Efficient– (of a system or machine) achieving maximum productivity with minimum wasted effort or expense.
“Efficient.” en.oxfordictionaries.com. 2017.
Sometimes a simple dictionary reference can explain better in a few words what may take me a paragraph or more. I think we can all agree on the definition of efficient, especially in this day and age, where cars, dishwashers and light bulbs are marketed for their careful use of natural resources. Nobody can argue the natural order of the world encourages intelligent use of energy, if not for ecological reasons, but at least for economic ones.
There is a lot of hypocrisy in the world of ecological activism, I must admit. I’ve never understood how it makes sense to buy a 50-mpg hybrid car from Japan when it must travel on an ocean liner that uses enough fuel to supply a fleet of Hummer H1s over the next millennium. Then again, if one was a true environmentalist, one simply would not drive. But I digress; the point of this article is efficient design in industrial hydraulics, not as an outlet to criticize hypocrites, which I myself can sometimes be.
Efficient use of energy is just plain smart, whatever your motivation may be. Given the option, there is no reasonable logic in avoiding intelligent selection and use of efficient hydraulic components for industrial machinery. I do understand capital allocation for machinery can be limited, which sometimes prevents machine manufacturers from employing the best components and designs, especially since free market dictates (mostly) the quality of the machinery itself. If you are an end user in the machinery food chain, it is your obligation to ensure you’re not shooting yourself in the foot by choosing a machine that ends up costing you more in the long run than what money you saved with a poorer design.
So what makes a hydraulic machine efficient? The first step is a deep understanding of the basic properties of nature … specifically the Second Law of Thermodynamics. This law states that total entropy in a closed system can only increase over time. In layman terms, it says energy can only get more disorganized over time, and the harder you try to get energy to behave itself, the more you’ll end up wasting it. This means no perpetual motion machines, and no getting something from nothing. Every single process in the universe results in some form of waste. No exceptions.
For any good engineer, physics comes as second nature. For others, it’s not so intuitive. If you think it’s a good idea to use a hydraulic power unit to run a hydraulic motor, which in turn is powering a generator … well, you are in the “others” group. Every time you must convert energy from one form into another, you waste energy and lose efficiency. The above example converts mechanical energy into hydraulic energy, back into mechanical energy and then into electrical energy. Meanwhile, you could have just used your prime mover to power the generator directly.
Okay, so what if there is an absolutely essential reason for your hydraulically powered generator? Like say your generator is stuck inside the belly of a giant mechanical snake, a circus clown is riding a unicycle with a hydraulic pump attached to the hub, and there was no direct way to mechanically link the unicycle with the generator except with a trail of hydraulic hose. Hydraulics now kind of make sense, but there would be a right way and wrong way to do it.
The wrong way to transmit energy far from its source is to use inefficient components. If you absolutely must transmit hydraulic energy deep into the belly of a monster, you absolutely must choose efficient components. This is where my own definition of efficiency, as it relates to fluid power, comes into play: efficiency is the difference between the input horsepower and output horsepower.
If your (non-clown) power unit runs with a gear pump, which itself is being powered by a 10-hp low duty electric motor, you’re off to a bad start. If your prime mover starts out with 75% efficiency, you’ve already reduced the hydraulic input power available to the pump by a quarter. Now your gear pump only has 7.5-hp to work with, and that’s before it’s done its own terrible job of converting energy. I should mention that an electric motor’s power rating describes how much energy it uses rather than how much it outputs. Creating a measly 7.5 mechanical hp still sucks back 10 hp worth of electrical energy, and that’s what your meter reads when it comes to your electricity bill.
Depending on pressure, gear pumps are lucky to be 80% efficient. As pressure rises, they become less efficient as pressure bypasses internally at an increased rate. Remembering that we’re now only working with 7.5 mechanical horsepower at the pump’s input shaft, we can reduce the hydraulic energy being delivered to the circuit down to 6 hp. The 1.5 hp missing from the equation is lost as pure heat (just as is the first 2.5 electric hp). If you’re looking for a space heater, give yourself more than 10,000 Btu/hour of heat being thrown off by our pump-motor combination … and we haven’t yet introduced our hydraulic motor to turn the generator.
