Although many detractors sneer at the idea of hydraulic efficiency, right-sizing components, proper system design and modern technology can go a long way to achieving system efficiency.
“Hydraulic efficiency” is a term alluding similar sentiments to “exact estimate” or “scientific belief.” It’s not that hydraulic efficiency is an oxymoron, per se, but these aren’t traditionally two words that make sense shoulder to shoulder. If efficiency was your top benefit on the list of machine requirements, fluid power wouldn’t have been on your short list of options, at least in the past half-century or longer.
Efficiency is a word now more commonly familiar to us, thanks to the escalation of green values—especially those defining the way we use natural resources. No longer can we take a limitless and inexpensive source of energy for granted, nor can we abuse the dirty sources of inexpensive energy at the expense of our precious environment. We must take full advantage of our energy resources to achieve the work required for maintaining our standard of living, while reducing associated waste along the way.
What is efficiency?
I define efficiency as work-in minus work-out. Essentially, it’s the differential between the energy your process requires and the energy input required to achieve that process. Your process could be stamping, rolling, injecting, moving, pressing or any other mechanical function capable of being achieved in a rotational or linear motion. If you’re running a punch press, for example, the machine efficiency is defined as the current draw of the pump’s motor minus the combined force and velocity of the punch die.
Most machines are designed to convert energy from one form to another, which can sometimes occur multiple times. Because of the Laws of Thermodynamics, you cannot change energy from one form to another without creating waste energy, and this is a fact regardless of the energy transformation taking place. In the case of a hydraulic machine, you must convert electrical energy to mechanical energy within the electric motor, resulting in partial waste. Then you must convert mechanical energy into hydraulic energy within the pump, resulting in partial waste. Then you must convert hydraulic energy back into mechanical energy at your cylinder or hydraulic motor, resulting in partial waste.
The amount of energy wasted in the above example could be staggering, especially if you’re using an old machine with old components. Let’s say you have a 10-hp electric motor—and keep in mind electric motors are rated on power consumption, not power output. Your old motor might have an efficiency of 85%, meaning it will produce 8.5 hp at its shaft, the other 1.5 hp being wasted as pure heat.
In your old power unit, you have a worn and tired gear pump. When new, a gear pump is lucky to have 80% efficiency, so I’ll be generous to throw 75% at this example, since gear pumps become less efficient over their lifetimes. So this pump can convert only 6.4 of the motor’s 8.5-hp shaft output into usable hydraulic energy. The rest of the energy is, you guessed it, wasted as pure heat. We’ve now lost 36% of the electrical energy inputted, and we haven’t even done anything yet.
Just to be intentionally derisive, I’m going to choose a hydraulic motor as our actuator; a gerotor motor to be exact. These motors come at a modest price and perform at a modest level. They were a clever design back in the day, but have high leakage to lubricate the myriad components, and they leak even more if you operate them outside their optimum torque and speed curve. Leakage, I should note, is a designed element of most hydraulic components, based on gaps and clearances with internal moving parts, which is required to lubricate that component. More moving parts or higher clearances means more leakage, and I should further note, any fluid lost to leakage carries with it pure heat equal to the pressure and flow of the leakage.
Now that I’ve blasted gerotor motors, I’ll back it up by saying they’re often incapable of reaching 80% efficiency. There are some versions of these “orbital” motors, like the disc valve variant, which can be close to 90% efficient, but it would be only within a tiny window of flow and pressure. I’ll stick with 80% for this example, which is generous. With the 6.4 hydraulic horsepower we have in our system, we’re left with 5.1 hp at the hydraulic motor’s shaft.
Why use hydraulics in the first place?
So with barely half of our input energy making its way to the output stage, it’s easy to see why I’m dubious of “hydraulic efficiency.” So why use hydraulics when we could have powered our machine straight from the electric motor and take advantage of 8.5 hp instead of 5.1? In that answer lies the reason hydraulics are awesome; with $300 worth of valving, you can infinitely vary torque and speed, and reverse direction. Our electric motor would require sophisticated electronic control to achieve the same features.
