Whenever I present hydraulic maintenance training, a concept I always enjoy explaining to my students is what I call the perfect hydraulic fluid. And it’s very simple. 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 cSt, Exhibit 1.

Exhibit 1. Temperature-viscosity diagram of the perfect hydraulic fluid.
Of course, no such fluid exists and I don’t expect that such a fluid will be developed in my lifetime. But if such a fluid was developed, its creator would have the key to a gold mine.

 Exhibit 1. A perfect fluid’s viscosity index would be represented by a horizontal line intercepting intercepting the Y axis at 25 cSt.

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

Exhibit 2 shows how this can manifest itself in an axial piston pump. Gold-colored varnish deposits are evidence that this hydraulic system has been operating over-temperature. Due to low fluid viscosity, the lubricating oil-film between piston and its 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 happens the tensile force pulls the slipper from the piston — causing catastrophic failure.

Exhibit 2. Catastrophic failure caused by low fluid viscosity
Exhibit 3 lists typical optimum and permissible viscosity values for an axial piston pump. Note the optimum viscosity range is 16 to 36 cSt. This is the viscosity range where the system will operate most efficiently — highest ratio of output power to input power. Stated differently, this is the viscosity range where fluid friction, mechanical friction, and volumetric losses are optimal for system performance.

 Exhibit 2. Gold-colored varnish deposits provide evidence that this axial-piston pump was operating a excessively high temperatures.

Exhibit 3. Typical viscosity values for axial-piston pumps
But exhibit 3 only tells us half the story. There’s critical information missing. We need to know what operating oil temperature equates to each of these viscosity numbers.

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 cSt, as shown in Exhibit 1, a significant variable is removed and problems arising from insufficient fluid viscosity are instantly solved. And that would we worth paying good money for!

Exhibit 3. Viscosity value cSt
Minimum permissible 10
Minimum optimum 16
Optimum bearing life 25
Maximum optimum 36
Maximum permissible 1000

Alas, such a “magic pill” solution is not available to us right now. So we can’t control the rate of change of viscosity with temperature — or not to the ideal degree at least. 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, 22°C (72°F) and regardless of 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 a computer the weight and viscosity index of the fluid you’re using and then select the viscosity you want the system to run at – say 25 cSt. Then, regardless of whether its summer or winter and the amount of heat load (internal leakage) on the system, the “climate control” would heat or cool the oil as necessary to maintain optimum viscosity. 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 has to do some leg work.

Some of the variables that must be considered include:
• starting viscosity at minimum ambient temperature,
• maximum expected operating temperature — which is influenced by system efficiency, installed cooling capacity and maximum ambient temperature, and
• permissible and optimum viscosity values for individual components in the system.
Once all these variables have been taken into account and a fluid with a suitable weight and viscosity index has been selected, an additional column of information can be added to Exhibit 3 so it looks like that shown in Exhibit 4.

Exhibit 4. Viscosity with corresponding temperature values
Having defined the operating parameters shown in Exhibit 4 for a specific piece of hydraulic equipment, damage caused by low or high fluid viscosity can be prevented by installing fluid temperature alarms or shutoff controls.

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