Simple Service Steps Save Hydraulic Systems

Stan Watkins

Until the day that refuse vehicles are immune from mechanical failures, those dependent on heavy trucks may find it helpful to understand the problems before attempting to correct them.

A refuse truck's hydraulic system frequently requires a mechanic's touch. It is important to identify the early signs of failure so that components such as pumps, valves and cylinders do not fail.

An Unavoidable Problem Contamination, or any foreign material in the hydraulic oil including water, diesel fuel and abrasive particles that chew away at internal components, is the number one killer of hydraulic systems. Virtually every defunct hydraulic system shows some signs of contamination - even those in which contamination was not the primary cause of system failure.

Since hydraulic systems will always experience some level of contamination, it's essential to determine which level is acceptable for your application. Generally, the higher the system pressure, the cleaner the oil must be. For example, a typical system which operates at 2,000 PSI requires a cleanliness I.S.O. code of 19/16 or 5,000 particles per milliliter larger than 5 micron and 640 particles larger than 15 micron (see table). While this may seem like a lot of contamination, according to the Fluid Power Association, it's a tolerable amount.

To find the I.S.O. particle level on a truck's hydraulic system, go beyond the oil sample analysis, which determines oil cleanliness. For example, at the time of the analysis, request an I.S.O. particle count. The majority of contamination-related failures occur between 40 microns or less (particles which are too small to be seen with the naked eye).

Water is the most common type of liquid contamination. Reportedly, as little as 0.2 percent of water contamination can cause the oil to break down and lose lubricity. To find the percentage of water in the oil, analyze or visually inspect an oil sample. Under extreme conditions, the problem can be spotted instantly. When water and oil emulsify, or blend under pressure, the oil will turn white and must be replaced.

Preventing System Failure Filtration is the key to keeping contamination within an acceptable level. The return line is the most common form of filtration on a refuse vehicle. When filtering the oil on the inlet side of the system, do not use anything finer than a 100-mesh strainer.

When shopping for filters, consider the following: * Do not purchase filters based on micron rating and price.

* Even the best filters are only as good as the service they receive. Maintenance serves as a gauge on the filter system - giving notice when it's time to change filters. In the long-run, service saves money.

* To ensure that you are getting the proper filter, ask the supplier if it's rated for five, 10 or 15 micron; 10 or less is recommended. Also, is it an absolute or nominal rated filter?

* Find the beta ratio/rating. Typically, the higher the number, the better. For example, a beta rating of four is 75 percent efficient, or one particle in four will escape through the filer on the first pass.

* Determine how much flow the filter can handle. For example, if it's on a 50 GPM system, a 100 GPM filter is recommended.

Heat also is a major contributor to system failures. Friction, which causes heat, is created from lost lubrication or when a large amount of oil is put into a small area. The heat affects the oil before other components fail. In refuse trucks, heat is commonly produced from flow saturation, or too much flow for the system to tolerate.

Too much flow is created by excessive operating RPMs, a pump that is too large or from components such as hoses and fittings that are too small. Since refuse trucks operate with fixed displacement gear pumps, the faster the RPMs, the more oil that will be produced. To solve this problem, reduce the RPMs to an acceptable level for the system's designed flow.

A new product on the market is a fixed displacement gear pump with a load sensing capability which reportedly can eliminate overspeed controls, dry valves, flow controls and electric air solenoids without fear of flow saturation.

Since the hydraulic oil is the most susceptible component to excessive heat, it's important to use the maximum temperatures guidelines provided by hydraulic oil suppliers. For example, one supplier recommends a maximum continuous temperature of 180 degrees Fahrenheit and a normal operating temperature of approximately 130 degrees Fahrenheit.

Heat will be produced even under normal conditions. However, most heat is rejected at a fast enough rate to prevent the oil from reaching a critical running temperature. When oil is exposed to an excessive amount of heat, expect to see the following results:

Varnishing. This sticky, gummy residue that clogs filters, valves and orifices is most evident as a collection of sludge on the bottom of the hydraulic reservoir.

Loss of Lubricity. Heat reduces the oil's film strength and permanently reduces lubrication. Lack of lubrication has a cumulative effect on all the components in the hydraulic system.

Viscosity. As the oil is exposed to high heat conditions, viscosity increases and creates additional problems for trucks operating in high ambient temperatures such as those in Arizona and Florida.

To determine if heat is a problem, several different tests can be conducted. For example, look at the oil. If it has been exposed to too much heat, it will appear dark or opaque. Also, use a sample from your bulk tank to discover how the oil feels when it's rubbed between the forefinger and thumb. The oil should be slippery, like the new oil from the bulk tank. Lastly, if the oil has been too hot, it will smell burnt. Once the body's three senses have confirmed that the oil has failed, properly dispose of the old oil and replace it with fresh oil and new filters. After a day on the route, check it again.

Unfortunately, hydraulic systems are vulnerable to immediate and irreversible complications. To get the most bang for your buck, take the time to evaluate your system's contamination level, pump and hose fittings and temperature levels.

Do your trucks suffer from a chronic hydraulic disorder? Following are typical problems and the conditions they can cause.

Replacement pumps are larger in displacement than necessary. As a result, they will require higher torque loads than necessary; operate at less efficient speeds; and create more heat.

Hydraulic lines and hoses are the wrong type or too small. As a result, they will require additional horsepower to compensate for the pressure losses in the system. They also will create heat, which damages hoses and oil, requiring both to be replaced more often than necessary. They also will restrict the amount of oil which can flow without turbulence, causing aeration and heat and will collapse or burst, making the system fail.

Systems often have the wrong oil. Consequently, the oil will need to be replaced more often. When the oil is too thick, it will create heat from high viscosity friction or from cavitation. When it's too thin, it will create heat from low viscosity and cause excessive slippage, poor efficiency and component wear.

Systems are not calibrated and often are set up improperly. Examples of this include relief valves that are set too low; a system that's not protected from contamination; and incorrect circuit continuity.

Improperly installed drivelines cause vibration, noise, seal leakage, contamination and pump shaft damage. In addition, this allows dirt to invade the seal area by distorting it and to abrade the shaft seal area, requiring shaft replacement.

Inadequately trained system operators. As a result, they will not know when the system needs servicing or if they are damaging the system by forcing flow over relief. System operators also need to know how much damage they can inflict.

Systems are operated without any oil, or with the supply valve closed.