Cylinder synchronization with flow divider 2

[RobertHasty]

Hello,

I was wondering if you could help me out with an advice regarding cylinder synchronization by means of a piston flow divider.

The thing that is concerning me is the fact that the pipe/hose length going to one and other cylinder is not the same.

Working principle of the flow divider is that it sends the equal amount of flow to each cylinder regardless of different pressure applied to each of them.

But I can't figure out how will the different hose/pipe length influence on synchronization. I'm guessing the longer pipe/hose means bigger pressure drop and therefore bigger opening on that side of piston divider.

I would like to read some of Your advice on this.

Thanks.

 

[Oldhydroman]

Hi Robert

Yes, you have it right. The flow divider will adjust itself so that the two outlet flows are equal regardless of the different resistances to flow on each of its outlet ports. The valve can't tell the whether the higher pressure on one outlet is caused by a longer hose or by more load on that cylinder/motor.

A much longer hose on one side will mean there is a larger volume of compressed oil on one side and that will make it a little more "spongy" than the other side. When the load increases you also have to push more oil into the spongy side than you do for the stiff side - this is because you have to make up the greater compressibility volume. Piston type flow dividers aren't so accurate that you would notice this effect (unless your one hose was much, much, much longer than the other).

DOL

 

[RobertHasty]

Thank You Oldhydroman for Your answer.

I will try that my hoses don't differ much in length.

So, to summarise it, the cylinders will move synchronously when all the hoses are filled with oil, and the divider will do his job.

I don't understand what do You mean when You say

Piston type flow dividers aren't so accurate that you would notice this effect

.

I'm planning to use Sun's FSBA-XAN and I would say that 3% is pretty accurate. Or maybe not?

 

[Jboggs]

Over the last year I have installed the following gear type flow divider in two similar applications. Results have been very good. It synchronizes two hydraulic lift cylinders that raise and lower an access door on a melting furnace. Without the divider, the door action was always sloppy at best. With the divider the two cylinders have always matched positions exactly, dead on.
"Haldex Barnes Gear Type Flow Divider, Two Sections, 10 GPM, 3000 PSI max, Viton Seals for high temp"

This was my first experience with flow dividers and I did a lot of research on them. I have worked on lots of hydraulic systems over the years and have a pretty good understanding of the principles and the practice. I can't say anything negative about spool type flow dividers based on experience, but honestly they still strike me as a bit of black magic. I seem to struggle with an operating principle for them that would be fail-safe in all normally anticipated conditions. My gut clearly understands the functionality of the gear type dividers, and I have learned to trust my gut over the years.

For what its worth.

 

[RobertHasty]

Over the last year I have installed the following gear type flow divider in two similar applications. Results have been very good. It synchronizes two hydraulic lift cylinders that raise and lower an access door on a melting furnace.

Hello,

Did You have a situation where pipe/hose length from flow divider to first cylinder and pipe/hose length from flow divider to second cylinder differed?

 

[Oldhydroman]

Robert, you don't necessarily have to use hoses of the same length to each cylinder. It's the job of the flow divider to equalise the flows regardless of the differring loads on the cylinders. The slightly higher pressure drop in the longer hose just looks like a little bit of extra load so the flow divider will compensate for that effect as well.

Regarding the effect of extremely long hoses: let's say, for example, your shorter hose was 1/2" dia x 1 m long and your longer hose was 1/2" diameter x 20 m long then the volume of fluid in the first hose would be 127 cc and the volume in the second hose would be 2534 cc. If the pressure were 210 bar and we assume that the dilation effect of the hoses cause the compressibility of the fluid in them to increase to 2% per 70 bar, then to build up pressure in the short hose you would have to add ~8 cc but to build up the same pressure in the long hose you would have to add 152 cc.

Now let's assume your cylinders are 50 mm bore and you are using the flow divider to synchronise the extension of the rods: when you start the movement the flow divider (assumed to be perfect) will push the same flow into each of its outlet ports. A fraction of a second after starting, the flow divider will have pushed out 152 cc: the closest cylinder will see 144 cc coming into its full bore port before the distant cylinder sees anything. With the cylinder sizes in this example the closest cylinder will move 73 mm before the distant cylinder has even started. This is an extreme example with the numbers chosen just to show the effect.

