hydraulic cylinder drawing

[greeswed]

Hello everyone,
I propose you a problem and I hope you can help me clarify my ideas.
Suppose you have a double effect hydraulic cylinder with a single stem that supports a certain vertical load. we say that the cylinder is facing with the stem upwards. the two cylinder connections are blocked for example as in my case by two retainers for which the oil cannot flow through the connections. this cylinder under the effect of the load drops (the stem naturally) some mm. the question is as follows:
is it possible that the cause is an internal thread due to the wear of the piston seals?
This topic was a source of debate between me and my colleagues. my answer was no because the volume of oil moved during the descent into the big room, can not be contained in the small room with stem (considering the incomprehensible oil). This problem for me can be seen (I want to correct myself if I'm wrong) in the same way as a cylinder without internal losses with the two connected connections. according to you applying a force down on the stem this goes down? and if we take it from the opposite side what happens?
as often happens from a discussion other arise: someone said that it can not descend because if the gaskets are consumed I have the same pressure in the two rooms and a resulting force would arise that pushes the piston always upwards...
thanks to all

 

[Er Presidente]

hydraulic plunger pistons, simple effect, work like that, have no seals inside and the stem does not descend until the oil is discharged. a double effect with internal losses becomes a simple diving effect and the stem does not go down.
But if there's any leak outside, the speech changes.

 

[cacciatorino]

This topic was a source of debate between me and my colleagues. my answer was no because the volume of oil moved during the descent into the big room, can not be contained in the small room with stem (considering the incomprehensible oil).

It is not entirely true that the oil is incomprehensible: it has its "elastic substance", like all the materials. Not to mention that it is possible that in the oil there are dissolved gases or air bags trapped somewhere. if you want to hold the cylinder locked at the end of the race I think you have to keep the pump on, or insert a pressure accumulator in the input line.

 

[Fulvio Romano]

I'm sorry, but if I connect the two exits between them is not like having a shock absorber? and if I have an important thread (as if I had left instead of the piston seals) I wouldn't have the same thing?

but the piston that does, drops a few mm and then stops, or continues its descent at constant speed?
It is true that the oil is slightly compressible, as well as the chambers and the cylinder and the pipes, but it is true also that if the cylinder is brought in position with pressure to regime, all the elasticity of the system, to that pressure, has already worked, so the system cannot yield further unless the pressure is varied.

but the cylinder lifts the load or this is applied after?

 

[MBT]

Hello everyone,
I propose you a problem and I hope you can help me clarify my ideas.
Suppose you have a double effect hydraulic cylinder with a single stem that supports a certain vertical load. we say that the cylinder is facing with the stem upwards. the two cylinder connections are blocked for example as in my case by two retainers for which the oil cannot flow through the connections. this cylinder under the effect of the load drops (the stem naturally) some mm. the question is as follows:
is it possible that the cause is an internal thread due to the wear of the piston seals?
This topic was a source of debate between me and my colleagues. my answer was no because the volume of oil moved during the descent into the big room, can not be contained in the small room with stem (considering the incomprehensible oil). This problem for me can be seen (I want to correct myself if I'm wrong) in the same way as a cylinder without internal losses with the two connected connections. according to you applying a force down on the stem this goes down? and if we take it from the opposite side what happens?
as often happens from a discussion other arise: someone said that it can not descend because if the gaskets are consumed I have the same pressure in the two rooms and a resulting force would arise that pushes the piston always upwards...
thanks to all

a couple of questions. . .
the load drops a few mm and then stops? or if I leave him there for a few hours he keeps coming down?
Have you tried to put a pressure gauge on the cylinder's minimums? What pressure do you see? is he "consistent" with the theoretical one? and the theoretical one, how much is it?
You said you had some lockers. What kind? and where are they mounted? directly on the cylinder mouth or a few feet away?
What fluid do you use? water-glycol or mineral oil?

 

[MBT]

hydraulic plunger pistons, simple effect, work like that, have no seals inside and the stem does not descend until the oil is discharged. a double effect with internal losses becomes a simple diving effect and the stem does not go down.
But if there's any leak outside, the speech changes.

? ? ? ?
and the oil seal with what do I do if I have no seals? ? ? ?

Look. .
These are made of hydraulic cylinders with a simple effect since leonardo was desperate for monnalisa. . .
use heart attack pressure, they know them all over the world

look for a random cylinder as the seals are.
http://www.enerpac.com/en/repair-sheets-and-instruction-manuals

 

[MBT]

It is not entirely true that the oil is incomprehensible: it has its "elastic substance", like all the materials. Not to mention that it is possible that in the oil there are dissolved gases or air bags trapped somewhere.

