UNEVEN LOADING WITH TWO CYLINDERS & FLOW DIVIDER

[INNEEDOFCOFFEE]

I have circuit question.
Circuit is as follows:
- approx. 3 ton uneven load (i.e heavier on one side versus other) two cylinders rotating load, powered up and gravity down circuit. PO (Pilot operated) checks mounted to blind end of cylinders. Standard parker A & B to T (Motor spool) driving. Using a cartridge style synchornizing flow divider/combiner between the valve and the cylinder. The standard D03 bang bang valve flows approx. 14 gpm wide open which is way to fast for this application. Currently have a heat generator/orfice in the A line which slows the assembly down and uses approx. .6 gpm to 1 gpm. (acceptable speed bad for the flow divider)

Three problems with this circuit: with the third that I am looking for a solution at this time.

1.) 3 port pilot check valves are bad with paired cylinders in this situation due to the fact that the least loaded cylinder check will open first thus transferring the combined load to the second valve at double the pressure. (this is one problem)

2.) Flow divider has a closed center function which allows a small amount of load holding capability. When the driving D03 valve shifts to center (flow stops) the flow divider cartridge shifts to center, thus holding pressure between the flow divider and check valve mounted to the cylinder. (No safety until the flow divider bleeds off pressure) Thus the cylinder walks and is dangerous.

3.) and most troublesome of them all. Uneven loading of cylinders without a mechanical timing device. The flow divider at a low flow rate divides great/good and keeps the cylinders in time. In the combining mode(gravity down) the heavy side travels faster then the lighter side. Thus the problem. The flow divider is synchornizing which once the cylinders bottom in either direction the cylinders will level back out. But I can not have the cylinders out of time due to other reasons. I am looking for a possible economical fix for this.

Possible cost effective solutions for the third so far.
a.) inline flow control with free flow check.. Free flow for the up(dividing mode) and meter back out on the gravity down (combining mode of the flow divider). Idea sounded possible and was better but still let the cylinders get out of time in the gravity down operation.

b.) size the heavier cylinder to the heavy load, have the ouput rod end oil be the fill for the blind end on the lighter side. Have not tried in this application but have in others.. Bleeding air out of the cross line from one cylinder to the other is not fun.

c.) turn circuit into powered up and down, place heat generators/orfices on all lines, size the orfices accordingly to get the desired speed out of both cylinders depending on the loading conditions??? Might work but kinda farmer like and produces alot of heat.

d.) Expensive method, two piston motors tied together with a coupler for a flow divider. Expensive but would work. $2000.00(ouch) for pistion flow divider

Does anyone have any ideas besides mechanically timing the system.

 

[hydromech]

My advice would be...

1) Have a cup of coffee.
2) Have another cup of coffee.
3) Big problem with P' O' check valves...throw them away and use overcentre valves or counterbalance valves to hold the load. Intergrated Hydraulics and Sun Hydraulics both have good quality, cost effective valves and manifolds.

4) Chuck the flow divider away and replace it with a rotary geared flow divider...Rotary flow dividers have a much wider operating range...the faster they run the more efficient they are. Delta Power do an economical one.

As with most hydraulic systems the degree of accuracy depends on the amount of cash you throw at the job.

Having said that...changing the P' O' checks and the flow divider will be a cost effective solution.

Drive the cylinders in both directions...flow dividers don't work as flow combiners if the cylinders are not mechanically linked together...the lighter cylinder will always move before the heavier cylinder or in vertical applications the heavier load will move first because gravity makes it easier.

You will need 2 directional control valves. Put the flow divider before the valves...divide the flow between the valves and operate the valves to change the direction of the cylinder.

If you can, you should remove the chokes and install a flow control valve before the flow divider...it's not the best way to control the speed but it will be the easiest to install.

I am assuming that you have a fixed displacement pump...you might want to consider resizing the pump to avoid excessive heat generation or even better use a variable pump.

