Hydraulic systems - the Inside Story

James Whittonin Technical

Hydraulic systems have now been commonplace on grounds maintenance equipment for many years, and not just on large equipment. From pedestrian machines to high-end ride-on equipment and tractor mounted mowers, hydraulic drive systems seem to have become the designers' choice. Jim Whitton continues his machinery series.

I can understand this because, generally speaking, hydraulic drives are very reliable, require far less running maintenance than conventional mechanical belt and chain drives and, maybe most significantly, are able to sustain more operator misuse without breaking. However, as with all things, they do have a "down side". Because these systems require very little day to day maintenance, their essential periodic maintenance can easily be overlooked or not carried out correctly.

Here's the problem with this: The effects of inadequate or incorrect hydraulic system maintenance are like a ticking bomb! They may not show up until well into the life of the machine, but believe me when they do strike, they can be catastrophic, unexpected and extremely expensive to repair.

I recently came across a fairway mower that had sustained an internal hydraulic component failure in the traction system. This is the worst sort of failure because debris is then circulated around most of the other components in the circuit, damaging them as well. In this case, the repair estimate was in excess of £14,000. From my experience, this type of repair is very difficult to accomplish successfully since it often requires dismantling of the entire system and even other subsystems that share the same hydraulic fluid supply. All hoses, tubes, valves and fittings need to be flushed thoroughly to prevent the expensive new components being damaged on start-up by debris hiding in some nook or cranny being dislodged. No matter how careful the flushing process is carried out, there is always the risk of debris particles remaining. From the repairer's perspective, it's also a huge gamble to provide a warranty with such a repair, so many don't.

It's not surprising then that major repairs such as this can easily render the machine beyond economical repair. Catastrophic failures are obvious; the machine simply fails to operate. The other symptom of not maintaining the hydraulic system properly is a gradual deterioration of machine performance. This happens slowly over time, but will eventually result in the machine being unable to operate effectively at all, especially when hot. This type of problem is normally associated with debris in the hydraulic fluid. The debris can be from internal wear of components or more likely introduced into the system from outside.

Here is a key fact: Experts say that 75-80 percent of hydraulic system failures are caused by fluid contaminated with dirt, wear particles and other foreign material. In today's high pressure systems, clearances in valves are very small making the control of contamination critical. With this in mind, it's easy to see how correct maintenance is also super important.

The second most common cause of system damage is overheating of the hydraulic fluid. To underline how important this is, some mineral based mono-viscosity hydraulic fluids can start to break down at 60 degrees centigrade. That's not a very high temperature when you consider that most modern car engines run with oil temperatures near 100 degrees centigrade. Running temperatures of systems must not constantly exceed 60 degrees centigrade for best reliability. Here is another important fact: For every 10 degrees centigrade above 60 degrees operating temperature, the hydraulic fluid service life is halved! So beware, even if you adhere to the manufacturer's fluid change intervals, the system may be damaged if it has been operating above the recommended temperature.

Running with system temperatures too high is likely to cause the fluid to begin to break down at a molecular level. Since the fluid that is being pumped around the system is also the primary lubrication, running high fluid temperatures can cause the lubrication film between moving components in pumps and motors to fail. This leads to metal to metal contact and internal damage.

Metallic particles from such a situation are the main source of internal debris contamination. When hydraulic systems are running too hot, they are also less efficient due to the fluid viscosity (thickness) being reduced. Whilst not normally a problem in the UK climate, running hydraulic systems too cold can also cause problems and internal damage due to the fluid being too thick to pump, leading to a phenomenon known as cavitation. The science of this is beyond the scope of this article and me! As an example of how operating temperature can affect system performance, I remember driving an 85hp tractor out to a verti-draining job. The tractor in question was fitted with a full hydrostatic transmission. What I didn't realise was that, prior to setting out, I had accidentally knocked one of the hydraulic spool valves into the "on" position. The system was now "pumping against a dead end" causing the relief valve to be permanently open. This causes the hydraulic fluid to heat up rapidly. So much so that, within a few miles, the tractor slowed until it wouldn't drive at all! Once the mistake was realised and the transmission fluid cooled down, the tractor operated as normal. The fluid was changed soon after as a precaution.

The third source of system damage is water contamination. Contrary to popular belief, oil and water can in fact mix, forming a milky solution which can cause corrosion of internal components if not rectified. Apart from water leaking into the system through open tank vents and caps, it can also be formed over time due to condensation on the inside of metallic fluid reservoirs as they cool down after a hard day's operation.

