Friday, December 26, 2008

Regenerative Circuit Question

Are the pressures on the Cap side and Rod side of a cylinder in a regenerative circuit equal?

6 comments:

pnachtwey said...

It depends

Acceleration=(Pcap*Acap-Prod*Arod-Ffric+Fload)/mass
were Fload is negative if it opposses extention

Chances are no.

FluidPowerWorks said...

Not if the Cylinder is EXTENDING. From observing many operating Regeneration circuits it appears pressure on the Rod End is less tha 25-50 PSI higher that that on the Cap End. The observations were reading 1500-3000 PSI gauges so it was hard to determine the exact pressure difference.

Hsve you read this article in Hydraulics & Pneumatics magazine for a circuit that can regenerate Double Rod End Cylinders and Hydraulic Motors.
http://www.hydraulicspneumatics.com/200//ArchiveSearch/Article/False/83400/ It is in the present months magazine and shows how a pumps flow can be increased at the hydraulic motors to obtain higher Rev's. from high CIR motors that were specified for High Torque. Basically a Hydraulic Motor transmission without any Gears.

JDK said...

As FluidPowerWorks is pointing out a "Regenerative" type circuit can be used in many ways. I had read those articles - very interesting. I will try to be more specific with future posts. The point of the question was not to explore every regenerative type circuit but to discuss what I would call a hydraulic principle.

JDK said...

The answer to the "Regenerative Circuit Question" is NO. Anytime oil flows from one point to another it takes energy in the form of a pressure drop. If one makes the assumption that the pressures are the same that can lead to problems.

Two examples:

1. The flow from the rod side to the cap side is 2 GPM and goes through only a relatively short 2" ID tube, nothing else. The pressure drop might only be .00001 PSI and be negligible.

2. More practical - the flow from the rod side to the cap side is 20 GPM goes through two valves and a 1" ID tube. As can be seen the resistance to flow from rod to cap is now delta P of tubing and delta P of valve one plus delta P of valve two.

Time to throw up the hands? No.
As "pnachtwey" points out there is an equation that can be used to determine pressure required at the cap in this type of regenerative circuit. If you recognize that the oil does not flow from rod to cap "free of charge" you can simply substitute for "Prod".
Prod= "Pcap+Delta P (rod to cap)"

Since you know the flow and therefore can estimate fairly closely each pressure drop between rod and cap, by adding them, Prod now equals Pcap + a constant.

Hence, the VCCM equation also works for an extending cylinder in regen.

The point of the question is this: If you assume that Pcap = Prod during regen, you stand a good chance of miscalculating the required HP to do the work required during the regen part of the cycle.

Anonymous said...

The pressures on the Cap side and Rod side of a cylinder in a regenerative circuit would be equal. What is causing the cylinder to extend is the differance in force between the cap end and the rod end.

JDK said...

"What is causing the cylinder to extend is the differance in force between the cap end and the rod end."

I agree 100% with this but oil doesn't flow anywhere without a pressure drop. The pressure drop may be negligible or may not be. It depends what the rod oil flows through as it makes its way to the cap. In most cases there is some type of valving that has pressure drops. Please read my second comment.