Gentlemen, please refer to the circuit attached (autocad 2007 file).

http://www.megaupload.com/?d=SUZ4J3NL

Here I have a hydraulic system which runs on 2 pneumatic pumps 4 lpm each. The pump supplies to the valve block which further powers 4 cylinders (which can be separately operated). While referring to a particular cylinder,with the down command the 2 DC valves (vickers make: spool type) gets deenergized and for the up command the same 2 DC valves gets energized. During the down command the oil flows via proportional pressure reducing valve.

The relief valve is set at 150 bar, while the proportional valve setting is at 60 bar. Attached to all the cylinders are a hanging load (forged roll + 3 phase induction motor) which have a weight of around 800 kgs (total for each cylinder). The application of the circuit is like this..two cylinder are required for an operation which make one set (one 450 and one 300 stroke cylinder). The circuit is used for continuous casting of billets in Steel Industry. One cylinder is called withdrawal (lower stroke) while the other one is called straightener(bigger stroke). The withdrawal roll is lowered during start of the operation sequence and remains as it is throughout the operation while the straightener is lowered at a specific time during operation and after that it also remained lowered. During operation the cylinders never take full stroke, and that's how they press the hot billet which goes under them.

Problem here is this..sometimes it happens that the lowering command for the straighter / withdrawal is given but the cylinder does not moves at all..now during the course of lowering command, if any of the other 3 cylinders (there are total 4 cylinders as can be seen in the circuit) are moved either up/down, the cylinder which was not earlier moving moves. By this I mean that we have two straightener, S-1, S-2 and two withdrawals, W-1, W-2. Now if S-1 is lowered, and it is not moving, I try to move any among S-2, W-1 or W-2 apart from their movements S-1 also moves which wasn't moving earlier. This problem comes usually after the system has been started and command is given only after some time. Problem is strange in the fashion that it happens to any of the rolls not specifically any one. Also it should be noted that once if the roll is lowered (which wasn't moving down earlier)...after lowering if you successively give up/down command to it the cylinder moves accurately as per the commands.

Another problem is this I need particularly down stroke of 450 stroke cylinders fast (4-5 seconds) Now it takes around 16 seconds for the full stroke to finish, while the upstroke takes 8 seconds. The flow control valves are fully open. I understand that fundamentally the system has low flow rate because of the low flow rates pump, but then there are 3 accumulators 24 litres each which should give me the required flow. Also the flow is not divided because at a time only one of the cylinders is operated. The 300 stroke cylinder speed are also slow..but that is not much of a concern. The down speeds are important for the process, not the up ones.

I have been facing serious problems with this system, can you guys help me with the existing system how can I increase the speeds and also the above stated peculiar problem of lowering the rolls (which don't come down on being lowered and on giving command to other cylinders instanly get lowered)

Please get back to me if you need any clarfications in understanding the system or the problem.

Thanks

Rishi Gupta

Dear Rishi,
I don't understand your Circuit description and
its functioning. However, in order to encourage
others,I attached your curcuit in a PDF format.

POWERPAK.pdf (67 KB, 77 downloads)

I believe that I'm correct in the following...

A cylinder with the same flow to both ends will retract SLOWER.

Something perhaps along the lines of ....
MAX velocity extend = 3.60 inches per sec (18 gallons/min or 82 liters/min for a 125mm bore cylinder )
MAX velocity retract = 3.15 inches per sec

Your retract speed is being slowed by back pressure caused by the cap end return flow.... INCREASE it..

Regards Woody

http://www.hydra-dyne.com/Tech...ces.php?_a=cylinders

This message has been edited. Last edited by: Woodygb,

quote:

Here I have a hydraulic system which runs on 2 pneumatic pumps 4 lpm each. The pump supplies to the valve block which further powers 4 cylinders (which can be separately operated). While referring to a particular cylinder,with the down command the 2 DC valves (vickers make: spool type) gets deenergized and for the up command the same 2 DC valves gets energized. During the down command the oil flows via proportional pressure reducing valve.

Seems very odd to me...why such a strange system when it would seem that ONE valve per cylinder would do the job?

Regards Woody

I'll try and make some sense of all the non return valves........

This message has been edited. Last edited by: Woodygb,

quote:

Originally posted by Woodygb:
I believe that I'm correct in the following...

A cylinder with the same flow to both ends will retract SLOWER.

A cylinder with has the same flow to the rod end while retracting and to the cap end while extending will be retracting faster.

However, it isn't the flow that makes it go. It would help if the cap end valves were bigger by a factor of 120^2/70^2 to the rod side valves.

I agree with woodygb about having two valves to coordinate. It is simpler to control one but when going down there is a tendency to cavitate on the cap side when using one valve. In both cases the control is from metering out on the rod side but when using one valve there isn't enough flow to the cap side.

It is not clear to me how the cylinders, straighteners and withdrawals are oriented.