It doesn't matter what type of flow divider you use; the effect is all to do with the effective compressibility of the fluid in the hoses. If your total stroke was 2 metres then this 73 mm error equates to 3.6% but if your stroke was only 100 mm then the error is 73%.

To mitigate this effect use mutli-spiral hoses rather than braided hoses (the dilation effect is greater in a braided hose) or use lengths of rigid tube on the long runs if you can.

DOL

 

[Jboggs]

Ditto. Unless you are talking about MAJOR differences (an order of magnitude), hose length should make no difference at all. Cubic inches in = cubic inches out. You aren't balancing pressure; you're balancing flow. That's why I'm more comfortable with the gear type unit. Its like a mechanical rack and pinion connection. The only way one cylinder can move is if the other cylinder moves with it, regardless of the load (or lack of it) on the other cylinder.

 

[hydtools]

You can eliminate the hose effect by using steel tubing instead. Maybe equal lengths of hose in each line where flexibility is required.

Ted

 

[RobertHasty]

Hy Guys,

I'm now experiencing some problems with this cylinder synchronization. We have tried 3 machines and they work well, but on one machine, cylinders are out of synchronization. On one cylinder, rod extends all the way to the end, but on the other cylinder, the extension doesn't make a it's full path.

Do You have any ideas what could be causing this? We bled the air and everything...

http://files.engineering.com/getfil...3-b6ff-4de6ca5694cc&file=Eng_tips_problem.pdf

 

[hydtools]

A leak in the inlet line. A blockage in the outlet line. Can you share cylinder pressure readings with us?

Ted

 

[byrdj]

unequal loading on piston (load equals rated pressure*piston area)
damamage piston/rod where friction causes above

 

[RobertHasty]

A leak in the inlet line.

I'm guessing that by inlet line you mean the lines that go to the each cylinder. No leakages were discovered there. As a matter of fact, there aren't any leakages anywhere.

A blockage in the outlet line.

I don't think this is an issue because the rod is doing both extension and retraction. Only when extending, it doesn't make it's full path.

Can you share cylinder pressure readings with us?

Unfortunately not at the moment. The guys at the site didn't provide me with this information. But what I can tell for sure is that the cylinders are "loadless", i.e. no load is being applied on them.

unequal loading on piston (load equals rated pressure*piston area)damamage piston/rod where friction causes above

As stated above, no load is being applied on the cylinders at the moment.


I myself came with the idea that perhaps one cylinder (the one that fully extends) has more oil in it. The reason for this suspicion is the fact that machine was commissioned at one place, then disassembled and moved to another place where problems occurred. I know it's not much, but it's all I've got at the moment.

 

[Jboggs]

I suspect you have a flow divider that is allowing some internal leakage. Switch the flow divider on the suspect cylinder pair with one from one of the good circuits. Does the problem follow the flow divider or does it stay with the cylinder pair? That will tell you if you have a defective (or improperly adjusted) flow divider or if the problem is with another component in the circuit.

 

[hydtools]

If the system was commissioned, disassembled, reassembled and exhibits your problem, was it reassembled correctly? What is different from the commissioning site to the installed site?

If one cylinder has more oil in it than the other, then 'bled air and everything' did not occur.

Oil will continue to enter the cylinders until: the cylinder(s) reach a hard stop, the pump runs out of oil, the flow to a cylinder bypasses and does not enter the cylinder, the system shuts down when one cylinder reaches its limit before the other cylinder does.

While the rods were both extending were they in sync? Are they in sync until one stops and the other continues?

Ted

 

[RobertHasty]

hello,

Believe it or not, the technicians at the site used a crane to extend the second rod. I was informed that the machine works fine now. I'm not sure about the damage that they could cause and still dont know why the second rod didn't want to extend....

 

[byrdj]

could it have been an alignment issue that was side loading the cylinder and the pull through with the crane deformed the connections enough to correct the alignment

 

[hydtools]

Well, if at first it doesn't work get a bigger hammer.

Ted