Fig 1A: A dumper working at a construction site SOS Lab
Colour change of any lubricating oil plays a very important role towards performance of the component. It clearly indicates that the lube oil is beginning to break down which has a direct consequence on the machine performance.
The intention of this article is to investigate any correlation between oil analysis data and change in colour of the oil. It will be limited to non-engine oils (differential, final drive, etc) because all used engine oils are black.
Experiment
Oil analysis is everyday activity in our scheduled oil sampling laboratory (SOS Lab). Our laboratory is equipped with Atomic Absorption Spectroscopy (AAS for wear metal analysis), Fourier Transformed Infra red Spectroscopy (FT-IR for oil condition monitoring). Both are manufactured by PerkinElmer. Ferrous Debris Monitor (FDM) measures iron and other magnetic particles irrespective of
Lube oil analysis and colour comparison Introduction
Fig 1B: A track type tractor working at a coal mining area
particle size. AAS is calibrated using calibration standards available from VHG Lab, UK. AAS has size limitation, so FDM test slate is used as a back-up support. Oil samples are extracted from machines working at site by our service technicians using standard procedures.
Results and discussion Table1 shows the results of differential oil samples of five dumpers of Caterpillar make. These are deployed at one of our remote construction sites. A striking relationship is observed when the results are compared for different oil samples. Extent of oxidation increases with increasing concentration of wear metals with corresponding darkening of colour. This means that extent of oxidation, deepening of colour and concentration of wear metals, are all directly related.
In order to understand the fact, we turned to another machine, a track type tractor, deployed in a coal mining area. It has two final drives, rear left & rear
right, RL & RR respectively. Pictures of dumper and tractor are displayed in Fig 1A and 1B. Two compartments of the same machine have different results. The same correlation between extent of oxidation, amount of wear metals and colour is observed. The results are depicted in Table 1, Fig 2 & Fig 3 respec- tively. These facts are strongly supported by FDM results which monitor both small as well as large particles. On the other hand, AAS monitors particles of size ≤ 10 microns. Dominant wear mode can be predicted on the basis of these data. Smaller size particles are predominantly rubbing particles. Silica is the main culprit. Iron & chromium generation is directly related with silica. It abrades critical components of metal surfaces generating small particles detected by AAS. When silica is trapped between two metal surfaces (three body abrasion), due to uneven load distribution, some large chunky particles are dislodged from the metal surface. This is the onslaught of fatigue wear.
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LUBE MAGAZINE No .103 JUNE 2011
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