Hydromech

 

[kcj]

-I agree on the PO checks: Basically openor closed with no modulation and control. i.e. to much 'gain' can make them unstable and chatter.
PO checks also can trap very high pressures if thermal expansion or load kicks back. Counterbalance/load control avoid both of these issues. Better modulation, and relieving functin beyond a certain point.

-Ditto on the gear divider/combiner. Spool type only throttles energy on the lightly loaded side. Gear types actually transfers it to the heavyily loaded side an intensifies pressure. Also solves the trapped pressure situation that is defeating your PO checks.

Delta, Haldex/Barnes, parker, and others. Two section gear ones are avilable at

http://www.surpluscenter.com

or at Nothern handyman

-Gear dividers do combine with single acting cylinders in lowering (assuming the unbalance is not enough to totally unload one corner. Pressure after all cannot go below 0). If you need float position in down, stay single acting. If you don't I would change to double acting as suggested above.

-Hydromech: I don't understand your thought on why needing two valves. Simple directional valve, then the gear divider split to each cylinders??

-Variable pump yes the best. Assuming also you have a gear pump existing, you will either have to dump excess flow across relief, wasitng nergy, and live with it, or change the pump flow to theis circuit.
If tthe pump is way too big, easiest is to put in a variable pump, or smaller gear pump. Alternately, you can put another gear divider ahead of the directional control valve. Say the pump is twice what you need. A two section, equal sized divider ahead of the directional valve will split the sflow in half, but double the pressure. A 14 gpm pump, 2000 psi load, that only needs 7 gpm will send 7 gpm across RV at 2000 psi, about 8 hp.
using a divider ahead of DCV, 7 gpm in one side goes to tank, but the energy goes to the other section. Pump puts out 14 gpm at 1000 psi, one section has 7 gpm at 0 psi, the other is 7 gpm at 2000 psi. No heat losses.

-I don't understand what you mean by heat gernerators/chokes. Any sort of throttling device, flow divider or flow control, is converting energy to heat.

-If this is the only device on the pump, you could also use a priority divider ahead of the DCV, or a bleed off flow control ahead of the divider. Both are wasteful, but if the load operates most of the time well below RV setting, they can save heat.

k

 

[hydromech]

K...I based the statement about using 2 valves with a flow divider on my experience on a boat davit we supplied to the U.S Coast Guard.

I inherited an old system that used a rotary flow divider to synchronise 2 independent double acting cylinders. Originally there was 1 DCV and 1 flow divider...the flow divider would split the flow when extending the cylinder and combine the flow coming back form the cylinder.

All worked well until...Moving the cylinders slowly the lightest loaded cylinder would move first...pushing oil trough the flow divider. Because the the pipes into the annulus of the cylinders were direct from the DCV,via a "T", the lighter cylinder was able to move without the heavier cylinder.

As far as the flow divider was concerned, it was seeing oil coming back from only 1 cylinder...that element of the flow divider was turning. The other element was able to turn but wasn't moving any oil.

The heavier cylinder would not move until the pressure drop across the lighter cylinder was equal to the force required to move the heavier cylinder. Once both cylinders were moving they were synchronised.

Rotary flow dividers will combine flow as long as the oil is forced into them. If a light cylinder can move by itself...it will.

In INNEEDOFCOFFEE's case the heavier loaded cylinder will need less pump pressure to to move against the OVC/counterbalance valve. With low flow and negligible pressure drop I think it is likely that the heavier load could...and I mean could, move on its own if the flow divider is used as a flow combiner.

My suggestion to INNEEDOFCOFFEE is; to get a safe reliable system is to use the flow divider to split the flow to each "P" port on the DCV.

The other advantage is that if one of the DCV fails to move for what ever reason...the flow divider cannot turn and it is not possible to move 1 cylinder without the other.

Hydromech...

 

[INNEEDOFCOFFEE]

Thanks for the reponses hydromech & kcj..

