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Reynolds |
I arranged the graph in order to see the curve flow/pressure as shown on the piston pump catalog (pressure compensated type). I have also corrected the position of the relief valve set at a pressure value bigger than the working pressure, acting only to protect the system. In reality, the parabola of "system curve" is only hypotetical. At the beginning the pump at rest is held in position of maximum flow by an internal spring. At the start you get maximum flow immediately and the pressure follow the curve from point "a" to "b". Throttling the system the pressure increase but the pump remain at maximum flow exactly like a fixed delivery pump until the pressure reach point "c". Reducing again the flow passage, throttling more, because the set point of the pressure regulator has been reached, the swashplate begin to reduce the angle and therefore the flow until point "d".  |
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Reynolds |
Real proportional/servo systems are equipped with battery of accumulators near the power unit,
It is sometime installed an accumulator along the pressure line. On the servovalve manifold is installed a small accumulator on the pressure line. A similar small accumulator is placed on the return line to eliminate the surge peaks due to the return oil coming back to tank So the main pumps remain constantly positioned around point "d" in the meantime the system is working Luciano This message has been edited. Last edited by: Luciano,  |
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Reynolds |
No one install co$tly accu for aesthetics reasons.
On big installations over 30 mt, when the high dynamic servo open, two things has to happen: - accelerate the long column of fluid - stroke the pump for the necessary amount When the servo open require immediately the necessary flow, so an adequate accu should be placed as near as possible to the servo. Physical law has not mercy for who ignore them. So the acceleration of the column of oil has to be evaluated; if: - Length of the oil column 30 m - Ramp time 5/100 s - density 870 kg/cubic meter - Final fluid speed in the pipe 5 m/s  |
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Reynolds |
Deltav supplied by the small accu depend from the column volume.
If internal pipe diameter is 50 mm, the surface is around 20 sq cm, the volume is V= 30*100 * 20 = 60.000 cu cm ---> 60 lt Deltav =26/12500 *60000 = 124 cu cm; so a little accu is required -12500 is the bulk modulus in bar The big accu has to be dimensioned taking into account the difference between the peak flow and the average flow generated by the pump, the calculation results generally give big volume. In mill application, it is possible forecast only an average cycle, so the reference has to be taken evaluating the performances of previous machines. The big accu, when placed near the pump, allow to reduce the swashplate oscillation. Moreover a micronic filter without by-pass placed on the delivery of the pump is not submitted to intermittent flow and is more efficient. These are rough calculations; if it is required a more accurate evaluation, has to be used a dynamic analysis SW simulating the complete system: connection, servo, cylinder, masses, friction, ecc. Luciano This message has been edited. Last edited by: Luciano, |
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Bourdon |
After all this heated discussion SOME POSTERS still say there is no need for TRAINED, DEDICATED Fluid Power Persons.
GO FIGURE! I imagine the Mechanical and Electrical, TRAINED/DEDICATED Persons are having a ball watching Fluid Power persons dilemna. Bud Trinkel FP Consultant Retired "It is incumbent on every generation to pay its own debts as it goes. A principle which if acted on would save one-half the wars of the world. "Thomas Jefferson" |
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Bourdon |
All I have are old price list' (1984), However the relationship to different sizes should be the same. These prices , are for Bladder Accumulators, Oil Service, 3,000 PSI,from OILAIR Industries, Inc. 1 Quart 269 1 Gallon 422 2.5 Gallon 579 5 Gallon 831 10 Gallon 1218 15 Gallon 1722 Bud Trinkel FP Consultant Retired "It is incumbent on every generation to pay its own debts as it goes. A principle which if acted on would save one-half the wars of the world. "Thomas Jefferson" |
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Bourdon |
Another way to keep pressure constant at an actuator is to set pump pressure that is filling the accumulators higher than the pressure at the operating valves.
