Cylinder synchronisation w/ Rotatory Flow Divider,... again 2

[jarel]

HI People,

This is my first post in the forum,... and English is not my Mother Tongue,... hope I get myself explained properly.

I come back with a long-discussed subject --> cylinder synchronisation with Rotatory flow dividers.

There are several comments and questions I would like to point out.

1.- In cylinder synchronisation, rotatory flow dividers should be placed in the "LOADED" chamber of the cylinder, i.e. the load bearing chamber. Typically this corresponds to the piston area of the cylinder. This one is clear for me, as putting in the annulus area chamber could cause one of the cylinders to cavitate... This is important for the money-savers who think that putting the synchroniser in the lower flow area (annulus chamber) will save them some money (actualy they end up gaining quite big headaches).

2.- When synchronising with rotatory flow dividers, it is important that the motion control valve (load holding valve, countebalance valve, etc...) to be placed in the COMMON LINE of the cylinders. This menas only ONE COMMON FLOW CONTROL valve for both cylinders. I've been told that putting two individual load holding valves in each cylinder will make the rotatory flow divider to operate worse,... is this true? If so, what is causing the miss-function?

3.- Final question: if we have to synchronise two cylinders with overcenter loads (the load changes from positive to negative),... how would we face this? Two flow dividers one for each side of the cylinders? Two prop valves with position tranducers? Any other idea?

Thanks in advance,



jarel

 

[PNachtwey]

2. The valves will not always open or close at the same time or one will leak more than the other. When open there will be differences in the flow due to mechanical tolerances.

3. The best option is to use a closed loop servo system that can synchronize actuators and adjust the gains as a function of the angle of the load.

Delta Computer Systems

http://www.deltamotion.com

 

[hydromech]

I have extensive experience...good and bad, of using rotary flow dividers to synchronise cylinders that have and overcentre load characteristics.

First off...Peter is correct that servo/proportion control would give the best response, but at a far higher and usually prohibitive cost. Not to mention lower reliability in less clean environments.

Motion control valves work very well with rotary flow dividers to artificially increase the pressures to keep the pressures in each leg more or less equal and maintain the same amount of leakage in each element of the flow divivder, thus providing better synchronisation. I cannot comment on having one motion control per system, but I have always used individual control valves as it allows for more options with the pilot ratios.

Flow dividers work best where the flow divider is placed in the pressure line before the directional control valves. The benefits here are that the cylinders are synchronised in both directions and should any of the solenoids on the control valves fail, none of the cylinders will move as oil must be flowing through each element for the flow divider to turn.

If the flow divider is placed between a single directional control valve and the cylinders, the cylinders will only be synchronised in one direction, in the other direction the flow will be combined. In this setup the lightest loaded cylinder will move first and because the other cylinder
is not moving any oil, the flow divider will turn and will cavitate. This occurance is worse where the cylinders are not mechanically linked in some way. I can still occur with linked cylinders and can cause serious damage.

Regards

Adrian

 

[PNachtwey]

Not to mention lower reliability in less clean environments.

Then use proportional valve with servo grade spools, otherwise use servo motors and be done with it.

I have questions:
1. Jarel, why do you ask and if this is for a real system then what are the specifications?

I have extensive experience...good and bad, of using rotary flow dividers to synchronise cylinders that have and overcentre load characteristics.

What made the difference between success and failure?

Did the failure cost more than a servo system if you consider extra time,parts and production lost?






Delta Computer Systems

http://www.deltamotion.com

 

[jarel]

Hi PNachtwey and Hydromech,

Thanks to both of you for the valuable help,...

Flow dividers work best where the flow divider is placed in the pressure line before the directional control valves. The benefits here are that the cylinders are synchronised in both directions and should any of the solenoids on the control valves fail, none of the cylinders will move as oil must be flowing through each element for the flow divider to turn.

Very interesting. It is not the most common arrangement though but I must admit that I see no drawbacks to it,...so far. Just wondering why it is not the most common solution....

