The results obtained in the starvation tests suggest that the natural supply conditions in grease lubrication are affected by a complex set of interdependent variables, including lubricant distribution, time, speed, and the dynamic nature of the lubricating film. Therefore it is very difficult to assess the way in which they individually influence the lubricant supply to the contact inlet. Grease lubrication under starvation conditions is a complex topic, and its proper treatment is beyond the scope of this study. These results were used to assess the performance of the custom greases in presence of different conditions of lubricant supply than fully flooded, and verify whether some of the trends observed in fully flooded conditions could be confirmed also in starved conditions. The aim was to simulate more closely the behaviour of the greases in a real bearing, where no mechanism of forced grease supply exists. However, it should be noted that the real conditions of operation of grease in a bearing are still rather different from this ball-on- disc starvation test, due to the presence of many factors that change the grease distribution and its supply to the rolling contacts152. Nevertheless, a basic attempt can be made to interpret the observed results. Several friction stages were identified under the nominally starved conditions (Figure 6.15 and Figure 6.16):
in the ‘severe starvation phase’, speed is too high to either retain the lubricant layer in the rolling track, or allow enough time for the bled oil to reflow back to the contact and feed it58. This condition leads to a rapid loss mechanism that soon makes the contact relatively dry. In accordance with literature data82,20, this occurs at higher speeds with greases of
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lower base oil viscosity. The maximum speed at which this phase was found is 500 mm s- 1 for the LP1 grease. This might seem a low value in comparison to typical real bearing speeds. However, it is important to consider that, despite the general absence of an external mechanism that forcedly pushes the grease into the rolling contacts of a bearing (i.e. the lack of an equivalent for the ‘grease scoop’ used in the fully flooded tests), the supply conditions, nevertheless, could be much less severe than in the ball-on-disc test employed in this study. The availability of lubricant in bearing contacts can be enhanced by a number of mechanisms, induced by the presence of a generally different contact geometry, the cage and the occurrence of vibration77;
The ‘controlled starvation phase’ suggests a monotonic thinning of the lubricant film. Since this occurs at constant speed, it is likely to be due to the displacement and removal of the deposited lubricant film by the overrolling action of the ball;
In the ‘recovery phase’ a decrease in friction can be associated with replenishment. This is likely to be in the form of either bled oil released from the grease, made available from the mechanical degradation of the grease which creates more mobile lubricant108, or bulk grease resupply;
In the ‘semi-starved steady phase’ a stable friction value is obtained. However, this value of friction is evidently higher than in fully flooded conditions, suggesting mixed lubrication conditions. This phase is likely to be governed by the presence of a constant film thickness, which could be due to the formation of a static layer of lubricant deposited on the rolling track, a stabilised rate of reflow of bled oil from the sides of the contact, or their combined effect3;
The ‘low-friction steady phase’ indicates the presence of a film which is sufficiently thick to completely separate the surfaces, and thus to provide friction values comparable to the ones obtained in fully flooded conditions. Therefore in this case the supply conditions are likely to be equivalent to the fully flooded case.
As previously found in earlier studies88, it is seen that a wide range of supply conditions exists between the extremes of fully flooded and severe starvation, probably encompassing replenishment mechanisms through bulk reflow and oil bleeding.
A starvation test was carried out also with LPE, since a remarkable difference in the fully flooded friction and film thickness behaviour was observed between this grease and the LP greases. In line with the results seen in the fully flooded tests, LPE gives higher friction than LP1 at the lowest speeds employed in the starvation test, i.e. 50 mm s-1 and 100 mm s-1. It was observed
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that, in fully flooded conditions, the friction behaviour of the base oils contained in LP1 and LPE is very similar. This suggests that, in starved conditions, the lubricant supply and consequent friction behaviour can be either driven by oil reflow, or by the low-speed thick films. In particular, oil reflow may be the main mechanism in LPE at 50 mm s-1 and 100 mm s-1, as this grease has already transited to the ‘high speed’ region at these speeds. Conversely, the lower friction observed in LP1 indicates that this transition has not occurred yet. The low-speed film formed in LP1 was retained in the rolling track for the relatively long duration of the test, and consequently provided lower friction. Oil reflow may be the leading mechanism responsible for the supply conditions and friction behaviour of both greases at 200 mm s-1 and 500 mm s-1, where both have transited to the ‘high speed’ operating region. Indeed at these speeds the behaviour of LP1 and LPE is practically the same.
While in fully flooded conditions the greases friction coefficient did not appear to be affected by oleic acid, under starved conditions oleic acid is seen to reduce the speed at which starvation occurs. With limited lubricant supply the oleic acid might encounter more favourable conditions for interacting with the metal surfaces, with consequent formation of a low-shear strength boundary layer at the contact interfaces. It has been reported that additives can be particularly reactive in starved conditions, and that they might limit the deposition of thickener in the track108’76. This might possibly explain the earlier starvation observed in the additised grease.