9 dumb mistakes people make when designing fluid power systems

1+1=3 as mathematical calculations on green blackboard

1+1=3 as mathematical calculations on green blackboard

Sometimes dumb is relative. Was Einstein dumb because he brushed off quantum entanglement as “spooky action at a distance?” He was wrong, of course, because two entangled subatomic particles can travel away from each other in opposite directions for a million years, and when one finally hits something, the other is immediately affected as well. Einstein theorized no action could happen faster than the speed of light, making quantum entanglement impossible, but for unknown reasons, it works. He was wrong. But was he dumb? Hardly.

There is a lot of dumb going around the world of fluid power. Part of it is because so many untrained individuals work on hydraulic and pneumatic systems. Sort like the backyard mechanic who’s been working on small blocks since he was fourteen, so too are farmers and millwrights repairing cylinders and replacing components with little or no formal education. It’s not their fault, more often than not, as they were just recruited to a task because they were the most suitable candidate on staff.

Less acceptable is when engineers do dumb things. Not only should a solid base of math and physics give them an intuitive sense of what is right or wrong in an application, their ability to learn new fields should exceed that of the average wrench-turner. Engineers also have more resources at their disposal, such as textbooks and professional associations. In spite of this, and just like anyone repairing or installing hydraulics, engineers still do dumb things. Keeping with this theme, here is my list of top 9 dumb mistakes people making when designing fluid power systems.

9. Confusing cylinder extension and retraction forces. A cylinder would do nothing without a rod attached to your device, which it can move to and fro (with the exception of pneumatic rodless cylinders, but that’s a different story). The rod inside your cylinder is attached to a piston, and it is this piston which pressurized fluid acts upon to move the rod. Because the rod must attach itself to one side of the piston, it takes up surface area on that side. The rod side of the cylinder has only a doughnut of surface area to work with, but the piston side has the entire puck. Because of the area differential, cylinders retract with less force than they do extending.

The dumb mistake comes when a designer only uses piston side area to calculate force requirements in applications requiring equal push and pull. Worse still, I’ve seen more than one designer calculate the piston side area on a cylinder only working in tension (pulling). Needless to say, they came up short wondering why their cylinder wouldn’t move. Depending on the size of the rod, a cylinder will retract with up to half the force it extends, sometimes more. This means your cylinder might push with ten tons and retract with just five tons. Unless you’re prepared to double your system pressure (rarely an option), do not make this mistake.

8. Confusing pressure and flow. This is a common error for neophyte designers. They will want more force—to lift a bigger load—and will feel they need a bigger pump. Some find it counterintuitive that a smaller pump will allow them to raise pressure slightly while preserving input horsepower. It will help the mechanically inclined to picture a pump to be like a sprocket. By reducing the teeth (pump displacement) on the drive gear, the output gear (hydraulic actuator) will have more force but move more slowly.

It’s worse when designers confuse pressure and flow when it comes to valves. If you add an inline flow control to a system and choke it down, upstream pressure will rise. One can see how this would lead them to believe they’re adjusting pressure. Conversely, if you put a relief valve in a sub-circuit controlling a motor, for example, it will create backpressure as it were an additive load to the work load. This could cause the system relief valve to start dumping fluid to tank, leading an operator to believe they’re operating a metering valve. I can’t stress how important it is to understand the relationship between pressure and flow; master it.

7. Undersizing a reservoir. This is a common mistake a designer may or may not make, depending how well they understand dumb mistake 9. Although enough reservoir volume should exist to help with cooling, contamination settling, aeration removal etc., catastrophic damage can occur if a reservoir isn’t sized appropriately for cylinders.

When a cylinder extends, a volume of fluid equal to the entire cap size space must exist to fill the cylinder. Some occasions, there is no trouble upon extension, especially if the cylinder rod is small diameter. A small diameter rod takes up little volume in the rod side of the cylinder, so plenty of fluid travels out the rod side port and back to tank, where it replaces the volume used to fill the cap side.

However, when you have a large bore cylinder with a large rod, there is very little volume on the rod size re-entering the reservoir as the cylinder extends. Should you overlook the math entirely, you risk reducing the reservoir volume to a level below the pump suction tube. A pump with no inlet fluid will simply not pump in the best case scenario, but will more likely cavitate first, causing potentially catastrophic damage.

