Tracing The Culprit In A Truck's Troubled Hydraulic System

Stan Watkins and Dave Douglass

Efficiency is the name of the game in the waste industry. For example, when a refuse truck packer is operating efficiently, the driver can pack while the hydraulics run at higher engine RPMs. To keep operations on the upswing, fleet managers should beware of three common, and serious, hydraulic afflictions: cavitation, aeration and overpressurization.

Cavitation occurs when a pump inlet is starved for oil. Excess vacuum can use oil molecules to stretch, forming gas bubbles which implode inside the pump cavity and create small pockets of intense heat. In the end, cavitation severely pits the pump housing and damages the wearplate surface (see photo on page 45).

An undersized or restricted inlet hose is one cause of cavitation. To prevent this affliction, size inlet hoses according to the system manufacturer's recommended flow; do not size them according to the diameter of the pump inlet port or succumb to temptations to use any available hose size. Eliminate or minimize suction strainers and filters, plumbing elbows, excessive bend radii, shut-off valves or other items that will restrict oil from flowing to the pump. Also, never use a pressure hose for an inlet line. Instead, it's imperative to use an approved, properly-sized suction hose. When in doubt, always use a larger hose for the inlet; never cheat on the inlet side.

A sticking or malfunctioning bleed valve in a dry valve pump system is another common cause of cavitation in refuse packer hydraulic systems. A dry valve pump depends on a functioning bleed line to prevent cavitation. When a dry valve pump is in the off mode, a small amount of oil (one to two gallons per minute) enters the pump to provide bearing lubrication. Next, the bleed line and bleed valve return this oil to the reservoir at low pressure (under 20 PSI).

Typically, the bleed line is a 3/8-inch diameter hose which runs from the pressure side of the pump to the reservoir and enters the reservoir at, or just below, the oil level. Cavitation will result and a pump will be ruined in short order if the bleed valve becomes clogged with contaminants and prevents the bleed flow-through, or if the back pressure increases to more than 20 PSI.

To test a bleed valve, follow this tool-free procedure:

1. Start the engine and advance the engine speed to 1,200 RPM. Do not engage the pump.

2. With the pump in the off mode, engage one of the control valves to simulate a "deadhead" pressure. Hold the valve open for 20 to 30 seconds.

3. Listen for the sound of pump cavitation. A growling or a high- pitched whining noise indicates that the bleed valve is clogged with contaminants, resulting in cavitation. Immediately replace it with a new valve.

Even if the bleed valve passes the first test, cavitation may still be present. Use a vacuum test to determine the pump inlet vacuum. This test requires plumbing a vacuum gauge with a one- to 30-inch Hg. scale to the test port on the inlet side of the pumps. Check the vacuum reading once the oil is at its operating temperature and the engine is operating at maximum RPM.

Typically, cavitation damage will occur if a gear type pump operates at vacuums in excess of five to seven inches Hg. If the system operates at more than five inches Hg., the oil needs to flow more freely to the pump inlet. Do this by increasing the hose size, eliminating 90 degree fittings and tight bends and cleaning or eliminating strainers. Remember: The goal is to make it easier for the oil to travel from the reservoir to the pump.

Additional Afflictions Air can enter the hydraulic system through a loose fitting, a pin hole in the inlet line, agitated oil or a damaged pump input shaft seal. An aeration-damaged pump looks similar to those damaged by cavitation, pitting and heat. If a dry valve pump seal is oozing oil, aeration is often the culprit. In the off mode, the vacuum inside a dry valve pump can reach up to 28 inch Hg. At these vacuums, a worn or damaged shaft seal allows air to be drawn into the pump. To prevent aeration, replace worn shaft seals and tighten loose fittings.

Overpressurization, or pressure spikes, is common in refuse vehicles. A truck that repeatedly blows hoses and cylinder packing probably has a misadjusted or undersized pressure relief valve. While newer systems may feature more than one relief valve to protect various circuits, most older vehicles have only one main system relief valve. Pressure spikes, or sudden increases in system pressure, can occur faster than a relief valve can react. In fact, pressure spikes commonly exceed 5,000 PSI.

Two types of relief valves are primarily used on mobile systems. Differential valves are typically slower and more susceptible to sudden pressure spikes. Pilot operated valves are more sensitive and re-spond to quick and sudden pressure increases quickly (see diagram on page 44).

A properly adjusted relief valve contributes to truck safety. A tampered valve, or one that has been set improperly, will not protect a hydraulic system. Therefore, it's important to inspect relief valves for broken seals and missing paint or other evidence of tampering. Also, set the relief pressures at the O.E.M. recommended setting. If the system has a secondary relief, set it at no less than 200 PSI over the main relief setting. This setting prevents the two relief valves from interacting with each other and affecting system op-eration.

When learning about a truck's hydraulic system, do not confuse "pressure" with "flow." For example, when a pump is losing efficiency, most maintenance workers will adjust the pressure relief valve to increase the pump output pressure. This process will make an existing pump fail sooner and also will contribute to the premature failure of the replacement pump. When maintaining a system, never adjust the pressure relief valve in a system with a failing pump. In addition, always check the relief valve setting when replacing a failed pump.

Understanding the mechanics behind a truck's hydraulic system can be a challenge. However, once mechanics learn how to maintain these systems, trucks will operate safely and efficiency will increase.