Apparatus for testing shear stability of lubricants

[57] ABSTRACT

An apparatus for rapidly determining the loss in viscosity which occurs when a lubricant containing a polymeric viscosity improvement additive is subjected to high shear is characterized by a tapered roller bearing assembly, pre-loaded to a constant torque, in which one race revolves at a constant and measurable speed in a cell containing the lubricant while maintaining the lubricant at a set temperature.

Claim, 5 Drawing Figures

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PAWTEOJAN151974

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PATENTED JAN 151974

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3,785,196

APPARATUS FOR TESTING SHEAR STABILITY OF LUBRICANTS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is concerned with an apparatus for predicting the loss in viscosity that occurs when lubricants containing polymeric viscosity improvement additives are subjected to high shear. Compounded oils of this type when used as engine lubricants and in automatic transmissions, hydraulic, gear and recoil mechanisms, show a permanent loss in viscosity after extended use. Since the viscosity of a lubricant is a major factor in determining its utility for a given service, a primary object of this invention is concerned with predicting in a rapid and economical manner the loss in viscosity that occurs when oils containing polymeric additives are subjected to high mechanical shear.

2. Description of the Prior Art

Many modern day lubricants, such as automotive crank-case oils, contain Viscosity Index (V.I.) improvers dissolved in the lubricating oil. These V.I. improvers are long chain polymers which modify the effects of temperature upon the lubricating oil. They are well known in the art and have been described in numerous U.S. Pat. such as Nos. 2,642,414; 2,666,746; 3,087,893; 3,076,791; etc., In general, in a lubricant these polymers tend to fold back on themselves, i.e., ball up, thus reducing their overall length, as the lubricant becomes cold, so that the viscosity of the cold lubricant becomes essentially that of the lubricating oil component. As the lubricant becomes hot during use, e.g., during operation of an engine, these long chain polymers tend to unfold and become extended thus tending to increase the viscosity of the oil. The result is that the effects of temperature, for example, cold temperatures during start-up and hot temperatures during engine operation, are minimized by the addition of these long chain viscosity index improvers. These polymers are frequently polymers of various unsaturated esters such as polyacrylates or copolymers of vinyl acetate and alkyl fumarates; or they are hydrocarbon polymers such as polyisobutylene or copolymers of ethylene-propylene, etc. Because of their high molecular weight, these polymers tend to break into fragments during operation of the engine due to a mechanical shearing action as the oil is churned, for example, between the cylinders and pistons. This results in the polymer chains breaking into smaller lengths, which in turn lessens the effectiveness of the polymer as a viscosity index improver. The purpose of the present invention is to provide a mechanical device in which a lubricant containing a V.I. type of polymer can be subjected to a shearing action so the resistance of the polymer to shear breakdown can be accurately and easily determined in a laboratory test.

A wide variety of devices and methods have been described in the prior art to determine the viscosity breakdown of oils containing polymeric additives. Some of the more useful procedures have been described in “ASTM Special Technical Publications” No. Ill (1950) and No. 182 (1955); Trans, of the Soc. of Auto. Eng’s. No. 680069 and No. 690058 (January 1969); Journal of the Inst, of Petrol. (U.K.) Vol. 55, (March 1969) and SAE Journal, Vol. 77, No. 5 (May 1969).

These methods have generally involved: multiple passage through a capillary or sharp-edged orifice; sonic degradation by subjecting the oil to a sonic oscillator or magnetostrictive transducer; diesel fuel injectors; power steering pumps and single cylinder engines. While correlations with field tests are good in some instances, most are time consuming, some require large voltimes of oil for testing and are only specific to a particular type of polymeric additive.

In contrast to the above, the present invention duplicates in a laboratory device the breakdown encountered in actual service, uses a small sample and provides a means for readily varying the temperature, speed and pressure at which the test is conducted.

SUMMARY OF THE INVENTION

This invention describes a laboratory type apparatus for predicting the loss in viscosity that occurs when lubricants containing polymeric additives are subjected to prolonged use in service involving high mechanical shear. The apparatus comprises a tapered roller bearing assembly having an inner race, outer race, and roller bearings held by a retainer, immersed in the sample of lubricant to be tested contained in a cell maintained at a preset temperature with a race of the bearing revolving at a variable R.P.M. under a force applied to the bearing. The sliding and rolling action of the bearing causes rapid breakdown in viscosity and correlates well with actual field use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the apparatus in elevation and partial section showing the principal parts of the invention.

FIGS. 2, 3 and 4 illustrate alternative means for subjecting the roller bearing to pressure.

FIG. 5 illustrates a cell for running the test at elevated temperature in contrast to the cell illustrated in FIG. 1 for conducting the test at ambient temperature or at temperatures below about 100°F.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the apparatus comprises a base plate 10 on which are circumferentially mounted a plurality of support rods 12 by means of threaded studs 114 in threaded sockets 112. Rods 12 are undercut at each end before threading so as to provide shoulders 110 which ensure a predetermined distance and parallelism between base plate 10 and motor support plate 14. .

Upper bearing plate 16 is slidably mounted on support rods 12, and while the extent to which plate 16 moves when the apparatus is assembled and pressure applied by means of springs 70, is hardly discernable to the unaided eye, plate 16 may optionally be supplied with low-friction bushings 18, of which Teflon impregnated porous bronze bushings are a representative example.

Variable speed motor 20, fitted with tachometer 22 is mounted on motor support plate 14 by means of extended studs 116 and is supplied with an externally splined shaft 24. Splined shaft 24 is slidably fitted into a mating internally splined coupling 26 attached to shaft 68 by means of a threaded connection.

