Hydraulics is easy to get wrong. In fact, I’d say there are more ways to screw up in hydraulics than there are to get it right. Not everyone finds fluid power concepts to be intuitive, so without proper education and training, mistakes, oversights and misconceptions occur all the time. I’d like to save you at least some trouble by giving you my “Top Five Hydraulic System Errors.”
1. Confusing flow and pressure valves. This error is somewhat common because of the dynamic relationship between pressure and flow. Installing a relief valve inline that is used to control flow will work in some cases, but is terribly inefficient and creates more heat than anything. Also, when a flow control valve is installed inline, and especially if you have a constant flow pump, setting changes have an effect on backpressure. To the untrained, this could appear as though the flow control valve is used to regulate pressure. However, these are two different valves for a reason, and if you don’t know how to apply them, ask someone who does.
2. Not realizing more flow requires more power. I’ve seen it a dozen times; a customer asks for a bigger pump because they want more flow. When you quote them a whole new pump/motor group, they’re shocked by the unexpected cost. You can’t get something for nothing, and any given combination of pressure and flow requires enough input horsepower to accommodate. If you increase one or both factor(s), you must also increase input power. More power means a new motor, and there is no way around this.
3. Not remembering how a rod affects the performance of double acting cylinders. I once had an argument with an engineer, who ironically enough taught night classes, about how a cylinder retracts with less force than it extends. The rod takes up area on the piston, reducing the surface to which pressure can act, reducing force. Furthermore, because the rod takes up volume within the rod side of the cylinder, the cylinder fills more quickly upon retraction. With a differential cylinder, always remember that it retracts more quickly but with less force than it does extending.
4. Thinking a bigger pump gives you more force. Pump size has nothing to do with how much force a cylinder is capable of producing. You could have a hamster wheel turning a miniature pump, and it would still be capable of producing enough pressure to move a mountain; it would just take forever to do it. If you want more force, you need more pressure or larger actuators. Bigger pumps produce more flow, and it should be remembered that flow is the time component of the fluid power equation. Force is only achieved through pressure, and in fact, pressure is force.
5. Thinking there are “standard” stroke NFPA cylinders. My current position at Higginson Equipment, which manufactures NFPA cylinders, sees me quoting many cylinders every week. I can’t count how many times a customer in a rush has asked, “What is your standard stroke-length cylinders?” To which I like to respond, “I don’t know, how long is a piece of string?” Here’s a tip; there are no standard stroke-length cylinders. Sure, in many cases “about this long” will do, but a customer can have ¼-in. stroke, 10-ft stroke or anything in between. It’s always application specific, and it’s hard enough to stock just the components that constitute an NFPA cylinder, let alone with myriad stroke options.