I need Help with running two Hydraulic Cylinders in series

[Andrew Lane]

Thanks in advance. Im a old Diesel mechanic and thought I was doing a simple Job but its got me a bit stuffed at this stage.
Have a larger Ag seeder Im setting up.
Two Rams double acting of the same size.
Tractor pressure is into one of the piston ends of the Cylinder out of that same Cylinder
we have a crossover hose to the shaft end of the second cylinder then out of the piston end back to the Tractor.
Im buggered If I can get it to lift equal .
I feel Like a idiot as I worked with a lot of Hydraulics but Im stuffing this up can someone please help?
I think Im missing something simple but maybe just lost my mind.
Thanks Andrew

 

[3DDave]

Possibly leakage and that the hose expands under pressure so not all the fluid volume leaving one is entering the other. In addition the bores might not be exactly the same.

I guess it depends on how closely you want them to track.

 

[akkamaan]

I am not 100% sure how you want to connect these two cylinders but I assume you want both to lift equally in two ends of a spread load. This can NEVER work with two DIFFERENTIAL cylinders in series if connected rod end to cap end. That because the capped end and the rod end have different displacement volumes.

If you use symmetric cylinders with rods on both sides (dummy rod) cylinders in series can "work".

Another option is to use 3 equally sized differential cylinders in series and use a "dummy cylinder" in the middle to convert the displacements.

You can also use a 50/50 flow divider with cylinders in parallel

Neither of the above will work "perfect" and consistent. For that, you will need some sort of "closed-loop" control with electronic or mechanic "feedback".

The problem with cylinders in series or flow divider is that by time internal leakage will make the cylinders "un-synched" and the will need a hydraulic reset into the default position.

 

[zeusfaber]

As I read it, you're connecting the annulus (rod) end of one cylinder to the annulaus end of the other - so you're already wise to the problem in akkaman's first diagram.

If there's any leakage past one of the piston seals, you will struggle to equalise the system.

A.

 

[Andrew Lane]

Thanks for the help,
Would a simple system in Parallel run a bit closer ?

 

[akkamaan]

If both cylinders have the same load-pressure (resistance) they will somewhat move "synched" when connected parallel.
What is the distance between the cylinders?
If one cylinder has lower load-pressure than the other that cylinder will get most of the flow until pressure is equalized between the two cylinders.
Please tell us more, that means drawings or pictures, about your application.

 

[Andrew Lane]

Thanks,
Both cylinders will have similar load.
We will use spacer blocks on the shafts to adjust the closed position,this is the most important position as it will adjust the sowing depth.
When lifting ( rods out) some variation is not a big issue.
Perhaps Im over thinking it as we have not fitted the spacer blocks yet,as long as the rods even themselves out when they bottom on these spacers it should be ok?
We have about 15' of hose between the cylinders

 

[3DDave]

See if this helps:

https://www.eng-tips.com/viewthread.cfm?qid=272324

You have cylinders with small rod-end areas and 12 feet of hose between them. Think of the hose as a rubber balloon that is stiffer than typical.

 

[akkamaan]

Great pictures
Have you tried to run the cylinders in parallel?
Test up and down and see how it acts. Maybe it works to satisfactory.
What is the approximate weight on each tire?

 

[akkamaan]

Think of the hose as a rubber balloon that is stiffer than typical.

What Peter Nachtwey is asking for in that thread you linked to is "rocket science" compared to this issue. It just makes it more confusing for Andrew Lane, the "OP"

 

[3DDave]

I don't care about that. It included links about hose expansion. Now akkamaan has really muddied this for no reason.

Maybe help instead of not help.

 

[hydtools]

If you T them in parallel, the lighter loaded one will move first. When the first one hits the stop the other will continue to its stop and then both will be at the same pressure.

Ted

 

[Andrew Lane]

Thanks everyone all your help is appreciated.
We are going to get some more fittings and try parallel.
By bleeding all the air we have been close to satisfactory in series but just are not happy with it yet this way.
Sorry Akkamaan I missed your question about weight ,total machine weight is around 2000 kg (4,400 lb) when full of fertilizer and seed.
running new 4" bore rams with 1,3/4" rods.

 

[akkamaan]

Why I asked about the weight was about how much weight there was on each side/cylinder. If it is exactly the same the cylinders will basically also follow each other synchronized if levers and internal friction is identical

So if these4400 lbs are distributed evenly, 2x2200 lbs there is a good chance a parallel connection will work fine. The more weight difference there is the more unsynched the cylinders will be. If then one cylinder is constantly lifting more than the other, try to add some weight on the "light" side.

 

[Compositepro]

Parallel cylinders are often synchronized with speed control valves, which are a specialized type of needle valve mounted on the cylinder ports. While speed synchronization cannot be perfect, they only have to be close enough to work over one or two stokes, as the cylinders come back to perfect synch when they come to end of stroke.

 

[3DDave]

The weight is not seen by the cylinders, there is considerable mechanical leverage being applied by the linkage.

For parallel plumbed cylinders, the weight/force distribution is only a factor once the torsional resistance of the joining structure is included as a factor. If there is no such resistance, then there is nothing keeping the cylinders in sync under even small load differences. Since there is zero connection from one wheel to the other, I'd say that torsional resistance is also zero.

 

[PNachtwey]

This will always work.

https://deltamotion.com/peter/Videos/SyncPosForce.mp4

An RMC75 will handle two axes.

What Peter Nachtwey is asking for in that thread you linked to is "rocket science" compared to this issue. It just makes it more confusing for Andrew Lane, the "OP"

Not rocket science, just an option not to use two servo motors in parallel.






Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[Andrew Lane]

Thanks everyone.
The parallel connection is working you are all champions