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While there are many laboratory techniques for determining the viscosity of hydraulic and lubricating fluids, many maintenance programs depend on information gathered on-site by less sophisticated instruments.
It is important that the accuracy and suitability of these instruments be determined before they are used to make critical decisions.
This article provides data gathered from a series of tests utilizing the Visgage, a viscosity comparator. The results show that reasonable accuracy can be achieved with the instrument when compared with the results obtained from a kinematic viscometer.
The most important property of a lubricating oil is its viscosity. It is important in the formation of the lubricating film in both thick and thin film regimes. It affects heat generation in bearings, cylinders and gears. It governs the sealing effect of the oil and the rate of consumption or loss, and it affects the power required under cold start conditions.
The basic concept of viscosity is shown in Figure 1.
Figure 1. Concept of Absolute Viscosity (µ)
When a plate is moved over a uniform film of oil spread across a stationary plate, the oil tends to adhere to both plates. This results in a zero velocity for the oil at the stationary surface and an oil velocity at the surface of the moving plate equal to that plate’s velocity.
The oil between the two plates can be conceptualized as being made up of a number of layers of oil, each being one molecule thick. A velocity profile results from the relative movement of the two plates as shown in the figure, where the velocity of each fluid layer is proportional to its distance from the stationary plate.
A force must be applied to the moving plate in order to overcome the friction force between the fluid layers. This force is proportional to the viscosity of the fluid. Thus, the viscosity of the fluid can be found using the equation shown in Figure 1 and can be defined as the fluid’s resistance to shear (or flow).
Viscosity determined as described above is termed the absolute or dynamic viscosity of the fluid. It is normally reported in the units shown in Table 1. Because the dynamic viscosity is a function of the internal friction of the oil, it is frequently specified in bearing designs and oil flow computations.
It is often more convenient to measure viscosity using techniques that are dependent on the fluid density, therefore lubricants are often specified by their kinematic viscosity.
The kinematic viscosity of a fluid is simply its dynamic, or absolute, viscosity divided by its mass density. The units for reporting kinematic viscosity are also included in Table 1.
Table 1. Viscosity Units
The Visgage uses the falling ball viscosity measurement concept. Its principle of operation is based on comparing the viscosity of the test fluid with a reference fluid of known viscosity. The viscosity reading is given at 100°F, even though the testing may be done at any temperature between 80°F and 100°F.
The viscosity comparator (Figure 2) is available in standard models, and can be custom-ordered for special applications. The differences in the models are the reference oils used in the comparator tube and the scale layout.
The model type used in the tests discussed in this article contains a reference fluid B-85-166 that has a viscosity index of 95. It is designed to be used with the test fluids with viscosities between 8.6 and 172.7 centistokes (cSt).
The procedure for determining the viscosity of the test fluid is simple. The Visgage test tube is filled like a syringe. The nozzle is inserted in the test fluid and the plunger is slowly withdrawn to pull fluid into the tube.
Bubbles must be eliminated and the test tube completely filled with the test fluid. The unit is then placed in its case which has a glass-covered window in its cover. The cover is closed and latched.
After allowing the temperatures of the test and reference fluids to equalize, the case is held with the Visgage in the horizontal position with the reference tube on top. The unit is tipped so that the balls in both tubes are at the plunger end of the instrument. The case is then tipped 30 to 45 degrees downward and to the left so that both balls run freely toward the left end of the tubes.
One ball will fall faster than the other. As the leading ball approaches the line on the left end of the scale, the instrument is gradually moved toward the horizontal position to stop the leading ball exactly on the end line. The viscosity of the tested oil is read from the scale position indicated by the other ball. This reading indicates the viscosity of the test fluid in centistokes at 100°F.
The temperature of the test oil should be between 80°F and 100°F, and the temperatures of the test and reference fluids should be equalized. Differences of greater than five percent in the readings obtained on successive tests of the same fluid indicate that the temperatures are not equalized.
If the viscosity index of the test fluid is greatly different from that of the reference oil (VI95), the Visgage should be warmed to 100°F before taking viscosity readings.
To evaluate the accuracy and applicability of the Visgage for hydraulic fluids, a series of tests were conducted on both new and used oils. The reference measurements for these tests were made using a kinematic viscometer. The test fluids included:
The 15 samples of MIL-H-5606 were extracted from hydraulic test benches.
Both the MIL-L-2104 and the Chevron fluid nominally fall into the ISO 46 viscosity grade. The MIL-H-5606 fluid was nominally a 16.8 cSt fluid, placing it between ISO grades 10 and 15.
Table 2. Kinematic Viscometer and Visgage Comparator Test Results
The results of these tests are shown in Table 2. All tests were conducted in accordance with the appropriate procedures. The results show that both new fluids gave readings on the kinematic viscometer that were close to their nominal values and well within the range of their ISO designation.
The Visgage readings for those fluids were slightly lower than the kinematic viscometer results. The reading for the MIL-L-2104 was well within the ISO viscosity grade, while that for the Chevron fluid was below the lower value for grade 46 but well above the next lower grade.
Comparing the results for the new and used MIL-H-5606 fluid samples illustrates that the used fluids had degraded considerably, probably due to the volatilization of the lighter fluid fractions. This is probably a result of the low utilization of the particular test benches, which allows the fluid to remain static for months at a time at temperatures exceeding 100°F.
The tests show reasonably close agreement between the kinematic viscometer and the Visgage comparator. Large deviations are thought to be due to variations in the viscosity indices of the used fluids or allowing the test fluid temperature to drop below 80°F before or during the Visgage test. Improvements in operator technique could reduce the deviations.
Notice that the average viscosity for these 15 samples of used fluid was found to be 21.6 cSt according to the kinematic viscometer, while the Visgage comparator readings gave an average of 21.2 cSt.
As a result of these tests, it can be concluded that while the Visgage comparator does not provide viscosity readings that correspond exactly to those obtained using the kinematic viscometer, it does present reasonably close agreement. On that basis, it is believed that the Visgage is suitable for use as an on-site viscosity measuring device.
The results of these tests and this conclusion are supported by numerous formal and informal comparisons in which fluids of a known viscosity grade (but not actual viscosity) were tested with the Visgage.2 The instrument gave viscosity readings that fell within the specified grade.
The Louis C. Eitzen Company, Inc. would like to advise readers that manufacturer’s operating instructions should be consulted for proper Visgage usage prior to accurate testing.
Two other Visgage models yield viscosity readings in centistokes at 40°C; model No. 38 and model No. 76. The Eitzen Company recently conducted in-house testing on multigrade engine oils and found the Visgage to be fairly accurate at 74°F room temperature. The company recommends these oils be tested near this temperature.
This article was based largely on a paper published in The BFPR Journal, Vol. 18, No. 1. Edited by I.T. Hong. Basic Fluid Power Research Program, 1985. Copyright 1984 by the Fluid Power Research Center, Oklahoma State University, Stillwater, Okla.