PSI - Issue 12

Francesca Curà et al. / Procedia Structural Integrity 12 (2018) 44–51 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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and an equally good stability against aging; these properties allow to guarantee an excellent protection against wear, corrosion and fatigue damaging; this oil is widely used in gear transmissions and in wind turbines. The third lubricant is the SKF LGMT 2 grease, distinguished by an excellent thermal stability within its operating temperature range.

Table 1. Parameters for Pin-on-Disk tests.

Test parameter Sliding distance [m]

100

Normal load [N]

5

Linear Sliding Velocity [mm/s]

100 13.5

Track Radius [mm]

GrapheneNano Platelets (GNP) have been added to the base lubricants in order to evaluate its effect as lubricant additive. GNP have grade C-500 properties, characterized by 2 nm particles thickness, with a diameter less than 2 µm and average surface of 500 m2/g. Different compounds have been made using a HeidolphHei Torque Precision 400 homogenizer, then mixed for 30 min at room temperature in order to achieve uniformly dispersion of GNP into the lubricants.

Table 2. Oils viscosity. Lubricant

Viscosity 40°C [mm 2 /s]

Viscosity 100°C [mm 2 /s]

Oil 1 - Mobil Jet Oil II

27.6

5.1

Oil 2 - Renolin Unisyn 320

320.0

24.0

Table 3. Lubricant compounds. Oil 1

Oil 2

Grease

Pure Oil 1

Pure Oil 2

Pure Grease

Oil 1 + 0.5% GNP Oil 1 + 2% GNP Oil 1 + 5% GNP

Oil 2 + 0.5% GNP Oil 2 + 2% GNP Oil 2 + 5% GNP

Grease + 0.5% GNP Grease + 5% GNP Grease + 10% GNP

Four type of compounds have been created for each lubricant to perform the tribological tests. Table 3 reports the percent of added GNP for each lubricant, starting from the pure oil or grease which constitutes the reference condition. For each compound three tests have been carried out. During the tests temperature and humidity were constant and equal to 21°C and 40% respectively. Fig. 1 shows the tribometer apparatus with flat specimen covered by Oil. Finally, characterization results of graphene added grease have been related to that available from previous studies performed by the authors on a mechanical component (a spline coupling), commonly used in many industrial applications, lubricated by the same graphene-grease compounds. Characterization tests have been performed by mean of a dedicated test rig (see Fig. 2). The test rig used for these tests, showed in Figure 2, is able to measure the friction of first detachment between shaft and hub; it is composed of a rotating flange, connected to the fixed part of the test rig, where one part of the spline coupling (the hub) is fixed to. The other part of the spline coupling (the shaft) is connected to a skid that can translate in axial direction by means of linear motion guides. The rotating shaft is connected to a bar where weights can be placed, generating the force W (see Fig. 2); the force generates the torque T that loads the spline coupling. Varying the weight it is possible to change the load applied on the spline coupling. The skid is connected through a