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The answer lies in the area！
Under the conditions of the same hydraulic cylinder diameter and speed, the oil inlet throttling speed control circuit can choose a throttle valve flow area larger than the oil return throttling speed control circuit, so that it is not easy to get blocked at low speed.
At first glance, do you understand what this sentence means?
The answer is revealed, see if it makes sense!
To understand why meter-in operation allows for the use of a throttle valve with a larger flow area, the key lies in the difference in effective area between the hydraulic cylinder’s inlet and return chambers, and the resulting difference in flow requirements.
In hydraulic systems, the most commonly used hydraulic cylinders are typically “single-rod piston cylinders” (one end has a piston rod, the other does not). This results in different effective areas for the two chambers:
- The rodless chamber (the inlet side, where oil enters the rodless chamber during meter-in operation) has a larger effective area Ap (because there is no piston rod occupying the space);
- The rod chamber (the return side, where oil flows out during meter-out operation) has a smaller effective area Ar (because the piston rod occupies some space).
When the hydraulic cylinder’s velocity v is the same, the flow requirements for the inlet and return chambers differ:
- The flow rate Q₁ on the inlet side (rodless chamber) = velocity v × effective area Ap (since a larger area requires more fluid to move the piston);
- The flow rate Q₂ on the return side (rod chamber) = velocity v × effective area Ar (a smaller area displaces less fluid).
Since Ap > Ar, Q₁ > Q₂ (at the same velocity, the inlet side requires a larger flow rate than the return side).
The flow rate formula for a throttle valve is: Q = K × A × √ΔP (Q is the flow rate, A is the throttle valve flow area, ΔP is the pressure difference across the throttle valve, and K is a constant).
In this formula, the flow rate Q is directly proportional to the throttle valve flow area A (when the pressure difference ΔP is similar).
Therefore:
- In throttling-in, the required flow rate Q₁ is greater (because Ap is greater). According to the formula, to meet the larger Q₁, when the pressure difference ΔP is similar, the throttle valve flow area A must be larger (otherwise, the flow rate will be insufficient).
- In throttling-out, the required flow rate Q₂ is smaller (because Ar is smaller), so the throttle valve flow area A₂ only needs to be smaller to meet the flow requirements. (If the throttle valve area A₂ is too large, the flow rate will be excessive, and the speed regulation effect will be lost.)
(When the extension speed is the same, the flow area of the oil inlet throttle valve is larger than the flow area of the oil return throttle valve)
Simply put: the oil inlet side requires a larger flow rate (due to the larger cylinder area), so a throttle valve with a larger flow area is required to handle this amount of oil. On the other hand, the return side has a smaller flow rate, so a throttle valve with a smaller flow area is sufficient. This is the core reason why a throttle valve with a larger flow area can be used for oil inlet metering.
What are the benefits of a larger flow area?
A larger flow area means a relatively wider throttle valve passage. At low speeds, the oil flow is low and the flow rate is slow. If the throttle valve passage is narrow, impurities in the oil can easily accumulate and become stuck, causing blockage. However, the throttle valve used in the throttle-in speed control circuit has a large flow area and a spacious passage, making it less likely for impurities to cause blockage, thus reducing the risk of blockage at low speeds.
(When the throttle valve flow area is the same, the piston rod extension speed is different)
