As I mentioned in my previous post, energy contamination, a.k.a. heat is THE biggest enemy of a hydraulic system. Bigger than particle and water contamination these days, mainly due to the widespread understanding and adoption of modern filtration technologies. And here are 3 reasons why a hot-running hydraulic machine is an unreliable hydraulic machine.
#1 Oil Life
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.
By way of illustration, if you pour some cooking oil into a glass, it'll take days, even weeks before it darkens in color - a sign of oxidation. But pour the same amount of cooking oil into a frying pan - which gives the oil a large contact area with air - then heat it up, and the oil will go black in a very short space of time.
#2 Oil Viscosity
The operating viscosity of hydraulic oil is crucial for both adequate lubrication and efficient power transmission. The higher the maximum expected operating temperature, the wider the temperature operating window becomes. And the wider the temperature operating window, the more difficult it is to maintain oil viscosity within both allowable and optimum limits.
For example, consider a system with a cold start temperature of 5°C and a maximum operating temperature of 110°C. To maintain viscosity between 800 centistokes at cold start and 25 centistokes at maximum operating temperature would require an ISO VG150 oil with a viscosity index of 229. And this is not the type of hydraulic oil you can just call in and pick up from your local oil supplier!
#3 Seal and Hose Life
Like everything else, 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 one seal manufacturer, operating temperatures 10°C above recommended limits can reduce seal life by 80 percent or more.
And the constant cycle of heating and cooling, a process known as aging, is more severe when temperature extremes are greater. Aging causes these polymers to lose their elastic properties. This eventually results in leaking seals and hoses.
Bottom line: allowing a hydraulic machine to run hot is a costly mistake, and one that a lot of hydraulic equipment users make. There are six more to avoid, which I explain in "6 Costly Mistakes Most Hydraulics Users Make…" If you haven't read it yet, download your free copy here.