HYDRAULIC FAILURES

Hydraulic pump life cut short by particle contamination

What is 'contaminated hydraulic fluid'?

Contaminants of hydraulic fluid include solid particles, air, water or any other matter that impairs the function of the fluid.

How does contamination affect a hydraulic pump?

Particle contamination accelerates wear of hydraulic components. The rate at which damage occurs is dependent on the internal clearance of the components within the system, the size and quantity of particles present in the fluid, and system pressure.

Particles larger than the component's internal clearances are not necessarily dangerous. Particles the same size as the internal clearances cause damage through friction. However, the most dangerous particles in the long term are those that are smaller than the component's internal clearances.

Particles smaller than 5 microns are highly abrasive. If present in sufficient quantities, these invisible 'silt' particles cause rapid wear, destroying hydraulic pumps and other components

How can this type of hydraulic pump failure be prevented?

While the type of failure described above is unusual in properly designed hydraulic systems that are correctly maintained, this example highlights the importance of monitoring hydraulic fluid cleanliness levels at regular intervals.

As in this case, if the high levels of silt particles present in the hydraulic fluid had been identified and the problem rectified early enough, the damage to this hydraulic pump and the significant expense of its repair could have been avoided


Hydraulic fluid - getting the viscosity right
Most hydraulic systems will operate satisfactorily using a variety of fluids, including multi-grade engine oil and automatic transmission fluid (ATF), in addition to the more conventional anti-wear (AW) hydraulic fluid - provided the viscosity is correct.

Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn't matter how good the anti-wear, anti-oxidization or anti-corrosion properties of the fluid are, if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system, maximum component life will not be achieved.

Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These are:

starting viscosity at minimum ambient temperature;
maximum expected operating temperature, which is influenced by maximum ambient temperature; and
permissible and optimum viscosity range for the system's components.

Once these parameters are known, the correct viscosity grade can be determined using the viscosity/temperature curve of a suitable type of fluid - commonly AW hydraulic fluid defined according to ISO viscosity grade (VG) numbers.

Automatic transmission fluid, multi-grade engine oil and anti-wear, high VI (AWH) hydraulic fluid are commonly used in hydraulic systems that experience a wide operating temperature range. These fluids have a higher Viscosity Index (VI) than AW hydraulic fluids due to the addition of VI improvers. The higher the VI a fluid has, the smaller the variation in viscosity as temperature changes.

In simple terms, this means that if you are running ATF(46) in your skid-steer loader, you can operate the hydraulics with a higher fluid temperature before viscosity falls below optimum, than you could if you were running ISO VG46 AW hydraulic fluid.

When selecting a high VI fluid, the component manufacturer's minimum permissible viscosity value should be increased by 30% to compensate for possible loss of viscosity as a result of VI improver sheardown.

VI improvers can have a negative effect on the demulsification and air separation properties of the fluid and for this reason some hydraulic component manufacturers recommend that these types of fluids only be used when operating conditions demand.

As far as fluid recommendations go, for commercial reasons relating to warranty etc, I always advise following the machine manufacturer's recommendation. But in equipment that has a history of satisfactory performance and component life, there is usually no compelling reason to change the type of fluid being used.

High hydraulic fluid temperature - how it causes premature failures
I was asked recently to conduct failure analysis on two radial piston hydraulic motors that had failed well short of their expected service life. Inspection revealed that the motors had failed through inadequate lubrication, as a result of low fluid viscosity caused by excessive hydraulic fluid temperature.

How does this happen?
As the temperature of petroleum-based hydraulic fluid increases, its viscosity decreases. If fluid temperature increases to the point where viscosity falls below the level required to maintain a lubricating film between the internal parts of the component, damage will result.

The temperature at which this occurs depends on the viscosity grade of the fluid in the system. Hydraulic fluid temperatures above 180°F (82°C) damage seals and reduce the service life of the fluid. But depending on the grade of fluid, viscosity can fall to critical levels well below this temperature.

How can this type of failure be prevented?
The above example highlights the importance of not allowing fluid temperature to exceed the point at which viscosity falls below the optimum level for the system's components.

Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal-combustion engine with high coolant temperature. Damage is pretty much guaranteed.

Therefore, whenever a hydraulic system starts to overheat, shut down the system, find the cause of the problem and fix it!

What is cavitation?
Cavitation occurs when the volume of hydraulic fluid demanded by any part of a hydraulic circuit exceeds the volume of fluid being supplied.

This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid. This results in the formation of vapor bubbles within the fluid, which implode when compressed.

Cavitation causes metal erosion, which damages hydraulic components and contaminates the hydraulic fluid. In extreme cases, cavitation can result in major mechanical failure of pumps and motors.

While cavitation commonly occurs in the hydraulic pump, it can occur just about anywhere within a hydraulic circuit.


What is the 'diesel effect'?
The diesel effect occurs in a hydraulic cylinder when air is drawn past the rod seals, mixes with the hydraulic fluid and explodes when pressurized.

How does this affect a hydraulic cylinder?
When a double-acting hydraulic cylinder retracts under the weight of its load, the volume of fluid being demanded by the rod side of the cylinder can exceed the volume of fluid being supplied by the pump.

When this happens, a negative pressure develops in the rod side of the hydraulic cylinder, which usually results in air being drawn into the cylinder past its rod seals. This occurs because most rod seals are designed keep high-pressure fluid in and are not designed to keep air out. The result of this is aeration - the mixing of air with the hydraulic fluid.

Aeration causes damage through loss of lubrication and overheating, and when a mixture of air and oil is compressed it can explode, damaging the hydraulic cylinder and burning its seals. As you have probably gathered, the term 'diesel effect' is a reference to the combustion process in a diesel engine.