Viscosity Viscosityit is probably bbl the single most important t property of a hydraulic fluid. It is a measure of a fluid's resistance to flow. When the viscosity is low, the fluid flows easily and is thin in appearance. A fluid that flows with difficulty has highh viscosity i and is thick in appearance. In reality, the ideal viscosity for a given hydraulic system is a compromise. Too high a viscosity results in 1. High resistance to flow, which causes sluggish operation. 2. Increased power consumption due to frictional losses. 3. Increased pressure drop through valves and lines. 4. High temperatures caused by friction. On the other hand, if the viscosity is too low, the result is l. Increased oil leakage past seals. 2. Excessive wear due to breakdown of the oil film between mating moving parts. These moving parts may be internal components of a pump or a sliding spool inside a valve. 20
Viscosity The CGS physical unit for viscosity it or dynamic viscosity it is the poise (P), named after Jean Leonard Marie Poiseuille. It is more commonly expressed, as centipoise (cp). Water at 20 C has a viscosity i of 1.0020 cp. 1 P = 0.1 Pa s, 1 cp = 1 mpa s = 0.001 Pa s = 0.001 N s/m 2. Kinematic Viscosity It has become useful and customary to use kinematic viscosity defined as absolute or dynamic viscosity divided by density. ν = µ/ρ Where the units of ν are m 2 /s (ft 2 /sec). Note that for a gas, the kinematic viscosity will also depend on the pressure since the density is pressure sensitive. 21
The SI unit of kinematic viscosity is m 2 /s. The CGS physical unit for kinematic viscosity it is the stokes tk (St), named after George Gabriel Stokes. It is sometimes expressed in terms of centistokes (cst). 1St=1cm 2 s 1 =10 4 m 2 s 1.1cSt=1mm 2 s 1 =10 6 m 2 s 1. Water at 20 C has a kinematic viscosity of about 1 cst. EXAMPLE: Express 0.00165 Ns/m 2 viscocity in cp and cst. Ans: 1cP=1mPa s=1cst 0.00165 Ns/m 2 = 1.65 mpa s = 1.65 cp = 1.65 cst 22
Saybolt viscometer The viscosity of a fluid is usually measured by a Saybolt viscometer. Basically, this device consists of an inner chamber containing the sample of oil to be tested. A separate outer compartment, which h completely ltl surrounds the inner chamber, contains a quantity of oil whose temperature is controlled by an electrical thermostat and heater. A standard orifice is located at the bottom of the center oil chamber. 23
When the oil sample is at the desired temperature, the time it takes to fill a 60-cm 3 container throughh the metering orifice is then recorded. The time (t), measured in seconds is the viscosity of the oil in Saybolt Universal Seconds (SUS). Since a thick liquid flows slowly, its SUS viscosity value will be higher than that for a thin liquid. A relationship exists between the viscosity in SUS and cs. This relationship is provided by the following empirical equations: ν (cs) = 0.226t 195/t, ν (cs) = 0.220t 135/t, t 100 SUS t > 100 SUS where the symbol ν represents the viscosity in cs and t is the viscosity as measure in SUS or simply seconds. 24
Capillary tube Viscometer A quick method for determining i the kinematic viscosity it of fluids in cs and absolute viscosity in cp is shown in the figure. This test measures the time it takes for a given amount of fluid to flow through a capillary tube under the force of gravity. The time in seconds is then multiplied by the calibration constant for the viscometer to obtain the kinematic viscosity of the sample fluid in centistokes. 25
Viscosity Index Oil becomes thicker as the temperature decreases and thins when heated. Hence, the viscosity of a given oil must be expressed at a specified temperature. For most hydraulic applications, the viscosity normally equals about 150 SUS at 100ºF. Itisingeneral rule of thumb that the viscosity should never fall below 50 SUS or rise above 4000 SUS regardless of the temperature. Where extreme temperature changes are encountered, the fluid should have a high viscosity index. Viscosity index (VI) is a relative measure of an oil's viscosity change with respect to temperature change. An oil having a low VI is one that exhibits a large change in viscosity with temperature change. A high-vi oil is one that has a relatively stable viscosity, which does not change appreciably with temperature change. 26
The original VI scale ranged from 0 to 100, representing the poorest to best VI characteristics known at that time. Today, with improved refining techniques and chemical additives oils exist with VI values well above 100. A high-vi oil is a good all-weather-type oil for use with outdoor machines operating in extreme temperature swings. This is where viscosity index is especially significant. For a hydraulic system where the oil temperature does not change appreciably, the viscosity index of the fluid is not as critical. The VI of any hydraulic oil can be found by using 27
The VI of an unknown-vi oil is determined from tests. A reference oil of 0 VI and a reference oil of 100 VI are selected, each of which has uniquely the same viscosity at 210ºF as the unknown-vi oil. The viscosities of the three oils are then measured at 100ºF to give values for L, U, and H. The change in viscosity of an oil as a function of temp. is represented by a straight line on an American Society for Testing and Materials (ASTM) standard viscosity temperature charts. 28
Pour Point Another characteristic relating to viscosity is called the pour point, which is the lowest temperature at which a fluid will flow. It is a very important parameter to specify for hydraulic systems that will be exposed to extremely low temperatures. As a rule of thumb, the pour point should be at least 20ºF below the lowest temperature to be experienced by the hydraulic system. 29