How 2 Hydraulic Cylinder Can Operate Simulteneously

There is a device called a flow divider that will split the flow evenly to each cylinder. It may accumulate sight inaccuracies over time, but there are some end-of-stroke relief valve schemes that can resynchronize things. I have forgotten the exact details, but yes, it can be done.

Can i use 4 hydraulic cylinder acting at one time???

plz sugggst something for doing it....

That would not be simultaneous.

Sir,

Thanks for the suggesstion.

Niranjan

Why not just put a valve in the line with the lightest load to restrict fluid flow to that cylinder?

this commonly done by using one double acting cylinder as the first one and a single acting cylinder as the second.

As you know a double acting cylinder does not push out the same volume as the input accepts, since it has the volume of the shaft in the secondary shaft.

So you use the secondary shaft as the input for fluid and the primary outputs a volume of fluid(if it is filled) that is then ducted to the second one.

Any leakage in the second will require re-balancing, but as long as you have a bleed, you can deal with this by adding fluid as needed and using the bleed for balance.

google gives this, among many. They are called series cylinders

http://www.hydraulicspneumatics.com/200/Issue/Article/False/21797/Issue

Excellent article!

Hi, you have posed quite a difficult problem issue to solve (hydraulically) but it CAN be solved dependent on how much money you want to throw at it.

As Tornado has suggested, a flow divider can be used. A GEAR divider is the best type as it is 'load independent'. However, it is an expensive component and does suffer from some leakage that will require the cylinders to be 're-synchronised'. There are other flow dividers that work with 'balanced spools'. These are not particularly accurate but will split flows well between circuits.

The simplest method is a mechanical linkage. This may not be possible due to the situation.

Another method is to pass the flow from one cylinder to the next thereby utilising the volume of the cylinder to meter to the next in line. This may require a dummy cylinder between the two that require synchronisation because of differing volumes of rod end to full bore end.

The most accurate method is to equip both cylinders with an LVDT mounted inside the rod. This gives displacement feedback to a PLC that, in turn drives a proportional valve and keeps both cylinders moving together (via a PID loop). This, needless to say, is the expensive but the best solution.

There are also other methods in-between those indicated above that are 'too numerous to mention'!

Food for thought?

The flow divider is the easiest method I have a system that uses this and I have one pump that pushes up 4 cylinders at the same time. Over time you will notice some unevenness in the operation, but it most likely is a bad seal. They do make these devices that can be adjusted for the flow to balance them out.

Niranjan,

There can be many ways in which it can be done but a practical solution can be easily suggested if you can post or show the jpeg image or a flow diagram of the existing Hydraulic circuit / system with details of cylinder/s stroke , single acting / double acting , flow rate, Force/load on both cylinders in Kgs, the way it operates at present (existing load cycle w r t time) and the way you want it to operate (intended load cycle w r t time).

Actual solution will depending on the the budget sanctioned for this work, criticality of requirement i.e. degree of synchrous movement of cylinders (absolute or some tolerance is permitted ) , space available to accomodate additional hydraulic accessories etc.

Thanks and regards

Aniruddha

Putting one of these or these, on each supply line or 'venting line' (if double acting) - will give you equal movement - irrespective of load - and re-zero at every end stroke completion.

They are remarkably accurate, set and forget, no power required, 'dead simple'.

If the load imbalance is great, or the cylinders not 'end homing' - then the same folk above have 'input metering' valves to provide stroke and position, independent of load, or force transients.

Such as used in hi-rise building jacking. However, all such 'tricky systems' require power/electronics.

A flow control valve will not cut it, they are intended for speed control not synchronisation. A fluid will always find the easiest path. The higher the pressure the more pronounced the consequences of this law of physics. You are assuming that both cylinders have the same constant load for the full stroke, but small differences in alignment, friction in linkages or the load, seal or cylinder bore imperfections, etc. mean that the load changes constantly. As soon as you get any additional load on one cylinder the other cylinder will take most of the flow. This cylinder moves faster, changing the friction characteristic to reinforce the problem, thus causing a runaway effect.

Mechanical linkage is not usually the answer either. If one cylinder is trying to move faster tan the other, the linkage is likely to lock up. The load will stop moving completely or will judder to its destination as the system repeatedly locks and unlocks.

Spool type flow dividers are fine but they need oil free of water and particulates contamination to operate consistently. Good for aircraft wing flap controls but not really suitable for industrial duty.

Gear type flow dividers are effectively two (or more) matched metering pumps connected to the same shaft. Their accuracy is quoted in the suppliers literature and from this you can calculate how far out of syncro the cylinders may be (accuracy is quoted as worse case) at the end of stroke.

If gear type flow dividers are not accurate enough you have to go for a feedback system. LDVTs are a good option for the sensors because they are not affected by oil, grit and muck. The proportional valves are complex so you need to talk to a specialist. Note that the comments about clean oil apply here as well.

If you thought I meant a needle valve, your comments would make sense.

But I didn't say needle valve - and they aren't - so your comments don't.

I did not assume the cylinders were of equal volume, or area, or stroke, or through-rod, or even in the same room.

IF I assumed anything, it was the OP wants the same speed of extension - irregardless of volume, or area, or stroke, or load.

Meaning a volume over time - independent of load - specific to each cylinder. Which is precisely what the valves linked to do, and why they are calibrated in liters/minute.

Nor did I mention mechanical linkage - but your dismissing that out of hand is equally ignorant and/or assumptive of the OP's unstated application. As is dismissing 'gear' flow dividers against accuracy - which you have no way of evaluating.

