A primer on hydraulics

March 1, 2003
AS truck-mounted hydraulic systems have gotten more complex, the challenge has increased for installers to know the intricacies of installation and for

AS truck-mounted hydraulic systems have gotten more complex, the challenge has increased for installers to know the intricacies of installation and for municipalities to know the intricacies of troubleshooting.

“A lot of the problems that can happen in hydraulics come from installation errors and lack of education,” says Jason Westad, director of training and education for Force America. “The technology is rapidly advancing, so training has to follow. Ultimately, the end goal is to get trucks up and running properly and save on down time. The better educated the customer is, the better off all of us are.”

Formulas are important

For those who manufacturer boxes and apply cylinders and those who make and sell hydraulic systems, formulas are critical. For example, someone who wants to raise a 40,000-lb load and has a valve that has 2000 PSI available needs to plug those numbers into this formula: area = force/pressure. The result would be 20 square inches, or a 5" diameter cylinder.

Westad says there are two factors that allow someone to do more work with a circuit or a function:

  • The diameter of the cylinder.

    “The larger the diameter, the more weight you can lift at the same pressure, because you're pushing over a larger area.”

  • The amount of pressure.

“With increased pressure on the same diameter cylinder, you can lift more weight with a little more pressure.”

“What we always have to weigh that against is the speed of the function,” he says. “You could put, let's say, a 7" diameter cylinder on a function with very low pressure and it probably is able to do the work because it's a large-diameter cylinder. But it might move slower. So you'd have to put a larger pump on there to make it move faster, because what affects the speed of a function is flow.”

System components

He says every hydraulic system has to have these five components:

  • Reservoirs that dissipate heat and store fluid.

    The baffle, between the inlet side of the pump and the returning fluid, allows oil to cool down and allows any oil that might have been entrained to escape through the filler/breather before it gets sucked out to the pump again.

    The filler/breather allows air to escape, removes large debris when fluid is being added. The drain port drains fluid as the magnetic plug catches steel particles floating in the fluid. There is a return port on the middle of the reservoir, one on each side, and a suction port on the lower side, one on each side.

  • Strainer/filters that clean fluid.

    “We recommend 10-micron absolute filters — that's filtering contaminants down to 10 microns,” he says. “The human eye can see a 40-micron particle. We really want to keep any contaminants out of the hydraulic system. Contamination accounts for 80 to 90% of the failures in a hydraulic system.”

    There are two types of return line filters: in-line, which are installed in-line with the return flow, usually as close to the reservoir as possible, with 15-25 PSIU check valve and gage port; and the most costly in-tank, which are mounted in the reservoir on the top and allow the filter to be changed without losing fluid.

    Suction strainers are designed to filter out the larger contaminants prior to the pump. They are rated in mesh, usually threaded into the reservoir. In most applications, it will be necessary to drain the reservoir before the strainer can be serviced.

  • Prime movers — electric motors or internal combustion engines.

    “A prime mover will turn the pump. You're taking rotary energy — the energy produced from the truck — and converting it to hydraulic energy.

  • Relief valve.

    It's a protection valve for all hydraulic components that relieves oil back to the reservoir once it reaches a certain pressure.

  • Pumps.

Types are positive displacement, fixed, and variable; classifications are gear, vane, and piston.

“More and more, the trend is moving toward variable-displacement piston pumps because of reduced heat and horsepower savings,” he says. “With a gear pump, you're always turning a pump at certain RPMs. At 1800 RPMs, a gear pump will always produce the same amount of oil because it has a fixed-displacement. A variable-displacement pump has the ability to produce variable amounts of oil at the same RPM.

“When you turn on a gear pump, you're always pumping, say, 20 gallons of oil. If you only need seven gallons of oil, you're still producing 20 because you turned on the pump and it's doing certain RPMs. That extra 13 gallons is going back through the system to the reservoir and creating heat and excess horsepower off the engine. If you're only using seven gallons but pumping out, that extra 13 gallons is wasting the horsepower of the truck. In extreme conditions, sometimes people will put 50-gallon pumps on and they may only use seven. So they're wasting 43 and creating a lot of heat.

“The variable-displacement piston pump has the capability, used in conjunction with a load-sensing valve, that allows you to produce only the flow you're asking for.”

