Hydraulic Systems

Written by admin

February 28, 2021


  • High output force
  • Standard components
  • Precise control
  • Very complex final movements are achievable
  • Fluid circulation ensures heat removal


  • Low overall efficiency
  • Considerable capital costs
  • General characteristics not favourable for rapid movements
  • Possible leakage could cause economic losses, environmental contamination, smell, fire hazard
  • Possible noise pollutant

Hydraulics Systems – Components:

  • Fluid holding tank
  • Pumps to convert mechanical energy into fluid flow
  • Motors to convert fluid-flow into mechanical energy
  • Valves to ensure control, regulation and safety
  • Accumulators to damp fluctuation in pressure and flow
  • Directional control valves to distribute flow
  • Heat exchangers to keep fluid temperatures within reasonable limits
  • Filters to trap impurities present in the fluid
  • Piping, tubing, hosing
  • Control devices such as transducers and meters.

 Hydraulics Fluid – Characteristics:

  • Adequate Viscosity Level
    – The viscosity level must be chosen on the basis of the working temperature
  • High Viscosity Index
    – Ensures optimum viscosity either in starting or running conditions
  • Low Pour Point
    – Avoids pump cavitation during cold start particularly at extreme ambient conditions
  • Good Demulsification
    – Easy and fast water separation
  • Anti-Foam properties
    – Avoids the presence of foam
  • Good Air Release
    – Reduces the effect of air entrapment which could affect the compressibility of the hydraulic medium.
  • Chemical Stability
    – Increase the life of the lubricant and reduces the effects of oxidation i.e. formation of acid products (corrosive), sludges, lacquers and varnishes
  • Anti-Wear properties
    – Prevents seizure of the pump (heart of the system) or complete breakdown of components under high load

Complimentary Properties

Particular applications require complimentary properties.

  • Bio-Degradability – In cases where the fluid could be lost into the environment.
  • Bio-Degradability is obtained with natural esters, synthetic esters and light poly-glycols
  • Non-Toxicity – In cases where the fluid could get into contact with plants, food or other living organisms
    Non-Toxicity is obtained through the use of special base oils and additives that are approved for this use (FDA, NSF, …..)

Fire Resistance – In operations where the fluid could be exposed to High Temperatures or Fire – Such as steel production.

Fire Resistance is made possible with different solutions

  • Phosphoric Esters (But toxic)
  • Synthetic Esters
  • Water emulsions
  • Water solution with polymers


Pollution / Cleanliness of the oil is a new important aspect of hydraulic fluids. Contaminants can cause many problems like valve stickiness and pump wear.

To prevent the problems in the circuit there are filters to block contaminants but, if the fluid does not have the proper level of cleanliness filters will be blocked quickly at high costs.

There are different methods to determine the levels and dimensions of contaminants:

ISO 4406

This Standard is using automatic particle counters comprising three scale numbers:

  • 1st number: particles of dimension superior to 4 µm
  • 2nd number: particles of dimension superior to 6 µm
  • 3rd number: particles of dimension superior to 14 µm

 Example: A code of 22/18/13 signifies that there are more than 20000 and up to 40000 particles equal or larger than 5µm, more than 1300 and up to 25000 particles equal or larger than 6µm, more than 40 and up to 80 particles equal or larger than 14µm in 1ml of a given sample.

The code for microscope counting (old ISO 4406) comprises two scale numbers using 5µm and 15µm.

Hydraulics and Pollution

Naval Aerospace Standard NAS 1638 –(Contaminants)

NAS defines the contaminant with reference numbers from 00 to 12 in 100cm³ of fluid for 5 ranges of dimensions of particles:

Example: NAS 00 has the following distribution:

-5 – 15 µm 125

-15 –25 µm 22

-25 – 50µm 4

-50 – 100µm 1

->100µm 0

  • Pollution could present in new and used fluids
  • New Oils Used Oils
  • Additives (polymers etc) Particles of wear
  • Humidity Humidity
  • Impurities of base oils (paraffin etc)
  • Dusts
  • Carbonaceous particles
  • Oxidation products



This is the tendency of oil to block filters.
This is determined in the presence of two conditions