Motion control valves work very well with rotary flow dividers to artificially increase the pressures to keep the pressures in each leg more or less equal and maintain the same amount of leakage in each element of the flow divivder, thus providing better synchronisation. I cannot comment on having one motion control per system, but I have always used individual control valves as it allows for more options with the pilot ratios.

Thanks hydromech. So based on your experience, you would even recommend using individual motion control valves and "play" with the setting to get the most equal pressures in both cylinders.
Would this still be yhe case if the two cylinders are mechanically linked? See recommendations from Rexroth valves,....

One final question for both of you: I would assume that when cylinders are mechanically linked somehow there are two type of applications:

1.- Flexible structure = weak mechanical link --> rotatory flow divider is a good tool to help synchronisation of both cylinders

2.- Rigid/Stiff structure = strong mechanical link. Rotatory flow divider can do more harm than good, due to errors up to 5% in stroke synchronisation, it can "impose" deformation to the structure. If the structure is very stiff this imposed deformations can lead to high loads and even locking of the mechanism ---> the mechanical structure and the flow divider could be working one against the other.

Do you agree?

Thanks in advance for your time,
Regards,



Jarel

 

[hydromech]

Jarel...

Very interesting. It is not the most common arrangement though but I must admit that I see no drawbacks to it,...so far. Just wondering why it is not the most common solution....

It is cost...one directional control valve before a 10 element flow divider can drive 10 cylinders and is much cheaper than using 10 directional control valves. The down side of using one directional control valve and then using the flow divider to combine the flows from the cylinders is that the lightest loaded cylinder will move first and the flow divider will not synchronise the cylinders.

So based on your experience, you would even recommend using individual motion control valves and "play" with the setting to get the most equal pressures in both cylinders

.

Yes, in my experience it allows a greater variance of load between the cylinders. Even the best flow dividers have a greater internal leakage at high pressures. If a cylinder has a light load, I find that that the synchronisation is better if the hydraulic resistance is increased to make the leakage the same in all elements of the flow divider.

Would this still be the case if the two cylinders are mechanically linked? See recommendations from Rexroth valves,....

That depends on the rigidity of the mechanical link. It could be that the lead cylinder tends to pull the other cylinders if there is enough rigidity in the link. This can cause instability inthe system as the loads in the cylinder rise and fall rapidly.

One final question for both of you:
I would assume that when cylinders are mechanically linked somehow there are two type of applications:
1.- Flexible structure = weak mechanical link --> rotatory flow divider is a good tool to help synchronisation of both cylinders
2.- Rigid/Stiff structure = strong mechanical link. Rotatory flow divider can do more harm than good, due to errors up to 5% in stroke synchronisation, it can "impose" deformation to the structure.
If the structure is very stiff this imposed deformations can lead to high loads and even locking of the mechanism ---> the mechanical structure and the flow divider could be working one against the other.
Do you agree?

Yes...in princple.

Flow dividers are a cost effective solution and can provide satisfactory performance. Where a flow divider manufaturer could not guarantee the required performance I dropped the flow divider and used proportional directional control valve with closed loop control via rotary encoders. This meant extra care with filtration as flow dividers are more dirt tolerant that servo/proportion valve.

This is my point with Peter(PNachtwey), yes servo/proportional control is better, but unless you keep the systems clean they can cause real trouble.

Regards

Adrian

 

[PNachtwey]

This is my point with Peter(PNachtwey), yes servo/proportional control is better, but unless you keep the systems clean they can cause real trouble.

If you can't keep your system clean you will have trouble sooner or later with any system.

The advantages of a servo system is that the loads can vary, the viscosity of the oil can vary, the valves and the mechanical parts will vary over time and the closed loop feed back will correct for those variances.

We have not been told what you are really trying to do so it is hard to know if the extra costs of a hydraulic servo system are justified. You need to look beyond the initial cost and think about the return on investment.






Delta Computer Systems

http://www.deltamotion.com