knots

Oil Storage for Hydraulic Presses

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Ge thanks Ptree, I'll run right down to the local store for a couple gallons of ethylene oxide. I remember some really cool videos from the Gulf war showing FAE weapons being used to flatten underground gun forts. One shot was from the fins of the bomb showing how the ethylene oxide was dispersed, like a really FAST sprinkler, then boom.

 

I hadn't thought about shock loading being determined by basic physics, How's it go,? Force = mass x acceleration, acceleration = "delta" x time. (I don't recall the correct term for velocity change so used "delta". It also explains why we hardly ever saw spikes above 3,000psi. the shockiest tool on the rigs were the powered drive hammers. WE ran a 350lb weight to drive casing and the penetrometer and used the hydraulicly driven hammer. This hammer was a steel tube with a 350lb weight that moved freely in it. the drive was a very simple chain drive mounted on the back side with a lift lug that passed into the steel tube to engage the bottom of the hammer weight. The lift chain was driven with a hydraulic motor. The chain was driven from the bottom and ran over an idler sprocket at the top so most of the time it was running with almost no strain on the motor but when the lug engaged the hammer weight the psi spiked to about 3,000psi and during the lift ran at about 900psi.

 

Almost everything on the rig was plumbed through hoses so that was where most of the pressure spikes got cushioned, every time a hose jumps it's pressure expanding the hose, shortening it which is work energy lost but really extends the life of motors, pumps, valves, seals, etc. I can still hear the sound of the hydraulics and hoses thumping with the rhythm of the hammer. Same with the sampling hammer, we drove that with a hydraulic cat head but it was a lighter hammer, 140lbs hanging from 1" manilla rope so the shock loading hardly twitched the pressure gauge.

 

Frosty The Lucky.

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Frosty, you came awfully close on the formula, I don't remember exactly but velocity, mass density, length of the moving fluid slug and rate stopped all were in there.

One to remember, These spikes took a recording oscilograph. That was a 1950's  recorder that burnt traces on thermal paper. The IR light was shot onto a Gravimeter, that is a mirror hung on a spring, and as the current that was sent by the transducer amplifier went thru the spring heated and expanded, twisting the mirror and moving the trace. The paper was moving at 800 mm/second. That is 48 meters/minutes or "more paper than you can catch in a blink".

The spikes are so very brief that most pressure gages don't even move, they just fail quick. But the spike do stress everything, including those all important metal wires in the braid that make the hose able to hold pressure, they nibble the O-rings and cause the bite at the crimp to slowly fail.

 

To get an idea, 60 Hz electricity made a 1/2" or so half wave and these spikes were almost a straight line up/down.

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Cool, almost is pretty good for me where math is concerned. <grin> I'm happy to say we didn't have to worry about metal to metal, near instantaneous shock initiators, just hoisting a 350lb hammer weight 30" so the gauge was more entertainment than necessary.

 

My use and understanding of hydraulics is almost completely applications in relatively low stress conditions. Heavy equipment isn't a real high stress application, everything is over designed to the point it just stalls the hydraulics over bypass and blowing a seal, cylinder, fitting, hose, etc. is almost always just a mess to clean up.

 

The most dangerous experience I've had with a hydraulic failure was when the high pressure line to the valve body blew and fired hotter than "boiling water" fluid straight at my face from about 4'. It was winter on Kodiak Island, not so cold but sleet and rain are way worse than sub-zero any day, so I was wearing my Refridgeware and had time to duck, taking the hit on the insulated hood. I hit the kill switch about the same time as the oil hit me so all I had to do was get out of dodge.

 

The guys exclaimed that rain drops were boiling off me when they landed. I was really excited to hear this but had to stand with as little of my gear touching me while it slowly baked me like a potato. that was on a Mobile B-50 drill which is entirely hydraulicly driven making the oil run really hot, we'd turn the water hose on reservoir tank regularly to keep it from foaming out of the system. Not our drill so there were no line driers on the hydraulic system, get it much above boiling and it'd blow foam out of the breather on the tank when the foam level got high enough.

 

It's a real rush knowing how hot the oil is and actually seeing the the broken fitting and end of the hose rear up from the valve body to point straight at your face and see the rush of hot oil coming for you. Oh yeah I was LUCKY. I was wearing gear that shielded me from burns beyond discomfort, had time to dodge the gush and just happened to be looking at the right place at the right time. Had it been a high pressure system rather than heavy equipment I probably wouldn't be talking about it.

 

Frosty The Lucky.

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Frosty, work with equipment long enough and we all get a story or three. A fair amount of my work at vOGT was forensic anaylisis of returned products, and the occasional defense against litigation type testing.

Things like the valve failed and blew 600# live steam at the ground from 2' up and when the guy trying to find the leak fell in the crater he was parboiled. Not our valve by the way, but we had to prove it was not ours.

Or the guy who lost much of his hand due to a check valve failure in a 50 year old press, that had the valve mis-applied and somewhere in the 50 years someone drilled holes in the piston check that cracks propagated from etc etc. After you have examined the 2 or third set of burned up or more correctly burned down shop in a year, you tend to gain respect for hydraulic fires. After you have hauled the third guy to the hospital for hydrostatic test water or oil injected into their hands you gain more respect for pressure.