So now we are aware of the potential causes of system damage, what can we do at a practical level to mitigate the risk? Quite a lot as it happens.

Let's take a look at fluid maintenance first. The fluid is the life blood of the hydraulic system. It has a very hard life transferring power from the engine to the cutting unit, lift system, power steering and traction components. Today's hydraulic fluids (and many other types of oil) are nothing short of amazing. Over my many years in the industry, I came to appreciate how much technology goes into the development of these fluids. I was fortunate indeed during my career to become friends with a senior chemist and technical sales manager from a well-known lubricating fluid manufacturer. I obviously can't mention the brand but, as a result of the business relationship, I truly now understand how oil can be termed as "liquid engineering"! To illustrate this, there were a number of occasions when I encountered problems with machine transmissions, wet brake squealing and repeat hydraulic pump failures where the engineering solutions seemed elusive. A phone call to my contact at the oil company brought a rapid solution to the problem, simply by changing to a more suitable off the shelf fluid or a specially formulated one designed to address the particular problem. Since then, I have never underestimated the importance of oil.

Blocked oil cooler matrix

Fluid Maintenance:

TOP TIP: Buy the best quality fluid you can afford, it's cheaper in the long run.

The new fluid MUST be to the manufacturer's recommendations, normally stated as an ISO or API specification together with a viscosity (thickness) rating. For example, a typical hydraulic fluid for UK operations might be "ISO VG 46". ISO is the European standard, VG stands for "viscosity grade" and the number is the thickness of the oil. Choosing the correct viscosity rating of the oil is very important and often overlooked. Oil with too high a viscosity will cause cold start-up problems in cool climates. I have seen triplex greens mowers stall on the first green after sitting overnight in unheated sheds! There is also a less obvious problem when the fluid is cold and thick. The machine's hydraulic filter incorporates a bypass valve which is designed to prevent damage to the filter when the fluid is thick. This valve opens to allow the thick fluid to go around the filter until it is warm (and thin) enough to pass through the filter without damaging it. Any time this valve is open, fluid is NOT being filtered, allowing damaging particles to be pumped around the system. Not good.

On the flip side, fluid that is too thin may break down at high operating temperatures, causing internal damage. If in any doubt, consult the machine manufacturer. Some manufacturers use a multi-viscosity fluid in their hydraulic systems to address the temperature issue. Much like multi-grade engine oils, these fluids offer the best of both worlds. They allow better performance in cold start-ups and still offer good system protection at high operating temperature. I can't understand why more manufacturers don't adopt this strategy given the expense of system failures.

TOP TIP: When draining the old fluid, use a clean container. Any metallic debris will be captured in the bottom of the container.

Close inspection of any debris can reveal lots about the state of the system and can give early warning of major component failure. Professional oil analysis is highly recommended as a predictive maintenance strategy. Many oil suppliers now offer this service.

Here's something you may not know. New hydraulic oil isn't necessarily clean hydraulic oil. Modern hydraulic system incorporate filtration systems to reduce the amount of debris suspended in the fluid. It's not uncommon for these systems to "filter out" particles larger than 10 microns. That's a millionth of a metre to you and me, which is extremely small. Considering a human hair is about 100 microns wide (or at least mine was thirty years ago), you can understand how well these filters work. The bottom line is that the oil in a system filtered to 10 microns will be cleaner than the vast majority of new oil from the drum. Indeed, I have known fleet users that changed the hydraulic fluids on their ride-on machines during end of season servicing only to find that on first start-up of the machine after the service, the machine's hydraulic filter (blocked) warning light immediately came on. Yes, the particles in the "new oil" had blocked the filter!

What's the lesson? Don't change the hydraulic oil in the machine until the manufacturer recommends it. The oil in your machine is quite likely to be cleaner than the new fluid you put in. There is a move in the industry to filter new oil before it is introduced into a machine.

That way immediate blocking of the machines filters is avoided. Let's not forget that even though the machines filters will eventually remove any unwanted particles from the oil, it may take many cycles of the fluid around the system until they are fully removed. During this time, some damage may have already occurred.

Keep the system clean

Changing filters

TOP TIP: Don't use non-genuine hydraulic filters.

This can be a false economy. Filters can be expensive but, as we saw earlier in this article, much cheaper than a full system failure. I have seen non-genuine filters which look identical to the OEM parts but their filtration particle size instead of being 10 microns is only 25 microns. That means that debris over twice the size is allowed to circulate in the system.

TOP TIP: Keep hydraulic tank vents clean.

