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Although gelling is reduced by these long-chain additive molecules, individual wax particles separating out of oil at low temperatures may still plug filters and impede circulation. Pour point additives that suppress this gelling effect of the wax are used in many automotive oils as well as in industrial lubes. With most mineral-based industrial oils (designated as turbine, hydraulic, industrial and machine oils), this pour point corresponds to the temperature that freezes the paraffin molecules of the oil into a white crystalline wax that will eventually immobilize the overall oil. This is the lowest temperature at which oil will flow when chilled under prescribed laboratory conditions (ASTM D97). The low-temperature limit for starting an oil- lubricated machine is often specified by the pour point of the oil. In other cases, grease or self-lubricating materials may reduce or even eliminate troublesome low-temperature problems. Avoiding high churning and splash losses at low temperatures in gear boxes developing remedies for more efficient lubrication of bearings and lubricated joints and implementing a reliable and low- maintenance technology that enables vehicle wheel bearings to safely operate over wide temperatures require careful lubricant selection.įortunately, specially compounded mineral oils or synthetics are available that match cold flow requirements.1 In difficult cases, heating is needed for piping, reservoir and filters. Industries and transportation in northern parts of the United States, Europe and Canada are vulnerable to harsh outdoor conditions during winter months. As a result, machines often cannot start or excessive friction causes a complete failure. Dropping below the pour point and the higher viscosity not only restricts oil flow to bearings and other machine elements, but also translates into high startup torque. It is worth emphasizing that the above expressions are not fundamental laws of nature, but rather definitions of viscosity.Low ambient temperatures affect the flow characteristics of a lubricant. In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u can be important is the calculation of energy loss in sound and shock waves, described by Stokes' law of sound attenuation, since these phenomena involve rapid expansions and compressions. Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation (the strain rate). For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. A fluid with a high viscosity, such as pitch, may appear to be a solid. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity such fluids are technically said to be viscous or viscid. Zero viscosity is observed only at very low temperatures in superfluids. So for a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.Ī fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls.

Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. The viscosity of a fluid is a measure of its resistance to deformation at a given rate.