By Carl Dyke, CD Industrial Group
Today’s engineer needs to understand the complexity of modern systems, from fuel consumption to load sensing.
Mobile hydraulic component and system designers have quite a challenge. Market competition compels them to keep up with a lot of change and innovation over time. If we think about the advances in mobile hydraulic systems design, what are we really talking about? Is it that the machine has become more complex with highly sensitive operator controls? Are these systems sophisticated because of the intentional engineering to make them more efficient, accurate and reliable? Is it also that the reliability of these systems makes a significant contribution to human safety? The answer to all of these questions is yes.
Let’s have a look at a few of these design parameters and the operating, maintenance and troubleshooting issues that can arise.
Safety systems require reliability
It is true that hydraulic systems in use on mobile and construction machines feature some of the most sophisticated circuit designs. In many cases, these machines feature braking and steering systems that must offer the highest levels of priority function and reliability. Hydraulic accumulators are often used in the larger machines as the main flow buffer for steering and brakes.
Mobile crane systems and aerial work platforms are additional examples where both high reliability and safety are paramount. A failure over the control of the boom or the stabilizing outriggers has serious implications. While electronic sensors are often involved, basic hydraulic components such as pilot-operated check valves and counterbalance valves have to function flawlessly.
Manufacturers of these valves work hard to produce a spool that will not seize and a poppet design that is least likely to trap a particle on the seat during valve closure. Perfect performance for all conditions and for badly contaminated fluid is impossible, so the use of a built-in, wire-mesh filter screen is typical for the pilot ports at the least.
Production and efficiency
A wheel loader that might have one hydraulic pump dedicated to the steering function may also have a control valve to share that pump’s flow with other functions—such as the boom lift or bucket tilt—to achieve maximum cylinder speed when steering is not occurring.
Excavators need to operate non-stop, for days and weeks while offering extremely accurate and consistently repeatable motions. The only reason to start the engine in an excavator is to power the hydraulic pumps. Therefore, the entire hydraulic system must be designed to be efficient with the fuel source.
Road graders are designed to save fuel. It is not necessary to heat fluid by unloading most of the hydraulic system during long travel cycles when hydraulic motion is not needed. Yet, the hydraulics must be ready to respond at the instant when the operator moves a valve lever. One way to accomplish this objective is with a load-sensing system.
Load-sensing systems are now found on a large percentage of mobile machinery, due to the efficiencies that they offer. These systems use pressure feedback—directly from the work-loaded cylinders and motors—to constantly modify and control the pump’s parameters.
The pump involved is generally of the variable displacement piston type, and the spool valves for directional and flow control appear to be typical. However, a special signal hose from the valve bank to the pump controller offers a clue to the presence of the load-sensing design. A network of small passages in the valve bank (and the use of tiny shuttle valves) sends the pressure from the valve section with the highest loaded, active motor or cylinder, all the way to the pump’s controller (compensator) through that special load sense signal line. The maximum system pressure is constantly modified using the variable displacement controller on the pump.
Using a variable displacement pump together with constantly adjusted maximum system pressure results in less wasted fuel at the prime mover. The maximum hydraulic system pressure is constantly adjusted to be only slightly higher than what is needed at any operational moment. When directional valves are returned to the center position, blocking off pump flow to the system, the load sense line is bled down, which sets the system pressure to a low standby value of only a few hundred psi. Pump flow is also automatically adjusted to near zero. In this standby state, the pump demands minimal power from the prime mover.
Load sensing control for accurate flow
The load-sensing system also offers greater flow accuracy. If the hydraulic system is operating motors that turn metering and mixing augers on a mobile concrete batch truck, then this flow accuracy has a direct impact on product quality.
An electrical generator needs to maintain steady speed regardless of the electrical loads that it is powering. Any serious fluctuation in rotor speed could cause problems for the electrical appliances supplied by the generator. If the generator is turned by a hydraulic motor, the hydraulic system must have a way to correct the speed of the motor where there would otherwise be fluctuation with changes in electrical power demand.
In both of these examples, as the pressure at the hydraulic motor changes, the pump responds to the signal on the load sense signal line, changing its displacement slightly to maintain a steady pressure drop across the flow controlling valve, thereby providing a steady flow rate to the actuator. This is a pressure-compensated, flow-controlling technique that comes built-in with most every load-sensing system.
Component and system designers have the challenge of making sure that this closed-loop control scheme can be properly tuned.
Features such as load sensing add complexity to the system. If one hears a machine owner express a wish for the simpler hydraulics from a previous decade, they are also wishing for higher fuel consumption, and machines that were not as responsive.
Maintenance and common problems
Hydraulic faults in mobile machinery have much in common with faults in any other hydraulic system. Overheating and fluid contamination are common causes for intermittent faults as well as for component and system failures.
Conducting proper inspections and carrying out basic preventative maintenance at optimum service intervals significantly helps to reduce the frequency of failures. The setting of service intervals may need to be examined and adjusted on a seasonal basis or as work conditions change in environments where harsh usage occurs.
The gathering of scheduled, careful measurements of fluid properties, system pressures, temperatures, cycle times and flow rates can help predict a problem or failure at an early stage when corrective action is still a relatively inexpensive option. This practice is known as predictive maintenance.
A service shop’s disciplined use of preventative and predictive maintenance techniques increases the mean time between failures (MTBF) and keeps the machinery at maximum levels of production availability.
Hydraulic system problems on these sophisticated mobile machines can also be caused by previous maintenance work. This is often the case for reports of erratic cylinder or motor motions on a load-sense hydraulic system. A pump controller adjustment—if done incorrectly, perhaps without proper knowledge of how load-sense systems work—can be the start of a whole new problem, and possibly a safety issue as well.
A replacement component installation, such as a changed-out pump, can be the source of a new problem if the work is not done correctly. In the case of a drain hose accidentally connected to a controller option port on a piston pump, the now non-functioning pump controller can rack up a hefty repair bill due to extreme system overheating.
One of the most common problems reported from users of one-off, custom-built, load-sense hydraulic systems is “hunting” or “surging.” That is, the motors and cylinders under certain circumstances will be speeding up and slowing down, all while the proportional, directional valve is apparently being held at one constant flow position. This is not unusual if the machine design has not been tested for all possible operational conditions. Adjustments to the system may have to be made to accommodate a unique accessory or implement to be connected to the machine’s auxiliary ports.
In some cases, the operator of a mass-produced machine may find a way to put the hydraulic system through its paces in a way that was not the intent of the manufacturer—and in doing so, causes faults and wear that were not at all expected. It is not always possible for the original equipment manufacturer to lock out this type of harsh usage; thus, a series of faults and problems to troubleshoot will arise.
Even more complexity: Learning is not optional
We have glimpsed into the precise nature of valve design and the use of load feedback to dynamically adjust a pump controller, but we haven’t covered all of the technology that makes for a sophisticated, modern hydraulic system in a mobile machine. As was mentioned earlier, electronics certainly play a role. Quite often, an engine speed sensor informs a control module about excessive engine load, with that same module then sending an electrical signal to the displacement controller on the largest pump. The pump is automatically downsized while the engine is under excessive load to prevent an engine stall.
This is only a basic explanation of a horsepower limiting control, as was the explanation for the load-sense control scheme. However, they do serve to illustrate that there are carefully specified design parameters at work on modern mobile machines. These parameters, while appearing a bit narrow at times from an operator’s perspective, are meant to improve fuel economy—or they are there to make the machine motions smoother or more accurate. Without a doubt, the overlapping systems compel a machine maintainer or troubleshooter to deepen their understanding of the overall engineering goals.
CD Industrial Group Inc.
cdiginc.com