As I often remind hydraulics users here and in my work in other places, energy contamination, a.k.a. heat, is public enemy #1 of every hydraulic system. And it's a bigger threat to system longevity and reliability these days than particle and water contamination, due to the widespread awareness and adoption of modern filtration technologies.
Adequate lubrication of hydraulic components and efficient power transmission are both dependent on appropriate oil viscosity. If system operating temperature is allowed to exceed that required to maintain viscosity at around 20 centistokes, the likelihood of boundary lubrication conditions occurring, resulting in friction and wear, increase dramatically.
The temperature at which this point is reached, depends on the viscosity grade (weight) of the oil used and its viscosity index, which is the oil's rate of change in viscosity with change in temperature. In other words, the critical, danger temperature as far as viscosity is concerned, can be relatively low or high depending on the oil being used in the system.
But when it comes to oil, seal and hose life, the top-end danger temperature is less of a moveable feast. According to Arrhenius's Law, for every 10°C increase in temperature, the rate of reaction doubles. The chemical reactions we're concerned with in so far as hydraulic oil life is concerned are oxidation - due to the presence of air; and hydrolysis - due the presence of water. So the hotter the oil, the faster the rate of these reactions, and exponentially so.
The elastomers used to make hydraulic seals and hoses are improving all the time. But oil temperatures above 82°C accelerate the degradation of most of these polymers. In fact, according to seal manufacturer Parker Pradifa, operating temperatures 10°C above recommended limits can reduce seal life by 80% or more.
Similarly, according to hose manufacturer Gates, exposing a hydraulic hose to an operating temperature 10°C above its recommended maximum, cuts its expected service life by 50%. As you can see, a single, over-temperature event of significant magnitude can damage ALL the hoses and seals, 'crack' the oil and result in scuffing and wear of lubricated surfaces.
So what IS the operating temperature danger number for hydraulic systems? Well for reasons explained above, to avoid compromising oil, hose and seal life, I always work on a maximum of 85°C (185°F).
HOWEVER, to avoid compromising viscosity, lubrication and efficiency, a much lower temperature may have to be respected; from 85°C (185°F) down to around 50°C (122°F) or perhaps lower, depending on the grade and type of oil being used, and by extension, the climatic conditions the machine operates in. If you own or are otherwise responsible for the upkeep of hydraulic equipment, not knowing what this (lower) number is can be a very costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.