Hydraulic cylinder problems: Identifying causes and improving reliability

Hydraulic cylinders are the heartbeat for industrial machines. They convert pressure from fluids into linear force, thereby driving everything from excavator arms and press brakes to injection molding equipment and farm machinery. When they're working in a quiet manner, they are efficient. However, if they do not perform, they can face a range of consequences, from expensive downtime to grave security risks.

The problem when dealing with problems with hydraulic cylinders is that the signs, such as sluggishness, visible leaks, and erratic behavior, can be easily identified. However, the root causes are rarely apparent from the first look. A cylinder that sways under stress could be blamed on a seal, but the actual cause is an old piston or a scored bore. A cylinder that is running hot may indicate a problem with the seal caused by a misaligned load or a relief valve that is set too high.

This article will discuss the most frequently encountered hydraulic cylinder issues, what they actually cause, and how you can stop them from happening again.

Why do hydraulic cylinders fail?

Before examining individual failure mechanisms, it is helpful to comprehend the larger failure spectrum. The majority of hydraulic cylinder issues result from the root of a problem in four ways:

Contamination - Particles in the air or water that are in the hydraulic fluid can degrade seals, nick internal surfaces, and speed up wear across all moving components.

Incorrect installation or alignment side loads, off-axis forces, and mounting issues put pressure on seals for rods as well as bushings and the bore in cylinders, which the design didn't intend.

Seal degrading seals can fail because of chemical incompatibility, the cycle of thermal energy, friction, or simply wear and tear. A seal that has been functioning perfectly for many years could fail rapidly when the fluid in the system changes or the operating temperature shifts.

Maintenance that is not properly performed—delayed changes to fluids, ignoring early warning indicators, and infrequent inspections—can cause small issues to become costly failures.

Knowing which category your problem with the cylinder is in will determine how you tackle the problem as well as whether or not the repair is successful.

Common hydraulic cylinder problems and their causes

1. External leakage around the rod seal

Rod leakage in the seal is perhaps the most obvious hydraulic cylinder issue. A thin layer of oil covering the rod is normal. A microscopic layer of oil is needed to keep the seal lubricated. If you observe oil accumulating under the rod or streaking down the rod, it is a sign that the seal has failed.

Common causes:

• Surfaces of the rod that are pitted or scored: Surface damage that is not too severe can cause a leak that cannot be bridged by seals. The scoring of rods is usually caused by abrasive particles that enter through the seal for the wiper.
• Cracks or hardening of seals Seals that are exposed to extreme heat or other incompatible fluids will get brittle and lose the ability to flex to the rod's surface.
• Damaged or worn-out wiper seal: The seal blocks outside contaminants from getting into the cylinder. When it is broken, dirt and grit get to the rod seal and speed up its wear.
• Rod misalignment: A rod subjected to constant pressure causes the seal to come into uneven contact, which causes premature wear on one side.

What to do: Check the surface of the rod carefully prior to changing the seal. A replacement seal on damaged rods can fail quickly. If there is scoring, the rod might need to be replaced or re-chromed. Determine the source of the side loading, if it's an alignment problem.

2. Internal leakage and cylinder drift

Leakage inside the piston—a flow of fluid moving the piston's chamber from one to another—does not cause an obvious mess, making it difficult to notice until performance decreases noticeably. The most frequent sign is that the cylinder is drifting: the rod is slowly retracted when under load, even with the valve in the hold position.

Common causes:

• Damaged or worn piston seals: As piston seals wear, the seal lip will not keep that pressure gap between the rod-end chamber and the cap-end chamber.
• Insufficient lubrication or abrasive contamination causes grooves to form in the bore that permit fluid to escape the seal of the piston regardless of the condition.
• Damaged or cracked piston In extreme overload conditions the piston may break, allowing an obvious bypass route.

Diagnosis tip: To verify leakage inside the cylinder, stretch the cylinder to its maximum length under tension, then unplug and plug in the cap-end port. In the event that the rod retracts while the port is plugged, internal leakage is proven. Test the drift rate to determine the extent of leakage.

What should you do? What to do: A bore scored over 0.05 millimeters usually requires sleeve repair or honing. Repairing seals damaged in a bore is only a temporary solution, at the very least.

3. Hydraulic cylinder creep

Cylinder creep is different from drift because the movement is irregular, slow, and is often directional. It affects loaded and unloaded cylinders.

