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Learning More About Viscosity Lubrication Fundamentals LLearningearning moremore aboutabout ViscosityViscosity There’s more than meets the eye which can impact the behavior of viscosity and even rheology. Editor’s Note: This article was written in collabora- rate with a rotary viscometer by rotating the tion with Dr. Phil Guichelaar, Western Michigan spindle at different speeds. For each of University. these speeds, we calculate the shear rate (related to the rotation speed of the spindle) and e frequently say that viscosity is measure the shear stress (related to the torque the most important property of needed to rotate the spindle). Then we make a W fluid lubricants. Previously, we plot of the shear stress and shear rate as discussed the fundamental concepts that go shown below (see Figure 1). The data might fit into our understanding of viscosity in last a straight line starting from zero. The slope month’s issue. of that line or its steepness is the viscosity You will recall that viscosity is equal to for that oil at the temperature of the oil. the shear stress/shear rate concept. Though, Remember that mathematically and, what happens if we stir a can of oil at differ- also, graphically: an oil whose viscosity ent speeds? Does it seem obvious that more remains constant as we increase or effort is required to stir the oil at a faster decrease the shear rate is a perfect fluid (see speed? More effort is required, but for some Figure 2). The term, Newtonian, is used to by Dr.Robert M.Gresham oils it is not proportionally more effort. designate this behavior. Water, gasoline and Contributing Editor We can determine the effect of stirring most unmodified mineral oils display very 24 JANUARY 2005 TRIBOLOGY & LUBRICATION TECHNOLOGY Figure 1 . Flow Curve nearly Newtonian viscosity curves. Most fluids, including motor oils, are not The correlation perfect, or Newtonian. The shear stress is between shear stress not directly proportional to the applied shear rate such that when the shear stress is and shear rate defin- plotted against the shear rate, the points ing the flow behavior are not in a straight line. Often, the points of a liquid is graphi- do not begin from zero shear stress and zero cally displayed.This shear rate. Certain other fluids are sensitive diagram is called a to the length of time that the fluid is being Shear Stress flow curve sheared, resulting in changes in viscosity Shear Rate with the duration of the experiment. These kinds of fluids have interesting names: • Pseudoplastic Fluids. The viscosity of pseudoplastic fluids decreases with Figure 2 . increased shearing (see Figure 3). Molten polymers have this characteristic, which is used to advantage in injection molding when the material flows through small cross-section gates. µ Paper pulp suspensions also display pseudoplastic viscosity behavior. 1 Shear Stress • Dilatant Fluids. For these fluids, the Viscosity = ---------------------- viscosity increases with increased Shear Rate shearing (see Figure 4). A mixture of (F/A) angential Stress water and cornstarch can have the T consistency of thick cream if it is Rate of Shear (dv/dy) poured slowly, but will form a thick paste if it is stirred briskly. Quicksand also has this quirky viscosity behavior, related to breaking bonds between par- with the valuable lesson that slow Remember that escape movements consume less ticles or molecules, or to changes from the at-rest shape of long molecules. mathematically and, energy than violent movements also, graphically: an against an instantly thick morass. • Thixotropic Fluids. Fluids that are oil whose viscosity • Bingham Plastic Fluids. The viscosity thixotropic are not simply small mole- cules aggregated together, such as remains constant as curve for these fluids does not go we increase or through the origin. As shown in Figure water. They include molecules that 5, the shear stress can be substantial have a longer range structure that can decrease the shear even at a small shear rate, but once be altered with mechanical shearing. rate is a perfect the fluid is moving, the shear stress is For most of these fluids, the viscosity fluid. directly proportional to shear rate in decreases at higher shearing rates (see exactly the same manner as Newton- Figure 6). Mayonnaise, has this charac- ian fluids. Water suspensions of rock teristic. It is not a Bingham plastic particles behave in this manner. More fluid because its viscosity decreases familiarly, mashed potatoes, a suspen- with higher rates of shearing, but only sion of tasty solids in a liquid, flow in after a minimum amount of shear is the bowl when you stir, flow harder reached. Yogurt (without the tasty fruit when you stir harder, and assume a addition) is also reputed to have this mountain peak shape if undisturbed. behavior. Finally, good quality wall paint and motor oils are carefully com- • Shear Time Dependent Fluids. For pounded to be