More about the system.
This is one circuit on 20 circuit system. The pump driving this system is 20 hp, 34 gpm HP limiting pump @ 2000 psi. A couple of other circuits in the system at various times totally consume the capacity of the pump output when actuated. The circuit in question does not require anything close to what the pump can produce but the two other devices due so I am stuck with the big pump at this time. When this device is actuated about 10 other hyd circuits are actuated at the same time. The actual available flow would be maybe about 10 gpm from the pump for this one circuit if wide open. For the speed that is exceptable for this device, the flow is less than 1 gpm/more truely around .6 to .7 gpm. Thus the problem with any flow divider that I have found or played with. Most flow dividers, at least that I have played with like flow. The more fluid you push through them the less the percentage of error. @ .6 gpm to .7 gpm I am on the lower flow range for the sun cartridge style flow divider which produces the greater amount of error.

I have in the past used the delta and heldex gear type flow dividers on circuits that basically have the same function, I.E two cylinders powered up and gravity down, one bang bang valve to drive with a delta type gear divider/combiner after the valve and before the cylinders. 4 port, sun vented P.O check valves mounted to the blind end of the cylinders. This works but also eventually gets out of time. For this example the only uneven loading on the assembly was a 500lb electrical motor hanging off one side of the assembly. If the cylinders are never stroked either full up or full down, meaning the operator of the device every 30 mins or so actuates the cylinders up 2 inches, down an inch, up and inch, down an inch in the middle of the cylinder. Over time the cylinder would get out of time. Because of the describe this circuit was changed from a delta gear type flow divider to a sun cartridge style flow divider. In this example the device still gets out of time, but with the synchornizing portion of the flow divider and more flow through the divider I was able to lessen the amount of error. But there is still error. I have never found a good way to take the error out except to mechanically time the system.

In my writing above what I call a heat generator is a orifice mounted in the A port of the valve. It is set in the port and is sandwiched between the valve and the hydraulic manifold. The orifice is drilled to a certain size to produce the .6 to .7 gpm. Basically the same as a flow control, but cheaper and no knobes for the operators to grab a hold of to turn up the speed of the circuit.

Like the ideas above, I am going to have some time in the next month or so to play and try a few different concepts on this circuit.

KCJ... Like the idea of the flow divider used to split the flow to the circuit. I have never tried that one before. 7 gpm to the tank @ 0 psi and 7 gpm to the circuit in your example. Have to keep that one in my back pocket to try or test sometime.

1.) going to yard the 3 port PO checks and use probably a counterbalance valve or maybe a 4 port vented po check. The 4 port is suppose to vent, not letting the added weight be additive to the pilot pressure as in the 3 port po checks. Either way a counterbalance valve is probably right for this application.
2.) I am going to pull the cartridge style flow divider out and test again another delta or haldex.. Have a delta on the shelf here.
3.) I am curious about the two valve idea, flow divder before the circuit, flow control etc etc. but you both are certain about the gear type flow dividers working better in your experience. That has not been the experience that I have seen, but I will give it another go. I think that this assembly is approx. 3 tons with maybe 700 to 900lbs heavier on one side versus the other. Was thinking maybe to up the bore size of the cylinders, more flow, which would then let me open up the orfice size or adjustable flow control thus lessing the percentage error of the circuit. (I.E more flow to fill the cylinders) (Kinda expensive in cylinder cost thought) when not really needed.
4.)another option, D03 proportional valves, one for each cylinder, linear tranducer to monitor its position. As long as the code was wrote decent enough it could monitor the position of the cylinders.

Any other ideas? all are great and really appreciate it.

Need to go get another cup of coffee..

 

[kcj]

tks for info.

Given what you have, and need, I'd go with option 4. Depending on how fast and how precise you need, possibly a bang bang valve may work, but most likely two prop valve and two feedbacks are the best shot. Usually feedback is built into the new cylinders, but for the existing, you can add the feedbacks in parallel, or piggy back, on the existing mechanism. We use a lot of Custom Actuators, Plymouth MN as they are close and work with us a lot. They are voltage and slider type, and can be built into a separate cylinder. We have also used Bimba strips, very cheap, under $500, but not real durable in rough environments. Temposonics/Baluff is of course the cadillac tachnology and non contact, but we can't use them in high emf environment.

