EN
In practical applications, hydraulic motors can achieve both forward and reverse rotation, which is one of the most fundamental and crucial functions of hydraulic systems. However, during daily use, many users encounter an abnormal condition: the motor operates at normal or even higher speeds in the forward direction, but when reversed, the speed noticeably decreases, sometimes accompanied by sluggishness or a lack of power.
This issue of "fast forward rotation but slow reverse rotation" is not a random occurrence but rather the result of multiple factors acting together in the hydraulic system. If not promptly addressed, it can further lead to motor overheating, reduced system efficiency, and even premature component failure. Below, we will systematically analyze the key causes of this phenomenon based on practical maintenance and operational experience.
1、 Insufficient oil supply or inconsistent forward and reverse flow of hydraulic pump
The speed of a hydraulic motor is essentially determined by the flow rate of oil entering the motor. In an ideal state, as long as the flow rate is the same during forward and reverse rotation, the motor speed should also be basically the same. But in actual working conditions, many systems cannot achieve this.
Common issues include:
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The hydraulic pump itself is severely worn, and it can barely meet the flow demand when rotating forward, but due to changes in system resistance during reverse rotation, the pump output is insufficient;
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Inconsistent throttling inside multi way valves or directional valves leads to natural differences in flow in both directions;
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The system is equipped with components such as one-way throttle valves and speed control valves, but the installation direction or adjustment method is incorrect, only limiting the flow in the reverse direction.
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Once the actual flow rate in the reverse direction is less than that in the forward direction, the motor will naturally experience a phenomenon of "slow rotation and lack of strength" when reversing.
✅ Key points of investigation:
Check the working condition of the hydraulic pump, the internal channels of the valve components, whether there is a one-way throttling structure, and whether the forward and reverse flow rates are consistent.
2、 Asymmetric force distribution in both directions of mechanical load
The hydraulic motor itself is only a "power output source", and it is the mechanical load it drives that truly determines whether it can rotate easily. On many devices, the resistance experienced by mechanical structures during forward and reverse rotation is significantly asymmetric.
For example:
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The forward rotation of the hoisting mechanism is "releasing rope", with a small load; Reversing is the process of "winding the rope", which requires overcoming the weight of heavy objects;
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There is structural dead end and increased friction in one direction of the rotating mechanism;
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The reducer, bearing or coupling has eccentric wear or jamming in a certain direction.
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In this case, the hydraulic motor itself is not faulty, but because the load increases during reverse rotation, even if the flow rate is the same, the speed will significantly decrease.
✅ Key points of investigation:
Disconnect the load and idle test the motor. If the idle rotation speed is consistent, it indicates that the problem mainly comes from the mechanical load.
3、 Air is mixed into the hydraulic system, and the impact is more pronounced when reversing
Hydraulic systems are highly sensitive to air. Once air is mixed into the oil or bubbles are formed, it will cause a decrease in system rigidity and severe pressure fluctuations, especially during the moment of reversing.
When there is air in the system:
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Due to high inertia and low resistance, there is no obvious abnormality on the surface when turning in the forward direction;
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Once reversed, the oil needs to re-establish pressure, but air is compressed and the true effective pressure is slowly established;
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The final manifestation is slow reverse start, low speed, and motor stuffiness.
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In severe cases, the following phenomena may also occur:
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There is a lot of foam in the oil tank;
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The motor produces a "hissing" cavitation sound during operation;
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The system temperature rise has significantly accelerated.
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✅ Key points of investigation:
Check whether the oil suction pipe leaks, whether the oil is emulsified, whether the oil level is too low, and standardize the exhaust of the system.
4、 The back pressure on the return side is too high, and the reverse resistance is artificially amplified
During the operation of hydraulic motors, in addition to the importance of oil intake, the smoothness of oil return is equally crucial. If the back pressure on the return side is too high, it is equivalent to "artificially increasing resistance" at the motor outlet, making it difficult for the motor to operate smoothly.
The difference in back pressure between forward and reverse oil return is often caused by the following reasons:
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The return oil pipeline is too thin or too long, resulting in significant flow resistance;
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The return oil filter is clogged;
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Wrong connection of return port, incorrectly connected to high back pressure oil circuit;
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Some systems have added balance valves and hydraulic control check valves in the reverse direction, but the adjustment is improper.
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When the back pressure of the return oil in the reverse direction is significantly higher than that in the forward direction, the motor is like "feeding oil while being held back", and the speed naturally decreases significantly.
✅ Key points of investigation:
Measure the forward and reverse return oil pressure and compare for any abnormal back pressure.
5、 Internal wear of hydraulic motor, internal leakage amplified during reverse rotation
As the usage time increases, the internal distribution pairs, flat oil distribution surfaces, and between the rotor and stator of the hydraulic motor will gradually wear out. Once the wear intensifies, high-pressure oil will directly leak from the high-pressure chamber to the low-pressure chamber, forming a serious "internal leakage".
In certain structures:
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The sealing in the forward direction is still acceptable, and the speed seems normal;
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When reversing, the direction of force changes, and the internal leakage channel is "opened", resulting in a significant decrease in effective pressure;
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Manifested as weak reversal, significantly slow rotation speed, and rapid increase in oil temperature.
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✅ Key points of investigation:
By comparing the pressure, flow rate, and temperature rise under forward and reverse operating conditions, it is preliminarily determined whether there is internal leakage in the motor.
Summarize in one sentence and explain clearly
The fast forward rotation and slow reverse rotation of hydraulic motors are not a single fault, but the result of the combined action of five factors: oil supply, load, air, back pressure, and internal leakage.
To truly solve the problem, we cannot just focus on replacing the motor, but should investigate the entire system of hydraulic pump valve group oil circuit load motor body in order to achieve precise positioning and thorough treatment.
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