Transmission fluid shear

Utility Hydrogenated styrene-diene and star-shaped diene VI improvers have found their widest utility in gasoline and diesel engine oils. They may also be used in other applications which have only moderate requirements for shear stability, such as tractor transmission fluids and aircraft piston engine oils. However, the shear stability demands of many of the specialty applications precludes the use of currently available products. 

The severity of different lubricant applications covers a wide shear stability range. Figure 5.8 shows that engine oils are the mildest application, followed by automatic transmission fluids, hydraulic fluids and rear axle lubricants [63]. Thus, matching shear stability requirements of the application with the selection of VI improver is a key formulation consideration. 

The torque converter is a type of fluid coupling device that hydraulically connects the engine to the transmission—analogous to a mechanical clutch. Used in conjunction with the torque converter is a stator, which essentially assists at low engine speeds, thus increasing acceleration. The vanes inside the converter alter the shape of the fluid path into the stator. The stator captures the kinetic energy of the transmission to enhance torque multiplication. This process will not only increase heat, but also increase shear of the transmission fluid. In addition to torque conversion, at every shift event, clutch packs generate heat, which must be carried away by the transmission fluid. 

In the equation referred to above, it is assumed that there is 100 percent transmission of the shear rate in the shear stress. However, with the slurry viscosity determined essentially by the properties of the slurry, at high concentrations of slurries there is a shppage factor. Internal motion of particles in the fluids over and around each other can reduce the effective transmission of viscosity efficiencies from 100 percent to as low as 30 percent. 

These reflection and transmission coefficients relate the pressure amplitude in the reflected wave, and the amplitude of the appropriate stress component in each transmitted wave, to the pressure amplitude in the incident wave. The pressure amplitude in the incident wave is a natural parameter to work with, because it is a scalar quantity, whereas the displacement amplitude is a vector. The displacement amplitude reflection coefficient has the opposite sign to (6.90) or (6.94) the displacement amplitude transmission coefficients can be obtained from (6.91) and (6.92) by dividing by the appropriate longitudinal or shear impedance in the solid and multiplying by the impedance in the fluid. The impedances actually relate force per unit area to displacement velocity, but displacement velocity is related to displacement by a factor to which is the same for each of the incident, reflected, and transmitted waves, and so it all comes to the same thing in the end. In some mathematical texts the reflection... 

It may be helpful to express the change of viscosity of a silicone oil in a different manner. If we compare a typical silicone oil with a standard hydrocarbon oil of viscosity index 100, the two having the same viscosity at 100° F., we find that after cooling to —35° F. the silicone oil has seven times the viscosity it had, whereas the hydrocarbon oil has increased 1,800-fold in viscosity. This relative constancy of viscosity of the silicone oil makes it particularly suitable for use as a fluid in hydraulic systems for the transmission of power. Silicone oils do not react with the common metals of construction, and they are so inert that even at 300° F. they do not discolor r become acid or form sludge. They are satisfactory lubricants in hydraulic pumps and in any other device where conditions of hydrodynamic lubrication prevail. When used as lubricants, methyl silicone oils do not suffer loss of viscosity through shear breakdown under continuous load at high speed. 

The basic equations of motion contain coefficients of shear viscosity for the fluids. This gives rise to a dependency of porosity diffusion on the rate of fluid transmission through the pore space. In low viscosity cases such as crustal fluids in deep interconnected fracture networks, transmission is relatively rapid in reservoirs largely saturated with viscous oil, transmission of pressure effects can be quite slow. For a 10,(XX) cP oil filling 88% of the pore space of a 30% porosity 2-3 Darcy sand, it took about 5 weeks to see a substantial response at distances of 300 m from the excitation well which was being aggressively pulsed at the right frequency (Spanos et al., 2003). 

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