Selecting hydraulic fluid with the correct viscosity is the single most important factor for optimum performance.

Why is so much spoken about selecting hydraulic fluid with the correct viscosity? Well there are many properties that additives fortify for a specific application; but no one specification is as important as choosing the right hydraulic fluid viscosity that’s suitable over the entire operating temperature range.

Not selecting hydraulic fluid with the correct viscosity has these implications:

Fluid viscosity is one of the
factors that determines whether full-film lubrication is achieved and maintained. However, 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 and elevated temperature. If a mistake is made in selecting hydraulic fluid with the correct viscosity in a piston pump, this will often manifest as a gold-colored varnish deposit; indicating a high operating 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.

What would the ideal hydraulic fluid viscosity range be for an axial piston pump?

The optimum viscosity range is deemed to be 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.

Typical viscosity values for axial-piston hydraulic pumps

The ideal viscosity range is, however only half the story. We also need to know what operating oil temperature equates to each of these viscosity numbers when selecting hydraulic oil. 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. For a perfect hydraulic fluid this graph would “flat-line” on a temperature / viscosity diagram at 25 cSt. Right here a significant variable is removed and problems arising from insufficient fluid viscosity are instantly solved. Worth it’s weight in Platinum!!

Unfortunately, this is not available to us, so we can’t control the rate of change of viscosity with temperature perfectly. But what about controlling operating temperature?

Climate control the hydraulic oil system.

Most us have driven or ridden in an automobile fitted with climate control. If you select, 22°C this is maintained - regardless of whether it’s snowing outside or hot enough to fry an egg; the climate control heats or cools the car’s interior at the selected temperature.

Now what if we could do the same in hydraulics? 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 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. 

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.

How do we apply this to hydraulic pumps and equipment?

Having defined the operating parameters for a specific piece of hydraulic equipment, damage caused by low or high fluid viscosity can be prevented by installing hydraulic fluid temperature alarms or shutoff controls.

But at the end of the day it all boils down to selecting hydraulic fluid with the correct viscosity to start with. Call on the professionals; Habot Synthetic Lubricants if you need help selecting hydraulic fluid with the correct viscosity!