Lube-Tech
- Impossibility to drain out the previous lubricant when switching lubricant types
It begins with a vigilant inspection, looking for symptoms of incompatibility, side effects and/or impaired performance. This is where oil analysis can play a vital role to evaluate the degree of incompatibility.
4.1 Signs of incompatibility The signs of incompatibility are: -
Stratification of the oil, flocculation, insoluble formation, deposit, gelling, silting
- Reduced filterability or filter blockage, stable foam, air release deterioration
- Loss of properties & performances, additives inefficiency. The equilibrium of the well-balanced selection of additives risks being disturbed
Integration of a higher quality lubricant can lead to a loss of properties and may have an unfavourable impact, meaning the mixture can be worse than the performances of the initial lubricants.
The laboratory approach is an evaluation of hydraulic fluids compatibility step-by-step. If one step fails, It’s not required to go further. Otherwise, it is recommended to check the next steps.
4.2 Heating and cooling cycle of mixtures The compatibility of mixtures of hydraulic fluids are examined using new lubricants and/or the effect of combining new (replacement) lubricant with in-service (original) lubricant in the system.
The proportion (% weight per weight (w/w)) when using two new hydraulic fluids is often :10/90 – 50/50 – 90/10. When the in-service fluid is used to check the compatibility, the mixtures often used are 2% (w/w) of original fluid with 98% (w/w) of replacement fluid and 10 % (w/w) of original fluid with 90% (w/w) of replacement fluid.
26 LUBE MAGAZINE NO.166 DECEMBER 2021
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.137 page 4 Other proportions may also be used.
As part of the procedure, the pure oils and mixtures are heated in an oven for a fixed period of time and then remove and cooled down before the selected tests are carried out.
In ASTM D 7752, the recommended temperature is 70°C +/- 2°C following by a cool down for 24 hours in the dark.
Some other conditions may be applied (from 24 hours to 336 hours and from room temperature to 204°C).
4.3 Step 1: visual inspection and filterability The first step after the heating and cooling cycle is a visual examination and filterability tests (the compatibility index).
The visual examination checks for separation, flocculation, stratification and precipitation, which are all signs of immiscible fluids.
A mixture of incompatible fluids may produce a substance that is markedly inferior to its constituents. Even in identical base stocks, the formation of a precipitate may occur as a result of additive interactions.
The ability of a hydraulic fluid to pass through fine filters, without plugging them, is known as its filterability.
In this practice, compatibility will be determined using ISO 13357-1 (ASTM D 7752) or ISO 13357-2 filterability test methods. Since hydraulic systems utilise fine filtration to protect components from wear, incompatibility often exhibits itself as premature filter plugging.
The membrane used must be compatible with the type of fluid.
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