A hydraulic system is designed on a fundamental premise: hydraulic fluid is incompressible. The pump pushes fluid, and the cylinder moves instantly and rigidly. When air, a highly compressible gas, is introduced into this closed-loop system, that fundamental premise is broken. The result is what many operators and technicians refer to as being “air locked.”

While the cylinder isn’t mechanically locked in the traditional sense, the presence of air can render it functionally useless, unable to build the necessary pressure to move its load. More than just a performance nuisance, trapped air is a destructive force that rapidly accelerates component wear. At EverPower-HUACHANG, we engineer our cylinders to withstand rigorous conditions, but even the best components cannot perform correctly in an aerated system. This guide delves into the mechanics of air locking, how to identify it, and the engineering protocols to resolve it.

Figure 1: A standard EverPower-HUACHANG double-acting hydraulic cylinder. Air pockets can become trapped in the cap end (rear) or rod end (front) chambers, preventing proper fluid transmission.

1. The Physics of the Problem: Why Air is the Enemy

To understand air locking, we must look at the difference in Bulk Modulus between hydraulic oil and air. Bulk Modulus is a measure of a substance’s resistance to compression.

• Hydraulic Oil: Very high bulk modulus. At 3,000 PSI, it compresses only about 1-2%. This allows for precise, rigid transfer of power.
• Air: Very low bulk modulus. It is incredibly spongy. You can compress a large volume of air into a tiny space with relatively little force.

When a hydraulic pump sends pressurized fluid to a cylinder containing a large air pocket, the energy is first used to compress that air, like compressing a mechanical spring. The cylinder piston will not move until the air is compressed sufficiently to equal the pressure required to move the load. If the load is heavy, the system may reach its relief valve setting (maximum pressure) before the air is fully compressed. In this scenario, the cylinder never moves at all—it is effectively “air locked.”

2. Symptoms: How to Tell if Your Cylinder is Air Locked

Identifying air entrainment early prevents damage. Look for these distinct signs:

A. Spongy or Bouncy Operation

If the cylinder feels “springy” when it hits a load, or if it bounces when the control valve is closed, you have air. Hydraulic fluid should be rigid. If you extend a cylinder against a stop and it “rebounds” slightly when you let off the lever, air is expanding back to its original volume.

B. Jerky Motion (Stick-Slip)

Air compresses and releases energy unpredictably. This causes the cylinder to move in jumps or jerks rather than a smooth slide. This is often called “stick-slip” behavior and is detrimental to precision machining or load handling.

C. Strange Noises

Air passing through orifices, valves, or the pump creates a distinct high-pitched whine or screeching sound. Inside the cylinder, you might hear banging or knocking sounds as air pockets collapse under pressure.

D. Foaming Oil

Check the hydraulic reservoir. If the fluid looks milky, cloudy, or has a thick layer of foam on top, it is heavily aerated. This air will eventually be pumped into your cylinders, causing air locks.

3. Causes: How Does Air Get In?

Ideally, a hydraulic system is sealed. Air ingress points must be identified to prevent recurrence.

• Low Reservoir Level: If the oil level drops below the pump intake pipe, the pump will create a vortex (whirlpool) and suck air into the system. This is the most common cause.
• Suction Side Leaks: Any loose fitting or cracked hose between the tank and the pump inlet will suck air IN rather than leak oil OUT, because this section is under vacuum. These “phantom leaks” are hard to spot because there is no puddle.
• Improper Maintenance: Changing a hose, valve, or cylinder introduces air. If the system is not bled correctly immediately after maintenance, that air remains trapped.
• Worn Pump Shaft Seal: Air can be drawn past a worn main shaft seal on the hydraulic pump during operation.

4. Engineering Solutions: How to Bleed an Air Locked Cylinder

Removing the air (“bleeding”) is the only way to restore performance. The method depends on the cylinder type and system design.

Method A: The Cycle Bleed (Easiest)

For many systems, air can be purged by cycling the cylinder fully.

1. Ensure the reservoir is full.
2. Run the system at low pressure (idle).
3. Extend and retract the cylinder fully 5-10 times. Do not “deadhead” (hold pressure against the stop) for long periods; just touch the end and reverse.
4. This pushes air-laden oil back to the tank, where bubbles can rise to the surface and escape.

Method B: Cracking the Lines (Manual Bleed)

If the ports are not at the highest point of the cylinder, air may be trapped at the top.

1. Position the cylinder so the ports are facing upward if possible.
2. With the system at low pressure, slightly loosen the fitting on the return side (the side fluid is leaving).
3. Operate the valve slowly. You should hear air hissing or see foamy oil sputtering out.
4. Once clear, solid oil flows, retighten the fitting immediately.
5. Repeat for the other side.

Method C: Integrated Bleed Screws

High-end cylinders, like those manufactured by EverPower-HUACHANG for critical applications, often feature dedicated bleed screws located at the highest points of the barrel or gland. These function like brake bleeders on a car and offer the safest, cleanest way to remove air.

5. Special Considerations: Double Acting vs. Single Acting

Double Acting Cylinders

These are easier to bleed because you have hydraulic flow in both directions. You can force air out of both the rod end and cap end by cycling fluid.

Single Acting Cylinders (Displacement Type)

These are notoriously difficult to bleed. Because they rely on gravity to retract, there is no pressure pushing the oil out of the return stroke. If air is trapped at the top of a single-acting ram (like a forklift mast), you may need to manually bleed it at the highest point while the cylinder is fully compressed.

Telescopic Cylinders

Telescopic dump truck cylinders trap air in the upper stages. To bleed them, the bed must be raised and lowered slowly multiple times. Some designs require a specific bleeding sequence outlined in the OEM manual to prevent the stages from extending out of order.

6. Preventing Air Lock: Design & Maintenance

Prevention is cheaper than repair. Here is how to keep your system air-free:

• Reservoir Design: Use a reservoir with a baffle plate. This separates the return line from the suction line, giving air bubbles time to rise to the surface before the oil is sucked back into the pump.
• Tighten Suction Lines: Regularly check clamps and fittings on the inlet side of the pump.
• Port Orientation: When mounting cylinders, try to orient the ports facing UP. If ports face down, air gets trapped at the top of the barrel and cannot escape.
• Fluid Quality: Use high-quality hydraulic oil with anti-foaming additives.

7. Frequently Asked Questions (FAQ)