Inner race 28 of the upper thrust bearing is mounted on the upper under-cut section of shaft 68 by means of a press fit. Inner race 46 of the lowe r test bearing is similarly mounted to the lower undercut end of shaft 68 by

3,785,196 means of a press fit. An oil-slinger disc 42 and spacer collar 44 is fitted to shaft 68 before fitting inner race 46.

Outer race 30 of the upper thrust bearing is mounted by means of a press fit in upper bearing plate 16, and oil leakage from the upper bearing is prevented by means of oil seal felts 34 secured by retainer ring 36 held by machine screws 38. While a ball bearing 32 is illustrated in FIG. 1, a roller bearing similar to the test bearing may be used, or any bearing designed to accept moderate radial loads and heavy thrust loads.

Double-walled test cell 54 is insulated from base 10 by means of a fiber, MICARTA or TRANSITE cup 86 and is positioned and prevented from rotating by a plurality of pins 60 fastened to base 10 and extending through insulating cup 86 into corresponding recesses in the base of test cell 54. The internal diameter of test cell 54 is bored to accept a push fit or slip fit of the outer race 48 which is secured against movement by set screws 52 mounted in a plurality of internally threaded inserts 50 traversing the inner and outer walls. Set screws 52 may be tipped with a hard plastic or flats may be ground on the outer race in order to prevent burring which would interfere with removal of the bearing from the cell.

Internal ports 58 aid in distribution of a circulating cooling medium fed to cell 54 through inlet 62 to outlet 64 upon demand of a thermistor 66 having a preset temperature response. Thermistor 66 is secured to dip into the lubricant being tested by means of cover plate 40 which may be fabricated from a polyfluoroethylene such as TEFLON or KEL-F. A drain tube 102 permits the sample after a test run to be directly aspirated into a capillary viscosimeter. During a test run a cap 108 is secured to the top of the drain tube. In order to minimize any sample hold-up in the drain tube which may not be subjected to shear, a flexible wire attached to the cap may be inserted into the tube.

Cell 54 may be fabricated from steel, aluminum, stainless steel or other suitable alloys. Where it is known that an alloy will exert a catalytic effect on the lubricant composition under the test conditions employed, it may be useful to construct the cell of this alloy to enhance such effects when applicable.

In certain instances it may be desirable to run the test while maintaining an atmosphere of nitrogen, carbon dioxide or added oxygen and in these instances the apparatus readily permits such variation. A gas ring seal 76 attached to upper bearing plate 16, communicates with transverse bore 92 and shaft bore 90 to admit the gas below the level of the inner race 46 of the test bearing. Gas is fed through inlet 88 and leakage around the shaft is prevented by shaft seals 78.

Springs 70, are cylindrical helical springs wound from wire having a circular or rectangular cross-section with the ends ground square to a precise length. When assembled on rods 12 between motor support plate 14 and upper bearing plate 16 they are in a state of compression exerting a torque on rest roller bearings 56. The torque which may be calculated in inch-pounds is a function of the metal or alloy from which the spring is wound, the cross section area of the wire, the helix pitch and pitch diameter and the extent of compression. The design and fabrication of a spring to exert a particular force is an old and well established art. By the use of a family of matched springs which will exert a torque on the bearing in increasing increments, it is possible to rapidly obtain a plot of viscosity breakdown for a given time for any lubricant, in which the torque is varied and the RPM held constant. It will be obvious that a large single spring may be used in a manner to encircle motor mounting bolts 116, and be placed between motor mounting plate 14 and upper bearing plate 16. Similarly, a spring can be placed between base 10 and the test cell 54.

While the use of springs assures that the same torque will be applied in test after test since they possess an “infinite” fatigue life under the conditions of use, alternative means may be used for applying force to the test bearing as illustrated in FIGS. 2, 3 and 4.

FIG. 2 illustrates the use of a longitudinally expandable double bellows encircling rod 12 to which is fed a hydraulic fluid through entrance tube 106 at a predetermined pressure.

FIG. 3 illustrates a simple method for exerting force on the bearing, in which a hardened steel spacer 80 is substituted for spring 70 and force applied by a plurality of lead weights 82.

FIG. 4 illustrates a means for applying force by the use of spacer 80 and nut 74 which is tightened by means of a calibrated torque wrench to a given degree.

It may be desirable in certain instances to conduct the test at elevated temperatures in which case “cold cell” 54 is replaced with “hot cell” 94 illustrated in FIG. 5. Insulated resistance wire heater coil 96 is cemented in place with a castable refractory 98 such as ALOXITE or ALUNDUM cement and insulated by an asbestos cement 100. A protective metal jacket 104 ensures maintenance of the insulation in place.

A typical use of the apparatus as illustrated by FIG. 1, is as follows: 98 parts by weight of a mineral lubricating oil of known viscosity has dissolved therein two parts by weight of a V.I. improving polymer, e.g. polyisobutylene of 20,000 number average weight. The viscosity of the oil-polymer blend is measured by conventional means (not shown). A sample of the oil-polymer blend is then added to the test cell 54, the nuts 74 tightened with a torque wrench to obtain a desired thrust load on the bearing. The temperature of the sample is measured with thermocouple 66 and maintained at a desired temperature by circulation of a heat-transfer fluid through lines 62-64. The motor 20, rotates shaft 68, which rotates bearing member 46 and rollers 56 under thrust. This rotation under thrust results in a shearing action on the dissolved polymer. After the desired test time, e.g., 30 minutes, the oil-polymer solution is drained through line 102 and the viscosity of the resulting sheared oil-polymer solution is measured. The difference in viscosities of the sheared oil-polymer solution and the original unsheared oil-polymer solution is an indication of the shear stability of the polymer.