WHAT DOES THE DROPPING POINT TEST REALLY MEAN FOR GREASES: AN IN-DEPTH LOOK


Nearly 50 percent of mechanical component failures result from wear and friction underperformance of the applied lubricants or simply just the mechanical component reaching the end of its lifespan [1].


Therefore, the constant, unwaning performance of lubrication is necessary for a reduction in these mechanical component failures. Mechanical components include nuts, bolts, bearings, and belts, components which are used extensively in internal combustion engines (ICEs). The main lubricating agent used in ICEs is grease, a semi-solid, which in this case includes both motor oil and lithium grease. These greases are combined with additives and thickeners to improve lubrication properties and enhance other desirable properties. Lithium-based greases are used to lubricate ball bearings and bearing systems, while engine oils (motor oils with additives) are used to lubricate the crankshaft, camshaft, and rocker arms of an ICE [2]. In these internal combustion systems, components and surfaces are constantly moving and sliding in contact with each other. The surface area where these components or surfaces interact is called the contact surface and is observed in multiple areas of the engine, specifi cally the crankcase, piston and liners and the main and rod bearings [3]. Failure in the main crankshaft bearing can lead to engine failure [3]. Effi cient lubrication is thereby integral to the effectiveness and longevity of the engine. The applied lubricants must be able to function optimally in extreme conditions, as an ICE coolant operates at temperatures of approximately 195-220 degrees F (90.6-104.4 degrees C) [4]. These high operating temperatures require applicable lubricating greases to have a high dropping point.


The dropping point is an indicator of the heat resistance of grease and can be defi ned as the lowest temperature at which the phase of grease changes from semi-solid to liquid [5]. When this dropping point is reached, the effi ciency of grease vastly decreases as the semi-solid grease loses its structure


Table – 1 Physical and Chemical Properties of Tested Greases. Adaptation from [12]


tribological properties of the original grease are lost and the altered grease may no longer be suffi cient for maintaining a proper lubrication fi lm. In short, exceeding the dropping point leads to a drastic depreciation in the quality of lubrication and the lubricant is rendered essentially useless. This newly formed product has a lower viscosity, which causes the grease to have a decreased adherence to the components of the lubricating system.


Figure 1 Koehler High Temperature Dropping Point Apparatus Reprinted from Koehler Instrument Company Inc. [10]


and desired viscosity [6]. Some greases can regain their original consistency when cooled, but not all greases share this property [6]. Therefore, the desired consistency and the inherent


PIN Annual Buyers’ Guide 2023


Grease is desirable for lubrication due to its high viscosity, which effectively decreases friction and generates heat at the contact surface. Viscosity is a liquid’s (or in this case, a semi-solid’s) resistance to a change in shape or movement [7]. Decreasing sliding or movement proportionally decreases the force of friction. Therefore, semi-solid grease reduces the friction force more effi ciently than lower viscosity grease, which results from exceeding its dropping point. This decreased viscosity is due to an increase in the movement of adjacent oil layers caused by a lack of structure, which is lost above a grease’s dropping point [5]. These reduced tribological behaviors lead to reduced lubricating ability and increased wear on the contact surfaces; in other words, the deformation and removal of material caused by rolling or sliding. a process that involves the interactions between surfaces and the removal and deformation of material caused by rolling or sliding [8]. This wear causes a decrease in the longevity of the contact surfaces. The dropping point is therefore fundamental to both the performance of the grease and the longevity of the surfaces to which it is applied.


The test method utilized for the determination of the dropping point is standardized by the American Society for Testing and


Materials (ASTM) and given by the designation D2265. This test method yield results useful for identifying grease as to a type and for establishing and maintaining benchmarks of quality control [9]. In this test, a grease sample is heated and observed until a drop of material falls from the cup to the bottom of the test tube. The Koehler Instrument Company Inc, apparatus for the Dropping Point Test is shown below in Figure 1 [10]. This instrument conforms to ASTM D2265 and D4950 specifi cations and features a six-sample testing capability [10]. It additionally features a microprocessor programmable for high accuracy temperature control, reaching temperatures of up to 400 degrees Celsius [10]. Using the viewing window provided in the instrument, the moment when the material falls into the bottom of the test tube can be clearly seen and the reading on the sample thermometer can be recorded to the nearest degree. This temperature reading is designated as the experimental dropping point. Simultaneously, the temperature displayed on the temperature display screen of the instrument is recorded to the nearest degree and a third of the difference in the two temperature readings is added to the observed value and taken as the dropping point [9]


This test is quite simple from a laboratory experiment standpoint, but the recorded dropping point provides valuable insight into the temperature properties of grease and the proper applications it can be applied for. Greases with relatively high dropping point temperatures (in the range of 170-180 degrees Celsius) are highly sought after in commercial vehicle applications, such as in internal combustion engines in cars and tractor engine lubrication [11]. Meanwhile, greases with relatively low dropping point temperatures (in the range of 90-100 degrees Celsius) can be used for valve, conveyor, and spring lubrication, as well as many


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