Nor do you add anything on proportional positioning systems past hire an expert.

So your net contribution is everyone is wrong, but "use clean oil"

Well done you.

Hi,

Before you go down any so called syncronisation route you need to answer the following questions. Cyclinder sizes, bore, stroke and rod. working pressure, flow. distance apart. What degree of accuracy do you wish syncronisation to acheive? How rigid is the system mechanically and what is it doing and how often. Flow dividers are crude devices but never intended for accurate division of flow. If you can answer the above questions I will give you an opinion as to whetehr you attempt a simple solution or whether you go down a closed loop feedback method.

Regards

Oliver Dunthorne

Hydraulic Engineer

I have seen a twin cylinder hydraulic press that had a lot of trouble "synchronising" the two rams. It was eventually solved by a 10 cm diam torsion bar joining the two together. Just as Beej50 said "a mechanical linkage."

I will send to you a circuit it could help

Hi Islam

The diagram clarified things. It is designed to make both cilinders move hydraulically sincronyzed.

Seems to me you have this equipment working for some time and now it goes out of sincronization. Please confirm. If this is the case problbly it has leakeges, then replace all gaskets on cilinders and valves, check for scratches on the moving surfaces.

Hello,

This is a good system, obviously designed by the OEM as a Master & Slave with an auto top up at the end of the inward (lower stroke). This is typical for a brake press or something similar.

I assume the system used to function correctly and with time is now malfunctioning.The LH solenoid valve causes the cylinders to raise and lower but the oil from one cylinder feeds the other cylinder in both actions i.e. up and down. The RH solenoid valve provides the top/vent service.

The limit switches 3 & 4 need to be +ve when the cylinders are lowered if either one is not made there is an auto top up system built into the control sequence. This brings in valve 2 which either energises to the LH solenoid to top up the volume between the cylinders or the RH solenoid which allows oil to vent to tank.

If this automatic system is not functioning then you should look into the control system and make sure (a) the switches 3 & 4 are functioning (b) if they are, are the solenids of valve 2 energising? If they are is there oil flowing to the correct port of the valve 2. Put some pressure gauges in the A & B ports of valves 1 and 2 and some lights on the solenoids of valves 1 and 2 plus the limit switches. You will then see what is going on or not going on.

If on the other hand the system is so badly out of sync. that it is mechanically jamming then you need to look at the cylinders themselves as they may need resealing or refurbishing if they are scoured. Remember they need to be the same diameter for the system to function.

It is a good system and you should be able to get it to work.

good Luck

Oliver Dunthorne

Actually I didn't try this circuit before, but for the loop to the right it used to make a make up for any leakage from the connection between the two cylinders and also with a connection of the two limit switch which can make kind of calibration of the cylinders and and this circuit is used for the fine synchronization .

Thank You

As you can see, the cylinders are locked in synchrony by the short loop between them. Note the dashed line off to the right. This is in there for position adjustment or if one lines/gaskets has a leak. You should make sure you check all seals, and purge this short closed loop on any air that may have been drawn in by a bad seal when there are occasional excursions to a partial vacuum. Ideally, there should be no leaks, but they happen

How are you getting on with your investigations?

Your system is a good system you just need to find out which bit is not working.

Suggest you check previous post . http://cr4.globalspec.com/thread/53715/Two-Hydraulic-Cylinders-to-be-Synchronized this has been discussed before.

From my experience in the injection molding field you will need a flow divider. Just google flow divider and take your pick. Parker makes a good one just match it to the requirmenhts.

How is this more complex than the loader on my 30 year old massey tractor. Dual lift cylinders, one on each loader arm controlled through a single manual valve with the cylinders just T'ed together?

Well, there you go!

Not quite knowing what the OP had in mind, we have had to cover all bases. Your point about the Massey loader (of which, I too, am a proud owner) is covered by a 'mechanical linkage'. The two cylinders T'ed together are to all intense and purposes mechanically fixed together (or almost). I have noticed on my loader that some of the pivots have worn and this means that one cylinder can lift a few mm's more than the other, presumably putting strain on the load arms. A job for another day!

The interesting dilemma comes when you have two cylinders that can not be linked together and have varying loads that need to move absolutely at the same velocity and arrive at the end of the stroke at the same instant.

Have YOU got any solutions that we have not already covered?

positive displacement high pressure dual common shaft gear/vane pumps will work for this. Since they are on the same shaft each pump produces the same output per revolution to each cylinder.

Wear,if unequal, will introduce potential variance, but all things being equal, they should remain the same for some period. There could be a self correcting fluid path at minimum travel stop where a valve is opened for this purpose, with closure once actuation resumes, so the shafts would be set to the same zero every cycle

A very worthy post AND something not previously covered!

I thought you had in #6 i.e. 'gear flow divider' is that. strange world.

Well, yes your right! I wondered where that good idea emanated from !

But, in fairness, there is a bit of difference between a gear flow divider and a "positive" displacement twin vane pump. Actually, I'm a bit windy about calling ANY vane pump positive displacement BUT, perhaps a twin gear pump would suffice! It would work ok for a while until the inevitable wear set in

I have used a similar system on a very large dockyard crane in the UK that had one large double acting cylinder for the luffing. In that case, I used a tandem gear pump that provided joint flows to the full bore end of the cylinder but only one pump to the neck end. As the crane was an old circa 1950 m/c that was 600vdc, I didn't have to mess with the (somewhat) dangerous control system! Perhaps I've over simplified this explanation and is probably off topic too!