What Happens When Air Gets Into a Hydraulic System?
Hydraulic systems depend on something very simple: hydraulic fluid is extremely difficult to compress. When an operator moves a control, pressure applied to the fluid can be transmitted through the system to move a cylinder, turn a hydraulic motor or operate an attachment.
Introduce air into that system, however, and things change.
Unlike hydraulic oil, air compresses easily. Even a relatively small amount of trapped or entrained air can change how a hydraulic system behaves. The result can be anything from sluggish or jerky movement to excessive heat, unusual noises and potentially serious component damage.
So what is actually happening inside the machine?
Oil Transmits Force. Air Acts Like a Spring.
In a properly functioning hydraulic system, movement at the controls produces a relatively predictable response because the fluid itself compresses very little under normal operating pressures.
An air bubble behaves differently.
When pressure rises, the air compresses. Instead of immediately transferring all of that energy through the hydraulic fluid, some energy goes into compressing the trapped air. When pressure changes again, the air expands.
In effect, the air can behave somewhat like a tiny spring inside the hydraulic circuit.
That can produce delayed response, inconsistent movement or a "spongy" feeling from hydraulic functions. A cylinder may hesitate and then move suddenly rather than responding smoothly.
On equipment where precise hydraulic control matters, the difference can be noticeable to an experienced operator.
How Does Air Get Into a Hydraulic System?
Air can enter for several reasons.
A low hydraulic reservoir level is one common possibility. If the oil level becomes low enough, the pump may begin drawing air along with the hydraulic fluid.
Leaks on the suction side of the system can also cause problems. Interestingly, these leaks don't necessarily have to visibly leak oil. Because the suction line operates below atmospheric pressure in certain areas, a damaged hose, loose fitting or deteriorated seal can allow air to be pulled into the system without producing an obvious external oil leak.
Air may also be introduced when hydraulic components, hoses or lines are replaced and the system isn't properly purged afterward.
Even the oil returning to the reservoir can contribute to aeration if reservoir conditions allow air to become mixed into the fluid faster than it can escape.
Aeration vs. Cavitation
Aeration is sometimes confused with another hydraulic problem: cavitation.
They aren't the same thing.
Aeration occurs when outside air enters or becomes entrained in the hydraulic fluid.
Cavitation occurs when pressure at the pump inlet becomes low enough for vapor cavities to form within the fluid itself. Those cavities can then collapse as they move into higher-pressure areas.
Both conditions can produce noise, poor performance and component damage, which is one reason diagnosing hydraulic problems based on sound alone can be difficult.
Why Can Air Make a Hydraulic System Noisy?
One possible warning sign is unusual pump noise.
Air bubbles traveling through areas of changing pressure can rapidly compress and expand. This can contribute to rattling, knocking or whining sounds that aren't normally present.
Foamy or milky-looking hydraulic oil can also indicate that air has become mixed into the fluid, although contaminated oil can have other causes as well.
Ignoring the problem because the machine "still works" can be expensive.
Air Can Also Increase Heat
A hydraulic system already converts some energy into heat during normal operation. Aeration can make matters worse.
Repeated compression of air bubbles, inefficient pump operation and turbulent flow can contribute to additional heat generation. Higher hydraulic oil temperatures can then reduce viscosity and accelerate degradation of the fluid.
That creates a potentially damaging cycle.
Poor fluid condition can reduce lubrication, increase wear and further compromise the performance of pumps, valves and other hydraulic components.
What Happens to the Pump?
The hydraulic pump is particularly vulnerable.
Hydraulic oil doesn't just transmit power. It also provides lubrication and cooling for many components. When the pump receives an oil-and-air mixture instead of a consistent supply of hydraulic fluid, lubrication can be compromised.
Over time, this can contribute to wear on internal pump components.
Severe aeration may cause enough noise and vibration that the problem becomes obvious, but smaller amounts of air may show up first as reduced performance, inconsistent operation or rising hydraulic temperatures.
Cylinders Can Behave Differently Too
Air trapped in a hydraulic cylinder or its associated lines can affect movement because the air compresses before full hydraulic force is transmitted.
An operator might notice:
• Jerky or uneven cylinder movement
• Delayed response after moving a control
• A function that feels unusually soft or unpredictable
• Difficulty maintaining precise positioning
• Unusual hydraulic noise
• Foaming in the reservoir
• Higher-than-normal hydraulic temperatures
The exact symptoms depend on where the air is entering, how much is present and the design of the hydraulic system.
Why Finding the Source Matters
Simply getting the air out of the system doesn't necessarily solve the underlying problem.
If a suction hose is cracked, a fitting is loose or the reservoir is repeatedly being allowed to run low, the air can return.
Technicians therefore need to determine why the air entered the system in the first place.
That may mean inspecting suction lines and fittings, checking the reservoir level, examining seals and hoses, confirming that the correct hydraulic fluid is being used and following the manufacturer's procedures for bleeding or purging the system after repairs.
Hydraulic systems vary considerably between machines, so the correct procedure should always come from the equipment manufacturer rather than assuming every system can be bled the same way.
A Small Amount of Air Can Cause a Big Problem
One of the impressive things about hydraulics is the ability to transmit enormous forces with extremely precise control. That capability depends heavily on the physical properties of the fluid inside the system.
Air changes those properties.
What begins as a little foam in the reservoir or an occasional hesitation at the controls can indicate a condition capable of increasing heat, reducing efficiency and accelerating component wear.
For operators, unusual hydraulic behaviour shouldn't simply become "something the machine does."
Sometimes the machine is giving you an early warning.
And in a hydraulic system, listening to that warning may be considerably cheaper than replacing a pump later.
