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in the laboratory or in a component or bearing application. For an NLGI #2 grease, it is clear that comparing greases by their penetration grades will not give any indication about how they will affect efficiency. As greases run-in, the apparent viscosity typically becomes lower and the losses to churning are reduced. All greases behave differently and take different lengths of time and shear cycles to stabilise when subject to shear. The apparent viscosity of the grease is important, but not the whole answer, as shear history also plays a role. What is clear is that testing greases when they are newly applied to bearings or components, before they have chance to shear soften will give a different outcome of any energy efficiency testing. Figure 3 illustrates the shear softening behaviour of grease.


soap results in the formation of larger fibres. These thicken better than the smaller fibres but do not have the shear stability or lower bleed of the small fibres. Greases that bleed small amounts of oil in a consistent manner have been shown to give better lubrication properties than those that do not readily bleed oil (12). Having steady bleed of oil contributes to thicker EHD films being formed. In mixed lubrication, this reduces the metal- metal contact and in turn reduces frictional losses and improves efficiency. Manufacturing simple lithium soap greases with a partial quench results in a balance of yield, bleed and rheological properties leading to good lubrication and better efficiency. Lithium complex greases with that have been made with a well-controlled complexing reaction typically shear soften slightly by up to 30 penetration points. Higher levels of complexing acids produce higher dropping points but also higher thickener contents for the same NLGI grade, which can negatively affect the energy efficiency.


Figure 3. Shear softening behaviour of grease


It has been observed that urea greases with “grains of rice” or “rice pudding” structures can undergo temporary loss of consistency when subjected to shear. They can soften by as much as 120 penetration points when worked for 100,000 double strokes in a grease worker. Leaving the grease to sit without shear allows the grease to recover up to 100 penetration points. Urea greases can also have fibre-like structures similar to those of lithium soap greases. In the case of these fibre urea greases they shear soften up to 120 penetration points, but do not recover on standing. These fibrous types of ureas typically have lower thickener contents (~10%wt) than “rice pudding” urea greases (~12%wt). When this type of fibrous urea thickener was used for CVJ greases (6) they contributed to energy efficiency. The greases started with a grade #2 consistency. When pumped and assembled into CVJ joints they softened to NLGI #1. When worked in the CVJ they softened to between 0 and 00 grades. Unpublished temperature data showed that the softening contributed to a lower running temperature, reduced plunging resistance and lower vibration transmission compared to greases that did not soften such as anhydrous calcium or lithium soap-thickened greases. Traditional calcium complex thickeners used in CVJ greases were developed which softened 100 penetration points and mimicked the behaviour of the fibrous urea greases. One challenge for both of these types of grease that readily softened was keeping the greases sealed in. Improved sealing systems were developed which also allowed the use of shear stable NLGI 0 grade lithium soap greases, which also contributed to energy efficiency and reduced noise and vibration.


Lithium soap greases are also slightly different, depending on how they are made. If rapidly quenched lithium soap greases will have predominance of small fibres. These do not thicken the oil very well with poorer yields and higher soap contents but have lower bleed and better shear softening resistance. In terms of efficiency the higher soap content of quenched soap greases leads to higher churning losses. Slow cooling of molten lithium


Grease Fill Effects Another issue related to greasing is the amount of grease that is packed into the bearing or component. It is customary to pack only 20 to 50% of the available free volume of a bearing with grease. In the case of CV joints around 30% of the volume is filled. Figure 4 illustrates what happens to the running temperature and efficiency of greased bearings and components. If the grease is significantly over-packed, a thermal runaway will result and the component will fail prematurely. If the grease is slightly over-packed or a grease with too high apparent or base oil viscosity is selected, then the bearing will run hot. When bearings are packed properly, the component will heat up as it is running-in. The grease thickener will shear down and additives will react to form a protective anti-wear layer. The component will then absorb less energy as it operates and it will cool down to a stable steady state running temperature. Towards the end of the grease’s life, it is no longer able to lubricate effectively and as is seen with most grease component tests, the temperature starts to rise as the grease breaks down. With energy efficient greases, the amount of running-in and temperature rise at the start of operation is minimal. The surfaces are protected and the components have settled into stable running.


Figure 4. Grease effects on bearing running temperature Component and Bearing Testing


The only true way of measuring the efficiency of a system is to measure the energy put in and the useful energy coming out. In the case of a bearing or automotive component it is torque in torque out. Most automotive components have air flowing over them when in service and this needs to be built into the test rig used to evaluate the components. For stationary industrial components, only the ambient air temperature would need to be controlled. A schematic of a test stand is illustrated in Figure 5.


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LUBE MAGAZINE NO.129 OCTOBER 2015


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