Analysis of reasons and solutions for the hydraulic cylinder’s self-descent in the hydraulic system?

Introduce

Hydraulic cylinders are commonly used actuators in hydraulic systems. Their operational stability directly impacts the performance of the entire system. Today, let's discuss common issues with hydraulic cylinder operation. Why do they often lower on their own when locked? First, let's discuss the structure and working principle of hydraulic cylinders.

1. Hydraulic Cylinder Structure and Working Principle

A hydraulic cylinder typically consists of a rear end cap, cylinder barrel, piston rod, piston assembly, and front end cap. To prevent oil leakage from the cylinder or from the high-pressure chamber to the low-pressure chamber, seals are installed between the cylinder barrel and end cap, the piston and piston rod, the piston and cylinder barrel, and the piston rod and front end cap. A dust guard is also installed on the outside of the front end cap. To prevent the piston from striking the cylinder head when rapidly retracting to the end of its stroke, a buffer device is installed at the end of the hydraulic cylinder. An exhaust device may also be required.

alt. Hydraulic cylinder structure

2. General Hydraulic Cylinder Circuit

In a hydraulic system, when a hydraulic cylinder extends to a certain length, it no longer needs to move and only needs to be held in a certain position for a certain period of time. At this point, a directional control valve or hydraulic lock can be used to hold the cylinder in place. The general hydraulic circuit is as follows:

alt.Hydraulic cylinder circuit

3. Analysis Of The Reasons Why The Hydraulic Cylinder Descends Automatically

The piston rod of the hydraulic cylinder retracts under the action of external force. According to the structure and working principle of the hydraulic cylinder, we can analyze that the fundamental reason for the retraction of the hydraulic cylinder is that the volume of the hydraulic oil in the rodless chamber has decreased. The possible reasons for the decrease in the volume of the hydraulic oil in the rodless chamber are the reasons why the hydraulic cylinder descends automatically:

(1) Internal leakage of the hydraulic cylinder. The oil in the rodless chamber of the hydraulic cylinder flows into the rod chamber through the piston seal.

(2) Leakage at the rear end cover of the hydraulic cylinder. The hydraulic oil in the rodless chamber leaks to the outside through the rear end cover. This is an external leakage and is easy to check.

(3) The hydraulic oil in the rodless chamber flows away through the oil port. This can be divided into several situations:

a) Oil leakage from the oil port pipe joint or hydraulic oil pipe. This is also an external leakage and is easy to check.

b) In the circuit using the reversing valve neutral lock, the reversing valve may not be fully returned to the neutral position, or the neutral seal may be poor.

c) In the circuit using the hydraulic lock, the hydraulic lock ring may be damaged or the reversing valve neutral position may be improperly selected. (4) Oil leakage from the hydraulic cylinder barrel is also considered as external leakage, which is easy to observe and the possibility is generally very small.

4. Solution

(1) Internal leakage of the hydraulic cylinder may be caused by aging of the seal ring. The seal ring needs to be replaced or a more reasonable sealing structure needs to be redesigned.

(2) The same is true for leakage of the rear end cover of the hydraulic cylinder. The seal needs to be replaced or a more reasonable sealing structure needs to be redesigned.

(3) When the oil pipe joint leaks oil, check whether the joint seal ring is damaged. Replace the seal or replace the joint.

(4) In the circuit using the reversing valve center lock, the center seal is not good. Generally, it is caused by oil stains or other impurities that cause the valve core to not move into place, or the spring is stuck or broken, causing it to be unable to return to the center position. Professional personnel need to repair or replace the reversing valve. If the pressure maintenance requirement is relatively high, you can consider replacing the hydraulic lock.

(5) In the circuit locked by hydraulic lock, the neutral function valve with closed ports A and B cannot be used, because the closure of ports A and B may cause the hydraulic control one-way valve to still open in both directions and fail to play the locking role. In addition, the hydraulic lock spring breakage or impurities may also cause the hydraulic lock to be unable to be completely closed, resulting in leakage.

Summary

Hydraulic cylinders are critical actuators in hydraulic systems, and their stability is essential for system performance. A common issue is the undesired self-descent of hydraulic cylinders when locked in position. This problem arises due to internal or external leakage, such as seal failure in the cylinder, leakage at the end covers, or hydraulic oil escaping through the oil port. Additionally, improper functioning of directional control valves or hydraulic locks—such as incomplete neutral positioning, seal wear, or incorrect valve selection—can contribute to the issue. Solutions include replacing damaged seals, repairing or replacing faulty valves, ensuring proper hydraulic lock selection, and maintaining system cleanliness to prevent contamination. Addressing these factors ensures reliable hydraulic cylinder operation and prevents unintended movement.