Complex Hydraulic Synchronising Problem
Complex Hydraulic Synchronising Problem
(OP)
I’m looking for some advice on finalising the hydraulic design on a cable tensioning system.
Four Cylinders are being used to pull a 160 strand cable to 1274t tension over approx. 840mm stroke, the cylinders are 300mm bore x 200mm rod x 700mm stroke. Buckling is ok.
From 0mm to 700mm the load goes to approx. 200te, the next 140mm is stretching the cable and the force rises exponentially to 1274t.
When the cylinder runs out of stroke or the final tension is realised a large jacking nut is threaded down the cable onto a bearing plate, the problem is that in order to install the cable (offshore) the cable is installed with this 840mm slack which as is passed through a U shaped inclined clamp. When the job is done two opposing cable cross in an X shape so the cable end mounting plates are fixed approx. 45 degrees. When we install the slack cable the catenary weight causes the bearing plate to tilt 8 degrees in the mounting plate, analysis has shown a force of 200t is required to rotate the bearing plate.
Link
In order to prevent thread damage it is important to ensure the cylinders all extend in unison, to complicate matters the two cylinders furthest away from the corner of the bearing plate in contact with the clamp are being put under tension “dragging” the rods out of the cylinder (see att. where the hatching indicates high pressure in the cylinder due to the induced load), the two cylinders adjacent to the contact point are being compressed. The hydraulic design must accommodate this up the c.200t load after which the 8 degree gap between the bearing plate and clamp is closed and the force applied by the four cylinders push in the same direction. Various concepts for this control are being considered from individual servo control of each cylinder to the use of an over centre valves with a single directional valve and flow divider, both concepts have yet to be considered fully.
If we pursue a solution with servo or proportional valve controlling each cylinder with each having a feedback loop fed from cylinder mounted distance transducers this could give us a problem. Any error correction in one cylinder will have an effect on the adjacent cylinder as they are all rigidly connected to one another via the jacking plate (cross head), this could give rise to a confused system that may be impossible to operate.
I’m also considering a simplified solution of over centre valves (as shown) in the annulus side of each of the cylinder and piloting open from the full bore, the load induced is approx. 15t equating to 37 bar in the annulus.
I’ve discussed with the leading Servo valve people and they are very nervous of the problem noted above.
Any help would be appreciated.
Regards
escapizm
Four Cylinders are being used to pull a 160 strand cable to 1274t tension over approx. 840mm stroke, the cylinders are 300mm bore x 200mm rod x 700mm stroke. Buckling is ok.
From 0mm to 700mm the load goes to approx. 200te, the next 140mm is stretching the cable and the force rises exponentially to 1274t.
When the cylinder runs out of stroke or the final tension is realised a large jacking nut is threaded down the cable onto a bearing plate, the problem is that in order to install the cable (offshore) the cable is installed with this 840mm slack which as is passed through a U shaped inclined clamp. When the job is done two opposing cable cross in an X shape so the cable end mounting plates are fixed approx. 45 degrees. When we install the slack cable the catenary weight causes the bearing plate to tilt 8 degrees in the mounting plate, analysis has shown a force of 200t is required to rotate the bearing plate.
Link
In order to prevent thread damage it is important to ensure the cylinders all extend in unison, to complicate matters the two cylinders furthest away from the corner of the bearing plate in contact with the clamp are being put under tension “dragging” the rods out of the cylinder (see att. where the hatching indicates high pressure in the cylinder due to the induced load), the two cylinders adjacent to the contact point are being compressed. The hydraulic design must accommodate this up the c.200t load after which the 8 degree gap between the bearing plate and clamp is closed and the force applied by the four cylinders push in the same direction. Various concepts for this control are being considered from individual servo control of each cylinder to the use of an over centre valves with a single directional valve and flow divider, both concepts have yet to be considered fully.