Tsk...

A cylinder with the same flow co-efficient to both ends will retract SLOWER.

Hopefully that's correct.

quote:

Originally posted by Woodygb:
Tsk...

A cylinder with the same flow co-efficient to both ends will retract SLOWER.

Hopefully that's correct.

Yes, if the supply pressure is constant or nearly constant in both directions.

Rishi Gupta, has stated there are accumulators in the system. These should keep the system pressure relatively constant and not get into the flow limited case but we can't be sure yet.

The VCCM equation and force makes it go always applies.

Your Autocad drawing shows the valves at their default position?...de-energised?...and thus supplying the extend side of the cylinder...but you've drawn the cylinders retracted.

Thus I'm confused about Extend ( ROD OUT ) and Retract ( ROD IN ).

quote:

Here I have a hydraulic system which runs on 2 pneumatic pumps 4 lpm each. The pump supplies to the valve block which further powers 4 cylinders (which can be separately operated). While referring to a particular cylinder,with the down EXTEND? command the 2 DC valves (vickers make: spool type) gets deenergized and for the up RETRACT? command the same 2 DC valves gets energized. During the down EXTEND? command the oil flows via proportional pressure reducing valve.

quote:

Another problem is this I need particularly down EXTEND? stroke of 450 stroke cylinders fast (4-5 seconds) Now it takes around 16 seconds for the full EXTEND? stroke to finish, while the upstroke RETRACT? takes 8 seconds.

So the cylinders are orientated rod end down - cap end up and the mass hangs below?

The default de-energised position is cylinder extended?

At the end of a days shift are the cylinders left in the retracted position ...valves energised and LEFT energised...but the supply pumps are turned off ... or...?

What is the purpose of the non return valves between the P line and the cylinder ports?

Specs on the Vickers valves?

Regards Alan

This message has been edited. Last edited by: Woodygb,

Thanks for the posts . I am sorry for the confusion that arose with the drawing. The valves when de-energized extends the cylinder [rod end down]. The default position of the cylinder is rod retracted. That is both the DC valves are left energized. Also the pump compressed air supply ball valve is closed, when we stop the system.

The purpose of using two different DC valves, the non return valves is not much known to me. Actually this machine underwent revamping for production enhancement. The revamping job was assigned to us and during the course of the job the cylinders were changed with new ones. all dimensions(bore, rod, stroke) were increased, but the hydraulic power pack wasn't changed as the customer didn't want to change it.

Since the system runs on pneumatic pumps with a very low flow rate, even for the system to achieve 150 bar relief pressure, it takes about 2-3 minutes, when the system is started.

Since flow rate is problem we always perform the cylinder operation only one at a time. Pressure drops max to 135 bar. But yes, if two cylinders are extended [lowered] simultaneously, the system pressure falls low as to 40 bar. Once the stoke finishes, pressure again rises to 150 bar.

I understand that with the same flow rate for both sides, the rod retraction (roll going up) would be faster since the cross sectional area is lesser, and it happens practically here as well.

Please find attached the sketch which may help you to understand the system in a better way. Please get back to me in case of any confusions.

quote:

The purpose of using two different DC valves, the non return valves is not much known to me. Actually this machine underwent revamping for production enhancement. The revamping job was assigned to us and during the course of the job the cylinders were changed with new ones. all dimensions(bore, rod, stroke) were increased, but the hydraulic power pack wasn't changed as the customer didn't want to change it.

What were the original cylinder dimensions?

Can you give the valve specs or numbers?

I would check the N charge on the accumulators if you have not already done so-you also mentioned having increased the cylinder size during the revamp-with no increase in pump or accumulator capacity the system will be slowed down.
You may be experiencing silting regarding the deenergizing the coil and depending on a spring to shift the spool-the spring does not exert a lot of force-the caster straightners that I am used to dealing with use a detent 2 coil DCV.
Hopefully some of this may help.

Maytag

Assuming that the accumulators are the source of flow needed to activate all the cylinders whether simultaneous and each actuator can be used at all times and your very small pumps can't really provide the recharging of the accumulators(sole function), it looks like you can't get your desired speed.

Your pump max pressure is 150 and your accumulator precharge is 100, how much fluid is stored to give you the necessary rate of flow? If your operating pressure is 60 or less, do you think the 2+ gpm pump combined can move the cylinders to desired speed? You have emptied the accumulator already for few strokes(since cylinders can move at 60 something), and your low flow rate pumps are the ones giving flow at lesser pressure than accu precharge.

Your circuit is quite not good by using meter in/out to rod side(will intensify). A counter balance is better.
The check valves are for anti-cavitation but became irrelevant with the flow controls as well as connected wrongly.
A flow control at the valve P(after the reducing valve) will be needed once you corrected your accumulator precharge pressure. You have to have a minimum amount of stored fluid to operate your system.

