3 BIG Problems Caused By Hot-Running Hydraulics

hydraulic power unitA colleague who works for a major hydraulic component manufacturer recently told me his company’s original equipment manufacturer (OEM) customers are demanding hydraulic components which can handle operating temperatures in excess of 110°C. And this number is not for margin of safety – it’s for continuous operation.

Trouble is, heat is the biggest enemy of a hydraulic system. Bigger than particle and water contamination these days — due to the widespread understanding and adoption of modern filtration technologies.

And so in my view, a hydraulic system running at 110°C is a hydraulic system from hell – figuratively speaking of course. And so to intentionally design such a system is like ‘dancing with the devil’. Let’s consider some of the reasons why:

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.

When asked about the issue of oil life, my colleague from the hydraulic component manufacturer admitted that when lab testing hydraulic components at temperatures of 110°C and above for extended periods, the oil had “turned black as ink and smelt unmentionable”.

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. In case these numbers don’t mean much to you, this is not the type of hydraulic oil you can just call in and pick up from your local oil supplier!

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 results in leaking seals and hoses.


Increasing the temperature at which a hydraulic system is expected to operate does nothing to enhance machine performance. It only compounds a host of reliability issues which cannot be solved with current technologies. So it’s not progress – it’s regress. And this false progress will certainly come at cost – to the hydraulic equipment owners who operate these hot-running machines. 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.

5 thoughts on “3 BIG Problems Caused By Hot-Running Hydraulics

  1. Thanks for lovely blog, If we control the internal leakage in equipments ,we can control the heat too, i think a big problem is internal leakage inside the pumps and motors ,cylinders,we must save the hydraulic oil from small particles which causes the wear between running parts , and this wear is cause of internal leakage, ,Aslam

    • While a single over-temperature event of sufficient magnitude may ‘crack’ the oil, high temperature operation over a prolonged period is more likely to destroy it.

Leave a Reply

Your email address will not be published. Required fields are marked *