  1. Without water
  2. In the presence of water

Methods used are:

  • DENISON TP – 02100
  • AFNOR NF E 48-690, 48-691
  • CETOP RP 124H
  • ISO DIS 13357/ 1-2

Additives in Hydraulic Fluids
Main Additives are:

  • Anti-wear
  • Anti-oxidants
  • Anti- rust
  • Anti-corrosion
  • Demulsifiers
  • Ant-foam
  • Viscosity Index Improvers
  • Others
Classification ISO Standard 6743/4
  • HH Mineral Oil
  • HL R&O Properties
  • HM AW
  • HR HL oil + Improved IV
  • HV H.V.I. + AW
  • HS Synthetic fluids
  • HG HM +ant-stick / slip properties
  • HFAE Oil in water emulsion
  • HFAS Aqueous chemical solutions
  • HFB Water in oil solution
  • HFC Aqueous polymer solutions
  • HFDR Phosphoric esters
  • HFDS Chlorinated hydrocarbons
  • HFDT Mix of HFDR and HFDS
  • HFDV Other types of synthetic fluids


Classification ISO Standard 6743/4
  • ATF Fluids (Automatic Transmission Fluids)
  • HA Automatic Transmissions
  • HN Hydraulic couplings and torque converters


R&O Fluids – ISO HL (Main Specifications)
  • ISO
  • ETOP 91H HL
  • AFNOR E 48603 HL
  • BS 4231 HSC
  • DIN 51 524 teil 1HL
  • Cincinnati P-38-54-55-57


Antiwear Fluids – ISO HM – HV (Main Specifications)
  • AFNOR E 48603 HM and HV
  • BS 4231 HSE (only HM)
  • CETOP 91H HM and HV
  • DIN 51 524 teil 2 HLP and teil 3
  • Cincinnati P68-69-70 (only HM)
  • Vickers M 2950


Unnecessary Cyclical Use of Hydraulic oil

In an effort to reduce cyclical consumption of oil and maintenance parts on machinery I have decided to educate users of oil to identify common mistakes and do proper preventative maintenance so oil consumption and expensive parts usage can be kept to a minimum.

Why should you take note?

My company is a manufacturer of some oils and a supplier to the users. Am I shooting myself in the foot by helping users to reduce consumption? In the short term maybe, but if everyone that are using oil don’t start acting responsible, we are all pulling the trigger.

Report – Hydraulic Mistakes

Using the wrong oil

The oil is the MOST IMPORTANT component of the hydraulic system.

  1. It serves as a lubricant.
  2. It removes contaminants and particles through the filtering system.
  3. It is the means by which power is transferred throughout the system.

For these reasons the viscosity of the oil is extremely important because it affects the performance and the service life of the equipment.

Expanding on a previous report, there is a section on Running too hot or cold and viscosity. It is referred to as the Temperature Operating Window (TOW)

Basically if you use oil with a viscosity too high for the climate the machines have to operate in, the oil will not have adequate flow and thus not lubricate properly during cold start.

If the viscosity is too low for a hotter climate the exact same problem occurs.

The above however is not the end of the problem of using the wrong oil or inferior quality oil where the additive packages are sub-standard or the base oil is of a low quality. Within the allowable upper and lower bands of viscosity required there is an even narrower viscosity band that affects power transfer. If you operate outside the “Green Band”

  1. Oil with viscosity higher than ideal – Power is lost due to fluid friction.
  2. Oil with viscosity lower than ideal – Power is lost due to mechanical friction and internal system leakages.

The loss in power also results in overconsumption of diesel or electrical power, an added cost.

Note: By just following the OEM’s recommended oil and viscosity guidelines does not necessarily result in optimum results. The conditions and external temperatures at which machines are used may require an oil with different qualities than the recommended oil.

To avoid costly oil selection, avoid #3 – Running too Hot, establish the oil grade and viscosity you are currently using. Establish the average temperatures you are running at (Winter and Summer), what conditions are you running under (Clean, Dusty, Wet etc.)

Send me the numbers at harold@xmt.co.za and I will send you a recommendation on the oil you should be using.