If we pursue a solution with servo or proportional valve controlling each cylinder with each having a feedback loop fed from cylinder mounted distance transducers this could give us a problem. Any error correction in one cylinder will have an effect on the adjacent cylinder as they are all rigidly connected to one another via the jacking plate (cross head), this could give rise to a confused system that may be impossible to operate.
I’m also considering a simplified solution of over centre valves (as shown) in the annulus side of each of the cylinder and piloting open from the full bore, the load induced is approx. 15t equating to 37 bar in the annulus.
I’ve discussed with the leading Servo valve people and they are very nervous of the problem noted above.
Any help would be appreciated.
Regards
escapizm





RE: Complex Hydraulic Synchronising Problem
Use 1 flow divider...ideally from Slack and Parr in the UK.
Use a directional control valve for each cylinder that you want to operate.
If you want to drive 4 cylinders, use a four channel flow divider and 4 DCVs.
Use over centre valve to hold the cylinders still and use double rod cylinders (if you have room) to keep the same speed in both directions. Don't use a flow control valve to control speed because even pressure compensated valves have a pressure drop that will cause leakage across the flow divider and will put the cylinders out of sync.
This set up works perfectly well in many marine applications where cylinder synchronisation is paramount. It is very robust.
Regards
Adrian
RE: Complex Hydraulic Synchronising Problem
What are the benefits of 4 DCV's instead of 1 into the flowdivider? Ive used 1 in the past.
Did you see the link to my circuit?
Regards
escapizm
RE: Complex Hydraulic Synchronising Problem
There are good reasons for using 4 DCVs.
1) It gives you synchronised flow in both directions
2) It means that you don't have to use the flow divider as a flow combiner, the tank lines from the DCVs go around the flow divider.
3) When using as a flow combiner, if the cylinders are not mechanically linked, it is possible to drive one cylinder on its own. With uneven loading on the cylinders the lightest loaded cylinder will tend to move first. With no oil coming in from the heaviest cylinder, the gear in the flow divider will turn in air.
It is more expensive to have 1 flow divider and 4 DCVs, but the payback is long service.
The system I designed uses this set up. There is a fleet of USCG cutters...all of their boat davits use this hydraulic system.
Cheers
Adrian
RE: Complex Hydraulic Synchronising Problem
Sorry to be a pain…
My system is rigidly connected with all four cylinders attached front and back via rigid crossheads within a 1.5m cube (albeit without sides), the cylinders stroke 700mm.
I can’t quite grasp what the benefits of individual valves feeding into separate section of the flow divider are. I would of put a DCV with 4 times the capacity feed one section as shown below.
Service life is of minor concern, the cylinders are going to stroke once or twice until we achieve maximum tension then the whole hydraulic system is going to be removed and possibly never used again!
Also, am I right in thinking despite the uneven initial loading due to the mechanics of the machine that by installing the over centre valves the cylinders will see an even load and thus equal pressure drop in each section of the flow divider. Uneven pressure drop could lead to the cylinders extend out of sink, although the rigidness of the system will aid this I think.
circuits here
Regards,
RE: Complex Hydraulic Synchronising Problem
If you use a decent flow divider, you'll get good performance out of it.
Your schematic shows proportional directional control valves, that would be proper expensive. Do you need accurate position control?
What are the PO check valves for?
Adrian
RE: Complex Hydraulic Synchronising Problem
Precision* is of major importance and one consideration prior to my joining the project was servo or proportional valves, the Servo people had reservation with it working due to that mentioned in my first post so i came up with the other solution. The system operates very slowly stroking approx 700mm in 30mins to 1 hour. The system may also be left mid or full stroke as such i dont want spool leakage to permit the cylinders to come down thus the PO check.
*This is why i asked about the over centre valve and flow divider leakage.
Thanks
RE: Complex Hydraulic Synchronising Problem
The divider will only leak when it's operating.
To reiterate, the over centre valves will stop the oil coming out of the cylinder so the internal leakage on the other system components can be neglected.
The flow dividers Slack and Parr make are super accurate, they are derived from dosing pumps and they are very precise.
