|MadSci Network: Physics|
Dear Anonymous: Your question, albeit very interesting, is hard to answer due to lack of details. In order to answer it properly, it will be necessary to know which kind of wood and what type of oil you are intending to use. Some kinds of wood are very oily and will slide over another similar piece of wood even if no additional lubricant is used. It is also important to note that some oils will become sticky after some time or at some temperatures. I checked the Internet and I found the following information. There is a very nice website (http://www.chevron.com/prodserv/NewOronite/library/li_dictionary_l.htm) containing the following definitions: LOAD WEAR INDEX (LWI) - measure of the relative ability of a lubricant to prevent wear under applied loads; it is calculated from data obtained from the Four Ball EP Method. Formerly called mean Hertz load. LUBRICATION - control of friction and wear by the introduction of a friction-reducing film between moving surfaces in contact. The lubricant used may be a fluid, solid, or plastic substance. For principles of lubrication, see boundary lubrication, full-fluid-film lubrication, ZN/P curve. LUBRICATING OIL - compounded or finished oil consisting of base stocks and the additives necessary for providing the required performance. LUBRICITY - ability of an oil or grease to lubricate; also, called film strength. Lubricity can be enhanced by additive treatment. See compounded oil. COMPOUNDED OIL - mixture of a petroleum oil with animal or vegetable fat or oil. Compounded oils have a strong affinity for metal surfaces; they are particularly suitable for wet-steam conditions and for applications where lubricity and extra load-carrying ability are needed. They are not generally recommended where long-term oxidation stability is required. CORROSIVE WEAR - progressive removal of material from rubbing surface caused by a combination of chemical attack and mechanical action. As an example of the problems found using the inappropriate lubricant, I found the following information contained in foxtail Mechanical Music Digest™ Archives Wooden Crank Rod Lubrication By Craig Brougher Bill Finch had a very nice experiment there, concerning crank rod elongation. I have also noticed that maple elongates less, but there are different qualities of maple, too. The self-lubrication of wood comes from its resins liquefying under friction heat, and you will then notice a varnish buildup on the cranks after awhile. This is what gets hot in minuscule amounts. The reason oak has less friction but more elongation is probably obvious -- the internal "wicking action" of the wood is stronger in oak because of the "sinew-like structure of oak with the large pores, but the compression of oak is greater because there is less wood contacting the crank, since the pores are big and hollow. However, the reason for the knocking of wooden rods is due to the stopping and starting of these pumps, thousands of times a year. Once the rod cools off and the resin gets hard, then you get a tremendous amount of wear and tear until the rods get hot again. (I'm not talking about the whole rod -- just a very thin contacting surface, so you can't measure it very well.). The grease must be there, or your orchestration or band organ will definitely start knocking again, and you will have to make new rods. The grease substitutes for wood resins, so, since you are either going to use wood resins or grease, you'd better pick a good grease. A little common sense will tell you that any test run continuously can only test the continuous operation factor. Testing engines that way produces absolutely wonderful results, too. But turning them on and off, letting them get cold, and then switching them back on, and you get terrible results. Even changing the speed, changing the loading factor, etc. makes enormous differences in your results. So when this happens all the time in band organs, you end up instead looking around for truly great grease. In the other hand, using water as a lubricant could be tricky, because dry wood could absorb a lot of water (it depends on how dry it is and what kind of wood we are considering). In the nice site natmus we find: Wood is typical of the water-containing materials. Wood in furniture contains roughly ten percent water by weight. The exact water content depends on the temperature and humidity of the surrounding air during the previous few weeks. The water content matters to conservators because wood swells and shrinks at it absorbs and desorbs water. Water is also an accelerator of slow decomposition reactions. Under what circumstances does wood exchange water with air? If we take a five-liter jar we can calculate, by reference to the Mollier diagram and the density of air, that the air within it, at room temperature, cannot contain more than about a tenth of a gram of water. If we now pour in a kilogram of wood chips the bottle will contain about 100 grams of water, almost entirely in the wood. We can therefore flush the jar quickly with dry air and wait to see what happens, knowing that the relative change of water content of the wood will be very small indeed. In spite of the overwhelming reserve of water in the wood, the air in the flask does not become saturated with water but comes to equilibrium, after a short time, at about 50% relative humidity. A graph of the equilibrium RH of the air in the bottle after equilibration with wood pre-adjusted to various water contents (EMC = Equilibrium moisture content) looks like this: isotherm The exact curve depends a little on the species of tree, but the shape of the curve is always the same. The temperature dependence of the water content of wood The curve is called the absorption isotherm, a jargon term which means that the curve joins a series of measurements made at a constant temperature, 15 degrees in this case. We can complete the diagram by conducting the same series of measurements at various temperatures. The curves lie close together. Usually one can ignore the slight temperature dependence of the equilibrium, but there are circumstances where it is important: a point I will take up in a later article. isotherm2 Many materials show similar behavior: cotton, linen, wool, silk, silica gel and even concrete. Because the equilibrium RH depends on the water content of the material, almost unaffected by temperature, conservators always refer to the water content of air in terms of RH. This is not strictly correct, because RH is just a ratio, which can only be converted into water vapor content if the temperature is also quoted. It doesn't usually matter but when it does, this casual simplification can cause a lot of misunderstanding. Furniture is not usually displayed in glass bottles, even in museums. In the real world it is the air that controls the water content of the wood, because its low water vapor concentration is compensated by the continuous flow of fresh air past the object. Although the air is now controlling the water content of the wood, the results derived from the sealed jar experiments still apply. One must get used to thinking in both ways. Occasions when the sealed jar concept applies are quite numerous: packing for transport, freezing or heating in a sealed bag to kill bugs, flushing bagged objects with nitrogen or carbon dioxide to kill bugs, enclosure in a showcase. In order to prevent water absorption (or water loss), there are several treatments based upon the use of different oils. As an example we found (www.eastag.co.nz/haarlem.html) For hardwood floorings it is recommended that both the second and third coats of Haarlem Oil be diluted with 10 - 15% of vegetable turpentine. Hardwoods are Eucalyptus, Kwila, Iroko, Rata, Pohutukawa, Oak, Beeeh, etc. Oil absorption rates for these woods are lower. And also (www.wipltd.com/html/w0600den.htm) WIPLAM – is a revolutionary product for applications in the transformer industry. It is a homogenous material manufactured from specified species of timber. It has exceptional mechanical properties coupled with oil absorption characteristics. After vacuum drying and impregnation with Pyrochlor or transformer oil, it acquires dielectric properties of transformer oil with enhanced mechanical properties. And finally (www.wolman.com/product_category_list.asp?CatId=80) Wolman® RainCoat® Clear Water Repellent A totally clear, mildew-resistant, oil-base water repellent that allows wood to gray naturally. Protects wood from damage caused by water absorption. Deep penetrating oil formula gives wood a rich, attractive glow. Some of the natural damage found in very old wood could be avoided, according to the following site oilwrfr.pdf using oil treatment: Treatments at low temperatures using vegetable oils (120 ?C to 140 ?C) avoids splits and reduce tannins leaching. I hope this helps. Sincerely Jaime Valencia
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