6. Underestimating filtration requirements. If you like supporting your local hydraulic repair shop, please disregard this one. But if you’re a designer and feel a small spin-on paper filter assembly is suitable to protect your piston-pump supplied circuit, maybe you should think twice.

A paper filter element is a poor choice for even a logsplitter. They are terribly inefficient, have poor dirt-holding capacity and can absorb water, reducing lifespan. The best case scenario here is that you’re changing filter elements every month, if you’re on top of maintenance. The worst case scenario with a cheap filter is that it goes into bypass within the first month, and the operator doesn’t even notice, allowing fluid to run dirty as it slowly destroys every other component in the system.

It’s worth the investment to run only absolute-rated synthetic filter elements, in assemblies with high-flow housings and bypass indicators. High-quality filter material is much more efficient at removing particles, and can actually hold more of the nasty stuff before needing to be changed. It’s been observed that higher quality filtration actually lengthens filter life, because they prevent fluid from acting as a lapping compound, further exacerbating contamination levels.

5. Not heeding cylinder column strength limitations. This one is for beginner and advanced designers, who can both make this mistake. The column strength of a cylinder is a function of rod diameter, length, and mounting configuration of the cylinder and rod attachment. I like to use the example of an extended tap measure, which bends slightly under its own weight. But as you extend it out in free air, touching it to pretty much anything will cause it to buckle. The same can happen with long stroke, smaller rod cylinders with pivoting mounts.

If you buy your cylinders from a reputable manufacturer, they will provide input on the type of protection you’ll need for a given bore, stroke and rod size, and recommend a stop tube, oversized rod or pressure limitation, or any combination of the three. They may not, however, know exactly how a cylinder is mounted and what load it is moving. If your load pivots, such as with a clevis mount, you may need to de-rate the cylinder further, because a pivoting mount provides opportunity for misalignment and buckling. If you’re unsure, ask the professionals.

4. Not using current fluid power symbology. I’ve seen enough schematics in my day to know the difference between a crossing and connecting line (junction) on a drawing, but you can’t assume the young technician on the shop floor will know, especially if he’s an end user. Here’s a hint: if your crossing lines are arched, you’re doing it wrong. Check the latest ANSI or ISO standard and you’ll find a crossed line just crosses with no drama. If there is a junction, there is a circular black node. The arched line-cross of yesteryear is dangerously close to the current representation of a flexible assembly.

Even if you don’t care how a junction looks, get the latest standards and use them. You’d hate for your drawings to just look outdated, like using the old servo valve symbol. Seriously? What is going on there?

3. Not using the vast resource of Fluid Power World. Enough said.

2. Using old technology because you’re comfortable with it. Hey, don’t get me wrong, a two-stage pump was a great idea fifty years ago, but why settle for two flow points when you can have an infinite number using a horsepower-limiting pump instead? A horsepower- (or torque-) limiting pump is set so it will always provide maximum flow rate, inversely proportional to pressure, to limit power at a predetermined level. If system pressure doubles, flow rate drops by half. I hope you’ve heard of this system, because it’s old technology itself. But wait, there’s more….

Every big player in the hydraulic pump game now makes electronic “pressure compensated” pumps. A mechanical pressure compensated pump uses a pilot pressure signal control pump displacement, increasing and decreasing relative to system pressure. Now instead, electronically controlled pumps in a network of pressure transducers can be controlled to provide flow on demand exactly as needed by controlling prime mover speed. A servo or VFD-controlled electric motor varies pump speed to meet flow demand plus leakage flow.

The variable-speed pump drive system is so advanced that it can control a cylinder with absolutely no control valves, and even hold a load in place while the pump only turns to compensate for leakage. But you already know all this, don’t you, because you don’t make dumb mistake number 3 … right?

1. Thinking “flow makes it go.” I was taught flow makes it go and pressure is resistance to flow. I’d love to say this is a rookie dumb mistake, but even well-experienced engineers still get confused here. Hydraulics is a system of force transmission, and not a vehicle for arbitrarily moving fluid. Pushing on a column of fluid is no different from pushing on a steel rod, other than liquid is a bit more compressible than a solid.

This misconception is so prolific, I took it upon myself to create Cosford’s Law, which states: Pressure makes it go. Flow is the rate in which you create pressure. Pressure is why flow happens. If the pressure originated at the resistance, then flow would go backwards to the pump. Energy can only move from an area of higher potential to lower, not in reverse. Along with dumb mistake 8, this is the most important concept to understand in hydraulics.



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