By Josh Cosford
Complete understanding of fluid power (or any vocation for that matter) really does require the general—yet finely honed—macroscopic lens, as well as intimate comprehension of details. So get out your “macroscope,” unfold your lawn chair, pop a squat and aim your optics this way, because I’m about to kick your hydraulic brilliance up a notch.
Two words: science & math. If your affinity towards mathematics and science is already strong, then I’m positive your excitement with getting your notch kicked up is just as strong. However, those of you with strong suits other than math and science just let out a collective groan. Don’t worry; I’m not going to drop quantum theory and differential calculus on your lap and then claim it’s going to improve your fluid power wisdom.
My recommendation is for you to educate yourself with, and then subsequently put into practice through regular usage and application, the most fundamental aspects of science and math. As lame as high school seemed while you were stuck in class at nine in the morning when you would have rather been sleeping, 80% of what you need to master fluid power was taught at the secondary school level.
One example of material learned in high school yet still used every day in our industry is geometry, and nearly every day I calculate the area of a circle or the volume of a tube. I calculate the area of a circle to figure out how much force a hydraulic cylinder makes, or conversely, how much pressure I need to move a given mass. I calculate the volume of said cylinder to arrive at figures for cycle time or flow requirements. It seems elementary to you or any eighth grader to punch πr^2 into a calculator (which is the formula to calculate the area of a circle), but if I told you the mass I have to move and the pressure I was limited to, could your run the equation in reverse to figure out the piston diameter we need?
Even using this level of geometry and algebra could be considered too specific for the point I’m trying to convey in this issue. A=πr^2 is an application equation. How can we use our macroscope to broaden our understanding of fluid power using the circular area formula as an example?
Humans have been using π (pi) for thousands of years to describe the relationship between a circle’s circumference and a circle’s diameter. Simply stated, we know that a circle is 3.14 times longer around its circumference than it is straight through its diameter in the middle. This theorem is often underestimated or overlooked entirely. On top of that, we work with exponentially increasing area in that we use the square of the radius of a circle, not just the diameter itself.
Think about this: If I have a 2-in. bore cylinder and we jump to a 4-in. bore cylinder, did I double the force my hydraulic cylinder is capable of pushing with? Not even close! We actually increase force fourfold! This may not be intuitive if you’re not into math, so go get into math! Take a couple night classes, visit the countless (free) websites on math education or borrow some books from the library.
Another example in which your geometric affinity could be beneficial is in understanding levers and trigonometry. Imagine diagnosing a hydraulic excavator that has the operator claiming the 5000 lb he is attempting to move should be easily overcome by the current system pressure. Doing the math for piston area as discussed previously, you calculate the cylinder is capable of the 5000-lb of force.
But it’s not that simple, is it? Because the load on the bucket is out past the point in which the cylinder is lifting, we have a class three lever, which is a mechanical disadvantage. We actually need 10,000 lb of force. But wait, there’s more! The cylinder isn’t acting perpendicular to the boom; it’s pushing on the boom at an angle, which is a further disadvantage because some of the force is wasted pushing outward not upward. We have to bust out some trigonometry to figure this one out, and the result is that it actually takes closer to 12,000 lb of force at the cylinder to lift that bucket.
I think you get the point how advantageous mathematics is to the world of fluid power, but what about science? Scientific mastery allows us to zoom out our macroscopic lens to an even wider angle of understanding, nearly bypassing annoying details altogether. If you understand the fundamental laws of the universe, it’s easy to affirm or dismiss an analysis of a machine design or a contentious troubleshooting resolution.
Laws are called such for a reason, in that they are universally irrefutable. Fluid power is not part of a yet to be discovered dimension of time and space where physical laws do not apply. The unfortunate and sad truth is there are still a number of individuals whom deny the connection between physics and fluid power. You will hear these persons claiming that “flow makes it go,” or that “pressure is resistance to flow.” Both of those expressions are, of course, utter bologna, and entirely devoid of any foundation in science.
We know that force makes things go. In fact, Isaac Newton understood this more than 300 years ago. I’ll say it again: Force makes things go. If you push on something more than it pushes back, it will move. The long-standing hydraulic misconception is that rate of flow makes cylinders and motors go. It’s hard to fault anyone for this belief, especially because the formula for steady state cylinder movement uses flow as a variable.
Higher flow rates do, in fact, results in faster cylinders and motors. But the flow isn’t doing the moving; force is. A higher flow rate simply allows us to create pressure (force) more quickly. In simple terms, flow is just the rate at which you can create force. Without flow, we are no longer able create the force differential required for movement; the oil simply maintains its volume or even decays.
Another useful physical concept to keep in your pocket protector is the Law of Conservation of Energy. This law states energy can neither be created nor destroyed, but can only change forms; energy in must equal energy out. It also means you cannot get something for nothing. If your current hydraulic system run by a diesel engine does not appear to be performing as well as you wish, simply trying to increase pressure may not be possible. More pressure requires more input power, which could be beyond the capability of the current output of your engine. Your power is limited, and increasing either pressure or flow will also require more horsepower.
The Law of Conservation of Energy can be used for diagnostic and troubleshooting purposes, too. Understanding that all of the energy being put in to a system is being accounted for is some form allows you to figure out why the energy input doesn’t appear to equal the work trying to be achieved at the output. For example, if you have a 10-hp electric motor running your pump, but you are only able to observe 3 hp at your hydraulic motor, you realize the power has to be going somewhere. Because any energy not being used to create useful work is wasted as heat (and a little bit of sound), finding a physical location—such as a relief valve—in the system appearing to be hotter than its surroundings may signal the energy is being lost through that component. Using a relief valve as an example, all flow over the valve is wasted as 100% heat, and you will have to diagnose why the oil is taking this path.
Josh Cosford, Certified Fluid Power Hydraulic Specialist, is with www.fluidpowerhouse.com.
