The base stock of a hydraulic fluid makes up the bulk of what the fluid actually is.
Roughly 99% of the fluid is made up of the base stock, with the remaining 1% consisting of additives.
Hydraulic fluid base stocks come in three basic types.
Petroleum based fluids are the original, and still by far the most popular choice in hydraulic fluids.
These fluids are preferred because of their relatively low cost and high availability.
They also tend to have good viscosity properties, are good lubricants, and work well with many additives.
Bio-degradable fluids are used in situations where the environmental impact of a fluid leak is a concern.
The most common bio-degradable fluid base stocks are made from canola oil.
While these fluids generally perform well, they are not as versatile as petroleum based fluids.
They break down faster due to oxidation, do not function well at low temperatures, and do not combine well with some additives.
Fire-resistant hydraulic fluids (FRHFs) sacrifice performance to gain additional protection against burning.
This is especially important in situations where there are readily available ignition sources, for example, in a foundry or steel mill.
These fluids come in a few different varieties.
The different formulations provide different levels of resistance to burning.
Some will not be able to start burning at all, and may even provide limited fire extinguishing properties.
Others can light on fire but will not be able to sustain burning. Once the ignition source is removed, the fluid stops burning.
Each variety has different trade-offs.
For example, hydraulic systems using fluids containing water suffer from higher rates of corrosion and oxidation.
And hydraulic systems using fluids containing esters need special seals and gaskets.
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