All hydraulic tanks serve as the bulk storage of the system fluid. It also acts as the primary means of keeping the fluid cool. Because fluid is moving in and out of the tank it requires an air breather to the atmosphere. This allows air into the tank when the fluid level drops, and out again as the fluid level rises. Clearly, as air is allowed to enter the tank it must be filtered to the same specification as the hydraulic filter. When was the last time you changed the hydraulic tank breather filter? The breather filter can be

incorporated in the tank filler cap or is screwed into the top of the tank elsewhere, or in some cases at the end of a tube mounted way above the tank on the ROPS bar. Needless to say, never operate a machine with a broken filler cap or with a rag attached as a make shift filler cap. It's just asking for trouble down the line.

Keep the system cool:

As I said earlier in this article, one of the main enemies of the hydraulic system is heat. Basic systems rely on the bulk volume of fluid in the system, the tank, metal tubes and even cooling fins on the pumps and motors to keep the system temperature in check. In most modern high pressure systems this is not enough and some form of fluid intercooler is incorporated. This normally takes the form of an air to oil intercooler mounted in the engine radiator airstream. Sometimes the engine radiator and oil cooler are in an integrated unit; their fluids of course are separate (or should be). We all know the consequences of the engine radiator becoming blocked with debris, but keeping the hydraulic oil cooler radiator fins clear is equally important. Manufacturers design easy access to the oil coolers which must be cleaned with compressed air, never high pressure water.

The problem with using water is that it's difficult to dry the cooler matrix fully before using the machine. Dust can then be drawn into the fins of the cooler, mix with the water and set pretty much like cement. The effect over time is to gradually reduce the effectiveness of the cooler, and it's then also almost impossible to clean.

It's not just the coolers that need to be kept clean either. Pumps, valves and motors should also be kept clear of debris such as grass clippings. These are effective insulators keeping the components nice and warm! The classic area which can be problematic is under the operator's seat where most of the hydraulic components are situated. Keep this area clean.

What about the future?

There are moves in the hydraulic component industry to eliminate conventional hydraulic tank filler caps on machines. These are the main point of external debris ingress. Specially designed "oil make up points" will replace conventional caps. Filter rigs will be commonplace to pre-filter new oil before it is introduced into a system via the special fill point.

Hydraulic temperature gauges and alarms will become more common on machines, as will instruments that alert the user of sudden increases in particle debris in the hydraulic fluid. Telemetry will communicate such issues to users and dealers as an early warning.

Oil sampling and professional fluid analysis will be more commonplace as a regular maintenance strategy.

Maintaining a hydraulic system is not complicated and, compared with the cost of system failures, it is actually very cost effective. Correct maintenance will ensure a long service life of your machine and dramatically reduce the chances of catastrophic failures that will render the equipment a "write off"!


The hydraulic system is the most expensive part of most modern machines. When maintenance is neglected, it can be the limiting factor to the machine's service life.

  • Follow the manufacturer's service schedules for the hydraulic system; they have been developed to ensure the maximum reliability and longevity of the machine.
  • Keep fluid levels correct. The volume of oil in the system is important for correct cooling.
  • Keep the system cool. Clean oil coolers regularly using compressed air and keep areas around pumps, valves and motors clear of debris.
  • Use good quality hydraulic fluid that meets the manufacturer's specifications including the viscosity. Don't forget this can vary depending on the ambient temperature of where the machine is being used.
  • Use professional oil analysis as a predictive maintenance strategy. This can help prevent costly catastrophic failures.
  • The use of bio-degradable hydraulic fluids can require more frequent fluid changes and some have more limited cold start-up and high temperature performance than conventional fluids. Check the suitability with the machine manufacturer before using.
  • When draining hydraulic fluid, use a clean container and inspect any debris is the bottom of the container, it is a good indicator of system problems. Drain oil from the old filter, debris here can also indicate system issues. Dark colouration of fluid combined with a strong unpleasant odour is a clear indicator that the fluid has been overheated. If the fluid looks milky, it's contaminated with water and the source needs to be investigated.
  • Use only genuine filters; it's just not worth taking the risk.
  • Don't forget to replace oil tank breather filters (if fitted), they can be a major source of debris ingress that's often overlooked.
  • Filter new oil before introducing it to the machine hydraulic system if you can. Or, be prepared to use an additional machine filter that may become quickly blocked by debris in the new fluid after start-up. Better still, change the fluid but leave the old filter in place until the machine has be run up for a few minutes. If the filter (blocked) light comes on, then fit your new filter. You have just saved £50-60.