**Abstract:**
A novel method for detecting the neutrality of a hydraulic control valve using a pneumatic gauge is introduced. This approach offers a fast, efficient, and practical solution for type testing of product prototypes and online quality sampling during mass production. The neutrality of the valve significantly influences both its static and dynamic performance. For example, in three-way valves, the quality of the neutral position affects the amount of median leakage. Similarly, for domestic flow control valves with L-type throttles or Q-speed valves, the neutrality directly impacts leakage levels, altering the static characteristics. In electro-hydraulic servo valves, poor neutrality can affect pressure gain and bias, making it a critical parameter in both type and factory testing.
Traditional hydraulic methods for testing valve performance are time-consuming, labor-intensive, and energy-heavy, while also suffering from inaccuracies due to small-gap flow restrictions. In contrast, the application of a buoyant pneumatic measuring instrument offers several advantages: it allows for rapid transient measurements, eliminates the risk of "plugging" due to its amplification capability, and ensures high accuracy. Additionally, it operates at low pressure, consumes less energy, and reduces testing time. This device functions as a buoyant flowmeter, where compressed air flows through an orifice into a conical tube, lifting a float to a specific height. The air then flows through an annular gap around the float and escapes via a measurement gap. If the pneumatic gauge probe is removed, the airflow can directly jet into the sealing gap of the hydraulic valve. Variations in the gas flow (leakage) through the sealing area cause different float positions, reflecting the degree of leakage and thus the valve’s neutrality.
The principle of using a pneumatic gauge to detect the neutral position of a three-way spool valve is illustrated in Figure 1. Both tubes of the dual-tube pneumatic gauge are calibrated against the same reference. When ports E and F are closed, and ports A and B are connected to pneumatic tubes a and b, with the inlet C open to exhaust, the buoy heights in both tubes should be nearly identical if the valve has good neutrality. Any discrepancy indicates uneven leakage, suggesting poor neutrality and increased total leakage. The buoy position can be translated into the amount of internal leakage, as detailed in reference [3].
Table 1 presents test data for three-way spool valves, showing how the buoy positions change after axial movement of the spool. For instance, Valve No. 5 showed a significant reduction in leakage by approximately 3.5 mL/min after adjustment. Similar improvements were observed in other valves. For electro-hydraulic servo valves like the FM-31 model, which have near-zero openings in the middle position, the same testing method applies, as described in reference [4].
Another application involves using the pneumatic gauge to assess the neutrality of throttle structures. As shown in Figure 2, a dual-tube pneumatic gauge is used, with one tube blocked during calibration and the other connected to a throttle. Adjusting the ratio allows for accurate leakage measurement. When the handle is in the dead position, the leakage should be minimal if the valve core and hole are precisely machined. However, any deviation in machining or assembly can increase leakage. Table 2 demonstrates how adjusting the handle position improves neutrality, reducing leakage from 22 mL/min to 19.5 mL/min in some cases, meeting industry standards such as JB 2146-77.
In conclusion, the use of pneumatic instrumentation for checking the neutrality of hydraulic control valves is a quick, cost-effective, and accurate method suitable for both type testing and online quality control. It not only enhances structural design optimization but also supports efficient sampling of product quality. Given that pneumatic measurement technology is widely adopted in hydraulic manufacturing, this method requires minimal investment and is highly practical for real-time applications.
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