thixotropic. Can you these fluids, the shear stress changes explain why users of these fluids with time of shearing. This behavior is CONTINUED ON PAGE 26 TRIBOLOGY & LUBRICATION TECHNOLOGY JANUARY 2005 25 CONTINUED FROM PAGE 25 Figure 3 . Pseudoplastic Flow: Figure 4 . Dilatant Flow: would be pleased to have thixotropic wax particles in lubricating oils can cause the behavior? non-Newtonian Thixotropic behavior of the oil to become even more pronounced. • Rheopectic Fluids. This behavior is How does a Pseudoplastic oil behave in not common and is not very useful, a hydrodynamic bearing? At startup, the because the viscosity increases with shear rate is slow and the oil has a high vis- additional shearing after the minimum cosity such that the transition to mixed Clearly, viscosity and amount of shear is exerted. Some clay lubrication is facilitated. As the shaft solutions in water behave in this man- rheology are very speeds up, the viscosity decreases but ner. Can you imagine the behavior of important to the prop- hopefully the increased speed compensates rheopectic paint? It thickens into er operation of and the system remains in the mixed lubri- clumps when you apply it, and when cation regime or transitions into the hydro- mechanical equip- you stop trying to smooth it out, it dynamic regime. On the other hand, with a ment. An equipment runs down the wall. design engineer Thixotropic fluid, at start up, if the shear The non-Newtonian fluids discussed rate is slow the oil initially supports the designing a bearing or before have important behaviors. For exam- bearing. If the shear rate remains constant, gearbox, or a lubricant ple, in Pseudoplastic fluids, flow does not the oil will eventually thin and not support dispensing device, begin until a minimum shear stress level is the bearing. needs to understand reached. This is important for printing ink A Dilatant fluid is the opposite of a the implications of rheology where we would not like the ink to Pseudoplastic fluid in that its viscosity rheology on his or her move until it is deposited on paper. Once increases with shear rate. This would be the minimum shear stress has been design. very bad for a paint system, as it would be reached, the fluid thins with increasing very difficult to spread the paint evenly. If a shear rate and penetrates the structure of lubricating oil was dilatant, its viscosity the paper. When the shear stress goes back would increase with speed and friction loss- to zero, the ink does not move. es would increase, not a good outcome Meanwhile, with Thixotropic fluids, flow when the general objective is to decrease can vary with applied shear stress and/or energy consumption. shear rate, becoming rapidly thin with Clearly, viscosity and rheology are very increasing shear time, but when the shear important to the proper operation of forces are removed the fluid rapidly becomes mechanical equipment. An equipment thicker or more viscous. Paint is usually com- design engineer designing a bearing or pounded to be thixotropic — it applies easi- gearbox, or a lubricant dispensing device, ly (high shear stress and shear rate) but it needs to understand the implications of does not sag or run when the brush is rheology on his or her design. If the engi- removed, yet has the time to flow and level neer knows the conditions under which the out before thickening. At low temperatures, “converging wedge” is operating, the rotat- 26 JANUARY 2005 TRIBOLOGY & LUBRICATION TECHNOLOGY Figure 5 . Pseudoplastic Flow: Figure 6 . Dilatant Flow: ing speed, the load, and the possibility of operate reliably. A Dilatant fluid is the any intermittent changes in pressure with Lubricant formulators keep all of these opposite of a Pseudo- time, and if they can predict the range of concepts in mind and under control when plastic fluid in that its operating temperatures, they can specify a developing new products.<< viscosity increases viscosity and viscosity index for effective lubrication. Conversely, if the engineer Bob Gresham is STLE’s director of professional with shear rate. knows the viscosity profile of a lubricant, development. You can reach him at rgresham they can design a mechanical system to @stle.org. + High-capacity sample track available + Dual-bath design permits kinematic + Excellent choice when high viscosity testing at two different throughput is desired temperatures simultaneously + Two viscometer tubes per bath + Low to moderate throughput + One viscometer tube per bath Designed for unattended operation, the 2000 Series CANNON Automatic Viscometers provide fully automatic kinematic viscosity testing within parameters specified by ASTM D 445. The CAV instruments measure flow rates within ±0.001 second by electronically timing the liquid meniscus as it moves between thermistor sensors. Bath temperature is controlled within ±0.01°C between 20° and 100°, meeting the criteria specified by ASTM D 445/446.
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