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24

2023

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04

Valve driven hydraulic actuator - Selection considerations 2

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【Summary description】The starting pressure and internal leakage of the actuator are related to the sealing quality of the moving parts. The sealing quality is also related to the mechanical processing quality, size, and sealing structure selection of the inner surface of the oil cylinder. Overall, if the seal is tight and there is less internal leakage, the friction force will increase and the starting force will increase; On the contrary, if the seal is loose, the internal leakage is large, the friction force is small, and the starting force is reduced.

As mentioned earlier, attention should be paid to the size of swing angle, output torque, and working pressure when selecting. Here, we will talk about starting pressure and internal leakage, buffering and braking, etc. Below is a brief introduction.

1: Starting pressure and internal leakage

The starting pressure and internal leakage of the actuator are related to the sealing quality of the moving parts. The sealing quality is also related to the mechanical processing quality, size, and sealing structure selection of the inner surface of the oil cylinder. Overall, if the seal is tight and there is less internal leakage, the friction force will increase and the starting force will increase; On the contrary, if the seal is loose, the internal leakage is large, the friction force is small, and the starting force is reduced.

The straightness of the actuator motion rack piston, cylinder hole straightness, cylindricity, and other geometric tolerances, as well as the sealing material and the sealing compression amount during design, greatly affect the starting force. Seals with high straightness, high cylindricity, and moderate compression have lower friction compared to structures with low straightness, low cylindricity, and high compression. This is determined by the manufacturing process and design of each enterprise.

There are actuators with both integral and split structures in the market, and overall, integral actuators outperform split structures in straightness. The overall structure is machined from the entire casting shell, while the split type is assembled by three sections. The previous article also discussed structural issues and will not elaborate on them. Previously, there was an article that briefly compared two types of sealing structures. If you are interested, you can take a look. The results prove that the U-ring sealing is superior to the Gly ring.

The starting force is relatively small compared to the system pressure, and generally has little impact on the output torque in non servo systems. But because friction exists all the time, it is one of the factors affecting mechanical efficiency. Whether it is necessary to control the starting force or friction force is determined by both the supply and demand parties. This parameter is different for each manufacturer's product. But the smaller the better. How to balance and test design skills with processing, assembly, and process capabilities.

Internal leakage determines the volumetric efficiency of the actuator. Internal leakage can cause changes in load speed and pressure, and pressure changes can cause changes in torque. Generally speaking, the selection of actuators has a safety factor of 1.3-1.5, so it will not have a significant impact on the output torque. In general non servo systems, the impact of internal leakage is minimal, but over time, leakage into the cavity of the gear shaft needs to be drained out. So some manufacturers will connect a visual hose to the shell for easy observation of leakage. During the testing of the first 2000 Nm actuator, Qunmei Electromechanical tested 160 kilograms of pressure and 58000 times back and forth, and found zero leakage. The test results verified that the design parameters were reasonable and effective.

The European standard EN15714-4-2009 describes the requirement for leakage testing to be conducted in two stages: the first stage pressure is at 15% -25% of the maximum allowable pressure, and the first stage pressure is at 143% of the maximum allowable pressure. In fact, a brief 3-minute factory test proves nothing, and product quality needs to be inspected in actual working conditions.