How to remove air from hydraulic cylinders
As a key component in the hydraulic system, the performance of the hydraulic cylinder directly affects the operating efficiency and stability of the entire system. However, during use, air is often mixed into the hydraulic cylinder for various reasons. This air will not only cause the performance of the hydraulic cylinder to deteriorate, but may also cause more serious failures. Therefore, it is necessary to remove the air from the hydraulic cylinder in a timely and effective manner.
Why does air get into the hydraulic ram?
It is not uncommon for air to get into the hydraulic cylinder. This problem is usually caused by the following:
Incorrect operation when filling or replacing the hydraulic ram
When filling or replacing the hydraulic oil, if the operation is not standardised, such as not closing the drain valve in time or filling too quickly, it is easy for air to enter the hydraulic cylinder. In addition, the filling tool may not be completely cleaned, which can also cause air to enter.
Seal ageing or damage
The seal may age, wear or crack after long use. When the sealing performance decreases, the outside air will enter the hydraulic cylinder through the gap, especially when the hydraulic cylinder is in a negative pressure state, this phenomenon is more obvious.
Loose pipe connection
The pipe connection of the hydraulic system is the area where air is most likely to enter. If the pipe connection is loose or not installed properly, the air will be sucked into the hydraulic cylinder with the flow of hydraulic oil.
Installation or maintenance problems
When the hydraulic cylinder is installed or repaired, if the exhaust operation is not performed correctly, residual air will appear, which will affect the normal operation of the hydraulic cylinder.
Poor oil quality
Poor quality hydraulic oil may contain a large amount of dissolved air or release bubbles under high temperature and low pressure conditions, causing air to be mixed into the hydraulic cylinder.
What problems does air cause in hydraulic cylinders?
Air in a hydraulic cylinder will have many negative effects on the performance and life of the hydraulic cylinder:
Unstable pressure
The compressibility of air makes it impossible for the hydraulic system to maintain a stable pressure, resulting in instability in the hydraulic cylinder during operation.
Interference with the normal operation of the hydraulic cylinder
Air in the hydraulic cylinder will cause the hydraulic cylinder to jam, shake or stop intermittently, and in severe cases will even affect the operating efficiency of the entire equipment.
Accelerated oxidation of the hydraulic oil
Oxygen in the air will react chemically with the hydraulic oil, accelerating the oxidation of the oil, thereby affecting the lubrication and service life of the oil.
Increased noise
When air is mixed into the hydraulic cylinder, it will cause abnormal noise when the system is running, especially when the bubbles burst, it will make a loud noise.
Accelerated wear of components
When air bubbles burst in the hydraulic system, they create an impact force on the pipes and metal surfaces, accelerating the wear of components.
What to do before bleeding
Before officially bleeding the hydraulic ram, carry out the following three preparations:
Clean the system
Clean the dust and oil from the surface of the ram, pipework and associated equipment to prevent secondary contamination during operation.
Check the hydraulic oil level
Ensure that the hydraulic oil level is adequate and the oil quality is good. If the oil is insufficient or contaminated, it should be topped up or replaced in good time.
Prepare the necessary tools
Prepare the necessary tools, including wrenches, protective gloves, clean towels and exhaust auxiliary equipment to ensure smooth operation.
Exhaust operation sequence
The operation of the hydraulic cylinder exhaust should be carried out according to these six steps:
Shut down the system
Stop the operation of the hydraulic system, ensure the system is in a static condition and release the internal pressure to ensure safety during operation.
Locate the exhaust port
Locate the bleed hole or bleed valve on the hydraulic ram, generally at the highest point of the hydraulic ram or pipework.
Open the exhaust valve
Open the bleed valve slowly to allow the gas to escape with the oil and to prevent the hydraulic oil from overflowing too quickly.
Operate the hydraulic ram
Accelerate the exhaust by repeatedly extending and retracting the hydraulic ram.
Check the air bubbles
Watch the liquid coming out of the exhaust hole until the bubbles disappear completely.
Close the exhaust valve.
After confirming that the air has been completely evacuated, retighten the exhaust valve to ensure a good seal.
Three common methods to remove air from hydraulic cylinders
Power circulation exhaust
Use a hydraulic pump to drive the hydraulic oil circulation and exhaust the air from the system through high pressure. This method is suitable for large hydraulic cylinders and has high efficiency.
Vacuum exhaust
Use a vacuum pump to reduce the pressure inside the system so that the air is automatically precipitated and exhausted. It is suitable for precision equipment.
Use special equipment to exhaust
Some hydraulic systems are equipped with automatic exhaust devices to quickly exhaust the air in the system. It is suitable for systems with a high degree of automation.
Inspection and adjustment are required after exhausting
The following inspections and adjustments are required after the extraction process is complete:
Check the pressure
Start the hydraulic system and observe if the pressure gauge returns to the normal range.
Observe that the operation is normal
Test the smooth operation of the hydraulic ram to ensure that there is no delay, jerking or abnormal noise.
Top up the hydraulic oil
Check the hydraulic oil level and top up if necessary to ensure that the system runs smoothly.
Check the seals
Concentrate on checking for oil leakage at the exhaust valve and pipe connection, and repair in good time if a problem is found.
To prevent air from entering the hydraulic cylinder in the future
Strictly control the operation of hydraulic oil replenishment
Use special equipment for replenishing hydraulic oil to prevent air from mixing with liquid in the system.
Check seals and pipework regularly
Check the integrity of seals and the tightness of pipework connections and replace them in good time if hidden hazards are found.
Optimise the exhaust system
Install automatic bleed valves at high points in the pipeline to reduce the frequency of manual bleeding.
Control system temperature
Keep the operating temperature of the hydraulic oil within a reasonable range to avoid bubble formation caused by temperature changes.
Common exhaust problems and solutions
Bubbles still present after exhaust
The vent may not be complete or the seal may be damaged. Further inspection of the exhaust process and seal is required.
Hydraulic cylinder does not move smoothly
There may still be small bubbles in the hydraulic oil which can be removed by changing the oil or bleeding again.
Repeated bleeding has no effect
Check if there are any inappropriate aspects in the system design and contact a professional engineer for optimisation if necessary.
How to exhaust in these three special scenarios
High pressure system
Operate the exhaust valve slowly to avoid high pressure safety hazards.
Very cold environment
The viscosity of hydraulic oil increases at low temperatures. Before bleeding, the system must be properly heated to improve oil flow.
Complex multi-circuit system
Bleed each circuit in turn to ensure that the air in each branch is completely exhausted.