A proportional DCV with a flow divider and cylinder position feedback to close the loop should work a treat.
Cheers
Adrian
RE: Complex Hydraulic Synchronising Problem
http://www.beckwoodpress.com/company/active_leveli...
The automatic leveling control will keep your cylinders synchronized even though the loads on the cylinders are uneven.
escapizm's application is very slow compared to a press. escapizm's applications should be easy of the hydraulic design is good.
I don't see how a flow divider will work unless the synchronization specification is very lax. The oil will compress a lot with long cylinders.
A lot depends on the specifications and your tolerance for risk. The flow divider idea may work well enough but the servo control will work.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Complex Hydraulic Synchronising Problem
I give honest and open support and ideas based on real experiences to give a cost effective and robust solution to a problem.
Peter, it is not appreciated when you give advice that contradicts and undermines the advice I have given, which as I say, is based on real applications involved with life saving and mission critical systems in harsh marine environments.
Nobody doubts that servo control will solve the problem, but there are aspects of servo control that make it somewhat tricky to purchase, set up and maintain.
I was giving balanced advice, please don't come in a late stage and dismiss or undermine my advice. As I said...it is not appreciated.
Adrian
RE: Complex Hydraulic Synchronising Problem
Attribute it to what motorcycle racers call "brain fade".
... and everybody have a cup of tea. Please.
Mike Halloran
Pembroke Pines, FL, USA
RE: Complex Hydraulic Synchronising Problem
What is the point of an open forum where anyone can contribute if only one person is allowed to express their opinion? This is an engineering advice forum, not your own personal consultation.
Whilst I agree with you on this, it really isn't fare to dismiss other peoples opinions because they differ from yours. If you really want to, just rationally explain the pros and cons of both ideas. Multiple ideas for solving a problem is always better than one.
RE: Complex Hydraulic Synchronising Problem
My opening comment was to state that, from experience of both options and from the info given...a flow divider gives the most cost effective solution to the apparent problem. It was a piece of balanced advice. My objection to Peter's approach was that it came late on is the conversation that was, as I see it reaching a conclusion. The original post had indicated that a servo control solution was not preferred. There seemed little point advocating servo control, let alone being so dismissive of my comments.
All are free to add their own opinion and advice, but, as is often the case, people weigh in with unhelpful and misguided advice simply because they did not read the thread fully.
I always read each thread thoroughly to make decision on whether I can make an alternative point or support someone else's advice. I would expect other professional engineers to do the same.
People need to read what is there before contributing!!!
I'm off for a cup of tea...
RE: Complex Hydraulic Synchronising Problem
... PROVIDED that the associated bearings are long enough to not lock up.
Predicting whether that will happen is a sophomore kinematics problem.
... that we lack data to solve, but given the stated envelope,
I'm guessing that binding is possible to some extent.
So flow dividers make sense. I'd suggest that they be sized to spin relatively fast, so internal leakage won't affect their performance so much.
I'll bow to Adrian's experience about the flow divider source and the number of directional control valves.
As for the servos, beyond the innate complexity, I think the crossheads would need to be articulated so that the cylinder/transducer units could move independently of each other, _or_ the transducers would need extraordinary resolution so that each servo loop would be mostly independent of the others.
I'd also be troubled by having four (delicate) servovalves on a piece of heavy duty rigging, so I'd want to build an armored cage around them, adding weight and cost. ... to a device that may be used just once.
I'm not saying servos wouldn't work, just that they wouldn't be my first choice for this job.
Mike Halloran
Pembroke Pines, FL, USA
RE: Complex Hydraulic Synchronising Problem
Thanks to the peace negotiator call Mr Mike Halloran...
Have a star!!!
RE: Complex Hydraulic Synchronising Problem
Well now escapizm has his answer then. A servo control system will be more expensive but the risk is smaller because a servo valve will adapt for the compression of the oil column and leakage.
Purchasing just cost money.
It only has to work for a short while. Get an expert to set it up. This looks like a it is a small part of a big project and it would probably cost a lot to screw it up.