That's true. In fact, I have a strong suspicion that the cylinder has not been well untied. . .

if you want to hold the cylinder locked at the end of the race I think you have to keep the pump on, or insert a pressure accumulator in the input line.

no, if you work well you can lock it without keeping the pump under pressure.
the accumulator is counterproductive, because it is still a "elastic" element in the system.

 

[cacciatorino]

? ? ? ?
and the oil seal with what do I do if I have no seals? ? ? ?

Look. .
These are made of hydraulic cylinders with a simple effect since leonardo was desperate for monnalisa. . .
use heart attack pressure, they know them all over the world

look for a random cylinder as the seals are.
http://www.enerpac.com/en/repair-sheets-and-instruction-manuals

the seals serve only on the "top" flange, but between piston and shirt do not think they serve. in this case the thrust is guaranteed by the difference of area of the section between the lower room and the upper one. above all on long-run cylinders (ascensors) this system allows to avoid rectifying the cylinder shirt.
http://www.generalmatic.com/gmh/gmhfrt.jpg

 

[MBT]

the seals serve only on the "top" flange, but between piston and shirt do not think they serve. in this case the thrust is guaranteed by the difference of area of the section between the lower room and the upper one. above all on long-run cylinders (ascensors) this system allows to avoid rectifying the cylinder shirt.
http://www.generalmatic.com/gmh/gmhfrt.jpg

Yes, okay
but that is the exception, not the rule

Anyway, there are seals. on the flange and not on the stem, but there are.

 

[Focus]

? ? ? ?
and the oil seal with what do I do if I have no seals? ? ? ?

Look. .
These are made of hydraulic cylinders with a simple effect since leonardo was desperate for monnalisa. . .
use heart attack pressure, they know them all over the world

look for a random cylinder as the seals are.
http://www.enerpac.com/en/repair-sheets-and-instruction-manuals

in the "sloping silve" cylinders, the seals are only on the stem, the manco piston exists.

Hi.

 

[Er Presidente]

hydraulic plunger pistons, simple effect, work like this, have no seals inside...

Yes, okay
but that is the exception, not the rule

Anyway, there are seals. on the flange and not on the stem, but there are.

in fact I said that there are no seals inside.

 

[MBT]

However, greeswed has specified that it is a double effect, so discussions on the seals of the single effect are "academic"!

we will see if you give us more information

 

[Er Presidente]

The Commission's proposal for a Council Directive on the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States relating to the approximation of the laws of the Member States. as I wrote the case would be assimilated to a cylinder with dip stem, without seals. I think the subject is relevant.

 

[greeswed]

hello to all and thanks for the interventions.
I'll give you some more information. the 2 connections to the ends of the cylinder are blocked because there is a rexroth lock valve of the type z2s (Annex) which is interposed between the distributor 4/3 and the cylinder connections. said in other words the oil coming out of the cylinder first meets the non-return valve (one for each of the two lines) and after the distributor. has been set up to maintain positions since a simple distributor even if at closed centers can present small drawings. when the pump pushes the piston upwards and this arrives at the end of the stroke, the oil pressure reaches the 250 bar. at this point the distributor switches in the rest position and the oil must remain "trapped" to that pressure (then the small room or the little one left is 0bar). the stem drops a few mm (2-5) and then it is called (with manual command) the position so I can not tell you if at the long come down more. a clarification: I opened this conversation mainly to have conceptual clarifications, if you want theoretical. It's clear that if there's air in the oil, this could come down. it could also be that there are some threads in the tubes that connect it to the valve block (not easily accessible). I was looking for confirmation that in the absence of external losses, in the absence of air, neglecting the compressibility of the oil, a possible wear of the gaskets of the piston can not bring down the stem. extinguishing the concept, we pretend that there are not exactly the gaskets of the piston. If I push the stem down even with important forces, for me this cannot come down because the volume of oil that would move if it fell cannot physically be contained in the small room. Now we suppose instead of wanting to pull the stem up instead of pushing it down. For example, we imagine having the stem all at the lower dead point. Suppose for simplicity that there is no applied load and that the pressure in the two rooms (oil feet) is 0. of course the two connections are blocked. We also pretend that the piston has no seals. If I throw the stalk, for me this should go up. the oil moved in the small room can be contained in the large one (instead it remains free volume). But then what happens? Maybe a depression is generated and tends to form the void?
Thank you.