He says piston pumps can conserve energy and avoid heating the system, since they supply only the required flow at a pressure level dictated by the load condition. By changing the angle of the swash plate, the pump now has the ability to determine the amount of output and deliver only what is needed by the system.

All piston pumps operate on the principle that a piston reciprocating in a bore will draw in fluid as it is retracted and expel fluid on the opposite stroke.

Westad says piston pumps are highly efficient, come in a wide variety of displacements, have medium- to high-pressure capabilities, and can operate at a higher RPM range continuously. Because of their closely fitted parts and finely finished surfaces, cleanliness and good-quality fluid are vital to long service life.

Strengths: they operate at higher speeds vs. a gear pump; operate at higher pressures; adjust flow to the demands of the system; low noise level; long service life; pressure compensation and load-sensing are standard, not an option; and they can operate with a constant mesh PTO.

Weaknesses: Contaminated fluid accelerates wear of pump parts and blocks critical lubrication passages; high vacuum conditions led to catastrophic failures; excessive case pressure causes shaft-seal leakage and rotating group failure; excessive speed allows pistons to separate from shoes.

He says the majority of piston pumps (4 cubic inches or larger) will be mounted on the front bumper or frame extensions, and connected to the harmonic balancer on the engine via a driveline. The pump will have a 1.25 straight-keyed shaft. The driveline is usually a solid type and short in length. It is important that the driveline angle not exceed three degrees vertically or horizontally.

He says the smaller displacement pumps can be direct mounted to a PTO. An anti-seize compound must be applied to the shaft of the pump prior to installation; this will prevent premature wear of the pump shaft and the PTO shaft. A main relief valve is recommended in the directional control valve. This will suppress hydraulic shock loads and provide additional system protection (200-400 PSI above high pressure setting).

He says all load-sensing type pumps will have a load-sense signal line that has to be installed from the directional control valve to the pump. In most instances the load-sense line will be a -4. When all the spools in the directional-control valve are in neutral, there will be no flow in the load-sense signal line. A load-sense signal will be present when a valve spool is shifted and the pressure of the signal will be equal to the pressure required in that valve section.

Before starting a piston pump, it is necessary to pre-fill the case with fluid. Failure to fill the case can result in undue wear on the rotating group, and in some cases cause immediate pump failure.

Westad says that when a component fails, it is due to contamination, aeration, cavitation, overload conditions, or abuse, and that components seldom wear out without contributing factors.

Replacing a component may not necessarily correct the performance problem, which can be compounded by the failure of more than one component. When a failed component is replaced, the system must be thoroughly flushed; strainers, filters, and the fluid must be replaced.

Troubleshooting

He says there are three major requirements to troubleshoot a system:

  • Understand flow and pressure concepts.

    “Flow controls speed and pressure controls the amount of work you can do,” he says. “That's an important relationship because a lot of times people will call and say, ‘My hoist wasn't going up fast enough, so I cranked up the relief. I gave it more pressure.’ We have to tell them, ‘That has nothing to do with the speed of your hoist. It only has to do with the amount of work it can do.’ By cranking up the relief, a lot of times they get the pressure too high in the system and burn up the pump.”

  • Understand the system components and their functions.

  • Have a systematic approach to problem-solving and a means of knowing each system intimately.

“You always want to isolate the problem,” he says. “Is it the pump? Is it the valve? Is it the electronics? Once you isolate it, we have steps we can give the person based on which component they have.”

He says the system will tell the troubleshooter a lot by the heat it develops, the noise it makes, pressure, and the speed at which the system is operating.

“Do not assume that leaks are mere nuisances, when actually they are warning signs of more serious problems,” he says. “Investigate all leaks and make the proper repairs.”

He lists the common errors: operators are not trained to recognize when they are damaging components; the system was not set up properly, with the pressure too high or too low; no filtration; the wrong fluid, wrong viscosity; undersized conductors requiring more horsepower to compensate for backpressure, etc.

Add-a-fold valve

He says troubleshooting the add-a-fold valve, the electronic load-sensing valve requires an understanding of the problems that could occur:

  • No sections will operate.

  • Section not holding a load, which is due to either cross port leakage in the neutral, main spool, the spool not in the center condition, or a loose spring to the center kits.

  • Load creeps up, which is due to the spool not being in neutral, with the PSI gauge higher than normal.

