In Search of the Perfect Hydraulic Fluid

Brendan Casey
Tags: hydraulics, viscosity, industrial lubricants

When conducting training courses on hydraulic maintenance, I always enjoy explaining to my students a concept I call the "perfect hydraulic fluid".

The perfect hydraulic fluid would have a viscosity index (the change in a fluid's viscosity relative to temperature) represented by a horizontal line intercepting the Y axis at 25 centistokes (Figure 1).


Figure 1. Temperature/Viscosity Diagram of The "Perfect Hydraulic Fluid"

Of course, no such fluid exists, and I don't expect such a fluid will be developed in my lifetime. For the industrial chemists out there, the word impossible might even spring to mind. But if such a fluid was developed, its creator would have the key to a gold mine.

Based on my experience in the hydraulic repair business, I believe the failure of hydraulic equipment users to fully appreciate the interrelationship of fluid viscosity and temperature while allowing their equipment to operate outside permissible viscosity limits is a major cause of premature failure of hydraulic components.

Fluid viscosity is a determining factor concerning whether full-film lubrication is achieved and maintained. If load and surface speed remain constant, but elevated operating temperature causes viscosity to fall below what is required to maintain the hydrodynamic film, boundary lubrication occurs with the possibility of friction and adhesive wear.


Figure 2. Catastrophic Failure Caused by Low Fluid Viscosity


Figure 2 shows how this can manifest itself in an axial piston pump. The gold-colored varnish deposits are evidence that this hydraulic system has been operating above the recommended temperature. Due to low fluid viscosity, the lubricating film between piston and bore has been lost. The resulting friction has super-heated the piston causing it to expand in its bore to the point of interference. Once this occurs, the tensile force pulls the slipper(s) from the piston(s) - also known as catastrophic failure.

Table 1. Typical Viscosity Values for Axial Piston Pump

Table 1 lists typical optimum and permissible viscosity values for an axial piston pump. Note the optimum viscosity range is 16 to 36 centistokes, where the system will operate most efficiently - highest ratio of output power to input power. In other words, this is the viscosity range where fluid friction, mechanical friction and volumetric leakage are optimal for system performance.

However, Table 1 tells only half the story. Critical information is missing. It is necessary to know at what operating temperature each of these viscosity numbers is reached.

To establish this, we need to consider the weight of the fluid in the system and its viscosity index - represented by its gradient on a temperature/viscosity diagram. The flatter the line, the wider the allowable operating temperature range - for both optimum and permissible viscosity.

Here's where the perfect hydraulic fluid comes in. If you could use a fluid that flat-lined on a temperature/viscosity diagram at 25 centistokes, a significant variable is removed and most of your problems relating to maintaining viscosity and the reliability issues it can cause if you don't, are instantly solved. And that would be worth paying good money for.

Alas, such a "magic pill" solution is not available right now. So we cannot control the rate of change of viscosity with temperature - or at least not to the ideal degree. But we can control operating temperature. So here's another ideal: the climate-controlled hydraulic system.

Most us have driven or ridden in an automobile fitted with climate control. You dial in say, 75°F (24°C) and regardless whether it's snowing outside or hot enough to fry an egg, the climate control heats or cools the auto's interior to maintain the selected temperature.

What if your hydraulic equipment had a similar system? You tell the computer the weight and viscosity index of the fluid you're using and then select the viscosity at which you want the system to run - say 25 centistokes. Regardless of whether it's summer or winter and the amount of heat load (internal leakage, etc.) on the system, the climate control heats or cools the fluid as necessary to maintain optimum viscosity. Best part: it's possible. It's just not very practical.

So with the perfect hydraulic fluid not available and the climate-controlled hydraulic system not feasible in most applications, human intervention is required. Someone must do some leg work.

Some of the variables that must be considered include the following:

  • starting viscosity at minimum the ambient temperature

  • maximum expected operating temperature - which is influenced by system efficiency, installed cooling capacity and maximum ambient temperature

  • permissible and optimum viscosity values for individual components in the system

Once all these variables have been considered and a fluid with a suitable weight and viscosity index has been selected, an additional column of information can be added to the table (Table 2).

Having defined the operating parameters shown in Table 2 for a specific piece of hydraulic equipment, damage caused by low or high fluid viscosity can be prevented by installing fluid temperature alarms and/or shutdowns.

In the absence of the perfect hydraulic fluid and short of installing climate control on all your hydraulic equipment, this is the only way to ensure failures similar to that shown in Figure 2 don't happen to you. 

About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his Web site,

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