I was waiting for the specifications and an estimate of how much skew there would be. I saw no calculations for the estimated skew. The skew will increase as the load increasing due to compression of the oil. Skew will increase over time because of leakage. How much leakage will their be with the best flow dividers over 30 minutes of moving time.
With no calculations or specifications I would say it is iffy if a flow divider would work.
As far as costs go. How much does failure cost?
I read it thoroughly. What is misguided about my advice? Why isn't it helpful?
This applications seems like a slow one, 30 minutes or more, where kinematics aren't a problem. The problem will be how much the oil columns will compress under load and how much the oil will leak if the move takes a long time.
quote]
As for the servos, beyond the innate complexity,
[/quote]
To an expert, servos are not complex.
I doubt the Beckwood press cylinders are articulated yet the press must accomplish much the same goal. The link I posted says the press control can handle uneven loads. I have actually been there and seen it in action. The total force of the press is probably much less than 1247t but the press is much faster and must react to uneven loads in milliseconds. These press machines use servo control to do the automatic leveling day in and day out stamping thousands of strokes per day! escapism needs only to do a few strokes.
The resolution of MDT rods is now down to micros which makes little difference in this application because of the forces involved. The metal will probably stretch or compress 10s of microns.
The press provides plenty of shock and the valves survive. Again, escapizm's system only needs to work once.
I would like to know who escapizm's servo people are.
It will be interesting to see which way escapizm goes and how his project works out.
This forum is technophobic.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Complex Hydraulic Synchronising Problem
It's not technophobia, it's balanced and free advice based on experience. I for one do not have a company to promote, in particular a company that has much to gain in advocating electronic control.
I am fully aware of the full benefits of PID control with proportional or servo control. I am also aware of the costs and fragility of the said systems.
If you get the chance, have a look at the code of conduct for Chartered Engineers as part of the Institute of Mechanical Engineers. That will explain my motives.
By all means, work as an advocate of servo control to support you business interests, but please don't be so quick to dismiss the ideas of your fellow engineers.
I apologise for detracting from the theme of the original question.
Cheers
Adrian
RE: Complex Hydraulic Synchronising Problem
No it isn't. Your prejudice has been revealed by using the word fragile over and over again. See below.
I didn't mention my company. It is simply in my signature.
Well maybe.
At first I only offered an alternative with a question about flow dividers meeting the specifications and this question still hasn't been answered.
We get involved with a few unique application like this one. This project looks like it is part of a big construction project. A simulation would be good for a proof of concept. It may take a few days but it might save time and money in the end. It would want to minimize risk.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Complex Hydraulic Synchronising Problem
Some clarity on the above.
Given the rigid nature of my existing (huge casting was purchased months ago) crosshead and the possible uneven loading along one edge, MOOG (the leaders IMHO) suggested the analysis/simulation to determine how the system would operate and the subsequent design of the software and programing could be as expensive as the valves alone. If I was to go with servo control of each cylinder individually that would require 24 servo valves plus digital controllers as there are 6 systems as my circuits. The valves would be stainless steel to handle the pressure and are made to order with a total package price of c. GBP £179,000 plus any analysis and programing!!! Let’s say GBP £360K for a temporary installation aid and this is not for an ATEX Exd Zone 2 system as I’ve now discovered the system must be adding another 10% perhaps.
If I have confidence in parallel operation (+/- 10mm over 700mm stroke) of the jacking plates with the flow divider this is my favoured solution, I can increase the speed of operation but would be nervous of moving such massive loads quickly, if gear leakage is an issue on the smallest high pressure flow divider I can find I may well increase the speed. The load is only uneven up to c.200t after which it becomes even across all four cylinders as the cable becomes taut. 70 Bar in the full bore gives me 50t in each cylinder so I can get the 200t with plenty in hand, we will prove the system at FAT before we procure the full quota.
