The ‘built-in’ inefficiency of every hydraulic system:
Compression of the oil.
A fluid’s compressibility is defined by its bulk modulus of elasticity – which is the opposite of compressibility. Meaning, as the bulk modulus of elasticity increases, compressibility decreases.
Bulk modulus is an inherent property of the oil and therefore an inherent inefficiency of a hydraulic system.
The fluid in the pipeline and actuator must be pressurized, and consequently compressed, before it will move a load.
Because this compression of the fluid requires work at the input – which cannot be converted to useful work at the output – it is lost work and therefore a contributing factor to the overall inefficiency of the hydraulic system.
The larger the actuator and the faster the response time, the higher the inefficiency attributable to bulk modulus.
And in high-performance, closed-loop electro-hydraulic systems, deforming oil volumes affect dynamic response, causing possible stability problems such as self-oscillation.
Unlike viscosity index, bulk modulus cannot be improved with additives. However,hydraulic equipment users can take steps to minimize the inefficiencies and potential control problems associated with compression of the fluid.
The first is to ensure hydraulic equipment doesn’t run hot.
Compressibility of the fluid increases with temperature. Mineral hydraulic oil is approximately 30 percent more compressible at 100°C than it is at 20°C.
Of course, there are many reasons why you should never allow hydraulic equipment to run hot – most of which we’ve already discussed. Reduced bulk modulus is another one.
The second is to prevent conditions that cause aeration.
Air is 10,000 times more compressible than oil. One percent of entrained air by volume can reduce the bulk modulus of oil by as much as 75 percent.
While controlling aeration is largely a design issue – for example, the amount of dwell time the oil has in the tank – proper maintenance also plays an important role.
Dissolved air comes out of solution as temperature increases, which is another reason to maintain appropriate and stable operating temperatures.
Also, oxidative degradation and water contamination inhibit the oil’s ability to release air, often resulting in an increase in entrained air and thus compressibility.
The BEST time to carry out a maintenance and reliability audit on a piece of hydraulic equipment is BEFORE you buy it.
By starting with the end in mind, you get the reliability outcomes you desire – before the machine even gets delivered.
You specify the contamination control targets you want to achieve based on your reliability objectives for the piece of equipment.
And instruct the manufacturer to deliver the machine appropriately equipped to achieve these targets.
Based on the weight and viscosity index of the hydraulic oil you plan to use, you determine the minimum viscosity and therefore the maximum temperature you want the machine to run at.
And instruct the manufacturer to deliver the machine equipped with the necessary cooling capacity, based on ambient temperatures at your location. Rather than accepting hydraulic system operating temperatures dictated by the machine’s ‘design’ cooling capacity – as is the norm.
And we could continue by specifying things like flooded inlet for all pumps and so on. But you get the idea.
So the next time you or the company you work for are looking to acquire hydraulic equipment, begin with the end in mind.
Define your maintenance and reliability objectives in advance and make them an integral part of your equipment selection process. Craig Cook
Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.
Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system, other components could also be at fault, but this would require the services of an experienced technician.
By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, and then if necessary, each component in that area can be tested or replaced.
STEP 1 – Pump Suction Strainer
Probably the trouble encountered most often is cavitation of the hydraulic pump inlet caused by restriction due to a dirt build-up on the suction strainer. This can happen on a new as well as an older system. It produces the symptoms described above: increased pump noise, loss of high pressure and/or speed. If the strainer is not located in the pump suction line it will be found immersed below the oil level in the reservoir (point A).
Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump. The suction strainer should be removed for inspection and should be cleaned before re-installation. Wire mesh strainers can best be cleaned with an air hose, blowing from inside out. They can slso be washed in a solvent which is compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum base hydraulic oil. Do not use gasoline or other explosive or flammable solvents.
The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced. When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints (points B, E, G, H, J, and K). There must be no air leaks in the suction line.
Check the reservoir oil level to be sure it covers the top of the strainer by at least 3″ at minimum oil level, with all cylinders extended. If it does not cover to this depth there is danger of a vortex forming which may allow air to enter the system when the pump is running.
STEP 2 – Pump and Relief Valve
If cleaning the pump suction strainer does not correct the trouble, isolate the pump and relief valve from the rest of the circuit by disconnecting at point E so that only the pump, relief valve, and pressure gauge remain in the pump circuit. Cap or plug both ends of the plumbing which was disconnected. The pump is now deadheaded into the relief valve.