Grading and Excavation Contractor | Sophisticated Systems Call for Sophisticated Maintenance

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	Photo: ©iStockphoto.com/Andrew Johnson

Preventing excess heat, keeping fluids clean, and plugging leaks are some of the ways you can help maximize performance of today’s hydraulic systems while minimizing downtime and repair costs.

By Greg Northcutt

Sitting in command of a 37-ton excavator, a 185-horsepower bulldozer, or any other machine that digs, lifts, pushes, hauls, or otherwise moves earth, it’s easy to take for granted the machine’s hydraulic system. But this system is the heart of your machine’s performance. In harnessing the power of the engine, it applies pressure to a confined fluid and drives a mechanical load. It’s what allows you to raise and lower a boom, adjust the angle and tilt of a blade, and, in many cases, drive and steer the machine.

Producing higher pressures and faster cycle times than ever before, today’s hydraulic systems feature closer tolerances in control valves and between all moving parts and greater system stress. This increasing sophistication, along with the desire of owners to maximize uptime, extends maintenance intervals and prolongs machine life, highlighting the need for proper maintenance in several areas to enjoy full performance and value from these systems.

For example, undermine the physical or chemical strengths of hydraulic fluid by letting it get too hot, too viscous, or too dirty, and you risk damage ranging from erratic operation due to sticky valves and loss of efficiency to costly catastrophic failure of pumps, motors, valves, and cylinders. What’s more, the components of hydraulic systems work together intimately. As a result, damage to one component can lead to damage to others. For instance, a leaky cylinder seal can cause fluid to overheat and break down, causing damage to other cylinders or pumps.

Hydraulic Advantages
The beauty of hydraulic machine control as opposed to the use of mechanical systems, such as levers, gears, and cables, is that any change in pressure applied to the fluid at any point is transmitted undiminished to the load. That allows you to adjust speed, torque, and power infinitely without any peaks or valleys and to control the machine easily and precisely. What’s more, should the load exceed the machine’s power, the system can be designed to stall without damaging the equipment.

The performance of the hydraulic system reflects both the amount and the pressure of the fluid flowing through the system. It includes a pump, which draws fluid from a reservoir and transmits it to the motor. The motor, in turn, drives a shaft or turns a load, providing the muscle for the system. Meanwhile pump-control, pilot, and relief valves control the pressure of the fluid. The speed at which these components operate is determined by the flow rate of the fluid, while the hydraulic pressure determines the force that this fluid exerts. Tying the system all together are filters, hoses, lines, and fittings, along with a cooling unit.

Another feature that favors the use of fluid over mechanical power transmission in construction equipment is what Thomas Watts, an ACE engineer with hydraulic system designer and manufacturer Eaton, calls the much greater power density of hydraulic systems. “Hydraulic systems have a very high power-to-volume ratio,” he explains. “To control the same amount of power as a hydraulic pump, the physical size of a diesel engine would have to be many times larger. A small-sized package gives hydraulic power a big advantage when operating mobile equipment, where efficient use of space can be critical. Power density and controllability make hydraulics very attractive.”

This size advantage continues to improve. Not long ago, for example, pumps commonly produced about 0.125 gallon per minute of flow per pound of pump. Today, that’s increased fourfold to about 0.5 gallon per minute per pound. At the same time the power output of hydraulic motors has been boosted from about 0.5 horsepower per pound of motor to 2.5 horsepower per pound and higher.
Hydraulic systems on today’s earthmoving machines typically generate pressure in the range of about 2,000 to 6,000 psi. That’s one reason for the high degree of precision control when operating hydraulically powered equipment.

“The higher pressures of today’s hydraulic systems require smaller flows from smaller components for the same-size vehicles than in the past,” says Michael Morge, senior market research consultant for Caterpillar.

In addition to its primary function of creating force and motion by converting flow to pressure, hydraulic fluid serves two other roles: It lubricates metal surfaces and helps cool system components.
Following manufacturers’ recommendations for inspecting and maintaining the system is the key to making the most of the precision and efficiency of your machine’s hydraulic power.

“Maintenance requirements are more important with today’s systems because of the higher pressures, tighter-fitting components, and increased complexity of such components as pumps, motors, and control valves,” says Morge. “Repair before failure has become commonplace to avoid unpredictable downtime, much higher costs of repair after failure, and the need to clean out the entire hydraulic system after failure.”

Here are some of the factors that can impair performance of hydraulic systems, along with steps you can take to control them.

Photo: ©iStockphoto.com/Andrew Johnson

It takes a lot of hydraulics to make a bucket do its job.

Control the Heat
Excess heat affects the performance and life of hydraulic systems by degrading the hydraulic fluid. By causing the fluid to oxidize, heat changes the fluid’s chemical composition and reduces its viscosity and ability to lubricate properly.

