How do I maintain a hydraulic cylinder?

Hydraulic cylinder maintenance is a combination of clean fluid and proper viscosity, examining the rod's condition and seal for scoring, inspecting mounting hardware and alignment, looking for external leaks and the possibility of drift under load, and a regularly schedule inspection instead of inspection in response to a failure. A cylinder that's been cleaned and protected and regularly serviced will see longer service than one that's only looked at when it begins to malfunction.

While they can seem simple in principle (a piston sliding within a cylinder with pressure on one end), hydraulic cylinders are incredibly sensitive to the cleanliness of the fluid, alignment, and exposure. Many of the reasons a hydraulic cylinder will fail prematurely are caused by preventable issues such as dirty fluid, scored pistons, worn or faulty seals, and improper mounting. Developing a maintenance schedule around checking and inspecting a cylinder can mitigate many of these causes.

Why do maintenance on a hydraulic cylinder?

A hydraulic cylinder sees pressure applied repeatedly and is generally mounted in a manner to allow the piston to extend and retract while resisting lateral force. On every stroke, the rod extends and retracts through the cylinder's rod-end gland fittings, where it is exposed to the environment. On retraction, any debris, water, or particles that have adhered to the rod surface are pushed back in past the rod seals and into the cylinder. Over time and thousands of strokes, this can abrade the seals and score the rod, leading to loss of pressure, internal leaks, and ultimately external leaks.

In addition to loss of pressure and the risk of damage to the cylinder, unclean fluid will also cause excessive wear in other hydraulic components. In addition to checking the cylinder, it's crucial to ensure that the fluid is clean and at the correct viscosity.

Daily and weekly maintenance

Rod surface

The exposed rod surface should be visually inspected for any sign of pitting, scoring, or corrosion. These issues can often lead to a buildup of dirt or particles that will get forced back into the cylinder with every extension, accelerating internal wear. Wipe the rod surface clean at the end of every day or during every shutdown in dirty or dusty environments.

External leaks

The rod and fittings should be visually inspected for signs of fluid loss. Leaks or weeping at the gland may indicate a faulty rod seal or wiper. A small amount of fluid on the rod at the point of the gland is the first sign of seal wear. This presents a much less expensive repair option than a leak that has worn a significant portion of the rod or gland.

Drift

When the cylinder is holding a load, observe the cylinder to see if the rod slowly extends or retracts. This can be a sign of internal leakage due to excessive piston seal wear. While this may be seen as a complete system failure, a simple seal replacement will generally fix this issue.

Mounting and installation

Mounting pins, bushings, and trunnions should be examined for looseness or unusual wear. Any movement in the mount or misalignment of the mounting hardware or trunnion will cause increased side loading on the rod and cylinder. This can accelerate seal wear many times beyond normal due to the increased stress on the seals from the lateral force. This is most evident on cylinders mounted with mounting hardware that apply a side load to the piston, such as a clevis mount.

Maintenance on a hydraulic cylinder

Fluid and cleanliness

The fluid should be sampled and tested at least monthly for cleanliness in accordance with the system's cleanliness standard. Particles are the leading culprit in accelerated cylinder wear because they act as an abrasive and reduce the life of the seal lips and cylinder bore. Filters should be serviced in accordance with the manufacturer's recommendation, and fluid should not be added directly from an open container that lacks an inline filter.

Seals and wipers

If drift or other seal wear is observed during other maintenance practices, it may be necessary to investigate further. Each cylinder has a recommended maintenance schedule based on the duty cycle and other factors. If the rod seals, piston seals, and wipers are being inspected during other maintenance, it's an opportune time to check their condition as well. Seal kits can be replaced during any other maintenance so scheduling them every 4,000 hours or according to the manufacturer's recommendation can eliminate unplanned downtime.

Cushioned cylinders should be inspected for excessive cushioning wear. If a cylinder is exhibiting roughness on end-of-stroke deceleration, it indicates that the cylinder's cushioning mechanism is worn at some point in the system.

Alignment

Alignment of the cylinder mounting to the load on the cylinder should be checked every quarter. If the cylinder is misaligned, the seals will experience additional wear across the seal lip surface. In extreme cases, a misaligned cylinder will cause a bent rod over time.

Environmental concerns should also be taken into consideration when determining the maintenance schedule for a cylinder. Any machine that's stationary for long periods between uses should generally have the rod retracted as far as possible to reduce the exposed surface area. Cylinders in dusty environments may require additional attention to rod seals and wiper maintenance. A light protective coating on the exposed rod surface of a machine that's been stored for a long period of time can prevent corrosion that can negatively impact the wiper or rod seals upon the next use.

Developing a maintenance schedule

A maintenance schedule can be developed based on the above information, using daily and weekly checklists to track the fluid, rod condition, and leaks, with monthly and quarterly service inspections to check the seal condition, mounting hardware, and fluid cleanliness. An annual major service or inspection can be performed to replace hardware and other wear surfaces as needed. Having a schedule and sticking to it helps ensure extended life and minimal downtime in any cylinder, regardless of duty cycle, application, or use. Keeping these practices in a computerized maintenance system also allows for tracking trends in maintenance and determining which cylinders begin to require more frequent service, requiring a closer inspection or altered maintenance schedule.

Overall, maintenance of a hydraulic cylinder is about controlling contamination, observing wear, and ensuring mounting practices are appropriate for the application. While none of the above checks are particularly complex, it is this combination that allows cylinders to see extended service and reduces total cost of ownership for a large number of machines.

How often should cylinder seals be replaced?

There is no hard and fast rule about how often cylinder seals should be replaced, as it's generally determined by the duty cycle of the machine. Many companies find that it's good practice to inspect seals every 2,000 operating hours and replace them as needed. A thorough inspection of the rod, bore, and seals should always happen when wear, leakage, or drift is suspected.

What causes a hydraulic cylinder to drift when holding a load?

A hydraulic cylinder should hold pressure when a load is applied. If the cylinder begins to slowly retract or extend, it's generally a sign of internal leakage in the system. This can occur in the cylinder if there is excessive wear on the piston seal or elsewhere. Isolating the cylinder from the rest of the system can help determine the exact cause.

Can I use any type of hydraulic fluid in a cylinder?

No. The correct hydraulic fluid should be used depending on the application. Using the wrong fluid or one that hasn't been adequately filtered can cause seal failure or excessive component wear. The correct fluid should be used and only added from a properly filtered, sealed source.

What are the differences between a hydraulic cylinder's rod seals and wipers?

The rod seals' primary function is to ensure that the fluid inside the cylinder does not leak out when the cylinder is in motion. The wiper or scraper, on the other hand, is there to remove any contaminants such as moisture or particulates from the rod on retraction so that they don't get in the seal area and cause failure.

When should a hydraulic cylinder be rebuilt instead of getting a new seal kit?

If a cylinder is exhibiting signs of excessive internal leakage, drift, or visible damage to the rod surface or bore, it should be rebuilt or have its seals replaced. If the cylinder's internal surfaces have been worn down beyond their tolerances specified by the manufacturer, a simple seal kit will not fix it.