RE: Complex Hydraulic Synchronising Problem
I couldn't get a better performance from any other device, but I could have paid more to have gears ground to match the housing to minimise the clearance.
Good luck
Adrian
RE: Complex Hydraulic Synchronising Problem
Link
Very entertaining.
RE: Complex Hydraulic Synchronising Problem
I’ve added a decompression valve (see circuit) in order to avoid and shocks as the assumed 12.5 litres of compressed oil (0.7% per hundred bar for compressibility of the oil and the same 0.7% for the expansion of the cylinder body/pipework = 1.4% / 100 bar = 6.3% @ 450 bar = 12.5 litres)
I am wondering though if I should add some lines to the over centre valve, if you see my original post with the link with the mechanical schematic you will see the full bore of two of the cylinders is always under positive pressure, whils the two furthers away from the contact corner are not.
On that basis will the two closest to the contact point open immediately I permit the oil to flow out of the annulus with a possible delay for pressure build up in the other two before they move? If this was the case I could put a line from each full bore of the cylinders furthers away to each of the over centre valves.
Thoughts?
RE: Complex Hydraulic Synchronising Problem
RE: Complex Hydraulic Synchronising Problem
What is the actual positioning tolerance and expected speed of movement under shock load?
I think a proportional valves with linear feedback will be better than 3% accurate and will provide temperature and viscosity compensation not possible with a gear flow divider. My personal experience with gear flow dividers would make it my last choice for positioning accuracy especially at your working pressure. Gear flow dividers also act as a pressure intensifier and the accuracy only gets worse with age.
Proportional valves are more robust than servo valves in many applications. Proportional valves can be closed center so there is no leakage like in a servo.
The most accurate if there is the possibility of reverse direction over load would be meter out. Your design with flow dividers will not stop the cylinders from extending if the rod can be pulled. The great part of a closed center cartridge proportional valve is they can be installed at each of the cylinder ports to eliminate dropping if a hose breaks.
To reduce the cost of the system, reduce the working pressure to 340 bar with larger bore cylinders or more cylinders and use off shelf components.
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Complex Hydraulic Synchronising Problem
Your right on the flow divider I now think this is not an option.
I'm now moving toward a solution like the below
The high pressure (actually now 500 bar) is dictated by the necessary load and the limited space we have, the max od of the cylinder we can accommodate is 355mm thus the 300 bore x 200 rod cylinder.
RE: Complex Hydraulic Synchronising Problem
You could use two tandem pumps (one clockwise rotation and one anti-clockwise rotation) and mount one double pump assembly at each end of a double shaft electric motor. Some of these radial piston pumps are not sensitive to direction of rotation – so that makes it even easier.
You could put a four way splitter gearbox onto your single electric motor but that’s probably an expensive overkill.
You could opt for a multi-outlet pump, either a radial piston or wobble plate pump. Hawe make some nice radial piston pumps with the option for individual outlets – you could use the five cylinder pump and leave one outlet disconnected (available as a spare if there’s a problem with one of the other outlets). Bieri have a similar option available on some of their pumps. Dynex-Rivett make a nice “check-ball” pump which has similar options. You could use the eight outlet pump and pipe together two diametrically opposed outputs for each of your cylinders.
Make sure you use leak-free valves everywhere in your circuit (direct operated relief valves, poppet type DCV's, overcentre valves with sealed pilot pistons).
DOL
RE: Complex Hydraulic Synchronising Problem
RE: Complex Hydraulic Synchronising Problem
Alternatively putting two pumps on each end of the electric motor also works (remember that two pumps will be clockwise rotation and two will be anti-clockwise rotation). You can get rid of the rotation problem if you choose a type of pump that works the same regardless of the rotation direction. Be aware that the motor frame size increases for any particular continuous power output when the motor has two flanges because the cooling fan is less efficient.
