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.