{"id":483,"date":"2014-03-31T12:49:50","date_gmt":"2014-03-31T04:49:50","guid":{"rendered":"http:\/\/www.hydraulicsupermarket.com\/blog\/?p=483"},"modified":"2014-03-31T12:49:50","modified_gmt":"2014-03-31T04:49:50","slug":"hydraulics-test","status":"publish","type":"post","link":"https:\/\/www.hydraulicsupermarket.com\/blog\/all\/hydraulics-test\/","title":{"rendered":"Hydraulics Test: True or April Fool?"},"content":{"rendered":"

\"cavitationSince 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.
\u00a0
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.<\/p>\n

True or April Fool?<\/p>\n

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.<\/p>\n

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.
\u00a0
The cavitating shrimp is TRUE.<\/p>\n

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.<\/p>\n

True or April Fool?<\/p>\n

\"hydraulic<\/a>

Exhibit 1. Circuit with drifting hydraulic cylinder.<\/p><\/div>\n

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.<\/p>\n

Our unsuspecting technician is an APRIL FOOL.<\/p>\n

Note that I said: “\u2026with ball valves (1) and (2) closed, the cylinder’s piston seal could be missing completely and the piston-rod would still NOT drift”<\/em>.<\/p>\n

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.<\/p>\n

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.<\/p>\n

\"hydraulic<\/a>

Exhibit 2. Circuit with ball valve (3) installed.<\/p><\/div>\n

So now the question can be rephrased: with ball valves (1) and (2) closed<\/strong> and ball valve (3) open<\/strong>, 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!)<\/p>\n

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.<\/p>\n

\"hydraulic<\/a>

Exhibit 3. Circuit simulated with valves (1) and (2) closed, and (3) open.<\/p><\/div>\n

\u00a0The 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.<\/p>\n

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.<\/p>\n

\"hydraulic<\/a>

Exhibit 4. Double-acting cylinder becomes a displacement cylinder.<\/p><\/div>\n

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.<\/p>\n

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.<\/p>\n

Editor’s Note: for more troubleshooting teasers like this one, check-out The Hydraulic Troubleshooting Handbook<\/a><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

Since 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.\u00a0A small shrimp-like animal has a ‘hammer’. It uses this hammer … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-483","post","type-post","status-publish","format-standard","hentry","category-all"],"_links":{"self":[{"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/posts\/483","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/comments?post=483"}],"version-history":[{"count":0,"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/posts\/483\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/media?parent=483"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/categories?post=483"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.hydraulicsupermarket.com\/blog\/wp-json\/wp\/v2\/tags?post=483"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}