Why hydraulic cylinders lose holding force under load

Hydraulic cylinders lose holding force under load primarily due to internal leakage past worn piston seals, external leakage at rod seals or fittings, valve drift or spool leakage in the directional/counterbalance valve, thermal expansion of fluid changing effective volume, and entrained air or gas compressibility within the system. When a cylinder is intended to support the weight of a dead or static load with no drift or spool leakage, any of these leakage or compression pathways allow fluid to bypass the piston or flow away from the circuit, which allows the load to settle slowly and eventually "cylinder drift" even though the directional valve is set and motion is not controlled.

For system designers and technicians For system designers and technicians, knowing the source of force is going to escape—instead of simply replacing seals and wishing—is an important distinction between a lasting solution and a recurrent service call. This article explains the mechanical and hydraulic reasons behind the loss of force in holding and explains how to detect these causes and what corrective actions are actually effective when working in the field.

What does "holding force" really mean in the hydraulic cylinder?

Holding force refers to the ability of a cylinder to resist a load without retracting or extending after that directional valve has been aligned and the flow is blocked. In a tandem-center or closed-center circuit, the force will theoretically be confined by the column of fluid to either end of the piston. In actual practice there is no way to ensure that the hydraulic circuit is completely sealed, and some amount of drift is common. A large amount of drift—enough to lower an elevated instrument, let an elongated press platen sway, or allow the clamped workpiece to move—could indicate a defect within the path of force retention.

Holding force isn't identical to output force. A cylinder could produce its maximum force when it is in active extension but not be able to hold the force after the flow stops because the holding force is dependent on sealing integrity instead of the pump's output.

Leakage of piston seals in the internal piston (Bypass)

The most commonly cited cause for holding force loss is an internal bypass of the seal of the piston, which allows fluid to move from the high-pressure end of the piston to the lower-pressure side within the barrel itself.

How do piston seals wear?

Piston seals - generally piston rings, U-cups, and polyurethane/ PTFE composite seals degrade due to the following:

• Wear from abrasive contamination of fluids, in which particulate matter gets stuck in the seal surface and cuts the bore
• Extrusion of the seal gap at high pressure, particularly when clearances are worn out or too large between bore and piston
• Chemical degradation resulting from incompatible fluids or additive breakdown, which causes the elastomer's structure to harden and expand or crack.
• Compression set, in which the seal will lose its elastic memory after a prolonged period of load, thus reducing contact pressure with the wall of the bore

If the bypass goes over the limit of a few drops per minute at the pressure that is rated when a cylinder is holding a static load, it will be able to visibly creep, even with the valve completely centering.

Bore scoring and surface finishing

Seals are only in the same way as the surface they are sealing against. A corroded or scored bore in a cylinder—caused by cavitation, contamination, or rod-side rust transferring beyond the wiper—leads to leak paths that a new seal cannot completely seal. Specifications for the surface finish of bores (typically within the range of 4-16 microinches according to seal type) are created because surface imperfections are converted into bypass channels when pressure is applied.

Leakage from the outside: rod seals, fittings, and glands

External leakage does more than just create an unpleasant mess on the floor of the shop—it also takes fluid volume out of the chamber that holds the load. Some common points are:

• Rod seal wears allows fluid to flow through the rod on the return stroke
• Failure of the head seal or the gland, including O-rings and backup rings near the static seal interfaces
• Fittings that are loose or damaged as well as hose connections. seat fittings for tubes
• Welds and barrels cracked on fabricated cylinders exposed to fatigue or side-loading

Even a tiny leak to be visible can leak enough fluid over a few days or weeks to permit a cylinder that is loaded to settle considerably, especially for cylinders with long strokes or large bores where a slight loss of volume will result in a greater linear drop.

Valve-side leakage is the most overlooked cause

Technicians frequently disassemble and seal a cylinder only to discover the same drift continues to exist because the actual leak route is through the valve itself and not in the valve itself.

Leakage in the direction of control valves

The valves for spools lap according to tight tolerances; however, contamination, wear, or a clearance that is not in accordance with the specifications permits internal leakage across the spool even when the valve is in the centered position. This is a normal, standard leakage rate in the majority of valves; however, as the spool wears and leakage grows beyond the design tolerance, the valve is unable to hold it in its position.