Position feedback gives you many options, error diagnostics, speed and acell control, knowing if within the normal working range limits, etc. It may be easiest to bite the one time extra cost and be done with it. Highly responsive underlapped prop valves will add some null flow leakage to the circuit though. Valves with some overlap and deadband won't add any leakage.

yes, gear dividers are like flow. The leakage is somewhat fixed, dependant on pressure only, and as a per cent of flow it adds up at low flows.

larger cylinders would reduce the working pressure, reduce leakage, and increase flow. All those help, but at the cost of replacing cylinders I'd focus on feedback and prop valve and closed lopp control.

k

 

[badasselectric]

I am not sure about the ins and outs of all things hydraulic but have worked on some pretty strange machines in the mining industry. here is an idea maybe you can run with it, on some drills they had a system called "paralleling automatics" which amounted to 2 identical cylinders installed at two articulation points. they were then hosed together, so when you moved the boom on the machine the slide would "automatically" stay parallel. the "pump" cylinder at the boom would make the slide stay in parallel simply by swaping oil with the other cylinder.

I say again I am not a hydraulic expert by any means, but what if you connect the rod side of your cylinder under the heavier load to a smaller cylinder of equal area on the piston side and control the table by adding or removing oil from the piston side of the larger cylinder.
just a thought, seems like it would be cheap, feel free to critique me, I am much more comfortable with wires than hoses. badass

 

[hydromech]

A short note for anyone considering using a rotary flow divider...Try to match the loads as closely as possible.

The element of the flow divider that sees a higher pressure will have more slippage; oil lost between the clearances in the flowdivider. That slippage translates as a lag on the higher pressure cylinder.

Always try to drive the actuator to its physical limits...that will allow the relief valves to operate and allow the actuators to realign.

Flow dividers will always have an amount of slippage...any loss of synchronisation will be accumulated until the actuators are forced into an identical position.

 

[kcj]

good points all.
yes, dividers do leak, just like the gear pumps they are based on, and it relates to pressure. taking that leakage related to total flow rate gives an accuracy. The loss is lower as a percent at higher flows.

The two cylinders in series may be a good choice. Don't know why, I just assumed he had more than two to synchronize. With good seals, they can be accurate. They don't rephase at end of stroke with out separate plunger operated valve inside or outside of the cylinders.

Area of outlet side of one cylinder must equal i\the inlet of the next one for equal speeds. Check for pressure intensification if all the load is on one cylinder circuit. (say one clevis pin gets disconnected from load). Also, with loads in series, the first one sees high pressures.

k

 

[INNEEDOFCOFFEE]

Thanks all.

In all the playing in hydraulics that I have done, the only true and complete way to time hydraulic cylinders with uneven loads is to mechanically time the system. Or at least that is what I am finding out. I sure wish someone could prove me wrong. (at a resonable cost) I have searched and searched and the best results hydraulically that can be accomplished is to lessen the amount of error between the cylinders. The higher the flow the less percentage of error. I have used sun cartridge style flow dividers, delta and heldex flowdividers, various motors coupled together and all have a leakage which amount to error.

Thanks badass for your suggestion. I agree that the solution suggested with swapping oil from the rod end of the heavier load to the blind end of the smaller cylinder on the lighter side would work. I was saving this idea for last and the only reason as to why is because I worked on a double roded cylinder system one time that was plumbed approximately the same way. I had a heck of a time getting the air bleed out of the transfer line from one cylinder to the other. When I did get the air all the way out, I had air trapped in the base of the cylider even when the piston was compressed(cushions). I changed the cylinders on another simuliar application so that the cylinders had bleed ports drilled next to the ports on the cylinders, so that you could crack the bleed ports thus eliminating the air from the line and the base of the cylinder. All was good, then leakage started across the seals of the cylinder. I couldn't win for nothing on that one. Although..... this idea has been used in the ag business for years with total success, or so I have heard. I would like to talk to someone that has had to maintain a system like that and see if their experience was the same as what I have encounter or if it worked good. I suppose with a good lipseal in the cylinder and proper sizing the idea seems sound with a resonable price. Again as kjc stated, you do need to have a relieve valve in the system for synchornization at the end of the cylinder stroke.