To be fair, I’m using one of the worst-case examples for hydraulic efficiency. Not only are there more efficient components available than gear pumps and orbital motors, there are ingenious approaches to using hydraulic components. Furthermore, recent advances in electronic control have not ignored the fluid power industry, and there are some tricks to further improve hydraulic efficiency.
Invest in better technology
I can’t stress enough that a hydraulic machine is really just an energy conversion device, and when you can convert your input energy into usable force with as little heat waste as possible, you’re on the right track. A pump converts the mechanical energy of the prime mover into hydraulic energy in the form of pressure and flow. If I were to recommend one component you blow the bankroll on, it would be the pump.
A piston pump, especially a high-quality one, can be 95% efficient at converting input energy into hydraulic energy. Not only does this pump provide 27% more available hydraulic energy than our old gear pump, it creates 80% less waste heat than it, reducing or eliminating cooling requirements.
Not only does an efficient pump help, an efficient design works wonders. If you have a fixed displacement pump on a flow control, any unused fluid is wasted as heat. For example, take even our 95% efficient fixed piston pump, giving us 9.5 gpm out of a theoretical 10 gpm. If your downstream priority flow control valve is set to 5 gpm, 4.5 gpm is bypassed to tank. However, all of the 9.5 gpm is still being created at full system pressure, and what’s dumped to tank is lost as heat. So now our 95% efficient pump is helping create a 50% inefficient system.
To get around this, pressure compensation was created. A pressure compensated pump is set to a particular standby pressure, and when this pressure is reached, the pump reduces flow until downstream pressure drops below that standby pressure. For example, if you have a 10 gpm pump set at 3,000 psi, and flow is restricted below 10 gpm, the pump will reduce its displacement to exactly match the downstream flow and pressure drop at 3,000 psi. Essentially, the pump only produces the flow being asked for, no more, but always at 3,000 psi.
But what if we only want 1,000 psi for a particular operation? Well, you could use a pressure-reducing valve, but the pump is still producing 3,000 psi, so you’re not saving any energy. To remedy this, the load-sensing pump was invented. A load sensing pump has an additional compensator that is plumbed downstream of the metering valve. This configuration allows it to measure load pressure and compare it to compensator pressure. The result is the pump will provide only the pressure and flow required of the circuit and actuator, with only a few hundred psi worth of pressure drop as the waste by-product.
Recent advancements in control technology have resulted in a similar concept of pressure and flow management, but using a combination of fixed displacement pumps, servo or VFD motors and pressure transducers. The pressure transducers measure pressure after the pump and after the metering valves, and PLC gives the signal to rotate the pump at a speed only fast enough to achieve the desired pressure and flow. It’s quite an advanced technology, and has progressed to the point a pump could hold a stationary load and rotate fractional speed just to compensate for leakage. Another advantage to this technology is that the motor doesn’t even turn when no energy is required, and then again only with the energy required by demand of the hydraulic system.
Aside from choosing efficient pump designs, using efficient hydraulic actuators is the next best place to continue. Not much can be said of hydraulic cylinders, because most are close to 100% efficient already, depending on sealing technology. But just like with your hydraulic pump, the hydraulic motor has many variations, each with their own contribution to overall efficiency.
Ranking popular hydraulic motors in terms of efficiency, they range from radial piston, axial piston, vane, gear and orbital, with efficiencies around 95, 90, 85, 80 or less, respectively. Of course, these motors would have the same ranking in cost, so the adage of “you get what you pay for” applies here. Other than just choosing an efficient motor design, there isn’t much you can do to enhance efficiency, other than eliminating return port backpressure, and applying motors with the same load-sensing techniques described with pumps.
So for the most part, hydraulics is not an efficient technology. But neither are gasoline-powered cars, and millions of those are sold every day, because there is no better option for their task. Regardless, efficiency in hydraulics is progressing, and advancements in materials and technologies will further that. As long as you are aware of what it takes to create “hydraulic efficiency,” the term won’t seem curious like “seriously funny” or “virtual reality.”
Filed Under: Fluid Power Basics, Fluid Power World Magazine Articles, Pumps & Motors