“One of the functions of hydraulic oil is to cool internal components of pumps and motors,” Morge notes. “Overheated oil can cause sticky control valves and premature failure of seals and bearings, resulting in very expensive repair of major hydraulic components.”

“Normally, the fluid regains its viscosity when it cools,” says Diego Navarro, service marketing manager for John Deere Construction and Forestry. “However, overheating destroys the molecules in the fluid, lowering the viscosity. Every 19°F above normal operating temperature cuts the life of hydraulic fluid in half.”

The loss of viscosity and lubricity increases metal-to-metal wear in pumps, motors, and valves, adding to heat buildup. Fluid can push past heat-hardened seals to create still more heat. Dirt around hydraulic fluid coolers and reservoirs can restrict airflow and raise the fluid temperature. Keeping a hydraulic function in over-relief can also cause fluid to heat up.

Keep Out Contaminants
The main threat to hydraulic systems with petroleum fluids is contamination from air, moisture, glycol, dirt, and metal particles. In fact, an estimated 70%–80% of all hydraulic system problems can be traced to these materials. Problems caused by contaminants range from accelerated wear of components and reduced life of fluids to slower machine operation and even complete failure of a component.

“Start with a clean hydraulic system and keep it clean,” advises Morge. “Contamination is the mortal enemy of hydraulic systems.”

For example, air—which can enter the system through a leak—can cause slow or jerky operation of the hydraulics. Foamy or milky hydraulic fluid can be another sign of air contamination.

Moisture in the air, which enters through the reservoir breather, can condense and lead to serious damage. In addition to freezing, blocking, or even breaking components, water can also promote chemical reactions that damage system hardware and adversely affect the viscosity and lubricating properties of the fluid. For example, water consumes corrosion inhibitors in the fluid, leaving components vulnerable to rust. Water in the system can also damage metal surfaces of pumps through a process called cavitation. Air in the fluid can also cause cavitation.

“Any water vapor in the fluid will form bubbles as it passes from liquid to vapor and vice versa with changes in pressure and temperature,” says Navarro. “As hydraulic pressure increases above a certain level, these bubbles collapse. This implosion creates tremendous microscopic pressure that can pit the internal surfaces of a pump and quickly destroy it. Air produces a similar effect and can cause wear to accelerate as well.”

The most destructive contaminants are the abrasive ones, such as alumina, iron, and silica. With the tight clearances inside pumps, valves, actuators, and other components, all it takes is a speck of dirt or a fragment of fiber to bring a piece of earthmoving equipment to a halt.

“Hydraulic loads are often balanced and carried by a very thin film of fluid,” notes Watts. To cool, lubricate, and carry the load, he explains, this fluid must be able to flow freely through clearances ranging from several thousandths of inch—as, for example, between a spool and a bore—to millionths of inch, such as between the moving parts of an axial piston pump.

“If a particle of metal, sand, or other contaminant gets wedged inside that clearance, it can destroy the lubrication, increasing the wear of components by creating metal-to-metal contact and damaging rubber seals,” Watts says. “If the particle is hard enough, it can act like a cutting tool and increase wear of metal components. It can also scratch sensitive surfaces, causing fluid to leak. This, in turn, degrades the system’s efficiency and increases cycle times.”

Some fluid-contamination problems take longer than others to significantly impact machine performance. For example, it may take quite some time for an operator to notice the loss of machine accuracy due to particle damage of valve metering surfaces. Some problems come and go. A particle that prevents a valve from closing completely may wash away only to be replaced later by another similar particle that causes the same problem before it, too, is washed away. A repeat of such episodes can lead to intermittent hydraulic system failure. Even worse, a particle that blocks the pilot orifice of a valve or causes a pump vane to become wedged in a rotor slot can cause catastrophic failure.

Sources of Contaminants
Contaminants can enter the hydraulic system through worn cylinder seals, faulty gaskets, and leaking valves, notes Navarro.

“If you see oil coming out of the hydraulic system, you can be sure that dirt and other contaminants are getting in,” he says. “Typically, fluid analysis of machines with leaking cylinders shows very high contaminant levels.”

Hydraulic-powered attachments, such as breakers or compactors, can also be a source of contamination. “Whenever you connect a hydraulic hose to your machine there’s a possibility of introducing contaminants,” says Navarro. “So can repairing or replacing a hose or pump in the field.”
Oxidation due to heat and exposure to oxygen in air can contaminate fluid with chemical reactions that adversely affect the lubricating properties of the fluid.

Mixing different hydraulic fluids is another source of contamination. “This is probably the most common fluid mistake that equipment owners make,” says Navarro. “The consequences of mixing different brands of fluid, producing a new formulation and different viscosity, are difficult to pinpoint because it takes time for the effects to show up.”

