Machinery Matters: By Larry D. Rumbol
I once had a charming aunt who used the word hydraulics in place of far more colorful language when something had gone unexpectedly wrong. Little did she know, as an expletive deletive, it was rather apt as hydraulic systems rarely fade into failure and often extract the worst language after a sudden and unexpected refusal to operate. Rather like a faulty parachute – it works or it does not. No half measures. One might shake a vending machine into submission but a hydraulic system? Never.
So why is that?
It is no secret that I love an analogy, and especially a human one. The (basic) vital components of a hydraulic system are hydraulic oil (fluid), a power pack (pump), valves, filters and actuators. (This is rather broad brush as there can be accumulators and heat exchangers and many other ancillaries.) Let us consider the power pack as the human heart pumping the oil/fluid (the blood) and the muscles as the actuators that make something move.
Hydraulic oil was a petroleum-based product developed in the late 19th century. This progressed into a much wider family of incompressible hydraulic fluids with base stocks that are water-based, vegetable-based, mineral-based or completely synthetic. Each of these has that key quality that it is incompressible and so under very high pressure will transfer that energy to move a cylinder or actuate a mechanical system. It also has additional properties relative to the application of work required of it.
Additives allow stability in the presence of heat, water, oxygen and other elements. Viscosity improvers exist, as do EP additives, where the fluid is under Extreme Pressure. As hydraulic oil is like blood touching moving parts, anti-wear additives play an important role to protect machinery as does its lubricity and heat-transfer properties.
We can easily see how hydraulic fluid has quite a role to play, and it is essential that the right hydraulic fluid is used for the purpose intended. Note well: There is no generic hydraulic fluid.
On yachts, hydraulic systems play a vital role in myriad equipment and can be a ship stopper when failure occurs: stabilizers, cranes, winches, hatches, ramps, lifts, thrusters, gearboxes, to name only a few. For each of those pieces of equipment, a manufacturer will have precisely decided upon a hydraulic oil to match the work required of it in the conditions expected to be encountered. The manufacturer will also indicate a fluid cleanliness code. Use the wrong oil at your peril. There is no one generic grade. I know I already said that, but it is worth repeating as quite a few seem to think otherwise.
When troubleshooting a system failure, look for obvious mechanical defects. At the same time, check fluid levels, then check fluid condition, and go through the circuit schematic. All this is obvious, of course, but can result in parts being changed when not actually required (unless obviously faulty). What is vital and oft overlooked is the fluid condition. Like blood, it is a vital indicator as to why failure could have occurred, especially if it is contaminated. Note that this may well be from an outside source.
Hydraulic fluid cleanliness is far more important than lubricating oil cleanliness. Lubricating oil is meant to carry impurities in suspension. Hydraulic fluid is primarily an actuating fluid with lubrication a secondary consideration. Consequently, there are cleanliness reporting codes applied to hydraulic fluids by the equipment manufacturers to ensure ultimate reliability. Specifically, let us consider the well used cleanliness reporting code ISO 4406.
But first, we must touch on how cleanliness and contamination are measured. First principles of elemental spectrographic oil analysis are essential as are analyzing viscosity, water, oxidation, insolubles, TAN and last but by no means least particle counting.
Particle counting is the least understood yet perhaps one of the most vital elements of hydraulic fluid condition monitoring. It is important to understand the basic methodology. Particle counting was developed in the 1970s, and there are three basic methods:
- Optical Microscopy (ISO4407). This is as it is written, viewing particles under a microscope and manually counting them. If time is of no consequence (and thereby cost), it can be an accurate method but is rather outdated and reliant on operator accuracy, which can rarely be calibrated.
- Automatic Optical Particle Counting (ISO 11500). This is machine-based technology and is available at every level of sophistication, accuracy and cost. Traditionally using either white light or a laser to create an interference, particles pass across the light source and thereby create a value (this is a highly simplified explanation). However it can be imagined that not all particles are a uniform size when creating what is effectively a shadow and so an algorithm is used (Equivalent Spherical Diameter) to overcome this. As always, sample integrity is key as such minor additions as air bubbles can adversely affect the result.
- Pore Blockage Particle Counting (BS3406). This method uses variable pressure and calibrated membranes at predicted particle sizes measuring aggregate levels linked to a software algorithm. The method does avoid certain false positives caused by air and especially if the liquid is dark, making optical measurement difficult. However, it does not have the same dynamic range as an Optical Particle Counter.
The cleanliness reporting code covered under the ISO standard ISO 4406:99 measures particles in three categories: greater than 4 microns, greater than 6 microns and greater than 14 microns. Each of these values is in one milliliter of fluid sample. The number of particles in each size category should be counted with an absolute value relating to an ISO code. (Table available on application to author.)
As with most things in life, other standards are available — for military, aviation and other specialist applications — but ISO 4406 is widely used. This cleanliness code will be specified by system manufacturers for end users to maintain and apply to the fluid.
So why particle counting and not just spectrographic elemental analysis for hydraulic fluid?
It is all about fluid cleanliness and the very high pressures these systems work under where the presence of contamination in the fluid spells failure. Once contaminant particles are identified, further analysis can be undertaken to establish their source. It cannot be over emphasized that fluid cleanliness and freedom from contamination are paramount. It should not be assumed that “straight from the drum” virgin oil is clean, either. There is a cleanliness standard for new hydraulic oil that is not always met. Be sure of the source and their storage conditions.
High-quality filtration should never be forgotten either, and we do get what we pay for. I once met an engineer who demeaned filters as just a “sock in a tin” and further berated XYZ’s premium-priced filters as budget breakers as they were always clogging. There was a look of incredulity when I explained that was a good thing … and maybe he should look at … . Well, that’s another story.
Maybe my dear aunt was wrong; hydraulics should never be a dirty word.
Larry D. Rumbol has 40 years of expertise in marine condition monitoring and is marine business development manager with Spectro | Jet-Care in the United Kingdom, United States and Switzerland. Comments are welcome below.