Although more costly, there are many reasons to use synthetic hydraulic fluid.

As the demands placed on hydraulic systems change, with industry demanding greater efficiency and speed at higher operating temperatures and pressures, more companies are beginning to use synthetic hydraulic fluid.

However selecting the best hydraulic fluid requires a basic understanding of each particular fluid's characteristics. An ideal fluid would have these characteristics:

• thermal stability 
• hydrolytic stability 
• low chemical corrosiveness 
• high anti-wear characteristics
•  low tendency to cavitate
•  long life 
• total water rejection 
• constant viscosity, regardless of temperature
• low cost. 

Although no single fluid has all of these ideal characteristics, it’s important to select one that is the best compromise for a particular hydraulic system application.

Arguments substantiating the use synthetic hydraulic fluid.

The main reason to use synthetic hydraulic fluid is when mineral hydraulic oil cannot meet the requirements, even with modified additive packages.

Synthetic hydraulic oils have the following benefits over mineral hydraulic fluid:

• Thermal stability 
• Resistance to oxidation 
• Stable viscosity across a wide operating temperature range 
• Good flow properties at low temperatures 
• Low volatility at high temperatures 
• Flame retardant 
• Reduced environmental impact 

Each of the following factors influences hydraulic fluid performance:

1. Viscosity - Maximum and minimum operating temperatures, along with the system's load, determine the fluid's viscosity requirements. The fluid must maintain a minimum viscosity at the highest operating temperature. However, the hydraulic fluid must not be so viscous at low temperature that it cannot be pumped these are qualities that attract many manufacturers to use synthetic hydraulic fluid. 
2. Wear - Of all hydraulic system problems, wear is most frequently misunderstood because wear and friction usually are considered together. Friction should be considered apart from wear. Wear is the unavoidable result of metal-to-metal contact. The designer's goal is to minimize metal breakdown through an additive that protects the metal. By comparison, friction is reduced by preventing metal-to-metal contact through the use of fluids that create a thin protective oil or additive film between moving metal parts. 
3. Foaming - When foam is carried by a fluid, it degrades system performance and therefore should be eliminated. Foam usually can be prevented by eliminating air leaks within the system. However, two general types of foam still occur frequently: 

• surface foam, which usually collects on the fluid surface in a reservoir, and 
• entrained air. 

Surface foam is the easiest to eliminate, with defoaming additives or by proper sump design so that foam enters the sump and has time to dissipate.

Entrained air can cause more serious problems because this foam is drawn into the system. In worst cases, it causes cavitation, a hammering action that can destroy parts. Entrained air is usually prevented by properly selecting the additive and base oils. Caution: certain anti-foam agents, when used at a high concentration to reduce surface foam, will increase entrained air.

1. R & O - Most fluids need rust and oxidation inhibitors. These additives both protect the metal and contain anti-oxidation chemicals that help prolong fluid life. 
2. Corrosion - Two potential corrosion problems must be considered: system rusting and acidic chemical corrosion. System rusting occurs when water carried by the fluid attacks ferrous metal parts. Most hydraulic fluids contain rust inhibitors to protect against system rusting. The tests used to measure this capability are ASTM D 665 A and B. To protect against chemical corrosion, other additives must be considered. The additives must also exhibit good stability in the presence of water (hydrolytic stability) to prevent break down and acidic attack on system metals. Companies use synthetic hydraulic fluid because of their oxidation and thermal stability - Over time, fluids oxidize and form acids, sludge, and varnish. Acids can attack system parts, particularly soft metals. Extended high-temperature operation and thermal cycling also encourage the formation of fluid decomposition products. The system should be designed to minimize these thermal problems, and the fluid should have additives that exhibit good thermal stability, inhibit oxidation, and neutralize acids as they form. 
3. Water retention - Large quantities of water in a hydraulic fluid system can be removed by draining the sump periodically. However, small amounts of water can become entrained, particularly if the sump is small. Usually, demulsifiers are added to the fluid to speed the separation of water. Filters can then physically remove any remaining water from the hydraulic fluid. The water should leave the fluid without taking fluid or additives with it. 
4.  Temperature - System operating temperature varies with job requirements. Here are a few general rules: the maximum recommended operating temperature usually is 150° F. Operating temperatures of 180° to 200° F are practical, but the fluid will have to be changed two to three times as often. Systems can operate at temperatures as high as 250° F, but the penalty is fairly rapid decomposition of the fluid and especially rapid decomposition of the additives - sometimes within 24 hours! Once again many operators use synthetic hydraulic fluid because of the ability to operate at extreme temperatures. 
5. Seal compatibility - In most systems, seals are selected so that when they encounter the fluid they will not change size or they will expand only slightly, thus ensuring tight fits. The fluid selected should be checked to be sure that the fluid and seal materials are compatible, so the fluid will not interfere with proper seal operation. 
6. Fluid life, disposability - There are two other important considerations that do not directly relate to fluid performance in the hydraulic system, but have a great influence on total cost. They are fluid life and disposability. Companies that use synthetic hydraulic fluid can achieve significant savings by extending oil change intervals. 