And no, the overcentre valve will not cause the cylinder pressures to equalise. When you are retracting the rods the pump pressures will be whatever they are needed to be to overcome the tensile load (the overcentre valves are out of circuit at this point). When you are extening the cylinder rods then the action of the overcentre valves will be such to avoid the tensile load from running away. A high tensile load will mean that you will only have a small (but always positive) full bore pressure. A lower tensile load will require a slightly higher full bore pressure to complete the opening of the over-centre valves.
DOL
RE: Complex Hydraulic Synchronising Problem
Below is side elevation with section cut away, cylinders extended.
Iso view, Cylinders retracted
RE: Complex Hydraulic Synchronising Problem
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Complex Hydraulic Synchronising Problem
My opinion is just .02, but I also doubt the flow divider or multple pumps or any form of open loop synchonization.
But, for a one time use, where synchonization is somewhat loose but it must hold over a long period of time, how about a hybrid thought: If cost and programming and controls is the perceived barrier, use a human controller. Put feedbacks on all four cylinders, digital readouts on a panel, and human operated prop/servo valve with manual pots or sticks? Watch the display and move things slowly in synch. Then sit there intently for 30 minutes and keep them in synch while holding the load.
Not press speed or accuracy, and it only has to work once, but it has to work the first time out the door.
RE: Complex Hydraulic Synchronising Problem
The equipment has now been specified and ordered but had yet to be manufactured, installation is not till May 2014.
The link below is link to the very innovative (IMHO) final circuit design. Its not the easiest to follow but when you’ve lived with it for a while it becomes clearer…
Link
Spec
A five section high pressure piston pump delivers equal fluid to all cylinders, one is used to set the max adjustable pressure and unload the system.
Directional control is via a bank of 4 port high-pressure ball valves connected with a common spindle. Equal and simultaneous operation of the cylinders is ensured regardless of the external load* via cylinder mounted counterbalance valve.
Due to the ATEX environment and long lead (or unavailability) of 500 bar rated explosion proof equipment the electrical content of the system is at a minimum with the control being “mandraulic” operation.
*Recent analysis has suggested rather that seeing the imbalanced load the tensioning system due to is self-weight is likely to sit flush, its self weight wants to fall into the clamp, cancelling out the cable catenary moment which was causing the gap mentioned in previous post above, this simplifies the operation significantly for us.
RE: Complex Hydraulic Synchronising Problem
escapizm, this is very interesting problem. I would like to ask if you considered using a single hydraulic cylinder instead of four units. Even a hollow piston cylinder of 1,300 metric ton capacity and 1,000 mm stroke is feasible. Do you think there are disadvantages in using just one big cylinder instead of a set of four units?
Your problem is relevant to me because we are working on a project to pull Dyneema rope. We are considering a hollow piston cylinder, our requirements are a bit smaller, so an 800 ton capacity hollow piston cylinder is sufficient, rod diameter is 400 mm. and central hole is 250 mm. We have quotes for that of around GBP 25,000.
I appreciate your feedback, I am a bit surprised you did not choose a single cylinder... Maybe I overlooked something?
Thanks in advance,
Aarne
RE: Complex Hydraulic Synchronising Problem
We are using 4 cylinders not a single one.
The original solution was to use an annular jack as you suggest with a hole in the centre, I changed it when I joined the team as I had little confidence it, we invented this machine!!!.
The proposed unit had a 50mm stroke and required multiple “re-sets” with mechanical spacer rings being inserted.
However the main concern was as the unit had to have a hole in the centre approx. 700mm in diameter to accommodate our cable and it was VERY sensitive to angular misalignment. As you can appreciate if one side moves out of sync with the other the “circle” becomes and “oval” and in workshop perfect environments we had failures of the seals and that was when we were attempting to test lift true and square vertically. Offshore we are at approx. 36 degree with the addition of a moment caused by the slack catenary of our cable.
The other issue was as you approached the 50mm stroke you had to stop as unlike a normal hydraulic cylinder nothing retained the "piston" on the annular gap and it could pop out !
If your cable is small in diameter and vertical you should be ok.
Your solution is approx. 40 times cheaper than ours! But we do have six machines.
escapizm