Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


External grease lubrication of a polymer/steel interface is used in Electric Power Steering (EPS) systems. EPS has displaced hydraulic assist steering systems in the small and medium passenger car market because of its smaller package size and weight, high reliability, low noise/ vibration and low level of ‘harshness’ feel to the driver. EPS systems are a low parasitic energy replacement for hydraulic power steering systems, which together with reduced weight, has rapidly penetrated the market.


One main design of EPS systems is the Column Drive system where the electric actuator is close to the steering wheel within the cabin of the vehicle, Figure 4a, set out diagrammatically in Figure 4b.


Figure 4a. EPS Column Drive Figure 4b. Diagram of EPS Column Drive


The EPS gearbox, Figure 5, has a composite steel hub/nylon gear on the steering column driven by a worm gear, seen at the bottom of the housing shown in Figure 5. The worm gear is driven by a motor instructed by the Electronic Control Unit (ECU) from directonal and magnitude signals generated by the


steering wheel, as shown diagrammatically in Figure 4b.


Figure 6. Friction Coefficients for Nylon/Steel Interface, Unlubricated, and Lubricated by Synthetics and Fluorinated Greases


Internal, or intrinsic, lubrication to reduce polymer friction uses, ideally, a polymer material with a low friction coefficient without external lubrication to which lubricants are added at the injection moulding, extrusion or casting stage. The melting point of the polymer limits the choice of the oil lubricant. Solid lubricants such as molybdenum disulphide, MoS2


, and small amounts of PTFE


are used. Solid and liquid lubricants are added at the injection/ extrusion or casting stages of final polymer production.


Figure 5. Internal View of EPS GearBox


For cast polymer nylons, the lubricant is added at the molten monomer stage before polymerisation. Efficient mixing disperses the lubricant as a uniform distribution within the liquid monomer which is captured and maintained by subsequent polymerisation. Liquid lubricants are dispersed as uniform microcapsules within the polymer mass. In use, wear and applied force within the polymer/metal contact will break a microcapsule of lubricant which lubricates that contact.


LUBE MAGAZINE NO.135 OCTOBER 2016 35


No.106 page 4


Figure 5 shows the internal view of an EPS GearBox, Looking down the Axis of the Steering Column. The Steel Hub/Nylon Tyre Composite Gear has a peripheral yellow layer of grease, (the steel worm is seen at the bottom of the gear).


The nylon polymer gear/steel worm interface in the EPS gearbox is lubricated by grease, the nature of which determines the friction coefficient, Figure 6. The baseline for consideration is the un-lubricated nylon polymer/steel interface with a friction coefficient of 0.32. Synthetic lubricant-based greases 1 and 2 reduce that friction coefficient to between 0.065 – 0.075. The use of fluorinated greases to lubricate polymers is complicated – one fluorinated grease increases the nylon polymer/steel friction coefficient to 0.15 whereas another fluorinated grease formulation, fluoro-2, reduces the friction coefficient down to 0.04.


A further point is that the fluoro-2 grease formulation has a smooth plot in contrast to the other friction-time plots, a significant consideration for the reduction of Noise-Vibration- Harshness (NVH) in the EPS gearbox. It is essential to test several types of grease to find the optimum formulation for lubricating polymers.


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