Using synthetic hydraulic fluid that has a long operating life brings added savings through reduced maintenance and replacement-fluid costs. The cost of changing a fluid can be substantial in a large system. Part life should also be longer with the higher-quality, longer-lived fluid.

Longer fluid life with the use of synthetic hydraulic fluid also reduces disposal problems. With greater demands to keep the environment clean, and ever-changing definitions of what is toxic, the problem of fluid disposability increases. Fluids and local anti-pollution laws should both be evaluated to determine any potential problems.

Use synthetic hydraulic fluid for extended life and special applications.

Synthesized hydrocarbon (synthetic) hydraulic fluids contain no waxes that congeal at low temperatures nor compounds that readily oxidize at high temperatures which are inevitable in natural mineral oils. Synthetic hydraulic fluids are being used for applications with very low, very high, or a very wide range of temperatures.

Synthetic fluids were initially represented by a class of chemical compounds known as phosphate esters, which are reaction products between phosphoric acid and aromatic ring-structure alcohols. These fluids are extremely fire resistant and have widespread industrial use, as well as military and aircraft service. However, their popularity has declined because of environmental, cost, and compatibility factors.

The other type of synthetic fluids in use are synthetic hydrocarbons, more specifically, polyol esters. These fluids are the reaction products between long-chain fatty acids (derived from animal and vegetable fats) and synthesized organic alcohols. These products contain additives to impart anti-wear properties, corrosion protection, and viscosity modification. Fire resistance results from a combination of high thermal properties and physical characteristics. This is the most recent category of FRHFs and has gained widespread and growing use in synthetic hydraulic fluid.
Three different base oils have been tried as environmentally safe hydraulic fluids. They are ester based synthetic oils, polyglycols, and vegetable oils (which are sometimes called "natural esters"). Synthetic esters can be formulated as biodegradable fluids with superior lubrication performance, but their high cost has limited their usage. Polyglycols — attractive because they have excellent lubricity characteristics and are usually less expensive than synthetic esters — have been used more commonly. However, polyglycols lack required biodegradability and are potentially toxic in water when mixed with lubricating additives. Vegetable oils, such as sunflower, soy, or canola oils, have excellent natural biodegradability, are in plentiful supply, and are inexpensive. They have become the most commonly used environmentally safe fluids in hydraulic systems.

The use of synthetic esters - typically polyol esters - provides better hydrolytic, thermal, and oxidative stability, and excellent low-temperature fluidity, while preserving the high biodegradability and low toxicity of the fluids.

For nearly 30 years, polyol esters have been used to formulate aviation gas turbine lubricants, which demand high thermal and oxidation stability at extreme temperatures. While a vegetable oil-based hydraulic fluid can perform between 0° to 180° F, a similar fluid based on synthetic esters can be used between 25° and 200° F. Similar to vegetable oils, synthetic esters have the tendency to swell and soften elastomers, although again, the swell should not be a concern for most hydraulic applications.

If you use synthetic hydraulic fluid it’s bound to save you time and money, but nevertheless it’s important to use only high quality products, such as those supplied by Habot Synthetic Lubricants. If you have any questions feel free to contact our professionals for expert advice.