The consequences of mixing the two basic types of hydraulic fluid—ZDDP, which contains zinc to protect against oxidation, and TCP, which does the same thing using a different technology—are much more serious, Navarro notes. “Mixing these two different types can deplete additives, cause formation of sludge, and generate copper,” he says.

Contaminants, such as paint and weld splatter in hydraulic components and bits of rubber and fibers remaining in hoses, originate in manufacturing processes. Even the fluid itself can contribute to the problem. It can become contaminated with metal and rubber particles as it flows through pumps and hoses at the refinery and with rust and scale while stored in drums or bulk tanks.

Plug the Leaks
Check your machine, including pumps, valves, and cylinders, for leaks and replace seals as necessary. In addition to improving hydraulic performance, this can also help protect the environment. As little as 1 quart of petroleum fluid can pollute up to 250,000 gallons of water.

Take special precautions when checking hoses. “Fluid escaping a pinhole leak under pressure can be almost invisible,” says Jason Caldwell, marketing programs manager for Gates Corp. “Under the pressure of today’s hydraulic systems, fluid shooting from a pinhole leak can penetrate skin, causing severe, even fatal injuries. Never check for leaks by running your hand over a hose or a hydraulic connection.” Instead, he recommends holding a piece of cardboard or sheet metal a few inches from the hose to locate a pressurized leak. For a drip or a low-pressure leak, clean the area with a rag to locate the source of the fluid.

Look for puddles of fluids on or around the machine and low fluid levels in the reservoir, which also indicate a leak.

Inspect hoses and couplings according to manufacturers’ recommendations and wear safety glasses, he notes. Check for blisters, nicks, cracks, cuts, or signs of abrasion or hardening, which can be a source of future leaks. Also look for fluid seeping or weeping from the interface of the hose and coupling. Replace hoses showing any of these conditions. “Always use new hoses, couplings, and crimping equipment from the same manufacturer,” Caldwell says. “These items are not interchangeable among different manufacturers".

Navarro recommends that, prior to replacing a hose assembly, you should first use a sponge cleaning projectile to remove any metal and fiber fragments from inside it and cap the ends to prevent contamination until it is attached to the machine.

Filter the Fluid
“Proper filtration is probably the easiest and most cost-effective way to lower hydraulic system maintenance expenses and help ensure trouble-free system operation,” says Bert Elfers, director of product management for Baldwin Filters Inc.

As he points out, standard hydraulic filters focus on removing particles of a given size, such as 10 microns or larger. They are not designed to remove sub-micron soot, molecular-level contaminants such as those causing odors, bacteria, or burned oil. They are also not designed to correct acidic levels. Special filters are required to remove water from hydraulic fluid.

Suction filters, located between the reservoir and pump, use atmospheric pressure to move the hydraulic fluid and screen out any particles before they can enter the hydraulic system. As the name indicates, pressure filters—usually placed between the pump and control system—use pressure to push oil through the filter.

“Both types of filters will remove particles larger than a certain size,” says Watts. “However, it’s important to understand that they don’t screen out 100% of those particles.”

Very small particles, such as silica and alumina, can pass through hydraulic filters repeatedly, adds Navarro. “If they enter the system at a regular rate, their numbers tend to increase over time,” he says.

Manufacturers rate the effectiveness of filters in removing particles using a Beta ratio—the ratio of the number of particles of a certain size entering the filter to the number that pass through. If one out of every two of the particles in the fluid passes through the filter, the filter’s Beta ratio is 2 and its efficiency is 50% for that size particle. Similarly, a filter with a Beta ratio of 4 (one of every four particles passes through) is 75% efficient in removing particles, while one with a Beta ratio of 20 (one of every 20 particles passes through) is 95% efficient. If only one out of every 200 of the particles passes through the filter, the Beta ratio is 200 and the efficiency is 99.5%.

Generally, suction filters have Beta ratios of about 2 to 3, Watts reports. Beta ratios of pressure filters are typically in the 20 to 30 range. This is because the clean-filter pressure drop typically increases with higher Beta ratios.

Equipment manufacturers use a pressure-differential indicator to determine when a filter is no longer functional. This device measures the change in fluid pressure between the upstream and downstream sides of the filter element. As the amount of particles trapped by the filter increases, the pressure drop—or difference in pressure across the filter—increases, and the remaining life of the filter decreases. Typically the OEM will specify a maximum allowable pressure drop of 25 or 50 psi for a pressure-type filter, Elfers reports. On some machines this is indicated by a light. Others use a gauge to show when that point has been reached.

“Use caution when interpreting high pressure drops as signaling a clogged filter,” Elfers says. “High fluid viscosity, caused by low temperatures, can cause the pressure drop to increase temporarily.”

