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Hydraulic fluids

Fluids used in hydraulic systems serve multiple functions for the system performance:
  • transfer power by flow under pressure acting on moveable parts
  • lubricate surfaces in contact and relative motion – hydraulic cylinder components and seals, as well as other system components such as pumps and valves
  • prevent corrosion of components
  • cool the system, by carrying heat from areas of high load, motion or turbulence and spread it to the entire volume of the system including reservoir tanks and cooling equipment
  • clean the system by carrying contaminants and wear particles to filter bodies or settling areas
The fluids used in hydraulic systems come in various chemical compositions and viscosity grades as suited to specific applications.
Viscosity is a measurement of the thickness of a fluid or the resistance to flow. Seal performance is affected by the viscosity of the fluid and changes to the viscosity during use. Most typical hydraulic fluids exhibit decreased viscosity with increasing temperature and increased viscosity with increasing pressure.
The most commonly used media in hydraulic systems are mineral oil based fluids with various additives. However, a variety of alternative fluids may be encountered in special applications. For example, biodegradable fluids such as synthetic (HEES) or natural esters (HETG) and polyalphaolefines (PAO) may be used to reduce environmental impact in the event of accidental spills. Flame retardant fluids based on water or synthetic esters may be safely used in confined spaces or where the hydraulic system is used in close proximity to ignition sources. The data, specifications and recommendations in this section are for common mineral oil fluids. For guidance on specifications of sealing systems for alternative fluids, contact SKF.
The chemical composition of hydraulic fluids can impact the seal life and performance depending on compatibility with the seal material(s). Absorption and reaction of the seal material(s) with non-compatible fluids can cause, for example:
  • changes in seal material volume – increased “swelling” or decreased “shrinking” and their respective impacts on seal contact force and friction
  • hardening and embrittlement of the seal material
  • softening, loss of strength or dissolving of the seal material
  • degradation of the polymer chains or crosslinking, causing the material to fatigue or lose resilience
  • discoloration of the seal material
Generally, these changes are accelerated by higher temperature. To avoid these changes and the resulting damage to seal function and life, careful consideration should be taken to ensure compatibility between the fluid and all seal materials, as well as the temperature and mechanical loads on the seal material. SKF has a long history and extensive database of test results concerning compatibility of various seal materials and fluids, as well as unparalleled expertise in developing materials to meet customers’ needs for chemical resistance of seal materials.
Table 1 summarizes the compatibility rating for the most important fluids and materials used in the fluid power industry. For materials not listed, contact SKF. Table 1 provides general guidelines for new, clean fluids. Fluids vary by manufacturer, additives and contaminant levels. Materials vary by specific compound. The guidelines cannot substitute for testing the compatibility of a seal in the actual fluid and under actual operating conditions. Temperatures higher than specified in table 1 can lead to degradation of the basic fluid or its additives. This can cause deterioration of the seal material. For applications where higher temperatures are required, contact SKF.
In addition to the specified hydraulic fluid, seal materials can be attacked by exposure to other fluids from other parts of the machinery (e.g. greases, fuels, coatings), environmental factors (e.g. humidity or radiation) and degradation and reaction with the fluids, additives and contaminants in the system producing additional chemicals.
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