How Can You Identify If a Hydraulic Cylinder Is Internally Leaking?

A hydraulic cylinder is leaky when fluid escapes the piston seal from one end of the piston and the opposite under pressure. The most reliable method to test for this is a cylinder drift test. Extend or stifle the cylinder with tension by using the directional valve, set at a central location, and observe the amount of creep that the rod experiences within a predetermined time. If the rod drifts beyond the tolerance for load-holding coupled with slow or spongy motion, no loss of fluid from the outside is a sign of internal bypass instead of the valve or leak being externally damaged. issue.

Internal leakage is among the most often misdiagnosed problems of hydraulic systems since it is not evident. There's not a puddle underneath the machine or wet fittings and no clear cause. The system is performing less than it ought to—and that's the reason why technicians are chasing the wrong problem for hours before they find the true culprit.

What does "internal leakage" really mean?

Every hydraulic cylinder depends on a piston seal in order to maintain an equilibrium pressure between the rod-end and cap-end chambers. This differential allows the cylinder to support the load or move in a predictable manner. If the seal on the piston wears out or is extruded or fails, high-pressure fluid obstructs the piston and flows to the lower-pressure side instead of performing useful tasks.

This is different from a leak that is external, in which the fluid escapes from the cylinder completely through an external seal, a port, and a cracked barrel. Internal leakage remains inside the system, and the level of oil in the reservoir isn't changing much or even changing at all. This is the reason it's a common mistake to miss in a regular visual examination.

Signs that indicate an internal bypass

1. Cylinder drift under load

The most obvious sign is a cylinder that can't remain in place. If a machine is designed to be stationary when its directional valve has been aligned and the rod then slowly expands or retracts when in response to load, fluid is flowing over the piston. This is evident in presses, lifts, or clamping operations where a hold that is stationary is an integral part of the cycle.

2. Force or speed reduced under the load

A cylinder that has a good piston seal will move at a pace that is proportional to the flow of the pump regardless of the load (within the pressure rating of the system). If cylinder speed is dropping significantly as load increases, or the cylinder is "spongy" and hesitates before moving, a portion of the flow is going through the piston instead of dispersing into the chamber in front.

3. Excess heat generation

Internal leakage transforms energy from hydraulics into heat, instead of mechanical work. Fluid pushed across an old seal at high pressure and speed produces localized heat. A cylinder barrel that is hotter than the rest of the circuit, especially near the piston's mid-stroke location, is a clear sign of bypass within the internal system.

4. Inconsistent or unreliable motion

Internal leakage that is partial doesn't always show a uniform pattern. If wear on seals is uneven around the piston's circumference or temperature fluctuations affect the seal clearances, the piston could show jerky, uneven, or inconsistent motion instead of an even, steady motion.

5. No corresponding external fluid loss

This is the defining factor that differentiates internally leaking from the other issues. If the reservoir's level remains steady, there is no oil on the floor. While the fittings remain dry and the cylinder performs poorly, the internal bypass should be moved up the list of possible causes.

How do you confirm internal leakage The test of drift

Drift tests are the most common method used to confirm piston seal bypass without removing the piston.

Static (load-holding) drift test:

1. The cylinder should be extended to the mid-stroke position, under a load representative of it.
2. Place the valve's directional control to ensure that both ports on the cylinder are blocked.
3. The rod's position should be marked on the cylinder or with the dial indicator.
4. The measurement of rod motion is done in a predetermined time frame, usually 5-10 minutes.
5. Check the drift distance against the manufacturer's acceptable tolerance for the bore size and pressure rating.

Any deviation beyond the stated tolerance means that the piston seal isn't sealing properly.

Testing for dynamic bypass (isolated technique for ports):

1. Connect one port on the cylindrical (commonly the return line for the rod's end).
2. Line up the cap or connect it to a container that is clear.
3. Pressurize the port opposite while leaving the side that is disconnected exposed to the atmosphere or in a graduated container.
4. The flow of any fluid from the port that is disconnected and the other side is pressurized indicates that the piston is not being pushed, and the flow rate provides an approximate indication of the severity.

This approach is more intrusive, but it gives an accurate bypass rate, which can be useful in making the decision of whether the replacement of a seal is best left for scheduled maintenance or requires immediate attention.

The distinction between internal leakage and other problems

Internal leakage symptoms may be interspersed with pump or valve issues So, it is important to rule out others as a must prior to removing the cylinder to replace the seal.

• Leakage in the direction of the control valve A damaged or worn spool could allow bypass inside the valve and cause drift-related symptoms that appear similar to an issue with the cylinder. The isolation of the cylinder using the test for drift (valve center, load-held) aids in separating leakage in the valve from leakage in the piston seal. In the event that drift is stopped after the valve is re-inserted by blocked hoses rather than using the valve, it is likely that the valve is the source.
• Wear and tear on the pump Wear and tear of the pump decreases the flow and pressure in the entire system, but it's not likely to cause a particular cylinder to move under a static valve-centered hold. These symptoms are more likely to affect the entire system and not just an individual actuator.
• Problems with relief valves A relief valve that's partially open could cause a reduction in speed or force; however, it will not cause rod movement even if the valve's direction is set because the relief valve's path doesn't lie part of that holding circuit.

What is the cause of a failed piston seal?

Understanding the root cause can help to avoid recurrent failures following the replacement of a seal:

• Contamination: Particles of abrasive within the fluid speed up seal wear and may even cut the bore of a cylinder, creating the possibility of leaks that a replacement seal can't repair.
• Overheating: Continuously high temperatures cause damage to sealing elastomers. They cause them to crack, harden, and lose the sealing geometries.
• Overpressurization: Operating above the pressure of the cylinder's rating could cause the seal to expand into the gap between the bore and piston and cause permanent deformation.
• Bore damage: Pitting, scoring, or corrosion within the barrel prevents an installed seal from properly sealing.
• The wrong seal material: Using an unapproved seal material that is not suitable to the type of fluid used in the system or operating temperature can reduce service life considerably.

Repairs instead of. replace

A minor deviation within the tolerance of an older cylinder in a less critical application could be ok to observe rather than fix immediately. In load-holding equipment presses, lifts, and clamps, where the drift poses an inherent safety risk, internal leakage must be dealt with when it's discovered. If the bore exhibits wear or scoring that is beyond the specification of the seal manufacturer's clearance, honing or sleeving the bore is essential along with seal replacement or bypass could recur.

Internal leakage can occur without a drop in the reservoir fluid levels.

Yes. Internal leakage holds fluid within the cylinder and within the hydraulic circuit, and the reservoir's level is typically constant even when the cylinder loses its holding force or speed.

What amount of cylinder drift is considered to be acceptable?

The amount of drift that is acceptable varies according to the maker, size of bore, and use, but the majority of industrial cylinder specifications only allow one millimeter of drift in a 10-minute static hold. Always verify the tolerance of the manufacturer instead of relying on an overall amount.

Does internal leakage become more pronounced in time?

Most of the time, yes. When a piston seal starts to wear or expand and the bypass path is likely to grow larger under continuous operation, particularly if the heat or contamination is accelerating the process of degradation.

Does a pressure test only confirm leakage inside?

A static test of pressure can reveal a decrease in pressure over time, which could indicate leakage within the circuit; however, it does not isolate the valve from the cylinder. The test for drift using the isolated cylinder is more reliable in confirming the seal of the piston in particular.

Is leakage in the internal cylinder repairable without taking the cylinder off the machine?

Most of the time, there isn't. Replacement of the piston seal involves disassembling the cylinder for access to the bore and piston, which usually means taking it from the machine, unless the machine has an on-site service allocation.