Common causes:

• Contaminated control valves Spool valves that are damaged or contaminated may not sit properly and allow tiny quantities of fluid to go through even in the neutral position.
• The check valve, also known as the counterbalance valve that fails The valves are used to maintain pressure for load while the system is in rest. A damaged or worn-out valve allows for the fluid to move slowly.
• Thermal expansion in systems that oscillate between high temperatures: thermal expansion of trapped fluid could result in slow, unintentional rod motion.

Creep is often mistakenly interpreted as a problem with the cylinder; however, the problem lies in the circuit for valves. When disassembling a cylinder, remove the circuit around the cylinder and then test it on its own.

4. Uneven or jerky movement

A cylinder moving with a thump rather than smooth indicates pressure or flow instability within the system. However, the mechanical reasons inside the cylinder are worth studying.

Common causes:

• The hydraulic circuit is awash with air that is compressed and expands in unpredictable ways, leading to erratic movement. Air can enter the system through a lack of fluid levels or loose fittings. It can also be caused by an inoperative seal on the pump shaft.
• Stick-slips made of degraded or dry seals Seals that are running dry because of inadequate lubrication or incompatible fluid cause high friction that is then followed by a sudden shift.
• Bowed or bent rods Rods that are even a little bent can cause the appearance of binding when they pass across the gland of the rod, causing an irregular motion and rapid wear on the seal.
• Overly frictional on-load guides. In the event that the object that is being moved is equipped with its own linear bearings or guide rails that are not aligned properly with the load, the side forces that result can result in stick-slip, even in a cylinder that is healthy.

5. Overheating

Hydraulic cylinders aren't able to generate heat by themselves; however, they are part of an environment that produces heat. If a cylinder is hot, the viscosity of the fluid reduces, the seal's life decreases drastically, and the possibility of a catastrophic failure is increased.

Common causes:

• Pressure set to high in the system When you set the valve to release to be above the cylinder's maximum pressure, the system is working more than it ought to, producing excessive heat.
• Regular operation of the relief valve: If the cylinder is consistently at the pressure of the relief valve The workload is higher than the cylinder's capacity, or the system is not sized properly.
• Fluid degradation: The old or contaminated fluid loses thermal stability. The base oil degrades, and its capacity to transfer heat away from the components decreases.
• Insufficient volume of reservoir: Small reservoirs do not allow enough time to cool prior to the recirculation process.

Improving hydraulic cylinder reliability

The proactive repair process keeps the cylinders operating. Maintenance that is proactive keeps them operating efficiently.

Establish a fluid monitoring program. The process of counting particles and measuring testing of water content every few weeks detects issues before they can cause damage to the surface. ISO 4406 provides a standard cleanliness classification for maintenance teams to monitor against.

Inspect rod surfaces during every seal replacement. Surface finish can have a direct effect on the longevity of the seal. A rod's smooth surface with Ra 0.2-0.4 mm is suitable for dynamic sealing. The rougher the surface, the more wear it will experience. Surfaces that are too smooth could decrease the thickness of the oil film that seals the seal.

Verify alignment of the cylinder when installing and after any load changes. Utilize dial indicators to confirm that the axis of the cylinder bore is in line with the direction of motion. A small amount of deviation at the clevis pin could cause significant side load on the stroke.

Replace the entire seal kit, not just the seal that is leaking. When a cylinder is taken apart, it is likely that all seals in the dynamic range will likely have been subjected to the same operating conditions. Replacing just the failed seal will leave the other seals close to failing, and a new disassembly could be coming soon.

Note the operating parameter and the failure histories. A cylinder that fails three times in the span of two years indicates that something is wrong with the system as well as the component. Monitoring the cause of failure, the condition of the fluid during the period of failure, and operating hours, you can gather the necessary information to make informed decisions about the selection of components and fluid specifications along with maintenance time intervals.

Problems with hydraulic cylinders are not always random. A cylinder that leaks, shifts, or moves in an erratic manner is responding to certain situations like misalignment, contamination, seal incompatibility, or overloading. The identification of the cause instead of just replacing the defective part can be the distinction between a fix that is successful and one that is repeated over the course of six months.

Reliability of hydraulic systems is dependent on a keen eye on the quality of fluid as well as the correct method of installation and a consistent examination. When you get those three things correctly, most hydraulic cylinder issues can be prevented instead of being inevitable.