pnachtwey , you are quite right. Of course some folks do make lucky guesses. But, I don't choose to "design by luck", there are too many ways to be wrong.
And of course the first step is to size cylinders based on force and velocity needs. Next, translate that into flowrate, volume, and pressure requirements. Then, if there is more than one cylinder or other device, use the sequence of operations to determine the maximum instantanoius flow at each instant of operation. At that point it is possible to accurately size the HPU and decide if accumulators would be useful.
You don't guess, ever!!! We see this too often. The first thing to do is size the cylinder this requires knowing how much mass must be moved how quickly. Once the cylinder(s) are sized then the cycle times can be use to compute the necessary flow.
I agree about the using accumulators at the valve where the energy is needs. Too many put the accumulators at the HPU and suffer pressure loses before the oil gets to the valve.
PNACHTWAY, adding up the flows makes it possible to size the system flow capacity properly. Until the required flow is known, anything else is just a guess. Experienced folks sometimes guess better than accountants, but luck should not be involved in hydraulic systemn designing. If the peaks are short then an accumulator can help a lot.
Servo valves are a special case and my advice is to add both the specified valve leakage and the flow for the maximum cylinder velocity. Often a servo system performance can be improved by adding small accumulators right at the servo valve.
I agree that the instantaneous flow should be know for each instance of the cycle. This makes it easier to size the HPU and accumulator. The HPU only needs to supply a little more flow than the average per flow per cycle. For servo systems the accumulator should be sized and pressurized so the pressure doesn't drop more than 10% during the cycle so the system gain remains relatively constant.
This is a Good post that contains good information. But more information is also needed. Even if a system will only have a single hydraulic actuator with simple extend/return control, it is still mandatory to know both the required force and the needed velocity, in oredr to determine the fluid delivery flow required. In addition, even for that simple system, the duty cycle needs to be known in order to provide the required performance. For systems with multiple actuators you must know the sequence of operation to determine the maximum required flow.
So if the system performance is important it is probably smarter to obtain the design assistance of a good hydraulic systems provider. The money spent will be far less than the amount spent to re-do a system that is not adequate, or does not last.
Excellent Post Peter. My company designs work cells to automate manufacturing process. In doing so, you would not believe ( or maybe you would ) the variety of existing "systems" I run across during one year's time. Rube Goldberg would have been proud to witness some of the "designs" that are almost working. Most of these have been assembled just as you mentioned in your post--cobbled up with components existing in the back room. Component make, vendor model and part number have long-since been eradicated from the part making it impossible to tell what has really been used. The real problem arises when the "designer" either dies, quits or gets fired and someone is expected to troubleshoot. That's when redesign and reconstruction most often occurs.
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