For this current system I am working on, the cylinders and most of the components are already purchased. So I am going to play with various setups to see how much error I can eliminate from the system. I am going to try the following.

1.) Plumb the system as described and test it like crazy. This will be using the sun .6 to 3 gpm synchornizing flow divider/combiner. I am going to open up the orfice sandwiched behind the valve, increasing the speed of the assembly, to figure out what vertical speed I can live with before the assembly starts to tear itself apart. Opening up the orfice will increase the flow and lessen the percentage of error through the flow divider. These sun style flow dividers require a certain amount of delta P across the divider to function properly. I need to insure that I am getting this.

2.) swap out the sun cartridge flow divider and replace it with a relieving delt gear type. I think I have one here. Going to test this system like crazy.

3.) Have on order some counter balance cartridges that will work in the T-11 cavity po check bodies. I am then going to plumb a couple more lines and turn the system into a powered up/down system. Going to test this situation with both the sun and the delta flow dividers. See how the control is and how the percentage of error changes.

4.) I also want to test these situations first thing in the morning and later in the day so that I can see the difference that occurs with oil temperature when the machine is first fired up in the morning versus being run all day.

I will let you all know the results.

Thanks again and if any more thoughts come about please let me know.

 

[ddp5993]

You present an intriguing problem. Could you use an intensifier to balance the load/ motion? I have used these in conjuction with unmatch cylinders to balance lopsided loads in work holding devices.

 

[kcj]

I think in step one you will find the slow speed/load drift to m\be much worse. Two reasons, the spool type requires some amount of pressure drop ie flow to work, and may not totally close off the orifice at no flow condition. Low flows, not enough delta P to work the spool, and drift.
Second reason, more inmoprtatnly, the orifice adjustment you refer to I assume is the balaning portion. If the spool is cut to 50/50, and you want 48/52, there is a bypass orifice inparallel to the dividing oportion. That is very sensitive to flows, and again at no flow, if it is open at all, there is uncontrolled drift leakage.
Spool types are typically better with motor flows continuously.

keep us informed, an interesting project.
kcj

 

[budt]

I can send you a .doc file of all the ways I know of to synchronize hydraulic cylinders. It is from a circuit design book I put together to teach a circuit design class.

The file is 550 K and comes in two files. One file has all the circuit pages in schematic form and shows the symbols shifted to their next position as the cycle progresses. The other file is the text for the schematics explaining the operation and giving information on advantages and problems of each method.

You can downlaod a brochire at

http://www.ifps.org

in the "Links" section under "For Profit Companies" and "Hydra-Pneu Consulting" There is also a brochure their for a basic Fluid Power trining book I use for beginners.

Also you can download a set of symbols for MSWord that can be used to make schematic drawings in Word. Word has an extensive drawing prgram and can produce drawings up to "E" size.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[PNachtwey]

One more way to synchronize cylinders. Why not use two servo proportional valves, two temposonic rods and a hydraulic servo controller? The up front cost will be offset by peformance an not having to continually adjust all those valves.

 

[unclesyd]

Take a look at this site’s rotary flow divider.

http://www.hbus.haldex.com/products/hpm.htm#anchor812499

 

[regnit]

This reply may be too late. I have used a series hydraulic circuit to achieve syncronized cylinder in a few applications. Rod area of cylinder 1 is equal to the pistom area of cylinder 2. cyl 1 and cyl 2 are connected rod port to piston port (series). As cyl 1 extents it pushes the oil in it's rod end to the piston side of cyl 2. They move in sync.