Hydraulic Quiz

[Sniper]

Here's a little problem solving involving hydraulics. And here is the senario:

We take a perfectly working, double acting hyd cylinder apart. (Double acting has a port at each end allowing you to power the piston and shaft back and forth. A single acting cylinder has a port at the 'bottom' end only, with a breather at the shaft end. You can power it out, but gravity or weight must be appiled to get it to retract.)
Now, with the double acting cylinder apart, we take a hacksaw and cut a 'v' groove into the edge of the piston, removing the metal sliver, plus the bits of seal that would be in that area.
Figure #1 shows what we mean, dark area is the remaining groove.
Figure #2 We reassemble the cylinder just the way it was, piston and shaft are assembled and slide into the cylinder approx 1/3rd of the way. Cap and seals are now slid over the shaft and tightened down on the end of the cylinder making it leak proof. We add hyd oil in through both upper and lower ports, completely filling the void in the cylinder. (Yellow area)With all air removed and full of oil, we plug the ports making them leak proof. The cylinder is now sealed.
Figure #3 We put the cylinder, standing up in a solid frame using pins at each end. The upper arm is on a pivot pin so it can move up or down. We let go of the arm and the full weight of the arm is now resting on the end of the shaft.
What happens and why?
Figure #4 The exact same set up as in figure #3, only we added 100 lbs. to the end of the arm. Now we let go of the arm.
What happens and why?
Sniper

 

[pistolpete]

Maybe I am missing something, but with the piston notched as such it will almost act as a mono shock will it not? Kind of a neat thought about making your own shocks from a hyraulic cylinder though.

With Fig 3 - Depending on the wieght of the arm, it could take a really long time for the piston and shaft to travel the length of the cylinder, as there will not be the great difference of pressures between the two ends of the cylinders (as there would be with weight added). I would think that left in that position over an exptended period of time would create the same effect.

With Fig 4 - The fluid below the piston will be compressed by the added weight, causing the fluid to bypass the piston, going to the area of fluid with a lower pressure (could argue a vaccum) and allowing the piston and shaft to move to the bottom of the cylinder...

But like I said I may be missing something as I tend to look at things from a simple mind, and sometimes over simplify things. (not sure I answered the how and why though...)

 

[Sam_Fear]

I go with Pete's answer. Now add a spring to it.

 

[FlatBroke]

Over my head....[S

 

[Grumpyoldman]

I may be way of base here but, I don't think it's going to move at all. There is isn't any place for the fluid to go, It'll just compress and stay there even with a slot.

Now if you added a couple of holes it would move,.[S

Just my 2 cents...<can i get change back?>

 

[CR55]

How will the piston move with a slot and the seal having a notch? If it moves at all, it will probably only travel in one direction.........CR

 

[wayway63]

therewould be drift as oil is displaced into the top.
happens all the time on tractors with front end loaders. the volumes of the top and bottom are not the same with the shaft included on top. arm would slowly drift to the bottom.

actually after thinking about it the shafts are mounted the other way on loaders so i'll have to think a little more on it.

 

[wayway63]

nope after thinking should move slowly down due to displacement of fluid the piston is just along for the ride.

 

[Grumpyoldman]

I get it Figure #3 will move because there is hardly no pressure.

Figure #4 will not move because there is pressure pushing it down and holding it back.


I think....Damn my head hurts now....[S

 

[Sniper]

Ok, Your all close in one part or another. Grumpyoldman, your right on the money when you say it won't go any where. It's not going up and it won't go down. But why? This is the part that everybody has a hard time getting their head around. Wayway63 mentions the reason why without knowing it, man your dancing around the correct answer so close. I was at a 4 day hyd seminar at the University Guelph over 30 years ago when this was presented to us by the professsor. I got it wrong!! Like Wayway63, I was using the info I knew about from the actions of a front end loader to state my case. Like I said, I was wrong. But, its a very close and similar situation, the loader drops, but for a different reason. This one won't drop. Ok, that's it for the hints, the whole answer comes tomorrow. And I'll tell you why a front end loader drops when the seals go bad, Just as if it had a 'V' notch in the piston.[ddev

 

[Barber]

your right on the money when you say it won't go any where. It's not going up and it won't go down. But why?

Because...

We add hyd oil in through both upper and lower ports, completely filling the void in the cylinder. (Yellow area)With all air removed and full of oil, we plug the ports making them leak proof. The cylinder is now sealed.

By filling it completely up the fluid has no where to go... and since hydraulic fluid is made so it wont compress, the piston it basicly locked in place... hydro locked if you will...

 

[Sam_Fear]

So how bout if it were filled with water... or air?

 

[Sniper]

Barber, your on the right track.
Sam, water would act the same as oil, it won't compress. Air however is a different situation all together, it will compress. The piston and shaft would slide right in, likely to the bottom of the cylinder. The air wouldn't compress enough to carry it on such a short stroke like oil will.

