Although far from new, global warming is a real and present issue right now. And whether you believe the science or not, we’re currently witnessing evidence of what climate-change scientists have been warning us about for some time: increasing incidence of extreme weather events.
This apparent trend towards wider extremes in temperature has implications for anyone who designs, operates or maintains hydraulic equipment: the real possibility of machines having to operate at much lower cold-start temperatures and/or much higher maximum ambient temperatures, than the long-run average.
In other words, more hydraulic machines may find themselves operating in temperature extremes for which they are not equipped. And this has negative implications for machine reliability.
Because the viscosity of hydraulic oil decreases as its temperature increases and conversely, viscosity increases as temperature decreases, limits for oil viscosity and oil temperature must be considered simultaneously.
Excessively high oil viscosity can result in damage to system components through cavitation. And if the temperature drops below the oil’s pour point, damaging pressure intensification may occur in hydraulic cylinders.
Low oil viscosity causes loss of lubricating film strength. If load and surface speed remain constant, but elevated operating temperature causes oil viscosity to fall below that required to maintain a hydrodynamic film, boundary lubrication occurs with the possibility of friction, scuffing and adhesive wear.
Exhibit 1 shows how this can manifest itself in an axial piston pump. The gold-colored varnish deposits are evidence that this hydraulic system has been operating at excessive temperatures. Due to low oil viscosity, the lubricating film between piston and bore has been lost. The resulting friction has super-heated the piston causing it to expand in its bore to the point of interference. Once this happens, the resulting tensile force pulls the slipper(s) from the piston(s) — a.k.a. catastrophic failure.
With the above problems in mind, the perfect hydraulic fluid would have a viscosity index (the change in a fluids viscosity relative to temperature) represented by a horizontal line intercepting the Y axis at 25 centistokes, see exhibit 2.
Of course, no such fluid exists. And I don’t expect that such a fluid will be developed in my lifetime. But an oil’s viscosity index (VI) can be improved by using high VI base stocks such as synthetics and/or by adding polymers called Viscosity Index Improvers to the formulation.
Beyond the type of oil used, the only other way to ensure hydraulic machines are equipped to cope with ‘climate change’, is to make sure the oil’s actual operating temperature is appropriately controlled.
This means making sure adequate cooling capacity and cold-start protection is installed in the system – considering the expected change in climatic conditions.
Very few hydraulic systems have on overall efficiency greater than 80%. And it is possible for an electro-hydraulic servo system to have an overall efficiency as low as 10%! So any hydraulic system that has a large, continuous power output requires a heat exchanger.
But heat exchangers are space-consuming and expensive, so they often end up being undersized for the job. And the problem of insufficient installed cooling capacity is only compounded if the machine encounters ambient temperatures higher than expected.
In addition to the possible loss of full-film lubrication, high-temperature operation also reduces the life of the oil, seals and hoses in the system. So in a ‘Global Warming’ world, it’s better to have too much, rather then too little, installed cooling capacity.
Similarly, at the other end of the scale, to avoid possible cold-start damage, a change to a low pour point synthetic oil and/or the installation of tank oil heaters may now be required in applications where they weren’t required before.
In summary, our climate is changing. So make sure the hydraulic equipment you design, operate or maintain is adequately equipped to cope – or its reliability WILL be compromised. 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.