More Cleaning Options
Keeping the fluid clean is more cost-effective than replacing it or the components that are damaged due to dirt and contamination. One way contamination gets into a system is through the breather on the reservoir. Various products, such as the Dirt-gate and H2O-gate from Eaton, will help keep contamination out of the reservoir and the system. Even when adding fluid from a drum or other storage container, it is best to filter the fluid prior to introducing it into the reservoir. Eaton’s Clean Cart filters particulates and water contaminants from the fluid as it is being pumped into the reservoir.

Kidney-loop mobile filtration units can be used in the shop or on the job site to augment onboard filters by efficiently removing very fine particles from hydraulic fluid. For example, Deere’s Super Caddy, available from the company’s participating dealers, allows a technician to check particle counts and to flush particles and water from different hydraulic fluid systems. Separate filters allow the technician to avoid cross contamination when cleaning diverse hydraulic fluids.

“Onboard sensors provide information about particle count and percentage of water saturation as the technician filters the oil,” says Navarro. “A variable-speed drive allows the technician to change the flow of the pump, depending on fluid viscosity or temperature. It even indicates if the fluid is too cold to give an accurate reading, which keeps the system from giving a false signal when the fluid is below 80°F.”

Some types of fluid-degradation problems, such as those caused by oxidation, excessive acidity of the fluid, or mixing different fluids, can be solved only by removing the contaminated fluid and replacing it with clean fluid.

One way is to replace fluid in the reservoir. However, this removes only some of the total fluid in the system, notes Navarro. For example, draining the reservoir of an excavator still leaves about half the contaminated oil in the rest of the system. In the case of backhoes, about 60% of the total fluid remains in the system after draining the reservoir. To replace the fluid this way requires draining the reservoir, refilling it with new fluid, cycling the machine, and repeating this process several times. “Depending on the hydraulic system’s capacity and the capacity of the reservoir, you may have to do this a total of five times to replace 90% to 97% of the contaminated fluid,” he says.

Another way to remove the old fluid and contaminants is to shoot projectiles through the hoses and lines. Participating John Deere dealers provide such a service. It offers a faster alternative to flushing hydraulic lines with solvent and air. The mobile Ultra Clean System, which can be used with any make or model of machinery, uses a pneumatic launcher to shoot a specially designed projectile through hydraulic lines and hoses. “The technician simply disconnects both ends of a line or hose and launches the cleaning projectile through it,” Navarro says.

Contaminated components should be disassembled, cleaned, and inspected for reusability.

Keeping nipples clean is important.

Keep Track of Contaminants
Periodic analysis of hydraulic fluid is another important part of keeping the hydraulic system operating at peak power and efficiency. It involves collecting samples of the fluid and having them analyzed in the lab. Various equipment manufacturers, oil companies, and independent labs provide this service. Typically, the fluid is analyzed for wear metals, additives, contaminants, viscosity, water, Total Acid Number, and particle count.

Labs vary in their ability to provide a complete analysis of the sample. Eaton, for example, can supply very sophisticated laboratory analysis that not only identifies the cleanliness and character of the fluid but also identifies specific contaminants. It uses spectrometric analysis and scanning electron microscope (SEM)/energy dispersive X-ray analysis to help identify the source of the contamination.
Alternatives to conventional lab testing include the Target-Pro 2 Portable Particle Counter. Available from Eaton, it uses fluid samples and gives laboratory-quality cleanliness-identification results in the field. Another is the TestMate Contamination Monitor (TCM) inline particle counter. Made by Schroeder Industries LLC, this palm-size unit connects to the hydraulic system and continually monitors fluid particle levels in accordance with ISO 4406:1999 as the machine is operated. Lights or codes on a monitor screen in the machine’s cab indicate the number and size of the particles. This unit does not identify the type of particle in the fluid. Instead it determines the size and amount of contamination in the hydraulic fluid.

“The lights or codes come on if the fluid is too dirty, based on the user’s setting,” says J.D. Funk, the company’s manager for oil service products. “That indicates that it’s time to change the fluid, diagnose the problem, and/or use a filter cart to prevent failure of hydraulic system components.”
This unit also detects performance problems before an operator does.

“Many operators don’t notice hydraulic performance has dropped off until the system has lost about 20% of its total efficiency,” Funk says. “The TCM can tell you if efficiency has dropped as little as 6% or 7%. Depending on the size of your machine, that can be a huge difference.”

The ability to continuously monitor the condition of the hydraulic fluid is another advantage. “It provides a movie of contaminant levels instead of a snapshot,” he says. “That can eliminate changing fluid every so many hours even though it doesn’t need to be changed. Also the unit can store up to a year’s worth of data to keep a record of certain maintenance events, such as when filters needed changing. That can be useful in troubleshooting problems.”

Greg Northcutt writes frequently on construction issues.

GEC - September/October 2007

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