Lube-Tech
Copper compatibility may also be assessed using a conductive deposit test. For this test, multiple test cells are placed in a temperature-controlled bath. Voltage is applied and resistance is monitored until a failure point is reached, either in the form of a short caused by deposits, or an open circuit due to corrosion of the copper. These tests can be lengthy, lasting up to 1000 hours, although recent work suggests shorter tests may be possible. Another option is the traditional ASTM D130 copper corrosion test. The subjective nature of the visual rating required to assess the results of this test can present challenges in its application, although modifications to length and vapour phase exposure have shown promise.
Fluid compatibility with structural plastics can be investigated using ASTM test method D638. The method can be used to evaluate the tensile strength of plastics following their exposure to lubricants. Volume change and hardness can also be assessed. Additionally, methods are in development by the AFEV Consortium to determine the degree to which EV fluids are compatible with the coatings used to protect motor windings in wet motor applications.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.141 page 3 High speed durability
Motor speeds in electric vehicles approach 20,000 to 30,000 rpm. Many platforms use a gearbox to reduce the speed from the electric motor to the wheels. Use of a single fluid in these applications means that the lubricant must work equally well in both the high speed, low torque condition at the motor and at the low speed, high torque condition at the output of the gearbox. Conventional test methods such as the FZG tests for scuffing and pitting, the L-37-1 (low speed, high load), and L-42 (shock load) axle lubricant tests could be applied to the reduction gears, but there are currently no standardised automotive test methods for evaluating gear or bearing performance at high speeds. Evaluation of high speed lubricant performance is one of the tasks of the AFEV Consortium.
Oxidation and aeration resistance Electric motors provide an ideal environment for EV fluid oxidation. Catalyst materials present in the motor (insufficiently coated copper leads or windings, for example) and elevated temperatures may accelerate oxidation reactions. This can lead to increases in fluid acidity, viscosity, sludge, and varnish formation.
There are a number of existing tests used to assess oxidation, including the Aluminum Beaker Oxidation Test (ABOT), DKA Oxidation Test (CEC L-48), L-60-1 (ASTM D5704), and Indiana Stirring Oxidation Test (ISOT). These methods were developed for conventional fluids and may not have direct applicability to the materials and operating temperatures of electrified vehicle drivetrains. It remains to be seen if additional methods will need to be developed that are specific to EV fluids.
Figure 1: Test methods are in development to determine the degree to which EV fluids are compatible with the coatings used to protect motor windings.
Aeration refers to the condition in which air is entrained below the surface of a fluid. This differs from foam, which exists as air present at the fluid surface. Aeration is undesirable in a drivetrain fluid, as it can cause reduced heat transfer, increased
26 LUBE MAGAZINE NO.170 AUGUST 2022
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