Changing the oil (Hour Use Intervals)

2 Reasons that necessitate the changing or replacement of Hydraulic Oil (or any oil for that matter)

  1. Degradation of the Base Oil
  2. Depletion of the additive package.

The above happens due to:

  1. Oxidation
  2. Contamination
  3. Oil time in service

Due to the nature of the application of Hydraulic oil the service change intervals are normally estimated and done in hours of oil being in usage. This primary method as an indicator is a major problem and should really only be used as a guideline for changing oil. If the OEM requires compulsory oil changes on certain hours and this affects your equipment warrantees, the following is an alternative:

Rather decide on a regular oil testing program and get oil analysed by an independent Laboratory. Based on this you might have to shorten intervals or you could extend intervals.

Given the high price of dumping oil both in the effect on the environment and the cost of new oil, changing of oil should really become a well-planned and thought through process.

You also do not just want to extend intervals as the use of degraded oil will in the long run compromise the service life of all the other components in the hydraulic system.

Also, contamination by particles or water may also not necessitate the changing of the oil. In most cases, except for severe contamination, the oil could be filtered and additives added to get the oil back up to standard.

Operating Temperatures

It is amazing that very responsible operators know that, when an engine is running too hot, they should not carry on running like this but should solve the problem immediately but, unfortunately the same operators do not think that running a Hydraulic system too hot could pose any major problems. Hydraulic systems are simple compared to an engine but just like an engine, the fastest way to destroy components (seals, hoses and the oil), is by running too hot.

Running too hot when it comes to the OEM’s prescriptions of the ideal operating temperature is one thing, the system may be running at a high temperature that is normal for the type of equipment or operation but it may be too hot for the oil viscosity used. As the operating temperature increases the oil viscosity decreases, so a system is running too hot when the system reaches a temperature that is too high for the oil viscosity and thus causing inadequate lubrication and a loss of power will occur.

As an example – If you system contains a vane pump, that requires a higher viscosity than a piston pump you should be looking to maintain a viscosity of 25 centistokes (cSt or mm²/s).

Mineral Oil with Viscosity Index of 100

= Operating Temperature of 35°C for a ISO VG 22 Oil and

= Operating Temperature of 65°C for a ISO VG 68 Oil

Operating Temperatures above 82°C damages most hose and seals compounds and oil is degraded at a faster pace. (Where systems operates above 82°C as a norm, special seal and hose compounds are required and there are oils available that operates at these temperatures)

So avoid running too hot by following recommendations at the end of #1 and send me the info at

Changing the Filters

As in #2 – Changing filters on hours could result in:

  1. Changing them too early, before their dirt holding capacity is used up – Unnecessary money down the drain.
  2. Changing them too late, filter starts bypassing- the particles in the oil decreases the component life and the oil life.

So you want to change the oil filters when their Dirt Holding Capacity is fully used but before they go on bypass.

The best way to accomplish this is to make use of Pressure Indicators that in shows a drop in pressure across the filter element by way of a clogging indicator or better yet, the use of a differential pressure gauge.

Wrong Filters locations

You may think that placing filters at perceived strategic points and anywhere else is a good thing right, the more filtration the better?

This wisdom is wrong and the one that is most commonly used and is probably the worst is the filter before the pump inlet, you have to protect the pump from trash, isn’t it so?

The placement of a filter at this point is the worst thing you could do; oil should be flowing without restriction into the pumping chambers. Any restriction could reduce the service life of the pump by 60%. This number is worse for vane and piston pumps where any vacuums cannot be tolerated due to the systems not being designed to “suck”.

The pump draws oil from a dedicated reservoir that should not have any garbage in it and the pump intake is at least 2 inches off the bottom.

Filters installed on the drain lines of piston pumps and motors causes a different set of problems but the results are the same as suction strainers – it reduces service life and can have failures in expensive components.

Go through your system and remove these filters, OEM’s may have placed them there but any pump or motor manufacturer will tell you that they should not be there.

Are Hydraulic components self priming and self lubricating?

This may be the general assumption – but is this correct in all cases. From experience it has been shown that many preventable problems could be avoided by following certain steps.

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