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Hydraulic valves, regardless of how they are classified, are essentially devices that can change flow passages and openings; therefore, they have at least one flow passage. As long as the flow passage is open and there is a pressure difference between the two ends, there will be flow; when the opening or pressure difference changes, the flow will change!
The opening of the valve and its variation are determined once the valve is manufactured. However, the change in actual opening area during actual operation cannot be directly measured. The displacement of the valve spool can be measured in principle, but generally requires performing surgery on the valve: installing a rod connected to the valve spool in the valve, connected to a displacement sensor. Therefore, the opening and variation of hydraulic valves are always expressed using pressure difference – flow characteristic curves.
All hydraulic valves have at least one pressure difference – flow characteristic curve; only the forms of expression differ, and the names given to them are sometimes different.
The openings of directional control valves and shut-off valves generally do not change with pressure difference and flow; therefore, their pressure difference – flow characteristics are roughly like fixed hydraulic resistance.
The opening of pressure valves generally changes with flow: the larger the flow, the larger the opening. Therefore, the pressure difference can remain roughly constant at different flows, and the characteristic curve is generally parallel to the flow axis.
The opening of flow control valves generally changes with pressure difference: the smaller the pressure difference, the larger the opening. Therefore, the flow can remain roughly constant at different pressure differences.
However, whether pressure valves or flow valves, their openings are limited, and after reaching the maximum, they will not increase further and become fixed hydraulic resistance. Therefore, the pressure difference – flow characteristics at that time are as shown in region A.
ISO 5598 “Fluid Power Systems and Components – Vocabulary” has the following definition: rated flow, confirmed through testing, at which a component or piping is designed to operate (the flow for which a component or piping is designed and confirmed through testing). The corresponding national standard GB/T 17446 translates it as “rated flow”. If it is understood as “specified flow”, it would be wrong. No hydraulic valve must operate at a specified flow. Therefore, it is better to understand it as “rated flow”.
Many manufacturers use this term in product manuals, but there is no unified definition for it.
For directional control valves and shut-off valves, nominal flow usually refers to the flow that can pass through under a certain pressure difference. This definition appears simple, but is actually not clear. 1) The pressure differences selected by major European and American manufacturers are often different; some choose 0.5MPa, some choose 0.7MPa (derived from the imperial unit 100psi). The flow values passing through at different pressure differences are, of course, not comparable. 2) Some valves, such as directional control valves, often have multiple flow passages and may have multiple different pressure difference – flow characteristic curves. According to which one should the nominal flow be determined?
For pressure valves and flow valves, it actually often refers to “rated flow”, roughly the maximum flow that can be achieved.
Therefore, the nominal flow given in product catalogs can only be used as a reference; its value is limited, and it is still necessary to compare pressure difference – flow characteristic curves. Some manufacturers, such as Hydraforce, sometimes simply do not give nominal flow, but instead say, go look at the curves.
Neither the 1985 nor 2008 versions of ISO 5598 “Fluid Power Systems and Components – Vocabulary” include “nominal flow”, probably also based on these considerations?
If nominal flow refers to the flow that can pass through the valve at a certain pressure difference, such as 0.5MPa or 0.7MPa, and almost all hydraulic valves nowadays can work at much larger pressure differences, then of course the working flow that can pass through is also much larger.
The flow that can pass through flow control valves is basically manually set. The flow that can pass through throttle valves or relief valves is theoretically unlimited: the larger the pressure difference, the more flow passes through, until the pressure exceeds the burst pressure and the valve is destroyed. However, solenoid directional control valves, due to the limited thrust of the solenoid, have a certain working range and can only switch normally at a certain flow.
But in any case, to work at large flows while hoping for smaller pressure losses, the actual opening of the flow passage must be larger, which is why nominal size, DN, DG are used to characterize the size of the valve.
Initially, generally referring to SAE (Society of Automotive Engineers standards), the millimeter value of the valve port diameter was used for designation, such as: 06, 08, 10, etc. But with technological development, in order to fully exploit the valve’s flow capacity and obtain better structural characteristics, the port diameter was changed. Therefore, now, nominal size, nominal diameter, DN, DG, etc. are only loosely related to the actual manufacturing dimensions of the port, as rounded values convenient for reference: an 08 valve is definitely larger than an 06 valve.
Because moving the valve spool is the only means of regulating flow, and flow determines the movement speed of the hydraulic cylinder. Therefore, the speed of valve spool movement affects the rate of flow change, thereby affecting the acceleration of hydraulic cylinder movement. And the acceleration of hydraulic cylinder movement determines the smoothness of load movement. Therefore, how to move the valve spool and control the speed of valve spool movement is very important for the smoothness of load movement!
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