 

I may be overcautious. I may also just have been exposed to several lifetimes of shop experience due to my job in only one life. Then again I may save a life or limb, so I will continue to caution when I get the chance.

 

Scientific myth they teach in High school. Everybody "knows liquids will not compress" that is how hydraulics work Right? Wrong! When that "Law" was writen they did not have equipment accurate enough to measure. Fluids do compress. Especially as the pressures go over 5000 psi. And the energy stored in the small percent of so per thousand psi of compression stores lots of energy. So does the gas that is disolved in the fluids and all the bubbles that did not get bled out.

 

Many learned engineers will tell you knowledgably that one drop leaked and all the pressure is gone in hydrostatic testing. Reality= at about 19,500 psi 2.5" by .375 boiler tube bent into a close return will make a sound like a half stick of dyaminte when it ruptures. Done that test hundreds of times.

 

Second Myth. pipe always splits along the lenght and opens a fishmouth when it ruptures from over pressure. Nope. Seen the pipes blow in half circumfrentially, seen hunks as big as you palm blow out, some flying 10' and chipping the concrete roof of the test cell.

 

To quote my old chief engineer and boss, "One test is worth a thousand expert opinions"

 

Ptree who once prided himself that he could break anything :)

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Water hammer in hydro systems is a good topic. I do have two porta-packs, one 5hp tandem center and a 10HP open centered. The tandem is snappier in performance but it really bangs hard when the spool shifts. From my own experience in home plumbing, a stub of pipe near the valves work sometimes, sometimes not. What's the best way to get rid of hammer, bigger lines? check valves?

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Check valves cause water hammer in most cases. A rated accumulator is the best cure for water hammer in a system that is already built. Install as close to the source of the fluid stoppage, IE close to the head end of a press, or close to the valve that is causing the hammer. Beware that accumulators are not cheap, and have a whole series of safety issues themselves. IE they need to be clamped rigidly so that if the pipe connection fails they do not become a fluid rocket. A line failure will deliver flow at pressure until the pump and the accumulator flow are exhausted, and a system that is turned off can often have stored pressure. They usually require maintenance from time to time as the bladders inside fail. They are usually charged to half system pressure with nitrogen, using a special Schrader charging connection that is not found in most blacksmith's shops..

 

While hose will soften some water hammer, it does fatigue them. The best way if possible is to design the system to not water hammer. In a press that goes solid, that requires an accumulator. Often valving can be changed to allow a slower stop time or shift to dump pressure to tank. Beware that sudden release of pressure makes shockwaves as well, but usually gentler then water hammer.

 

Remember that iin engineering like life every choice is  compromise

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What are the pro's and con's for the use of welded vs. bolted hydraulic cylinders in a forging press ?

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The pros/cons of welded vs tie rod. The welded does eliminate 2 joints. The barrel to head and cap joints on name brand cylinders that have not been disassembled are very trouble free, and usually designed to a safety factor of 6. In many cases the tie rods will stretch and safely relieve pressure before a rupture. Not a best choice for a cylinder in a forge press. Welded does not have these joints. BUT the more critical and most likely failure is the rod seals. Garbage gets on the rod, and eventually scores the scraper, and then the rod seal fails and you first get weep age, then drip then spray. Many cylinders for use in very dirty applications use a simple clamp on rubberized bellows to keep the crud off the rod.

 

I would not have any problem designing a forge press with a properly rated tie rod cylinder

 

Another compromise that must be considered is the rod diameter and too slim a rod can jackknife or collapse caused all sorts of problems like rod seal failure. In presses I try for a cylinder that is about 35% longer than the possible stroke and go for the largest rod diameter I can get, usually a "4:1 rod. That is a rod 4 times bigger than std. Gives way better rod bearing and seal life, is much stronger, makes for a naturally faster retract stroke. and has much better column collapse strength.

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Agreed on rod strength. I've seen two inch rods bend like twigs from side loading- not cool. On my main press I have a 7" double ended cylinder with a 5" rod. Yes- I lose a lot of working pressure taking up so much area but the stiffness is unbeatable. NO side deflection at all that I can tell. Just picked up a 5" double ended cylinder with a 2" rod for a new press. I just like the idea of having more guides essentially built into the system.

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With a double rod you do lose some area. You also eliminate the possibility of overpressure on the rod side from intensification. When controlling hydraulic cylinders, you control flow INTO the the cylinder, unlike pneumatic cylinders where you control OUT of the cylinder. If you put a speed control on that throttles flow out of a hydraulic cylinder and actually slow the flow out, then the ratio of blind to rod side in area is the ration of pressure intensification. with a large rod, one can operate a cylinder at rated pressure, throttle the outgoing flow of the rod side and fail the cylinder through over pressure from intensification.

On the other hand a big rod due to the volume displaced can easily give twice the retract speed as the full pump flow can give. useful in a forge press where you want that tooling to be out of contact after the push as soon as possible to limit tool heating.

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I agree with Ric about a separate cart. It doesn't have to take up much space. I do not like putting the pump, reservoir and high pressure hoses on the top. If there is some kind of a failure it is at the level of your head.  Below are two cart designs. The one is mine and the blue one is from the Claiborne press. These you can even put in another room for more safety and for removal of the noise. Yes, then there is the hose problem, but protective sleeves and or pipe to run them through can take care of that.

 

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