Continued from page 14 Friction Force = Normal force x friction coefficient F = µ.N Equation 1
Both static and dynamic friction need to be considered and understood in order to manage the friction between the surfaces, especially in reciprocating friction tests and applications such as CV joints
Friction Testing As reported in 2013 (2), there are many different standardized ways of way of measuring friction in greases. Historically the sliding 4-ball wear tester was used to measure friction. One important issue with using the 4-ball machine to measure friction is the bearing used in the apparatus influences the friction values. Some machines have air bearings and others have tapered roller bearings. The latter have higher motion resistance and this makes comparative data difficult to interpret.
Cameron-Plint instruments were developed in the 1960s to measure friction and wear. The main two instruments used are the TE70 Micro Friction machine (commonly known as the HFRR) and the TE77 High Frequency Friction Machine (commonly known as the Cameron-Plint). Both machines were designed to have sinusoidal reciprocating motion. Improvements in instrumentation led to the development and use of electrical resistance / capacitance methods for the amount of metal-metal contact. Around 1990, PCS instruments introduced their version of the HFRR.
The SRV test method was developed in the 1980s by Optimol in Germany. The latest instruments have many features that older machines did not have and the upgraded drive and control systems allow for higher loads and better data acquisition. The general principle was a “Square” wave motion driven forward and springing back. The tester measures the reaction force from the motion under load and from this the maximum, minimum and average values of friction during the test run are calculated. The test method was standardized as DIN 51834-6 (9). The software produces a friction coefficient-time trace. The output from the force transducer can be captured and digitized to give static and dynamic coefficients. A comparison of the testing parameters of the four types of test machines is included in table 1 below.
greases. The first selection issue was type and viscosity of the base fluid. Many low friction greases have base oil viscosities between ISO VG 68 and 150. The laboratory urea grease had an MDI-fatty amine thickener (~12%wt in the finished grease) and was made in a 3-litre resin flask. The base oil was a mixture of naphthenic and paraffinic mineral oils with 10%wt added PAO to give a final ISO VG of 100. The lithium complex was made in a laboratory grease kettle. It used azelaic acid as the complexing agent along with 12-hydroxystearic acid in a group I paraffinic oil with a viscosity of 112 mm2/s at 40 °C. Additives were tested at 2wt% on top of a model non-friction modified fully formulated package, which included zinc and phosphorus anti- wear, sulfurized extreme pressure, anti-oxidant, and corrosion inhibitor additives at normal treatment levels. Standardised SRV tests at 80 °C, 200 N load, 0.5 mm stroke amplitude and 50 Hz using a 10 mm polished bearing steel ball running against the standard surface lapped discs were carried out. The friction data were reported as being the average value over the test after running-in and the end of test (EOT) in which the friction data are averaged over the last 30s of the test run. The ball wear scar diameter was measured along the direction of travel and at an angle 90° to this and averaged. The friction and wear data for the baseline greases are in table 2.
Table 2. SRV Test results for the base greases
The summary of the testing of the 25 different friction modifiers (2) showed that only organic molybdenum complexes gave low friction coefficients (<0.08). It also showed that they do not work on their own and need other additives present such as zinc dithiophosphate and or sulfurized extreme pressure additives as found in fully formulated greases. A summary of the results of SRV testing of fully formulated (FF) greases with molybdenum complex additives is in Table 3. Moly1 is a molybdenum dithiocarbamate (MoDTC) and Moly2 is non-phosphorus molybdenum complex (MoX).
Table 3. SRV Test results for the fully F greases with molybdenum complexes Table 1. Comparison of testing parameters
As reported in 2013 (2), a whole series of commercial and experimental friction modifiers were tested in two different base greases. Based on published data (8), the most common globally used thickener systems for low friction greases are urea derivatives and simple lithium soaps. For some higher temperature applications, lithium complex greases are also used so it was decided to use lithium complex and urea-thickened
From this testing it was clear that the two molybdenum complexes reacted differently with the two different thickeners and that further optimisation of the formulations was needed. An additional 10 combinations of additives and molybdenum compounds were formulated (LF4 to LF 13). These were tested in lithium, lithium complex and urea-thickened greases. The best two candidate packages, LF4 and LF9, both containing mixtures of MoDTC, MoX and organic friction modifiers. The friction and wear scar data for these two packages are included in table 4.
Continued on page 18
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LUBE MAGAZINE NO.129 OCTOBER 2015
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