Modern mobile machinery has changed quite a bit. Here is a look at how construction equipment hydraulics have changed over the last couple hundred years.
By Josh Cosford, Contributing Editor
On a road near my home, there exists a hand-laid stone fence, perhaps 4 ft high and a hundred times as long. Crafted from locally-sourced rocks some century ago, I drive its length in awe as I imagine the physical and time resources used in its construction. The machinery to excavate, haul and lay heavy material was uncommon in the 1800’s, so I can’t reason it was constructed using anything but many strong hands.
The construction industry is as old as farming, and as societal needs grew, so too did the requirement for improvements in construction. The industrial revolution grew our capacity to construct buildings and infrastructure exponentially. Light and moderate construction techniques built our homes and offices, while heavy and intense construction made the factories and the roadways to get there. The hand-laid stone fence was obviously a light construction project, but it’s the heavy and intense construction so well suited to hydraulic motivation that has been important to civilization.
Modern construction gathers steam
Steam power is a form of fluid power energy transfer, but instead of pressurized air or hydraulic fluid, heat energy is added to water until it turns to its gaseous form. This transformation creates pressure as gas volume increases, which was captured in actuators to power large machinery. This technology gathered steam, as it were, in the early 19th century, but records show as early as 1796 a steam-powered dredge was used to clear the beds of waterways in England.
In 1835, William Otis, cousin to American industrialist Elisha Otis of elevator fame, applied steam energy to create a single-bucket land excavator. Accepted as the first self-powered, land-based machine used for heavy construction, it revolutionized the building of railway lines. This patented machine was able to move 300 yd3 per day, where two men and a wheelbarrow would drag this task out to a fortnight.
Some fifty years later, Sir W. G. Armstrong built the first excavator using hydraulics, where it was used in the construction of docks. It was steam powered, but also employed cables with only hydraulic actuation on one function. A semi-interesting aside: Armstrong’s company eventually merged with Vickers Limited, but disappointingly after much research, I could find no link to the Vickers of hydraulic fame. Regardless, Armstrong’s machine didn’t work very well and left the door open for others. The first machine to use only steam-powered hydraulic actuators without the aid of wheels and cables was the Kilgore 2-1/2 Yard Steam Railway Shovel. This machine was productive, but like the Armstrong machine, it was limited to rail line construction.
Creating a modern standard
It would take nearly another century before excavators looked and operated as they do today. For most of this span, excavators would remain cable-operated or some type of steam, mechanical, cable and hydraulic hybrid. Demag (now Komatsu) created the first 360°, all hydraulic, track-driven excavator as we know it today. The 1954 Hydraulikbagger, Figure 1, was powered by a 42-hp, 3-cylinder diesel and capable of 2.5 mph while carrying about a half yard of material. It was compact, efficient, agile and productive, especially for light and moderate construction projects.
So effective was the B504 that its construction features are now standard for the industry. Once excavators were gifted fully-hydraulic operation, construction equipment was capable of utility and productivity not previously possible. Decades earlier, the Ford Model T’s domination would pave the way (that’s right, I went there) for the development of interstate highways. The B504 was timed perfectly because the development of Eisenhower’s Interstate Highway System started shortly after. I’m not claiming the events were related in any way, but their timing ensured the construction industry in America would expand as never before.
Mobile construction equipment took form because of the inherent advantages of hydraulics; power density, controllability and reliability. Step one for hydraulic machinery was getting it all to work reliably and efficiently, but because construction is a competitive, low-margin industry, advancements came fast and hard. Productivity was chased, which needed the puzzle pieces of power, control and reliability to fall into place.
Early machinery was moderate pressure open loop, consisting of mostly gear and vane pumps running 1,000-2,500 psi. Even in the 1960s when hydraulic excavators were dominating their cable-operated counterparts, the technology advance was slow. OEMs saw the benefits hydraulics provided, so they applied the technology to loaders, scrapers and dozers, making them powerful and effective. But in the 60s, machining technology wasn’t able to provide the close tolerances required to make high pressure pumps, valves and actuators.
Higher pressures, sophisticated controls
As applied knowledge advanced, manufacturers realized high pressure was the key to productivity – and by “high pressure,” I mean 3,000 psi. Piston pumps can produce high pressure with efficiency, but they had to master tighter clearances and differing coefficients of expansion. Early variable displacement piston pumps used a swashplate with lever operation to control flow, providing an efficient speed control alternative to metering valves, which wasted energy.
The 1970s could be considered the decade of hydraulic creativity. To increase control and productivity, engineers were inventing clever ways to control hydraulics. The first hydrostatic drives were mastered and applied to loaders, enabling them to transition quickly and smoothly between forward and reverse motion. Caterpillar had the pressure compensated axial piston pump patented, and torque limiting was also developed in the decade of disco.
