If I could time travel back to my early 20s to my job at the car wash, I’d get a kick out of my awe regarding the little hydraulic motor that could pull a track loaded with twelve cars so easily and smoothly through the wash. I was so clueless about hydraulics that I once took the power unit’s pump to a local hydraulic shop to be serviced. Instead of removing the pump, I also brought the entire vertical mount bell housing and tank lid along!
I made mistakes, and it’s expected that all beginners make mistakes. But since my official fluid power education began in 2006, I’ve steadily grown and shed the frequency of mistakes. My mistakes now are either advanced or entirely palm-to-forehead worthy (like forgetting to join lines on a schematic). But there are mistakes it seems beginners only make, and they’re more common than I’d like. By highlighting these mistakes beginners must stop making, you can kick your hydraulic knowledge up to the next level.
- Confusing pressure and flow: Let’s get this out of the way and relate pressure to force and flow to speed. Pressure is simply force applied to a defined area (pounds per square inch), so increasing pressure will increase the force at your actuators.I once helped a customer who purchased a pump from an industrial retailer and complained that it now stalled out his gas engine. He admitted he was looking for a larger pump because he wanted his float trailer to lift more weight. Of course, I explained to him that larger pumps make “larger” flow, and he was entirely confused when I recommended a smaller pump.A smaller pump allows you to increase the pressure slightly while using the same input horsepower. Of course, the other components in the system must be rated for the increase, but this tale shows you how this mistake leads to wasted time and money.
- Confusing force and pressure: The difference here is subtle, and the result of this mistake is generally benign. This mistake affects your reputation more than your machine because it just makes you appear as if you don’t know what you’re talking about. As mentioned above, pressure is simply pressure over a defined area, while force is the energy potential of one object’s interaction with another.In conversation, be sure to describe pressure as pressure and force as force. Don’t be the guy who says your hydraulic press makes “fifty tons of psi.” On the other hand, avoid the misleading exclamation that your power unit operates at “3000 psi of force.” Force requires a vector (direction), while pressure is a scalar quantity (direction is irrelevant).
- Believing that flow makes it go: This mistake comes with a jingle and is probably the most common mistake even more advanced fluid power professionals still make. I, myself, was taught that “flow makes it go, and pressure is resistance to flow.” It’s quite the opposite, and Sir Isaac Newton agrees that all motion results from force, and oil molecules are no different.All pressure starts at the pump, and saying pressure is resistance to flow implies that pressure is higher at the resistance than the source. Pressure can only move from a direction of higher potential to one of lower potential.I’ve written many times about Cosford’s Law, which states: “Pressure makes it go; flow is the rate at which you create pressure.”
Once you eschew these three common mistakes, you can open your mind to the true nature of fluid power. Don’t feel bad if you were guilty of making any or all of these mistakes; it just means you’re at the beginning of your journey while on the right path.
Filed Under: Engineering Basics