With all the fanfare of a Renault Le Car at the Pebble Beach Concours d’Elegance, I introduce to you the gerotor motor. The gerotor motor is a type of internal gear hydraulic motor using a spool valve to distribute fluid to the rotor set, which itself rotates the shaft. These are also called “low-speed, high-torque” (LSHT) motors. Although they can produce adequate power over limited ranges of speed and pressure, they’re less efficient than even a gear motor. The problem is due to many moving parts and large clearances between them; they leak like a screen door, wasting upwards of 40% of incoming fluid as heat.
With merely 6 incoming hp, your motor is applying only 60% of that to the generator. The generator is left with only 3.6 hp available to it. And you would be correct to guess the generator itself doesn’t entirely convert that 3.6 hp completely intact. When all is said and done, the electrical power available to run a smoke machine in the mechanical monster snake is lucky to be one-third of the original power input! So what’s a circus clown to do?
Bozo needs to respect the laws of nature, and I’m not just talking about his capacity to squeeze two dozen of his colleagues into said Le Car. I’m talking about the respect he and everyone else must have for the Second Law of Thermodynamics. If you want an efficient machining running in your plant, you have to first ask for it.
Once you ask for it, here’s what can be done….
The choice of electric motor, pump and hydraulic motor must be of the highest quality and most efficient design. Some jurisdictions have already started to regulate electric motor efficiency, requiring certain ratings of power be only sold as premium efficiency or better. Standard and high efficiency motors (IE1 and IE2) are quickly becoming obsolete, which themselves are limited to 75-85% efficiency, depending on horsepower rating.
Premium and above-premium efficiency motors (IE3 and IE4) can supply 95% or more efficiency, meaning our 10-hp example provides a whole 9.5 hp to the hydraulic pump.
Speaking of hydraulic pumps, there is just as much a range of efficiencies in pumps as there are in electric motors. Of the major common types of pumps, bringing up the rear is the external gear pump. They’re inexpensive and reliable, but they are typically in the 80% efficiency range.
Coming in second place is the vane pump. The vane pump is quiet, which is why it’s the staple in an industrial environment. Anyone spending time working around a hydraulic power unit for an extended period of time can be brought to tears by the high-pitched harmonics of a hydraulic pump, so it’s understandable that the vane pump is used frequently. However, they hover in the mid to high 80 percent range in efficiency, leaving much still to be desired when efficiency is concerned.
Hitting the highs of efficiency are the piston pumps. Although there are a few major designs of piston pump, I’ll skip over those details and just say a piston pump is good for 90-95% efficiency. Sure, they’re expensive, but within the cost of ownership for a pump, only a few percent of the money you’ll spend is on the purchase price, with the remainder being gobbled up by energy usage.
To run your snake’s generator, we are obviously going to skip the gerotor motor, which doesn’t even have a baseball glove, let alone belong in the ball game. Just as with pumps, hydraulic motors are available in the same gear, vane and piston varieties, all with similar efficiencies. There are some variations of each with improved performance, but it’s safe to say a piston motor is the most efficient, and should be chosen.
So if we use an above-premium efficiency motor (95% efficient), a high quality piston pump (95% efficiency) and a piston motor (95% efficient), total system efficiency (comparing input energy to output energy) = 0.95 x 0.95 x 0.95 = 85.7%
Now with 10 hp worth of electrical power being used, the hydraulic motor turning the generator has nearly 8.6 hp available to it. That’s a lot more juice to supply robotic functions to our mechanized snake. Our gerotor motor was only able to supply 3.6 hp in the previous example, which means if that level of power is adequate enough, you could actually make due with a 5 hp motor, and save over 3,700 W on incoming electrical power!
Understanding and respecting the physical laws of nature is the first step in ensuring your industrial hydraulic system is designed efficiently. Other tricks can be used, such as hybrid pump drives, closed-loop pump control and load-sensing circuits, but those things are like splitting hairs compared to simply selecting well-made, efficient components. Better yet, start with your efficient components and add the tricks to those components to achieve total efficiencies well above 90%. After all, nobody wants to be the clown trying to power a giant snake with a unicycle.
Filed Under: Fluid Power Basics, Pumps & Motors