The dcv's are probably DG4S4, but as woodygb said why not use just one valve in each cylinder? Either a 4/2 or 4/3 can be used and do the job(if there is no need of center position a 4/2 is cheaper).

Btw you must have the pump flow rate sufficient enough to recharge your accumulator, which I think at present is very small and slow enough to do it.

Have a nice Sunday evening guys...

quote:

Problem here is this..sometimes it happens that the lowering command for the straighter / withdrawal is given but the cylinder does not moves at all..now during the course of lowering command, if any of the other 3 cylinders (there are total 4 cylinders as can be seen in the circuit) are moved either up/down, the cylinder which was not earlier moving moves. By this I mean that we have two straightener, S-1, S-2 and two withdrawals, W-1, W-2. Now if S-1 is lowered, and it is not moving, I try to move any among S-2, W-1 or W-2 apart from their movements S-1 also moves which wasn't moving earlier. This problem comes usually after the system has been started and command is given only after some time. Problem is strange in the fashion that it happens to any of the rolls not specifically any one. Also it should be noted that once if the roll is lowered (which wasn't moving down earlier)...after lowering if you successively give up/down command to it the cylinder moves accurately as per the commands.

Just some thoughts on the random cylinder "fault".

Moving more than ONE cylinder at any time causes a pressure drop and eventually the system pumps back to pressure.

At the end of a working shift ALL cylinders are retracted and the pump shut off in fairly swift sequence?

So it seems possible to me... after looking at the circuit diagram ... for each cylinder retract side to have a different pressure ( no waiting for the pump to bring the system back up to pressure at working day shift finish)...the first to close will have the highest pressure the last the lowest...this would give a randomness to the "sticking" cylinder/valve IF this imbalance is the reason.

Unfortunately I cannot see why this initial out of balance at start up would cause the "sticking" cylinder / valve.

Are there any electronics or sensors on this machine that could be confused by the initial pressure imbalance?

Regards Woody

Thanks for the posts gentlemen. The DC valve make is
Eaton | Vickers (Made in England )
Rated Pressure P, A & B 300 bar, T 100 bar
Model Code DG4V 3S 2A MU H5 60
Vickers H 507848
24 V DC 30 W P03 / 07

I would like to draw your attention that there is a proportional pressure reducing valve also. The make is

Proportional Pressure Reducing Valve

Make:ATOS (Made in Italy )

RZGO - A - 033 / 210 31

It has a driving circuit card which is

Make : ATOS

Model : E-MI-AC-01F11/1


@ Maglub: The proportional valve gives me around 60 bar pressure but at the same time relief valve pressure does not drop when movement of one of the cylinders is done. I have checked with pressure gauge on the accumulators in which pressure drops max to 135 bar from 150. I believe during idling the accumulator fills again. Also I would like to tell that the withdrawal cylinder which has a stroke of 300, it takes only 4-5 seconds for it to expand, but for the straightener which is having 450 stroke, it takes 16 seconds. I am sure you all will agree that this is strange.

There is one Pressure transducer just after the proportional valve which is taken in the PID loop for pressure regulation. during course of movement of billet pressures changes initially from 60 bar to 25 bar and then it continues with this pressure. Another strange problem with the circuit is that everytime I start the system I have different pressure in even the same straighter cylinder even at the same reference. Say I have given 25 bar set pressure, but lesser pressure is achieved and then PID hunts and results in 100% opening which ideally should result in 150 bar system pressure but still pressure remains below 25 bar. Also pleas note that that this happens sporadically.

DG4v3 is too small for a fast actuation, and you have no flow control before the P ports. Are you sure you are not exceeding the maximum flow rating of this valve?

My point in the last posting was about your accumulator pre-charge pressure which is very high, considering that you have a very low minimum operating pressure. It is not the pressure dropping of the stored fluid, but rather the volume of the stored fluid needed to actuate the cylinder/s at the desired speed, since the pump cant catch up. If the pressure drops and maintained at 135 bars, that means you have a delta P of 35 bars on the accumulator since your precharge is 100 bars. How much is stored fluid volume of each accumulator? Is it sufficient enough to fill the cylinder at a desired time? I am just imagining your cylinder(5" bore). Remember that the pump is just recharging and probably just enough to compensate for proportional case drain.

If the result of your computation is correct, I dont think a 24 liter accumulator is needed, for what?

If the system can operate at 25 bars(prop reducing) and your max system pressure to the cylinder is 60, is it not possible to precharge the accumulator at around 54 bars? I cant give you any exact values since I dont compute. With this possible precharge pressure(lower), you can store more fluid.

What is your minimum operating pressure???!!!
How much flow is needed to obtain the desired speed???!!!
What is the maximum rated flow allowed in the DG4V3 valve???!!!

Just an opinion.... cant think much after playing Rise of Nations T/P... will re-read and recorrect tomorrow...