Torque limiting (also known as horsepower control) is a method to automatically limit flow inversely proportional to pressure. As pressure rises, flow drops, and when pressure drops, flow increases. This method gave the best of both worlds, allowing an excavator to behave as if its prime mover was twice the rated horsepower. The swing, boom, arm and bucket functions could all move rapidly with no load, but then the pump would cut flow as pressure rises, supplying the force needed for heavy work.
By the 1980s, cable operation was nearly extinct in the construction industry. So effective was hydraulics, that even the control functions were hydraulic pilot operated, which was an older technology. The brakes, the steering and the machine functions could be worked from the cab using pilot valves. Try to explain to your teenager that a joystick used to have oil running through it, and the distance and vigor the joystick moved would push fluid at the spools of the directional control valves with the same effort.
Enter load sensing
However, the proliferation of load sensing technology in the 1980s freed up horsepower, and in combination with improving machining tolerances, pressure (and therefore power density) rapidly increased. Load sensing allows the hydraulic pump to provide the exact flow and pressure required by the actuators, adding only a little extra energy to create pressure drop. It wasn’t uncommon to now see standard 4,000 psi for the implement functions and more than 5,000 psi for the travel circuit. With load sensing, running 5,000 psi doesn’t cripple flow when you’re limited with input horsepower.
Although mobile construction equipment had the most advanced hydraulic systems in existence, they fell way short when it came to electronic control. Even electrical control was not a trusted method of operating pumps or valves. The 1990s didn’t see a lot of advancement with construction equipment, especially in the way that hydraulics were controlled. Digital machine monitoring existed, but most of the technology was supplied for operator comfort — climate control, stereo systems and 12-V chargers.
The advent of electronic controls
The turn of the century saw machine OEMs strong-armed into progress. The looming Tier 4 emissions standards forced manufacturers to rethink the design and implementation of construction machinery. Machine functions were increasingly controlled electronically, where hydraulic joysticks were replaced by proportional control, cabs were fitted with LCD digital displays, and machine maintenance intervals were monitored electronically. However, pressure hadn’t increased in three decades, remaining in the 5,000 psi range well into the late 2000s.
Electronics are now prolific in the construction industry. Just as with your car, your excavator has programmable performance modes. You can run in “eco” mode, or with the adjustment of a convenient dial, ramp it up the high-power mode. GPS navigation, automatic grade compensation, traction control and hybrid drive systems are working their way into modern construction machinery.
The crawler dozer, pictured, is a machine of surprising technological advancement. High-end models have individually controlled hydrostatic drives for left and right tracks, themselves each closed-loop electronically controlled. The dozer’s path is maintained based on operator control, and the software accommodates regardless of load, turning angle or traction. They are available with software applications, real-time data logging and customizable machine responses. If one operator prefers feather-touch, high response from her controls, while another prefers a slower, attenuated control method, both can save their user profile preferences. The machine can’t be started until the operator inputs their login, at which point the profile is loaded.
The value of power density is not lost on dozer manufacturers. New machines are closing in on 7,000 psi, allowing higher torque from smaller, lighter machines and realizing improved fuel economy. Lighter machinery also makes transportation to and from worksites much easier and provides a side benefit of reduced ground compaction.
What does the future hold?
So, what does the future hold for construction equipment hydraulics? It’s obvious that pressure will continue to rise, enabling smaller, lighter machines to achieve productivity previously enjoyed by only large, high-powered equipment. Advanced materials will permeate machinery, using both carbon fiber and 3D-printed metals to increase strength while reducing weight.
Digital control with increased saturation of cyber-physical systems will be commonplace. A construction sight workday will be planned from a computer control station, where all worked is carried out remotely with operator-less machinery. As well, the continued electrification will see engines replaced with electric prime movers and battery packs. At some point, machines will be fully autonomous, where a digitally scanned topographical map of the territory is inputted, and the machine is told how to grade or excavate to match the desired output.
Industrial environments increasingly see electric actuators, eschewing fluid power altogether. However, electric actuators will never replace hydraulic actuators in construction machinery. I make this bold prediction because electric cylinders and motors can never be made so small yet so powerful as to replace hydraulics. A 100-hp, bent-axis piston motor can fit into a shoebox, and that’s at current industry pressure levels.
Where I see electric actuation expanding is with power delivery. Instead of central power units and distribution through hydraulic control networks, actuators will be self-contained integrated actuators. The servomotor and pump combination will be built into the hydraulic cylinder, which will include a small reservoir and manifold containing all hydraulic controls. These units will be modular, configurable and controlled via wireless networks, while still providing the high force that makes hydraulics king.
The modern mobile construction machine has come a long way from the steam-powered machines of the industrial revolution. Continued advancement will see machines become more productive, efficient and powerful, while the reduction of machine operators will see worksites become safer, especially as robots replace construction workers. But I doubt I’ll ever see another newly constructed stone fence at the hands of robots.