A colleague and I have been discussing desired hydraulic oil operating temperatures. He is new to hydraulics and wondering about them. For the most part our systems are 50 to 150 HP industrial hydraulic systems, using either ISO 46 or ISO 68 oils.

I favor running around 110 - 120F (~50C), and I prefer to keep it user 130F (~55C). However we have high temperature switches that range from 130F to 145F on different pieces of equipment. The 130F switches seem prone to nunissance alarming. We seem to start having some trouble with a couple of servos if the oil temperature approaches 150F.

Just curious, what are the prefered temperatures used by the forum members?

I consider I have a good circuit if the temperature stays between 100-120 Deg.F. After all most Viscosity is rated at 100 Deg.F. so the oil flows well but is not thinned out so it passe through every clearance like water.

I always specify oil heating in cold climates with Off-Line Filters fed by a pump that changes the total system volume every 1-3 hours. This oil movement keeps from having hot spots in the oil when using Electric Tank heaters.

On most circuits I use a Solenoid Operated Relief and a large enough Electric motor on the Filtration loop to heat the oil by raising its pressure when teperature drops below 100 Deg.F. and allow free flow when temperature reaces 120 Deg.F. When necessary a Heat Exchanger can be installed in this Off-Line Loop to take care of excess temperature from a circuit that has extra Wasted Energy.

The following is quoted from the Vickers Mobile Hydraulics Manual (one of their text books):

On page 372 under the section headed - Heat or thermal contamination.

"Hydraulic fluids are typically not considered 'high temperature' fluids. Their useful life, as well as maintenance of viscosity, chemistry and the other attributes discussed in Chapter 16, is based upon continuous operation below a critical temperature. This critical temperature is 140 degrees F. Every 18 degree F increment higher than 140 degrees effectively doubles the oxidation rate of the hydraulic fluid (petroleum based) thus cutting its useful life in half, e.g. running a system at a consistent 176 degrees F would reduce the useful life of the fluid by 75%.

Most fluid manufacturers specify optimum ranges of temperature for their prodcuts - typically 90 degrees F to 120 degrees F or 100 degrees F to 130 degrees F. even though many fluids are operated in excess of these temperature ranges. The effects of temerature on the fluid are the same, regardless of the source of heat."

From NORIA
"For most mineral oils, a general rule of thumb is that the rate of oxidation doubles for every 10°C (18°F) rise in temperature above 75°C (165°F). Because of this, synthetic oils are often required in high temperature applications to
prevent rapid oil oxidation."

In reality it depends on what is limiting the life of the oil. In mobile applications the limit is often contamination from dust or water so strict temperature control won't gain much.

ISZ