An old house is being demolished close to where I live in northwest Sydney. The house was flattened quickly, but the swimming pool took a big longer to be removed. Last week while I was walking by the site, an excavator with a ripper tooth was busy making a final assault on the pool. It was 30 degrees outside.  This was pleasant compared to the 47 degree day a week earlier.  As I was passing, I found it strange to see a worker with a hose following the excavator.  I stopped to take a closer look (see photo). The worker was dousing the engine enclosure at the back of the excavator with water. I noticed that the excavator would run for a period of time before powering down. Then it would start working again after a relatively lengthy break. It seemed that the cooling system of this hydraulic machine was grossly inadequate for the heat load being generated.  I thought to myself, this is a very inefficient way of working. How often does this type of situation occur?  This incident motivated me to write this article about the consequences of high oil temperatures in hydraulic systems.

First of all what is the main function of hydraulic oil in a hydraulic system? Well, oil in a hydraulic system transmits power but it also lubricates, cools and cleans.  Proper conditioning of the oil is essential in ensuring a healthy system and long component life.  And one of the most important factors in maintaining the oil in good condition is mitigating the effects of heat, which raises the temperature of the oil.

What are the effects of high oil temperature?

1. Low oil viscosity

As the temperature of oil increases it’s viscosity reduces. If the viscosity of hydraulic oil reduces too much it may be insufficient in maintaining the required lubrication film thickness between moving parts. This may lead to material pickup, cold welding and component wear. Manufacturer’s data for many hydraulic components will give information on the recommended viscosity range of hydraulic oil for normal operation.  It will also give figures for maximum viscosity which is acceptable for short periods at startup and the minimum allowable viscosity which is also acceptable for short periods. For example, for the Parker Gold Cup series of pumps and motors, the following values are recommended:

– Maximum viscosity at cold start – 1,600 cSt

– Maximum viscosity at full power – 160 cSt

– Optimum viscosity for max life – 30 cSt

– Minimum viscosity at full power – 10 cSt

See below a viscosity chart which shows the change in viscosity with temperature for various grades of hydraulic oil. According to the chart, the viscosity of grade 32 oil is approximately 10 cSt at 74 degrees Celsius. 

The operating efficiency of the system is also affected by the viscosity of the oil. If the viscosity of oil is below what is recommended, increased leakage will occur which reduces operating efficiency.

2. Deterioration of Oil Properties

At higher temperatures hydraulic oil begins to deteriorate.  As a rule of thumb, at higher operating temperatures (eg. 75 degrees, but this figure depends on oil grade) for every 10 degree increase in temperature, the deterioration rate of the oil doubles or the life of the oil halves. High temperatures cause oxidation, additives to deplete quicker, corrosion to increase and the formation of sludge and varnish. 

3. Damage to Seals

Many hydraulic component seals have maximum operating design temperatures not much higher that 80 degrees Celsius. At temperatures above this they will deteriorate or harden.  

4. Downtime

As in the example of the excavator above, a machine may cutout due to the tripping of a high temperature sensor.  If this happens regularly as in the above example, it will severely affect the efficiency of operations.   

5. Increased Pressure of Confined Spaces

Another issue of concern related to temperature is what happens to oil in a confined space when it is heated. An example would be oil trapped in an enclosed space such as a hydraulic cylinder. As a rule of thumb for every 15° increase in temperature, the pressure will increase 100 bar.  Pressures can increase considerably if the sun shines on a cylinder for example.

What causes oil to heat up?

1. Mechanical Inefficiency

Sliding, rolling or rotating mechanical parts such as reciprocating pistons, bearings and shafts generate heat in hydraulic pumps and actuators due to friction. Hydraulic oil lubricates these mechanical movements and cools the components by transporting the majority of this heat away from the pump or actuator. This transported heat raises the temperature of the oil. The heat created by mechanical inefficiency can be estimated as follows:

The mechanical efficiency is a function of operating pressure and will also decrease with age as components wear and working tolerances increase.

Although most of the heat generated is transported away by the relatively cool hydraulic oil, the components themselves also heat up and some cooling takes place through heat radiation from the components and convection to the surrounding environment.

2. Pressure Drop

Pressure drop which does not do useful work creates heat. Some examples are pressure drop in pipework, fittings, across valves and orifices. Pressure drop is also created internally in hydraulic pumps and motors as oil leaks to the case of the pump or motor.  

The heat generated due to pressure drop is as follows:

The heat generated due to internal leakage in hydraulic pumps and motors can be estimated as:

or alternatively as:

As an example, if an open loop system is used to drive a winch hydraulic motor (gearbox not considered for simplicity), the useful work per second is:

Note:   the system boundary in this instance is at outlet of hydraulic motor, not at outlet of winch. 

Also:

The power delivered by the pump to the system is approximately:

The total heat generated can be estimated as:

or alternatively as:

The above is a simplification and does not consider additional losses such as oil flow lost due to internal valve leakage. It illustrates just how much heat can be generated in a system due to pressure drop.

3. External Environmental Influences

If a hydraulic power unit is situated in an enclosed space without adequate ventilation, the space itself can minimize the cooling effects due to convection and radiation to the surrounding environment. If the HPU is situated in a factory close to a kiln for example, this is obviously another heat source that needs to be considered. As in the situation at the top of this article, a warm environment can reduce the effectiveness of air coolers. If the HPU or hydraulic components and piping are exposed to direct sunlight, this is another factor that must be considered when estimating the heat load on the system. 

Learning about the causes and effects of heat in hydraulic systems is the first step in minimizing the financial impact that overheating can have on your hydraulic machinery and equipment. In the next article I will discuss how the adverse effects of heat in hydraulic systems can be minimized.  

