countarbalance valve with prv???

[RobertHasty]

Hello,

I have a question regarding counterbalance valve. I've received a tip to install a pressure relief valve together with a counterbalance valve.

So I'm wondering what do I achieve by that?

Do I use a PRV next to the CB valve when it is set to high, i.e. higher than maximum cylinder working pressure? In such situation, when load reaches the PRV setting, it would simply remove the CB load holding function, or?

But then again, why would I need the PRV? It would be easier to set the CB lower, right?

Are there any recommendations regarding the CB valve setting, considering maximum working pressure of a cylinder?

 

[hydroman247]

The PRV is normally going to be near the control spools, perhaps incorporated into the spool valve block itself rather than on the cylinders.

 

[hydromech]

The "tip" you received is unhelpful.

A counterbalance valve is effectively a pressure relief and putting two valves together that have similar modes of operation will create problems.

Don't do it!!!

 

[RobertHasty]

Hello,

I spoke with the guy who gave me the tip.

He told me to do it because I have a gear flow divider beneath and that I should "expect pressure increasing for piston side of cylinder which could be released via counterbalance valve but the flow divider will block the further flow".

Now that seems helpful.

Sorry I didn't give all the necessary information.

 

[hydromech]

Your initial question suggested that you were told to put the counter balance valve and pressure relief valve next to each other. That would have made no sense.

You should use a counter balance to support the cylinders and in each leg of the flow divider there needs to be a pressure relief valve that allows the gear set of the leading cylinder to spill, allowing the lagging cylinder or cylinders to catch up.

The pressure increase you were told to expect would be from pressure intensification of the spur gears trying to displace oil into a closed circuit. That clearly wont work, so most good flow dividers have relief valves built into each element.

Who's flow divider are you using, there are some good ones and some bad ones.

 

[RobertHasty]

HELLO, I'm not quite sure we understood each other so I attached my schematics.

http://files.engineering.com/getfil...43-4d53-b74e-6da26a10ba5a&file=2012-11-22.pdf

 

[hydromech]

I see now...that's one way of doing it, but it's not a good way.

You could take the counter balance valve out of the system and get the same result. The check valve lets the fluid in and it can't come out until the relief valve opens. Let's say one of the cylinders is overloaded. The relief will spill and the cylinder will move in fast and uncontrolled manner...very dangerous!!!

Systems like this work better if you put the directional control valves after the flow divider. You get the same control for a bit more money, but you don't have to put additional valves in that make the system unsafe.

 

[RobertHasty]

You could take the counter balance valve out of the system and get the same result.

If I do that, what will do the "load holding and motion control"?

Systems like this work better if you put the directional control valves after the flow divider. You get the same control for a bit more money, but you don't have to put additional valves in that make the system unsafe.

After? You mean downstream of the divider?

 

[hydromech]

The check valve stops the reverse flow and the relief valve will stay closed until the pressure in the cylinder reaches the setting of the valve. However, unless you have some sort of control after the relief you'll get a noisy and jerky response on the cylinder or uncontrolled motion.

Yes...if you put a directional valve in each line between the cylinders and the flow divider, you'll get better control.

 

[hydromech]

I'll qualify my advice by saying I applied this type of system to a US Coast Guard boat launch system.

The arms were synchronised to within 99% of each other and as far as I know, the system continues to work effectively...in all weathers.

 

[pob786]

Hi, looking at your schematic, what is the purpose of the flow divider?, Why not just a"Tee".
Your schematic suggests a Counterbalance valve with an integral Relief valve (shock loading)on either side of cylinders for control in both directions.
The additional relief, downstream of flow divider is to allow one cylinder to "catch up" if it "lags behind" the other cylinder, if the other cylinder achieves either full stroke or relief valve setting!!!!!!!.
If one of the cylinders had damaged piston seals allowing by-pass through cylinder or damaged supply hose, then you could get pressure intensification on other cylinder because of the rotary flow divider acting like a pump.
Regards,
JP

 

[RobertHasty]

Yes...if you put a directional valve in each line between the cylinders and the flow divider, you'll get better control.

Something like on my revised schematics?

http://files.engineering.com/getfil...01-4a36-9fb9-555eec227ef8&file=2012-11-23.pdf

I'll qualify my advice by saying I applied this type of system to a US Coast Guard boat launch system.

I didn't doubt it works

 

[RobertHasty]

Hello,

Hi, looking at your schematic, what is the purpose of the flow divider?, Why not just a"Tee".

Because the divider has to split the flow in a 50:50 manner so that each cylinder can get the same amount of flow.

 

[pob786]

Hi Rob,
Probably my mistake, but I assumed that the supply to both cylinders was common from a direction control valve and the cylinders were mechanically linked together (like the cylinders attached on either side of an excavator arm)thus requiring no flow divider!!!!!.
Regards,
JP

 

[hydromech]

Yes Rob...I mean exactly that.

Me qualifying my advice is just assure you that there is some method in my madness.

This setup also ensures that the cylinders are in sync on retraction.

Nothing will move unless both solenoids are energised, so you cant move one cylinder without the other.

Adrian

 

[hydromech]

One more thing...

The motion control valves you have drawn are over-centre valves. I assume you'll be piloting them to open from the other side of the cylinder.

You know about the pilot ratio and what can happen if its too big right..?

 

[RobertHasty]

The motion control valves you have drawn are over-centre valves. I assume you'll be piloting them to open from the other side of the cylinder.

Exactly. I forgot to draw that.

http://files.engineering.com/getfil...55-4559-a4d5-6cbb0bfa0ea5&file=2012-11-27.pdf

You know about the pilot ratio and what can happen if its too big right..?

Small pilot pressure opens it?

 

[hydromech]

You'll be all good then...

A point to watch out for is when whatever it is you are pushing goes over the centre position, the load is vertical and so the cylinder pressure is at its lowest. In these cases it is the pilot pressure that keeps the motion control valves open. With a high pilot ratio the system becomes too responsive and that familiar but unwelcome rocking occurs.

Good luck!!!

 

[RobertHasty]

Hello, has anybody used EPV valve combination?

By looking at it, it could be used as a substitute on this schematics to avoid two counterbalances and two check valves.