This requires a Reducing Valve after the accumulator or separate reducing valves at each actuators DCV inlet. This way pressure fluctuations, as the pump comes on stroke, can be eliminated at the DCV. Bud Trinkel FP Consultant Retired "It is incumbent on every generation to pay its own debts as it goes. A principle which if acted on would save one-half the wars of the world. "Thomas Jefferson" |
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Reynolds |
The arrangement with the big accumulator near the servo, leave to the pump the duty of the acceleration of the fluid column every time the servo stops
If the piston pump has sufficient time to completely fill the accu, the flow stops. Because the servovalve open in very short time, the fluid column has a big acceleration. This stress the pump, filter and all the other components in the middle. There are hundreds of applications in which the servo is mounted on a manifold over the cylinder equipped with a small diaphragm accu on P, on T and even on pilot line. Detail of interest is the different type of accu. The screwed high pressure type accu, more costly, placed on pressure and pilot line, with machined internal surfaces, release less rubber particles and the servo last longer. On the return line a cheaper low pressure accu is of welded type Luciano This message has been edited. Last edited by: Luciano,  |
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Pascal |
Luciano,
Is this a steel mill application?? AGC? You are in Italy-do you use any Atos products? Maytag |
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Reynolds |
It is a mold oscillation application for continous casting mill, you can find more information on http://www.boschrexroth.com/Rexroth-IHD/Home.cfm?Page=R...9921&DirectDisplay=1 The circuitry in Automatic Gauge Control is similar to mold oscillation. The cylinder+manifold works in very dusty/moisty environment and it is protected by a stainless steel cover. I don't have any experience with Atos products and I find these components very seldom on steel mill application. Luciano This message has been edited. Last edited by: Luciano, |
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Pascal |
Rexroth is the dominant product line for steel mills in the US also. Our mold oscillation is electro-mechanical.
Maytag |
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New User |
I haven't read all the discussion on this subject and don't even know that any conclusion was reached but I found the original question, I believe asked by Bud Trinkel, an interesting one and one that I never think about, always accepting that flow is the element that causes a cylinder to move. That was something that was instilled in me when I first got into the hydraulic business many years ago, but!!!!!!.....you know when I think about it, most laymen only relate pressure to hydraulics and never do they talk about flow!!.. probably the reason why the uninformed technician or engineer or whoever invariably looks for the first pressure relief valve to adjust when he has a hydraulic problem.
Since I'm a nube to the forum here's my take on the subject. Flow is simply a medium for transmitting force through a containment device, a pipe, a hose whatever and if the force isn't great enough behind that pipe full of fluid then nothing moves at the other end - in this case a cyliner. If anyone of you has a boat with a manually operated hydraulic steering system, the helm pump to which the steering wheel is attached is usually a relatively high displacent bi-rotational axial piston pump. For all intents and purposes the ports of the pump are connected directly to a cylinder which in turn drives the rudder or outboard motor. Now if the cylinder is disconnected from the rudder or motor and free to extend and retract, bearing in mind that the inlet and outlet ports switch depending on CW or CCW rotation, the steering wheel becomes easy to turn. The force that you apply with your hand times the radius of the steering wheel equals the torque that is put into the pump shaft. I think most hydraulic people know that a pump used in a fluid power system is a pressure generating device and usually but not always a constant displacement device and the flow output is a function of the amount of fluid it outputs in one revolution. The faster you turn it the more fluid it outputs, hence more flow and subsequently more cylinder speed. Its pressure capability is a function of its ability to displace fluid without by-passing internaly at its rated rotational speed. Gear pumps generally have more internal slip than piston pumps. Now start applying a resistance to the cylinder and discover how much more hand force has to be applied to overcome that resistance. In effect you are simply applying more torque!!! Block the cylinder completely and she ain't going nowhere and if you do manage to turn the steering wheel when the cylinder is blocked, its only because of inherant volumetric deficiencies in the pump. Axial Piston Pumps are the best there is for use in a continuous displacement, manually operated hydraulic system, not that there are many but boat steering happens to be one. There are several other factors in this simplified explanation but at the end of the day Force is indeed the element that moves a cylinder and not flow. |
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Bourdon |
Great post Micahael, Welcome to the board. Hope you stay and join the discussions.
Bud Trinkel FP Consultant Retired "It is incumbent on every generation to pay its own debts as it goes. A principle which if acted on would save one-half the wars of the world. "Thomas Jefferson" |
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Pascal |
I was really fighting this "pressure makes it go" earlier in this thread....Peter N fought back...and he opened my dumb "lock up" with his persistance. For me it is obvious now that force makes it move, force in a liquid is pressure....Basic Newton's law... I also posted an opinion poll about this in this thread...... http://forums.hydraulicspneuma...?r=99810142#99810142 For me, flow is only a motion indicator...no flow, no motion.....if flow, then motion, except when 100% flow is high pressure by-passed...leaks not mentioned... This message has been edited. Last edited by: AKKAMAAN, Per A aw come on.....force makes it go....or slow.... |
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Bourdon |
Bulaga!!! Aloha!!! Eureka!!!... The Rebirth!!!...
I thought this thread is finnished and Peter is convinced that pressure alone nor flow alone cant make it go... If we define hydraulics as means of transmitting "force" alone, then maybe it is pressure makes it go... but it is defined as a means of transmitting "power"! Therefore pressure and flow make it go... as simple as that!!! If the cylinder moves, Work= Energy= Force x distance, isnt it? No need of Newton in this case just yet... Bernoulli's soul might kick your butt if you insist on pressure makes it go... Maglub Active Hydraulic Clown |
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