  • Pump always under a load, because the unloader's stuck.

  • Valve shifts manually.

  • No flow at any section. If the unloader is not closing, remove and clean because debris or contaminants may be lodged in the valve. If there is relief bypassing, remove and clean debris or contaminants. If there is no load sense signal, install a pressure gauge in the LS port in the inlet section.

More troubleshooting

The add-a-stack valve, designed for snow plow applications, has a V40 section for high flow and V20 sections for lower flows. Here are Westad's troubleshooting suggestions for various V20 problems:

  • No response at cylinders or motors (all spools).

    Determine which type of system it is: load sense or unloader. Check the pressure gauge on add-a-stack. Check the system main relief or hydraulic pump. Manually operate the spools when possible. Check the electrical power to solenoids or the unloader to make sure it's not open.

  • One or all spools jerky or erratic.

    Check pilot pressure (pressure-reducing valve). Check for bare or loose connections. Check the pressure on the port being affected. If the pressure that is required to move the load is lower than the pilot pressure, this can cause the same problem.

  • Section will not meter or it is jerky.

    Check solenoid or cartridge valve, which may be sticky.

  • One section will not operate, but all others do.

Operate a section to the left or right of the bad section and operate the bad section at the same time. If the bad section operates now, there is a problem in the load-sensing portion of this section.

Under control

Westad says that in the past, hydraulics on municipal trucks were generally operated with levers or cables that would allow the operator to engage the valve and move the function. But now, instead of stringing 12 cables for 12 functions, there is one electric harness and electric joysticks that operate the functions.

“Automated ground-speed-oriented controls are big,” he says. “Traditionally, municipalities have run hydraulic hoses up to the cab of the truck and had hydraulic flow control in the cab. That affected the rate of granular application for salt and sand trucks. So the driver would have to look in the rear-view mirror and adjust the knobs to see how much granular he was putting out.

“Now, with ground-speed-oriented spreader controls, you don't need to run hydraulics up to the cab. It's in the main harness coming from the valve, and there is an electronic spreader control in the cab that is calibrated.”

Westad says the CommandAll has certain wires pre-wired to the enclosure when it is removed from the shipping container. Other cables and wires that need to be installed will be packaged and labeled in a separate container.

He says the speedometer sensor is important because the automated spreader controls are ground-speed oriented, and truck manufacturers typically put it in a variety of places. With Navistar, it's now Option #012VVW. With Sterling, it's Post J on the passenger-side circuit panel.

Looking at the enclosure from the passenger side of the vehicle, the relay circuit board — which controls the valve functions — is located on the back wall. The spreader control board is located on the front wall. The switch panel assembly contains three items: back light circuit board, power distribution circuit board, and switches. The Switch panel is secured to the enclosure by removing 10 buttonhead screws to gain access to the circuit boards.

System power is obtained through the red 8-gauge wire. Connection is made to the battery and terminal, and an 80-amp circuit breaker is installed in this circuit, as close to the batter as possible to protect the wire to the enclosure. The system neutral wire is a white 8-gauge wire, which should not be connected for other auxiliary equipment.

The CommandAll enclosure contains a constant duty relay that can be controlled from the vehicle's dash key. The switch panel already has been removed and the dash key disconnect wire is visible. The wire color is orange and is distinguishable by the in-line fuse attached to it. That wire should be connected to the ignition switch, using a compatible connector and shrink wrap. It should only be hot when the ignition is in the “on” position.

Back lighting — terminology used to describe the illumination of the labels — allows the labels to be dimmed when the dash lights are dimmed. It requires the installer to provide the necessary wire from the dash lights of the vehicle to the back light terminal board.

The back lighting circuit board, located above the fuse board, is attached to the switch panel assembly. It has two or four warning-light terminals. The warning lights will have power provided to them from the control relay. The wire installed from the dash lights should be connected to the proper terminal that is provided on the circuit board.

The warning devices usually are open electric switches, with power provided from the control relay. A wire should be connected from the proper warning label to the sensor or switch.

About the Author

Rick Weber | Associate Editor

Rick Weber has been an associate editor for Trailer/Body Builders since February 2000. A national award-winning sportswriter, he covered the Miami Dolphins for the Fort Myers News-Press following service with publications in California and Australia. He is a graduate of Penn State University.