If you need support with the issues discussed in this article, you can contact us here: http://antares-global.com/contact/

Research has shown that more than 75% of hydraulic system failures occur from system contamination. Machine builders, equipment maintenance managers and operators must pay close attention to factors which affect the cleanliness level of hydraulic oil in order to minimize lifetime costs. However, even if operators are diligent and have rigorous maintenance procedures in place,  it is almost impossible to remove all contamination from a system. For example, contamination may lodge in a corner of a fitting during machine assembly.  After flushing prior to startup, this contamination may remain in the system if it is located in an isolated area away from the main flushing flow. Contamination such as this can work its way into the main system flow throughout the life of the machine due to different reasons such as machine vibration, the heating and cooling of the system, movement of the machine etc.  Contamination is also constantly being generated internally during machine operation.  For example a piston motor can generate metallic particles while starting under a heavy load before a lubrication film forms between moving parts.

The most effective way of minimizing system contamination and lifetime costs is through intelligent system design.  Proper selection, sizing and placement of the numerous types of filters and oil conditioning equipment available can contribute enormously to the health of the hydraulic system.  A well designed system will maintain high levels of oil cleanliness despite the numerous sources which can produce contamination.

The critical sizes of dirt particles for hydraulic systems lie in the range from 0.2 to 100µm.  In comparison, the minimum particle size that can be seen with a human eye is approximately 40µm and the width of a human air is around 75µm. The ever increasing operating pressures of hydraulic systems beyond 350 bar leads to tighter dynamic tolerances of the internal working parts of hydraulic components.  As tolerances become tighter the cleanliness level of oil needs to increase. Smaller dirt particles become more problematic in relation to the gap between moving parts.

What are the effects of contamination in the oil? Contamination wear generally falls under four categories:

• Abrasive wear – dirt particles between reciprocating surfaces leads to an abrasive wear of the surfaces.  This can affect component tolerances, lead to increased leakage and generate additional particles which leads to more wear.

• Adhesive wear –  high loads, low speed or reduction in fluid viscosity can lead to contact of metal surfaces which it turn can cause cold welding or component wear. 

• Erosive wear – where there are large pressure drops and as a consequence high localized oil flow velocity, particles entrained in the oil can cause an erosive wear pattern on metal surfaces.  As a result parts lose their working tolerances and form.

• Surface fatigue – particles between two surfaces can cause high local contact forces causing pitting, leading to further particles being entrained into the system.

What are sources of contamination?  There are many sources of system contamination including:

• During initial assembly dirt can be built into the system.
• While maintenance is being carried out, dirt can migrate into the system while components are being replaced or adjusted.
• Dirt can enter the system through leaking suction lines.
• Dirt particles are present in the air which is worse in certain dusty environments. Poor selection of air breathers can allow this dust to make its way into the hydraulic tank.
• Cylinders can be a significant source of contamination as the rod surface is exposed both to the external atmosphere and to the oil in the hydraulic system, as the cylinder is extended and retracted.
• Refilling the system with dirty oil. In general, bulk hydraulic oil delivered from the refinery needs to be filtered before it goes into the hydraulic system.
• Moisture in the air can make its way into the system through the tank breather.
• Internal wear of hydraulic component parts.

As stated above, intelligent system design can go a long way to minimising contamination problems. Some important considerations when selecting filters and designing the system are:

• The use of pressure filters is advisable when the system contains sensitive components such as servo valves.  They should also be considered when a system shutdown will have high economic consequences.
• Pay close attention to filtering of boost, flushing and servo flows. Filtering of case drain flows should also be considered. Some manufacturers strongly recommend this for certain products, while other manufacturers advise against filtering case drains or to filter with extreme caution.
• When selecting return filters, care should be taken to size the filter according to the maximum possible return flow.  This may be larger than expected due to differential areas of hydraulic cylinders for example. 
• The use of offline filters can have a major effect on the cleanliness level of a system. Even though the flows involved are typically very low, the constant flow through this type of filter can significantly reduce system contamination. 
• When designing the tank, ensure the return connections are relatively far away from suction connections and that they are separated by baffles. This allows air and particles to settle out from returning oil before it is pumped back into the pressure lines.
• Use of minimess points to allow connection of measurement instrumentation or to allow an oil sample to be taken. This minimises exposure of the system to external contaminants. 
• Use of quick connect couplings to fill the system with new oil. Again this minimises particle intrusion. 

If you need support with minimizing contamination in hydraulic systems, you can contact us here: http://antares-global.com/contact/

A dropped mast incident is described in the following safety alert: mast raise cylinder failure.  Another incident is described here: telescoping cylinder failure. One drill rig manufacturer recommends that a drill mast should be raised and lowered gradually from the mast rest.  The manufacturer recommends that the mast is initially raised approximately five degrees from the mast rest and lowered.  This is repeated at ever increasing angles until the mast is fully raised.  This ensures that if air is  present it will gradually be removed.  A better solution would be to install a bleed valve at the highest point in the cylinder and bleed air from the cylinder prior to lifting the mast.

In the example in the above link, the technician thought he was working “by the book”, but was unaware that the pump was sucking air through it’s suction and constantly supplying air into the system.  He would have noticed the problem before anything serious happened if the cylinder was connected to the mast and if he followed a procedure similar to that described above.

Another serious failure occurred when a motor in a closed loop winch system failed catastrophically due to cavitation.  See a picture of the failed motor below.   This occurred during load testing of an offshore crane to its  maximum rated capacity.  The load fell to the deck and luckily no one was injured. During the test, cavitation occurred in the suction of the motor due to an incorrect hose size fitted.  As air was being entrained into the system, the motor lost its load holding capability and rotated freely.