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Continued from page 13 Source: Kuldeep Mistry and The Timken Co., modified


Table 1: Selected Test Specifications to Wheel bearing Lubrication in Electric Vehicles


Table 1 depicts specific properties that must be tested. Other screening methods are used to ensure that a broad range of performance requirements are met: copper wire corrosion testing, high-speed foaming testing, dielectric strength and breakdown, electrical conductance and impedance, and technology for high-speed driveline test rigs. These methods are used for both dry and wet electric motors and are stated in specifics in Table 2. For EVs with wet electric motors, lubricants must be able to lubricate the gearbox and cool the electric motor.


The key question for EVs is whether low friction for range extension or cooling of the batteries and motors for safety and fast charging is the more crucial consideration? Hydrolubes, a mixture of water with thickener by polyalkylene glycols (PAGs), were shown to offer high load carrying capacities and low friction for the gears, with derived mean coefficients of friction exhibiting “superlubricity”, reducing friction by up to 82% compared to conventional gear oils and little to no increase in gear tooth bulk temperature with pitch-line velocity according to Research Centre for Gears and Gear Manufacturing (FZG) efficiency tests. In a monofluid concept in the vehicle, hydrolubes can additionally cool the batteries and motors.


Many exotic proposals to enhance the tribological profile of lubricants are under evaluation. Recent research for the development of EV oils and greases incorporates nanotechnology to strengthen the product. Recently, spherical alumina nanoparticles have been tested to aid friction reduction and copper nanoparticles have demonstrated reduced coefficient of friction of SAE 10 mineral base oil by 49%. These nano-additives aid in lubrication via the ball-bearing effect, the protective film effect, mending effect and polishing. Notably, nanoparticles slightly increase the thermal conductivity and decrease the heat capacity of fluids but especially enhance the heat transfer coefficient [W/(m²K)] by enhancing convection providing better cooling for the batteries and motors.


Table 2: Laboratory tests for electrical properties


Lithium and lithium complex grease dominate with 72% grease applications. Lithium greases have been ubiquitously used as lubricants for ICE vehicles due to their many beneficial properties such as high dropping point, corrosion protection, and moisture resistance. Lithium greases can commonly be used in EVs for lubricating wheel bearings and the powertrain. However, the inclusion of an electric motor requires dedicated lubricants to take into consideration properties such as noise reduction, electrical fields and currents generated from magnetic fields impact on the tribocontacts.


14 LUBE MAGAZINE NO.162 APRIL 2021


As well as nanotechnology, the addition of synthetic oils and titanium complex thickeners have been viable in reducing friction torque and increasing the lifespan of greases. More specifically, sulphur and phosphorus gear oil additives are commonly used to lower viscosity. Other than incorporating additives and polymers, laser surface texturing has been implemented to coat the gears.


For corrosion protection, boron compounds have been promising in enhancing anti-wear and extreme pressure properties as well as oxidation and thermal stability. Figure 3 illustrates tribo-components to monitor.


With growing concerns regarding the environment, biodegradability is a trending development. Bio-lubricants are prominent for accelerated biodegradability and low aquatic toxicities and can


Continued on page 16


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