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Friction |
R. W. Stuart Friction is a force which has been used by man for thousands of years. From the beginning it has allowed him to move around, climb, hold, push, strike, and perform all of the various mechanical functions of life. He warmed his hands by rubbing them together. As his intellect and manual skills improved friction allowed him to make stone tools and start fires by spinning wooden sticks in wooden bore holes. As man became involved in the industrial age his intimacy with friction and its characteristics required an increasing understanding of friction. Considerable engineering effort went into the study of materials, lubricants, and what friction really was. He still rubbed his hands together to warm them. Recently friction has been examined at the atomic level and some new and different concepts have resulted which are very interesting and are counter intuitive. For instance, rough surfaces may be more slippery than smooth surfaces, friction may be dependent on speed, and dry surfaces may be more slippery than wet surfaces. Man still rubs his hands together to warm them. Friction is a force- the resistance you feel when you try to slide a weight across a surface. Force through a distance is work and the work shows up as heat between the two surfaces involved in the friction. The friction is independent of the area of the surfaces. Classic engineering states that the friction is proportional to the weight of the sliding object- 100 pounds has ten times the friction that 10 pounds has for the same materials and this principle extends to rotating shafts, oscillating pistons and all other goodies that move in and around our RC airplanes including the air slipping past the wings. Friction limits ultimate rpm and how much turn down (idle) can be accomplished in an engine. Various materials and combinations of materials have different frictions expressed as coefficient of friction ranging from 0.05 (wet ice on wet ice) to 99.99 (a complete weldment). Normal bearings and bushings have coefficients of 0.10 to 0.15. It was intuitively believed that rough surfaces had a higher coefficient of friction than smooth surfaces and great machining effort went in to producing "micro" surfaces. Experience supported the idea that friction was not a function of speed even though more force was required to start a slip than to support it after motion started which was called slip-stick. This is basically the present state of the art. Man still rubs his hands together to warm them. Suitable explanation of these classic engineering observations included the idea that friction resulted from contact between points on the surface of materials and increased weight caused the points to distort into each other. Slip-stick was explained as a decrease in this mutual distortion as speed increased causing bounce and less point wedging with increased speed. Lubrication was supposed to fill the space between rubbing surfaces and thus reduce friction. It was believed that friction was independent of area in contact. A few little "bugs" plagued these ideas, but most of the engineering was valid at the nuts and bolts level. As we progress into the "atomic" age several new and interesting concepts evolve which modify our prior thoughts about friction. Destructive wear friction, wherein particles of one or more surfaces are sloughed off, continues to behave in the old classic way. Some modern systems are wearless but still show friction and this friction may be imagined as atomic cohesion between surfaces that are intimately connected with each other. This explains why friction is proportional to weight (more intimate contact) and is independent of area (more area- less pressure- but the same effective contact). It also explains why some material surfaces have different frictions with each other (coefficient of friction)- the atomic surfaces have different cohesions. Cohesion friction comes from one material's atomic lattice harmonically vibrating another material's atomic lattice when the two materials rub together and the degree of vibration varies with the two materials- explaining coefficient of friction. The vibration ultimately produces heat as with any friction. The secrete is to form the bearing from two materials whose atomic latices are not harmonically sensitive to each other or to reduce particle size which reduces harmonics. Further research into atomic surface relationships between materials will result in better lubricants. It has been found that long chain molecules (most oils) cannot resist pressure as well as cross chain molecules (castor oil) which might explain why we think castor is such a good lubricant for our glow engines. At the atomic level there is a difference between the apparent contact area and the true contact area. Looking at the true contact area, the sum of the point areas in actual contact, it becomes apparent that friction does vary with area and increasing weight causes more friction by putting more area in true contact by distortion. Some application of this idea may verify the automotive industry's conclusion that rough surfaces may have less friction than the super- fine finishes. At the atomic level it has been determined that dry friction sometimes is less than wet friction because the fluid allows more intimate contact between the surface and the fluid resulting in a lot of adhesive friction. Very delicate atomic level testing shows slightly less force is required to stick a particle than to unstick a particle and this phenomena amplified to practical machine size can represent noticeable friction. By cleaving mica atomically flat surfaces of 50,000 or more atoms (sq cm) can be prepared and these surfaces are part of the modern studies on friction. This rather new science, called nanotribology, will gradually input our existing engineering and modify present friction sensitive systems. A hint of what may develop has been the application of teflon and related products to machine design. Man will still rub his hands together to warm them. |
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