Is it possible, in one Any way or another, to run a double action cylinder, with a Kap/rod ratio >1, in a Klosed loop, with a "reversible", variable Axial piston pump, the type that is used in a hydrostatic transmission??? And that way eliMinating the deltA p losses over the eliminAted directioNal valve.HUVVA
Yes, not only possible but I know of several hydraulic presses that use Bi-Directional (Hydro-Static) pumps to power them. If you search for Hydraulic Press Manufacturing (HPM) they made some presses of 50" Bore with 48.5" Rods that were cycled by two 370 GPM Bi-Directional pumps. Off hand I can't think of another example that you might be able to find on the Net. The other place to look is manufacturers that make Pre-Fill Valves. They are often used on these Bi-Directional pump circuits.
3 areas of caution need to be recognized. 1. Typical charge pump pressure is 300-400 PSI so this would be the back pressure on the return side of the cylinder. Calculations must be made to make sure that you can do the work with that back presuure; 2. Charge pump must have enough flow to make up the difference in areas when extending the cylinder and to operate the pilot system of the pump; 3. Charge pump relief must be large enough to handle charge pump flow plus difference in area when retracting the cylinder.
AnyMouse, take a look at Fig. 8-18 at this Ebook on the Hydraulics & Pneumayics magazine site. This explains a typical Bi-Directional circuit for a Single Rod Cylinder. It is from a book I wrote to teach circuit design classes to anyone familiar with Flud Power symbols or who had completed the basic Fluid power class.Look in the Basic book, Ch. 8, Fig. 8-18 amd the explnation for your subject circuit. http://www.hydraulicspneumatics.comGo to the EBook Link and open the Basic Book.
This is the link to the circuit that FluidPowerWorks is referencing. http://www.hydraulicspneumatics.com/200/eBooks/Article/True/43639/It is a nice circuit in that it lowers the back pressure on the return side of the cylinder. Other Considerations:1. Depending on heat factors often times some charge pump flow is routed through the case of the pump for cooling. In a motor circuit this is usually shuttle flow.2. The ability of the suction check to keep the pump from cavitating during extension needs to be evaluated for position of reservoir, size of check, size of charge pump.
Take a look at this web site for a circuit for a hydraulic press using Single Acting Rams for both directions of travel and a small Fixed Volume pump since the low force Advance and Return need little pump flow to maintain a fast cycle time. A Bi-derectional pump could be used for this setup and would offer some distinct advantages relating to smooth controllable action of the cycle. http://www.gidravlika.kiev.ua/hawe/pdf/D6960-en.pdf The circuit is on page 8 The Single Acting Rams offer faster Trouble Shooting stints while acting exactly the same as a Double Acing Cylinder with an Oversize Rod to reduce Rod End volume for a fast, low force return stroke.
This is a nice circuit. I have also seen applications where a proportional relief valve is used in parallel with SV1. The proportional valve can be used to vary max. pressing pressure and provide some speed control during pressing (ex. drawing operation) and press return. The bi-directional pump would be another option. One neat thing about hydraulics: there is usually more than one way to "skin the cat". I appreciate your comments because this site is about discussing principles along with applications. That way you, me, and all visitors will be in a better position to 1. figure out what work needs to be done and 2. the best way to do it.
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