Fluid power tips
1st Quarter 2012
Hydraulic systems & components
How much money does your maintenance department have in its budget to pay for replacing hundreds, if not thousands, of litres of hydraulic fluid every year?
Is there room left in that budget to pay for pumps, valves and other hydraulic components?
There are three general causes of hydraulic fluid degradation: water contamination, particle contamination and heat contamination. Although considered a property of matter, and not physical contamination in itself, heat is the most detrimental to hydraulic fluids. Generally, reasonable amounts of water and contamination can be removed from oil, but the effects of cooking your hydraulic oil are irreversible.
The most drastic effect of hot oil is the loss of viscosity, which essentially means the fluid thins out and no longer lubricates. This can cause catastrophic damage, but can be avoided by switching to a higher viscosity fluid. It is not going to lower the temperature of your oil, it will just allow it to stay thick enough to lubricate when it gets piping hot.
Oxidation is the key effect that ruins your oil. For every 5°C the oil temperature increases, the rate of oxidation doubles. As the rate of oxidation increases, so does the acidity and viscosity of the oil. The oil can form sludge, gum or varnish which is also harmful to your hydraulic system. At this point, your hydraulic system could be in ruins, costing tens of thousands of rands in damage and maintenance resources.
The most obvious solution is to design the system to be efficient and produce very little heat. The design of an hydraulic system is one of the key factors in determining how efficient the system is, and how much energy is wasted as heat.
The hydraulic rule of thumb is that any energy being put into the system that is not being used to create useful work is wasted as heat. So if the system has 10 hp at the shaft of the electric motor turning the pump, and the hydraulic motor is putting out 5 hp, then there is 5 hp worth of energy wasted as heat. That equals over 13 Mj/hour of added heat.
There are some general tips that always work. First choose an efficient pump design, such as a load sensing piston pump. Not only is the architecture of the pump highly efficient, but the load sensing option wastes very little energy because it only puts into the system the amount of energy that is being asked of it.
On top of this, ensure the various valves in the system are efficient and sized appropriately. This way, little energy is wasted to move fluid through those components. Lastly, pick a motor design that is equally efficient, such as a piston motor. Bosch Rexroth’s new versions of the A4VG pump and A6VM motor have a combined efficiency in the 90% range. This means 9 hp output from the hydraulic motor is realised from the 10 hp electric motor input.
Oh, and if all else fails – install an oil cooler.
Josh Cosford is a Certified Fluid Power Hydraulic Specialist at The Fluid Power House in Canada. He runs an e-zine called Fluid Power Tips and is a guest columnist for an international engineering publication. He can be contacted at [email protected]
Further reading:
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