Hydraulics Test: True or April Fool?

cavitation erosion on valve plateSince tomorrow is April Fools day, here’s a couple of trick questions to test and entertain you-and perhaps, for you in turn to spring on your unsuspecting colleagues at work.
A small shrimp-like animal has a ‘hammer’. It uses this hammer to break the shells of the crustaceans it eats. But the hammer does not exert enough force to break the shell on its own. So it creates a small air bubble on the end of the hammer. When the hammer hits its victim’s shell this air bubble implodes and breaks the shell.

True or April Fool?

Leonardo da Vinci, the genius painter, sculptor, scientist and inventor urged his fellow artists and scientists to “go straight to nature” in the search for knowledge and understanding.

Taking da Vinci’s advice, if you wish to understand why cavitation eats away the innards of hydraulic components, look no further than the cavitating shrimp. If a tiny shrimp can use an imploding gas bubble to destroy the otherwise impenetrable defences of its prey, it’s little wonder air or vapor bubbles which implode under high pressure in a hydraulic system can erode case-hardened steel (see pic above) and even softer yellow metals.
The cavitating shrimp is TRUE.

The cylinder in the circuit shown in exhibit 1 is drifting. In an effort to isolate the problem, the technician has installed ball valves (1) and (2). When both these ball valves are closed, the piston-rod stops drifting. This proves to the technician that the cylinder’s piston seal is not leaking and therefore he should replace the directional control valve.

True or April Fool?

hydraulic cylinder arrangement

Exhibit 1. Circuit with drifting hydraulic cylinder.

This conclusion assumes that if the piston seal was leaking, the piston-rod would continue to drift with the ball valves closed. But with ball valves (1) and (2) closed, the cylinder’s piston seal could be missing completely and the piston-rod would still NOT drift. In other words, it is wrong to conclude the piston seal is not leaking.

Our unsuspecting technician is an APRIL FOOL.

Note that I said: “…with ball valves (1) and (2) closed, the cylinder’s piston seal could be missing completely and the piston-rod would still NOT drift”.

This statement is a great conversation starter-not at a cocktail party perhaps, but whenever I teach a basic hydraulics class it certainly is. Because there’s always at least one student, and usually several, who refuse to accept that the cylinder will hold its load without a piston seal. But it’s easy to prove it to them with a simple simulation.

The first thing I tell my sceptical students is that we don’t need to remove the piston seal to prove the point. Installing a third ball valve across the cylinder ports and leaving it open will achieve the same result–see exhibit 2.

hydraulic cylinder testing

Exhibit 2. Circuit with ball valve (3) installed.

So now the question can be rephrased: with ball valves (1) and (2) closed and ball valve (3) open, will the piston-rod drift? Raise you hand if you think the answer is yes-the piston-rod will drift. OK. Raise your hand if you think the answer is no-the piston-rod will not drift. (Relax, I can’t see your hands, and besides, I’ve already told you the answer!)

As the simulation shows-see exhibit 3, the cylinder does not (and cannot) drift. Notice cylinder velocity, v = 0 in/s, in other words, the piston-rod isn’t going anywhere.

hydraulic cylinder with ball valves installed

Exhibit 3. Circuit simulated with valves (1) and (2) closed, and (3) open.

 The reason for this is, the fluid volume in the cap-end of the cylinder cannot be accommodated in the rod-end, and so once pressure equalizes on both sides of the piston, the cylinder becomes hydraulically locked. Once this occurs, the only way the piston-rod can move is if fluid escapes from the cylinder externally-via the rod seal, for example.

Simply said, the double-acting cylinder becomes a displacement cylinder, a.k.a. a ram. To illustrate this point, the circuit could be re-drawn as shown in exhibit 4.

hydraulic cylinder -- displacement type

Exhibit 4. Double-acting cylinder becomes a displacement cylinder.

Of course, it’s important to consider that because of the loss in effective area – due to load-induced pressure now acting on the annulus area, the static pressure in the cylinder-come-ram must increase to support the same load.

And if you grasp the theory at work here, you’ll realize there are two exceptions to this load-holding-without-a-piston-seal party trick. The first is a double-rod cylinder – where volume is equal on both sides of the piston. And the second is when a load is hanging on a double-acting, single-rod cylinder. In this latter arrangement, the volume of pressurized fluid in the rod-end of the cylinder can be easily accommodated in the cap-end.

Editor’s Note: for more troubleshooting teasers like this one, check-out The Hydraulic Troubleshooting Handbook

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