Measuring Oxidation of Lubricants V. R. DAMERELL, Western Reserve University, CIeveland, Ohio
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turned on its side, on a level surface, and the sample allowed t o flow (the first time) past the horizontal mark t o the edge of the dish. The line perpendicular to the dish edge marked the lowest point of the dish-i. e., an extension of this line would bisect the advancing wave of lubricant. The dish was set u p right for about 2 seconds, then turned on the other side, until the sample flowed t o the edge of the dish. These preliminary operations were for the purpose of wetting the dish with the lubricant. Now a wrist watch (a pocket watch or stop watch would do) was held t o the ear with the left wrist, and the dish was set upright for just 2 seconds (8 watch ticks, including the 0.5 second required for turning), then turned on the side first wetted. The instant the wave of sample reached the ink line parallel t o the edge of the dish the dish was set upright for just 2 seconds; then it was set on the other side, and the watch ticks were counted until the wave of liquid just reached the junction of the other two lines. The dish was then set upright for 2 seconds, back t o the first side, etc., in a rhythmical manner. The elapsed time was measured by counting watch ticks, while the liquid traveled from the mark on one side of the container to the mark on the other. The dish was held by the tips of the fingers, near the edge, to minimize the thermal effect from the hand. With an oil of viscosity about 90 seconds at 210" F., the following consecutive readings were obtained, at 75' F.
NCREASE in viscosity has long been used as a measure of oil oxidation. The common procedure is to measure
the viscosity of the sample in a viscometer before and after oxidation, which is usually accelerated by an elevated temperature. Simplification of apparatus and saving of time are possible by determining viscosity change in the dish used for heating the sample. This can readily be done by measuring time for the lubricant to flow from side to side of the dish under carefully controlled conditions. This procedure permits the use of small samples, with a large ratio of surface to volume, so that the oxidation is greatly accelerated and the time of test correspondingly shortened. As an example, using &gram samples, the writer obtains in 2.5 hours results which normally require 100 hours with the usual (200-gram) sample.
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Number of Turn
Flow Time Quarter Seoohds ,.
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43 42 43 42
The difference in readings was due to a slight variation from a level position, either in the dish or the desk top. The recorded flow time for this sample would be 42.5 quarter seconds, with an accuracy of a t least "1 quarter second. The readings for the first two flow times were not included, since the liquid was allowed to flow to the edge of the dish. The glass was wet with the lubricant right to the edge to permit the smooth flow of the very thin film of liquid which occasionally was seen to precede the main wave upon which measurements were being taken. If this thin film was checked by a dry glass surface beyond the ink mark the main wave was in turn slowed up, leading to erratic results. 6 The reproducibility of this measurement depended upon the smoothness with which the dish was turned with the right hand. With a little practice the writer was able to do this in a rhythmical manner, allowing just two watch ticks (0.5 second). The turn should be done in such a manner that the sample runs down an axis marked by the two lines perpendicular to the dish edge; any motion of the flowing liquid sideways'will increase the flow time.
FIGURB 1
The writer has also been using this test for some time to measure the thickening of hypoid lubricants. The effect of heat on these lubricants may be more than just an oxidation, since even in the absence of oxygen certain extreme-pressure compounds may react to form a sludge (such as lead sulfide) which n7ill thicken the lubricant. According to the writer's best knowledge the tendency of a hypoid lubricant to thicken during use is not measured directly, a t the present time, but is arrived a t indirectly by separate determinations of sludge formation and oil oxidation. The method proposed below shows thickening tendency simply and directly.
Apparatus The apparatus consisted simply of an oven, a constant-temperature paraffin bath, and several glass crystallizing dishes, 80 mm. in diameter and 40 mm. deep. A straight ink line, several centimeters long, was drawn with carbon ink on the outside of each dish about 0.5 cm. from the top and parallel with the edge. A perpendicular line was drawn from this to the top of the dish, making an inverted T (Figure 1). This was repeated on the other side of the dish at a point just opposite. The horizontal line marked the stopping place for the flowing lubricant. The vertical line was used to guide its direction.