 

[Er Presidente]

to extremize you can think of a hydraulic piston without internal gaskets, dive as I said.
Forklifts often have lifting columns with this type of stem (the advantage and have stems with diameters close to barrels therefore very robust and stable). clearly if the movement of the stem involves changes in volume if oil does not enter and does not leave the movement there is.
2 or 5 millimeters doesn't seem to me a negligible movement, so a quantity of oil must have moved.

 

[MBT]

hello to all and thanks for the interventions.
I'll give you some more information. the 2 connections to the ends of the cylinder are blocked because there is a rexroth lock valve of the type z2s (Annex) which is interposed between the distributor 4/3 and the cylinder connections. said in other words the oil coming out of the cylinder first meets the non-return valve (one for each of the two lines) and after the distributor. has been set up to maintain positions since a simple distributor even if at closed centers can present small drawings. when the pump pushes the piston upwards and this arrives at the end of the stroke, the oil pressure reaches the 250 bar. at this point the distributor switches in the rest position and the oil must remain "trapped" to that pressure (then the small room or the little one left is 0bar). the stem drops a few mm (2-5) and then it is called (with manual command) the position so I can not tell you if at the long come down more. a clarification: I opened this conversation mainly to have conceptual clarifications, if you want theoretical. It's clear that if there's air in the oil, this could come down. it could also be that there are some threads in the tubes that connect it to the valve block (not easily accessible). I was looking for confirmation that in the absence of external losses, in the absence of air, neglecting the compressibility of the oil, a possible wear of the gaskets of the piston can not bring down the stem. extinguishing the concept, we pretend that there are not exactly the gaskets of the piston. If I push the stem down even with important forces, for me this cannot come down because the volume of oil that would move if it fell cannot physically be contained in the small room. Now we suppose instead of wanting to pull the stem up instead of pushing it down. For example, we imagine having the stem all at the lower dead point. Suppose for simplicity that there is no applied load and that the pressure in the two rooms (oil feet) is 0. of course the two connections are blocked. We also pretend that the piston has no seals. If I throw the stalk, for me this should go up. the oil moved in the small room can be contained in the large one (instead it remains free volume). But then what happens? Maybe a depression is generated and tends to form the void?
Thank you.

The answer lies in the question.
inside the cylinder there is a certain amount of oil, divided in a certain way between upper and lower chamber.
Obviously this quantity is fixed, the distribution of oil is according to the position of the stem.
It is true that the pressure in one room will be different from the one in the other and that in the middle there will be the seals
However, we reason for absurdity and imagine that the seals on the piston are not there.
pressure will tend to equalize, of course
at equalized pressure (and however, as the phenomenon happens) there will not be enough pressure to support the load that inevitably, inexorably, will descend
the purpose of the seals is obviously to prevent the pressure from getting equal.
If they do the cargo will remain firm in his position, otherwise. . .
all this giving for good (though not true) the incomprimibility of the fluid

 

[radio]

if we consider the incomprehensible oil and all the system (net, fittings, pipes, valves etc.) infinitely rigid, no movement can occur, even in the case of imperfect seal of the piston.
in reality the whole system has its flexibility, and then in case of piston seals no longer intact happens as it says mbt, the increase of the pressure side stem (due to the movement down of the piston) is not instantaneous (setting 0) but will be so “gradual (+ move) as “flexible” the stem side of the pencil (initial counterpression very low, long piping).
However, as greeswed explains the operation of the system, the problem could simply originate from a valve response delay; in time that puts us to close a part of oil could escape to the distributor and, especially in case it is an open center, towards the discharge.

 

[marcof]

if we consider the incomprehensible oil and all the system (net, fittings, pipes, valves etc.) infinitely rigid, no movement can occur, even in the case of imperfect seal of the piston.

I would say too, the amount of oil that should go up for any downward shift of the piston is always greater than the available volume in the upper chamber then.. .

 

[rinato]

If I may: oil real, if the plant does more cycles, or other jobs, it could have warmed up and then cools with closed valves.

there is also a fact, which I think is important: room diameters and stem, to understand how much oil "eat" actually.
and radio quoto, for the delay of response of the valves (maybe there is hysteresis? ).

 

[PierArg]

I would point out that I am ignorant of this.
but, logically, I would say that the piston does not come down precisely because of the difference between the areas that were hunting.