Josh Cosford • Contributing Editor
With the barrage of information, we’re exposed to daily from television, newspaper, radio, magazines and online media, few hours in the day involve freedom from being reminded how important it is to be mindful of our energy consumption. Let’s face it, we’re not any less reliant on energy than we were six weeks ago, let alone six decades ago. If you’re an environmentalist, you understand how difficult it is to extract energy resources from our planet and the repercussions involved with some of those processes. If you’re an accountant, you understand how expensive energy is, and it’s not getting any cheaper.
Waste is common
Industrial hydraulics has not traditionally been a champion of efficiency. If you list the top three reasons to
use hydraulics for an application, efficient use of energy is not on that list. Much of the history of industrial hydraulics was written to reflect the advances made in power density, reliability and responsiveness, all areas where advanced hydraulic systems excel. Each of those factors had at least some detriment to efficiency, and only in the past decade have they been systematically addressed.
Power density describes the amount of force capable of being achieved with the smallest possible actuator. Hydraulics has no peer in this regard. No other system of force transfer is capable of creating so much force from such small devices. The key to hydraulics’ high force density is the high pressure of the fluid used to transfer force. By using a small pump to stuff fluid into a large actuator, the level of force that can be achieved is monstrous.
But because of this high pressure, the potential for energy waste is high. High pressure means fluid will bypass and/or leak with greater intensity. A key fundamental of hydraulics is that any fluid not being used to create useful work is wasted as a heat by-product. This higher pressure equates to a higher pressure drop, and higher pressure drop means a higher waste of energy. You can imagine fluid leaking past a piston in a hydraulic pump will create a drop in efficiency proportional to the rise in pressure—for example, 5,000 psi leakage wastes more energy than 2,500 psi leakage.
Equally important is considering the energy lost over a pressure valve at high pressure. Just as more energy is wasted through leakage when pressure is high, the energy lost over a relief valve, for example, is also directly proportional to its pressure setting. Once again, 5,000 psi blowing over a relief will waste twice the energy as 2,500 psi, if flow is equal in both cases.
Intelligent circuit design is key
To remedy the waste of energy related to high-pressure hydraulic systems, intelligent circuit design is a must, and proper selection of efficient components should be heeded. A hydraulic circuit that relies on relief valves to control force will just result in wasted heat as fluid jets back to the tank, having done nothing but heat up the return line. But if an actuator is appropriately sized, it will operate the loads it was designed to handle without ever going over a relief valve. Also, not all hydraulic valves are created equal, and some have internal clearances tighter than others, mostly due to how refined they are in their quality and manufacturing process. When in doubt, use only the highest quality components.
Reduce internal leakage
Hydraulic cylinders, motors, pumps and valves require lubrication and sealing to operate effectively and efficiently. The piston shoe of a variable pump will melt quickly if it were sliding across the lens plate with no lubrication. Lubrication is one of the fundamental qualities of hydraulic fluid and is required to allow operation of all the various components used. Without lubrication, the metal-to-metal surface friction of many hydraulic components would make their function impossible.
The problem with lubrication is that it’s required to be consumed as leakage, and does not provide any
useful work to the machine. Leakage fluid flows through clearances, providing boundary lubrication and sealing, but always moves to an area of lower pressure. For this reason, and as previously mentioned, only high-quality components should be used if you care about efficiency. They will require less leakage to maintain lubrication due to their tighter tolerances.
Because leakage is such a concern for efficient hydraulics, top-tier manufacturers have addressed the rates of leakage from their components. Most of the advances are in metallurgy and refined machining processes. Advanced metals used in the pistons, rotating group and wear plate of a piston pump, for example, allow for a harder surface finish with lower friction properties. As well, the improved machining of these components creates clearances that result in more consistent lubrication and less required leakage to achieve it.
Take care in valve selection
Improved sealing for valves is also a result of those same advances in material quality and machining processes. Spool valves, for example, have traditionally been known to exhibit high leakage due to large clearances between the spool and body. But with a tighter and consistent clearance between the spool and body, and improved material quality that won’t warp or expand when heated, they will waste little fluid in sealing and lubrication.
Servovalves have long been the gold standard in hydraulic valve performance and represent the most responsive design of their kind. They are expensive and tricky little guys, able to flow rivers of hydraulic oil belying their size. They are easily the most responsive valve you can get, with rapid transient and frequency responses. However, they require absolutely ridiculous pressure drops to achieve this. In other words, they consume anywhere from 600 to 4,000 psi of fluid at their given flow rate, just to achieve this high performance. They’re the equivalent of a full-throttle Formula 1 car, which is clearly king of the hill in their domain, but perhaps a Corvette would suffice.
The Corvette of hydraulic valves is the proportional valve. In their simplest form, they are simply spool valves with variable coils that can vary their magnetic field to position the spool within the body, rather than just bang it from side to side. This allows it to vary flow to the work ports. The problem in the past is that they haven’t been that accurate or responsive, and were susceptible to pressure and flow forces.
The industry has responded by creating its Corvette “Z06,” which is a proportional valve using onboard electronics and spool-position feedback. The valve recognizes where the spool is supposed to be, then the electronics tell it where it really is and adjusts position accordingly. High-performance “prop” valves are so advanced as to nearly match the performance of servovalves. Because they are so close in performance now, they are an excellent option over servovalves in improving machine efficiency because they require minimal pressure drop to achieve that performance.
Variable speed reduces energy waste
The newest technology to hit the industrial hydraulic market is that of servomotor controlled pumps. Traditionally if you wanted to vary flow to a hydraulic circuit, the best option was with variable displacement pumps. However, when these pumps are pumping anything less than full flow, they’re not at their optimal efficiency. By using a fixed displacement pump, but varying its speed with a servo or VFD motor, the pump can provide as little or as much flow as needed, with minimal leakage and energy waste.
This technology requires sophisticated electronics with closed-loop control, but current projects and studies are resulting in massive energy savings. When in standby mode, the pump will spin at fractional speed, using only enough energy to maintain leakage and pressure. Although currently an expensive technology as well, it still provides enough return on investment to be justified for high-end applications, and just as with any new technology, it will become more economical as it proliferates the market. When this prediction is realized, hydraulics will enter a golden age of efficiency and re-invigorate its relevance as a primary choice for machine actuation.
Filed Under: Cylinders & Actuators, Fluid Power World Magazine Articles, Pumps & Motors