8/16/2023 0 Comments Oil viscosity chart in fahrenheit![]() ![]() Simply put, the higher a fluid’s VI number, the less change there is in the fluid’s viscosity over a given range of temperatures as shown in Fig. ![]() Viscosity Index or VI is an arbitrary number that characterizes the variation of viscosity of a fluid with variations of temperature. The viscosity of virtually all fluids used in hydraulic systems is affected by variations in temperature. Most hydraulic systems run with oil in the range of 150 to 300 SUS with typical ISO viscosity grade (ISO-VG) range of ISO-VG-22 to -68. Among these are: absolute or dynamic viscosity (centipoise/cP), kinematic viscosity (centistoke/CST) and relative viscosity as measured in Saybolt Universal Seconds (SUS or SSU). There are numerous methods for defining viscosity. ![]() Internal leakage, causing an increase in operating temperatures.This could cause increased cycle times or slower machine operation. Decreased pump efficiency (volumetric) due to increased leakage and possible cylinder blow-by.Seizure, particularly of pumps, could occur under heavy load of a breakdown in lubrication film between the clearances of moving parts. Inefficient separation of air from the oil in the reservoir.Increased pressure drop ( Δ P) due to increased resistance to flow.High temperatures created by power lost to friction.Increased energy consumption due to increased friction, and increased input requirement at the pump.Too high, or too low a viscosity for a given system, could present problems of performance, leakage, energy usage, and so forth. The selection of proper viscosity is often a compromise in order to optimize system performance. Viscosity affects the fluid’s ability to be pumped, transmitted through the system, carry a load, and maintain separation (lubrication) between moving surfaces. It has more resistance to flow than the lower viscosity or “thinner” fluid. A fluid that flows with difficulty would be said to have high viscosity. If a fluid flows easily, its viscosity is low. The viscosity of a fluid may be defined as its resistance to flow at a given temperature. In general these properties include: viscosity and viscosity index, pour point, lubricating ability, oxidation resistance, compatibility with system elements, rust and corrosion protection, and demulsibility. Consequently, when selecting fluids it is sometimes necessary to compromise some properties in favor of others that may be more important for a specific application requirement. Not all fluids will have all of the necessary attributes in equal strength. To adequately carry out the previously stated purposes, the fluids used in hydraulic systems must possess, to varying degrees, specific desirable characteristics. Note: In a straight petroleum product for every 18☏ rise above 140☏, the life of the fluid is cut in half. The right pump and/or motor must be chosen for the fluid being used or the right fluid must be chosen for the pump and/or motor that is specified. Some pumps and motors are limited by fluid viscosity and lubricity and must be de-rated if low viscosity or high water content fluids (HWCF) are used. Mild fluids become corrosive when hot stable fluids break down into useless new compounds. Most problems are magnified as temperatures rise. However, both closed systems and proper bearing and seal design help solve both problems. Viscosities are low at the higher temperatures, and oxidation will deteriorate the oil in open systems. The truth is that the designer must sacrifice some “important” characteristics to achieve “essential” characteristics. No single fluid meets all the performance requirements in even a single range of temperatures. Temperature is often the controlling parameter (when choosing a fluid). A fluid must be chosen that has a viscosity range and proper lubricity for the environment in which it will be working. The same fluid may become useless at cold temperatures because of high viscosity or freezing. At elevated temperatures, high water based fluids may lose water as it evaporates and will need to be continually monitored and replenished. T he temperature at which the fluid is to be working must be considered. Posted on by Fluid Power Journal in Features 0 Comments ![]()
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