The load-holding valve and counterbalance drift

Circuits intended to support suspended loads—booms, presses, lift arms, or booms—usually depend on a pilot-operated check valve, or counterbalance valve, rather than relying on the directional valve by itself since the directional valves aren't considered genuine load-holding devices. If the seat of the check valve is damaged, worn out, or set incorrectly, it can leak fluid even when properly set, which mimics an issue with the cylinder seal.

This is the reason why diagnostics should always be able to trace the entire circuit—including the cylinder valve, cylinder, or any load-holding checks—prior to identifying an element.

The effects of thermal and compressibility of fluids

Hydraulic fluids are not completely incompressible. Furthermore, temperature fluctuations influence the apparent force of holding on two levels:

1. The thermal contraction of the trapped fluid as the temperature of the system or ambient decreases following shutdown could cause an occasional deficit in volume, which allows the load to rest until pressure is restored.
2. The air that has been drained, whether through the reservoir's low level or suction-side leak or aeration resulting from turbulent returns flow, is extremely compressible in comparison to oil. Even a tiny amount of air volume inside the chamber that is trapped allows the piston to move in a measured manner under load before the pressure is sufficient to impede further movement. This is usually referred to as the sensation of a "spongy" or delayed holding reaction rather than a continuous leak.

Finding the leak's source

A well-planned diagnostic sequence can avoid the needless teardown

1. Isolate the cylinder. Shut off a manual valve (if installed) at the ports of the cylinder under the load. If drift stops, then the leak is downstream of the plumbing or valve; if the leak persists, it's internal to the cylinder.
2. Examine for any external leaks at rod seals, glands, and fittings prior to assuming an internal bypass.
3. Pressure-decay test: Block off the circuit, and monitor the pressure drop as time passes by the port for the gauge. A rapid drop is a sign of leakage inside the circuit, not mechanical creep.
4. Examine the fluid for signs of aeration (milky or foamy look) and confirm the level of reservoir and suction line quality.
5. Check the counterbalance/check valve's setting in line with OEM specifications before replacing the seals on the cylinder.

Preventing recurrence

• Maintain the cleanliness of the fluid in accordance with the ISO code that is required for the valves and pump componentsAbrasive contamination is the most significant cause of bore and seal wear.
• Seal material and operating temperature and type of fluid instead of settling for the generic kits for replacement.
• Re-inspect and re-certify the bore's surface finish at the time of resealing, not just following visible scoring.
• Maintain and size counterbalance valves for any use that has an induced gravity load instead of depending on the valve's directional function for its position.
• It is recommended to bleed air regularly after each maintenance work that can open the circuit. Also, verify the reservoir's levels to prevent suction-side aeration.

Does a cylinder's drift under load normal? Could it be a sign of an issue?

A slight amount of drift is typical in the majority of hydraulic circuits due to internal valve leakage as well as minor thermal effects. This becomes an issue when the drift rates exceed the app's tolerance, for instance, the press platen falling quickly in a matter of minutes or a higher load that settles enough to become visible within minutes.

How do I know whether the leak is in the cylinder or the valve that is not disassembled completely?

Install a shutoff by hand or install existing isolation valves on the ports of the cylinder and shut the valves when under pressure. If the drift ceases and the leak is not visible, it's directly downstream of the cylinder. However, in the event that it persists and continues, the leak is within the cylinder.

Does a brand new seal kit fail upon installation?

Most often, it is due to a damaged or out-of-tolerance bore, which the new seal does not fit into; a contaminated fluid creating damage rapidly; or an incorrect sealing material or orientation during installation. The condition of the bore should be examined prior to replacing new seals.

Why does my cylinder hold fine in cold temperatures but start to drift when running for a few minutes?

This is usually due to the expansion of trapped fluid's thermal properties in conjunction with a slight wear on valves or seals that gets more severe when viscosity decreases at operating temperature. This is because more fluid that is thinner at a higher temperature is more easily able to pass through small leak pathways.

Counterbalance valves negate the requirement for seals on cylinders that are good.

No. Counterbalance valves serve as the primary protection against load-holding in gravity-loaded systems; however, they do not take into account internal bypass of the cylinder. A cylinder that has worn piston seals is still losing holding force, even if it has an operating counterbalance valve because the leak route is on the opposite end of the circuit.