 

[Sniper]

Ok, as mentioned earlier, I had this problem presented to me along time ago at a seminar. At the time I was sure the piston and shaft would just slide in until it hit bottom. I thought the oil would pass through the 'V' notch from the bottom chamber to the top chamber allowing the movement of the piston and shaft. Nope, won't happen.

Here's why:

If the piston were to drop in farther, more shaft would be introduced into the cylinder. That's the key right there. There is no room for anything else, it's already completely full with the existing piston, shaft and oil.

To visualize this better, take a regular drinking glass, and hold a finger over, and in the glass, to the first knuckle. Now fill it full of water right to the rim. This is the same as the completely full cylinder, and your finger is the shaft. Now shove your finger in, to the second knuckle. What happens? The water over flows because we displaced it with more finger, or shaft. The difference here, is we have sealed the cylinder so no oil can escape like the water did. By sealing it, it won't allow more shaft to enter the space of the cylinder. So it will just sit there, unless oil leaks past the seal in the cap making more room for more the shaft. If there is no leak, there is no movement. Now you know! Sniper

Ok, ok, so the front end loader that was mentioned will leak down. It's the exact same situation, so why the different results. The seal pack on the piston has blown out, leaving an open path just like the 'V' notch in our model. So what's different? The loader hyd system has a relief valve that our model didn't have. "So what" you say. It held up the loader before, just because the seals blew out, why would it relieve pressure now? The pressure would have to increase to more than the relief valve setting to do that. Better yet, how could it do that?

Good question 'Grasshopper', let me explain.

We need a couple of examples to make this clear as to what's happening. The figures are at rest, tractor not running, loader in the air. The orange is the weight of the loader applying pressure.

In figure #1 we show the piston, shaft, and cylinder walls. The seals on this piston are good. The 'yellow' is oil under pressure and the arrows show where the pressure is being applied.

In figure #2 the same set up, but the seals are damaged and blown out letting the oil bypass just like in our model. The yellow is again oil under pressure and the arrows show where the pressure is being applied.

Figure #3 a & b Show the difference between the area that is now the 'piston'. As you can see, the area of the piston on the bad cylinder is greatly reduced. This is an important note.

Now the situation were looking at, is the same as the brake system we use on our vehicles. The load in the bucket is us, the driver. Applying pressure to the master cylinder, through the linkage of the brake pedal. The loader frame is the linkage, and master cylinder is the hyd cylinder. Now just like our brake systems, if we install a manual master brake with a large bore piston, the brake pedal is really hard and we can't apply enough brake pressure for the brakes to work. If we look at our loader with a good cylinder and seals, the piston is large enough that the weight of the loader won't produce enough pressure to unseat the relief valve and the loader will stay where it is. But, when the seals blow out the piston area becomes smaller, in fact that of the exact diameter of the shaft. Now back to our brake example. Since the large bore gave a hard pedal, we switch out the master to a smaller bore, and we can produce a much higher pressure to our brakes with the same or less effort to the pedal on our part. Same thing happened to our loader, once the piston seals blew out, the effective area of the piston became smaller. This produced a much higher system pressure at rest. High enough to lift the relief valve off it's seat and cause a slow leak of oil to the sump. This leak allowed more shaft to enter the cylinder letting the loader slowly lower to the ground.
PS. I messed this up, additional figures are on the next post. Sniper

 

[Sniper]

Additional Figures #1 #2 #3 a & b

 

[CR55]

Wow, only here on RRR can you get entertainment and an education! I love it Sniper, that's why I come here. Good lesson about hydraulics, gotta help out when doing a complete brake install. I wasn't aware of the "big bore" syndrome!...[S.....CR

 

[pistolpete]

Like I said I look at thinngs from a simple mind... Never thought at all that the displacement of the shaft itself would be the issue. Thanks, got my thinker turning, it needs that now and then.

 

[Sniper]

CR55, A while back one of the magazines, can't recall if it was Rod & Custom or Street Rodder that did a two part artical on brakes. Had some real good info, I picked up a couple of tips from it myself. The bore size was mentioned in it as well, large bore is usually power assist. Lose the vacuum, not much in the way of brakes.

Pistolpete, Shoot, I thought it was going to the bottom when I first seen it too. Hardly anybody thinks of the shaft being involved.

Manlifts use this very senario as a saftey item. They have installed holding valves at the ports that are locked off by spring pressure, and hyd released. (it's called a pilot operated valve) Blow a line and it lock the cylinder solid and won't allow it to drop a man to the ground.

Ok, that was kind of fun....somebody else's turn. Teach me something.[P

 

[Sam_Fear]

How about if I tried to lift the handle? [S

 

[wayway63]

That made my brain itch