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Measurement of Flow Time The dishes were weighed, and exactly 5 grams of samde ( 1 5 mn.) were weighed into the bottom pah of each &sh. Each sample was allowed to come t o room temperature; then the dish was
FIGURE 2. BATHFOR HEATING SAMPLES 265
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Heating the Samples The temperature in the writer's oven varied markedly from point to point. At 300" F. variations in a horizontal plane 30 cm. (1 foot) above the heating coils were as much as 50" F. The use of a bath was therefore indicated.
calculated from the initial and final readings, indicated the behavior of the liquid towards oxidation, or in the case of certain hypoid lubricants, towards heat and oxidation.
Duplication of Results To test the reproducibility of the method, six samples of a, hypoid lubricant, having a viscosity of about 90 seconds a t 210" F. were heated in two batches of three samples each. The initial and final flow times a t 75" F. and the percentage increases in viscosity were as follows:
Sample
Initial Flow Time
Final Flow Time
40 39 42 41 39.5 40
60 59.5 63.5 63 61.5 60.5
Viscosity Inorease
% 1 2 3 4 5 6
50 53 51 54 56 51
These indicate that the method gives sufficiently reproducible results for many purposes.
Discussion
HOURS OF HEATING AT
300°F.
FIGURZI 3. INCREASE OF FLOW TIMEWITH HE~ATING
A 22.5-cm. (9-inch) aluminum cake pan was filled with paraffin to a depth of 2.5 om. 1 inch), and a 20-cm. (8-inch) aluminum pie pan floated on this. he samples being heated were placed in a symmetrical position on this upper pan, assuring a level surface with the maximum interface between sample and air. If the sample dishes were floated directly on the bath they invariably tilted somewhat, giving a sample-air interface of varying area. A weighing bottle, containing oil, was placed on the center of the up er pan, and a thermometer bulb was immersed in this oil t o in8cate the temperature of the samples (Figure 2). The samples were preheated to approximately oven temperature before they were put into the oven. Most samples were heated for 2.5 hours at 300' F. since results obtained under these conditions seemed to be similar to those obtained in the usual 100-hour test, ernploying 200-gram samples. The 2.5-hour test seemed to give results higher than the 100-hour test, however, on those lubricants which oxidized excessively or formed a large amount of sludge.
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Measurement of Flow Time after Heating The dishes were taken from the oven and allowed to stand until they came to room temperature, and the flow time was measured as before. The percentage increase in flow time,
The use of flow times in the manner described has a t least four advantages: The change in flow time of the entire sample is determined, showing in a simple manner the tendency of the lubricant to thicken in practice. The method is time-saving. The apparatus is exceedingly simple. The sample may be reheated as many times as desired, giving a curve (flow time or percentage increase in flow time us. time of heating) instead of a single result. In Figure 3 are given three examples of such curves, obtained by heating samples for an hour, measuring flow time, reheating for an hour, remeasuring flow time, etc. A and B were oils of viscosity 50 seconds and 75 seconds a t 210" F., C was a hypoid lubricant having a viscosity of 90 seconds a t 210" F. These curves do not correspond to any test commonly used at present, to the writer's best knowledge. The directions given above assume a constant room temperature during the measurement of initial and final flow time. The writer was fortunate in having a constant-ternperature room in which to measure these flow times. However, i t is probable that most laboratories have available a spot which has a constant temperature for 3 or 4 hours, or sufficiently constant so that a small temperature correction can be applied to the results. If the temperature does not remain constant, two samples can be weighed out and only one heated. The initial and final flow times can then be determined a t essentially the same time and temperature. I n place of the glass crystallizing dishes, various metals could be used. These should prove valuable in showing the effect of various metals and alloys upon the rate of oxidation of lubricants, as measured by the increase in flow time. The writer makes no claim that the above method is the last word in this type of measurement, but wishes rather to call attention to the advantages of such a procedure. The method can be improved by mechanizing the manual operation of tipping the dish back and forth, and the writer hopes that some one with mechanical ability will be interested in working on such an improvement. The method can be used with oils having a wide range of viscosities by varying the weight of sample-for example, the writer obtained satisfactory oxidation results on a series of light household lubricating oils using a 2-gram sample.
