A hydraulic cylinder is one of the main actuating components in a hydraulic system. It can convert hydraulic energy into mechanical energy, producing either linear reciprocating motion or rotary oscillating motion, which makes it widely used in the mechanical industry. The quality of hydraulic cylinders significantly impacts the performance and lifespan of the machinery.

Leakage in hydraulic cylinders can be classified into two types: external leakage and internal leakage. External leakage refers to hydraulic oil leaking from the system into the environment, while internal leakage occurs when hydraulic oil flows from the high-pressure side to the low-pressure side within the system due to factors such as pressure differentials and seal failures. External leakage is easily detected through careful observation, whereas internal leakage requires precise measurement using instruments.

According to the performance requirements specified in the Chinese standard JB/T 10205-2010 "Hydraulic Cylinders," both type testing and factory testing of hydraulic cylinders require the measurement of internal leakage. The method for measuring internal leakage is stipulated in the national standard GB/T 15622-2005 "Test Methods for Hydraulic Cylinders," where the working chamber of the hydraulic cylinder under test is pressurized with oil to the rated pressure or a user-specified pressure. The amount of leakage through the piston into the non-pressurized chamber is then measured. However, this method does not provide detailed instructions on how to measure the leakage through the piston into the non-pressurized chamber or specify the position of the piston stroke during the measurement. Therefore, it is particularly important to explore methods for detecting internal leakage in hydraulic cylinders at any stroke position.

1.Common Methods for Detecting Internal Leakage

Volume Cup Method:This method involves moving the piston to the end or another position within its stroke range. The rod-less chamber or the rod chamber is then pressurized to the rated working pressure or a pressure specified by the user. A volume cup is used to measure the amount of oil leaked from the unpressurized chamber over a certain period of time. The result obtained through this method is the average internal leakage over the specified time period. However, it does not allow for the detection of internal leakage at any arbitrary stroke position of the hydraulic cylinder.

Pressure Drop Method:In this method, a pressure sensor is used to measure the pressure drop in the rod-less chamber over a unit of time. The internal leakage is then calculated based on the relationship between the pressure drop and internal leakage. However, due to the multiple variables involved in the relationship between pressure drop and internal leakage, as well as the presence of minor leaks in the hydraulic system of the test bench itself, it is difficult to accurately measure the internal leakage. Moreover, this method also does not allow for the detection of internal leakage at any arbitrary stroke position of the hydraulic cylinder.

Figure 1: Piston Settling Method

A method for testing internal leakage within any stroke position of a hydraulic cylinder is illustrated in Figure 1. Taking the measurement of hydraulic oil leakage from the rodless chamber to the rod chamber of the tested hydraulic cylinder as an example: The oil port of the rod chamber of the tested hydraulic cylinder C is not connected to any oil pipe, and the directional valve is set to its neutral position. The tested hydraulic cylinder C is loaded by a loading hydraulic cylinder D, causing the pressure in the rodless chamber of the tested cylinder to reach the rated working pressure or a user-specified pressure and maintain stability. A displacement sensor is used to measure the displacement S of the piston rod of the tested cylinder, while a timer measures the time, assumed to be t. If the piston area of the tested cylinder is A, then the internal leakage flow rate Q can be calculated as Q = AS/t.

2. New Method for Internal Leakage Detection at Any Stroke Position of a Hydraulic Cylinder

A new method is proposed to fix the tested hydraulic cylinder at any stroke position. Taking the specific gravity method as an example, the working principle is shown in Figure 2.

Figure 2 Working Principle

In this method, the piston of the hydraulic cylinder to be tested is stopped at any position within the cylinder body by loading the hydraulic cylinder and the return oil overflow valve. During the measurement process, internal leakage of the loading hydraulic cylinder itself will cause a slight forward displacement of both the piston of the loading cylinder and the piston of the hydraulic cylinder under test. This displacement will increase the oil volume discharged into the densitometer. By measuring the displacement of the piston in the loading cylinder using a displacement sensor, multiplying this displacement by the cross-sectional area of the inner cavity of the hydraulic cylinder under test gives the error discharge volume. The real internal leakage volume of the hydraulic cylinder under test over a certain period can be obtained by subtracting the error discharge volume from the total discharge volume. This ensures accurate and reliable measurement results, making it possible to achieve precise internal leakage measurements regardless of the position of the piston within the cylinder body.

The specific testing process is as follows:

(1) Installation and Adjustment: Connect and fix the piston rod of the hydraulic cylinder under test with the piston rod of the loading hydraulic cylinder. Adjust the directional valve and the flow control valve of the control circuit of the hydraulic cylinder under test to make it stop at the position where leakage needs to be tested. Adjust the return flow relief valve connected to the loading hydraulic cylinder to ensure that the loading force of the loading hydraulic cylinder is greater than the output of the hydraulic cylinder under test during testing, thereby ensuring that the hydraulic cylinder under test remains at any position within its cylinder body.

(2) Measurement: Inject high-pressure oil into one chamber of the hydraulic cylinder under test, with the pressure set to the specified pressure for testing leakage. Open the oil port of the other chamber and connect it to a densitometer. At the same time, start timing the loading duration. Measure the total amount of oil overflow from the open oil port using the densitometer and measure the piston displacement of the loading hydraulic cylinder with a displacement sensor.

(3) Calculation: Calculate the internal leakage of the hydraulic cylinder under test using the formula: Internal Leakage = (L - DS) / T, where L is the total amount of oil overflow from the oil port, D is the piston displacement of the loading hydraulic cylinder, S is the cross-sectional area of the inner chamber of the hydraulic cylinder under test, and T is the loading duration.

In the aforementioned method, a hydrometer is used to detect the total oil volume 𝐿 that overflows from the open port of the hydraulic cylinder. The internal leakage volume can be calculated based on the mass and density of the oil, and then measured using a high-precision hydrometer. Considering the effects of hydraulic oil cleanliness and temperature on its density, it is necessary to calibrate the hydraulic oil density before testing. The hydrometer allows for precise measurement of the hydraulic oil density at a specific temperature and cleanliness. By adding automatic timing and data storage functions to the hydrometer, the entire setup can achieve continuous online detection of internal leakage, which is more accurate than using a measuring cup. This method, which fixes the test hydraulic cylinder at any stroke position and uses the hydrometer method to detect internal leakage, is more suitable for detecting internal leakage at any stroke position of short-stroke hydraulic cylinders. For long-stroke hydraulic cylinders, there may be efficiency issues with the inspection. However, this control method can be used in conjunction with other inspection methods to address this issue.

In summary, this control method of stopping the test hydraulic cylinder at any stroke position can be combined with different inspection methods to achieve internal leakage testing at any stroke position of the hydraulic cylinder, featuring high detection accuracy and a high degree of automation. This is of significant importance for future improvements in hydraulic cylinder internal leakage detection methods and the enhancement of hydraulic cylinder product quality.

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