Running Cylinders in Series for Synchronization

[danhelgerson]

I saw a discussion started about running cylinders in series for synchronization. Budt responded but now I cannot seem to find the thread. Bud knows his stuff. I just want to clear up a point. When hydraulic cylinders are run in series, the first cylinder sees the entire load while each successive cylinder sees a smaller portion of the load.

For example: If four double rodded cylinders were being used to lift a balanced platform and the cylinders where run in series, the pressure in the first cylinder would be F/A where F = the total weight of the platform and A = the annulus area of the cylinder piston. 25% of the force would go to lifting the platform and 75% would go to pushing the other cylinders. The pressure in the second cylinder would be .75F/A. The pressure in the third cylinder would be .5F/A and the pressure in the forth cylinder would be .25F/A.

One of the beauties of hydraulics is that the load does not need to be balanced to synchronize the cylinders. Hydraulic fluid is relatively incompressible (about .5% for every 70 bar). If 90% of the weight were on one corner of the platform, the cylinders would still move in a synchronous manner. The pressure in the first cylinder would still be F/A but he pressure ratios in the successive cylinders would be different.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[kcj]

I just want to clear up a point. When hydraulic cylinders are run in series, the first cylinder sees the entire load while each successive cylinder sees a smaller portion of the load.


Not sure what you meant by this. First cylinder sees the entire pressures, but load is still only the weight applied. High pressures on both sides of the piston cancel out, and pxa of one side - pxa other side = the applied load.

kcj

 

[danhelgerson]

In effect, the first cylinder is acting against the entire load. It has to push its own portion of the weight and at the same time apply enough force to move the other cylinders. You are correct that the opposing pressure on the other side of the piston cancels out the additional force that will be used by the other cylinders.

In my example of four double rodded cylinders lifting a balanced load, let’s say the annulus piston area is 1 and the platform weight is 10. To hold the load, a pressure gauge at the inlet to the first cylinder would read 10. A pressure gauge on the exhaust port of the first cylinder would read 7.5 which would be the inlet pressure to the second cylinder. The exhaust pressure of the second cylinder would be 5 and the exhaust pressure of the third cylinder would be 2.5. The force exerted by the first cylinder rod would be the inlet pressure (10) x the area (1) minus the exhaust pressure (7.5) x the area (1) or 2.5 which is 25% of the load.

It is the piston that sees the whole load. The rod only sees its portion of the load.

Does that help??


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[kcj]

tks. well put. I already understood this but was fishing a bit for further explanation. Readers may have misunderstood that the first cylinder actually lifted the entire load in some way.

kcj

 

[budt]

Another way of saying this Kevin is:

All the actuators (Cylinders or Motors) in a Series Circuit must be capable of overcoming the entire load. I need to pick up 10,000# at 1,000 PSI with 4 cylinders requires each cylinder to have 10 Sq. In's. of area to do the work.

Load position is unimportant since resistance to flow will set pressure at each cylinder as required.

Another point on synchronizing with Double Rod Cylinders is the cylinders must return to a hard mechanical stop at regular intervals, each cycle is best but it can be every 10th cycle or other choices according to load imbalance, seal type, etc.

In fact any type of cylinder synchronizing, only using some hydraulic means must be re-synchronized at regular intervals to keep the system level. This also applies to motors that are moving something between two set positions.

In a book I use to teach circuit design I have 6 other ways to synchronize cylinders. All the circuits show a schematic that includes methods for automatic re-synchronizing anytime the cylinders go out of phase.


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

http://www.fluidpower1.us

 

[zepp]

please let me start of by begging forgiveness on my ignorance here, but, if we are talking about synchronized lifting, with hydraulic cylinders, why would you connect them up in series?
Ive found the best way to carry out a synchronized lift, was to actualy have one pump per cylinder. this is the simple terminology, but basicaly if there are four cylinders, there would be four pumps, all attached to the same driver, ie electric motor.
most of the lifting I do is in the high pressure range, (10,000psi/700bar) so for me, Im not worrying about extensive strokes, maximum would be 18 inches, but when used as pushing, ie pipe jacking, then the strokes go up considerably, but one thing we have found out is that the only way to combat the tolerances on the cylinders having effect, and also the altering load shift, one pump per cylinder is the norm, otherwise you have to go through constant flow alterations and such to combat an uneven lift.
Like I said, im not quite sure if I am off base here, but for the high pressure hydraulics, the method mentioned is somewhat dangerous,

 

[danhelgerson]

The primary advantage of running cylinders in series is the inherent volumetric efficiency of cylinders. The amount of internal leakage in most cylinders approaches zero. This means that for any given motion on the master cylinder, there is a known volume of liquid moving all subsequent cylinders regardless of the load on any cylinder. This ignores the compressibility of the liquid which would be about .5% for each 70 bar differential among the cylinders.

Hydraulic pumps are designed with internal leakage (volumetric inefficiency). This leakage will range from 1% to 5%, depending on the design and quality of manufacturing. The internal leakage is in effect an internal orifice which will pass more or less fluid as the pressure varies.

As a general rule, running cylinders in series will give greater positional accuracy than using separate pumps, gear flow dividers, or restrictive flow dividers.

The best positioning is done with electronic feedback but this is relatively expensive and often provides more accuracy than is required by the application. Series cylinders provide the best positional accuracy in terms of installed cost.

In your case, however, this might not be true. If you are operating at 700 bar on each cylinder, it would likely be impractical to run the cylinders in series because the first cylinder has to be capable of moving the entire load. If you do not use double rodded cylinders, getting the right combination of bore sizes and rod sizes could be a problem.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com