors of bottles and antiques, but what the heck): http://www.glass.co.nz/links.htm
The University of Virginia has a How Things Work article with glass trivia questions: http://rabi.phys.virginia.edu/HTW/windows_and_glass.html
Force = Pressure x Area.
Therefore, in the large area side, F = 1000 x 6, or 6000 pounds of force during extension.
In the small side, F = 1000 x (6-2.5), or 3500 pounds of force during retraction.
Extend speed = flow rate per area, or 300/6 = 50 inches per minute.
Retract speed = 300/3.5 = 85.7 inches per minute.
See you on the next update of the Construction Millwright Feature Page!
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