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Hydraulic Cylinder Seals: Know the Basics

Hydraulic Cylinder Seals: Know the Basics

Hydraulic Cylinder Seals: Know the Basics

Reading time: 6 min

Hydraulic cylinders are a key component of hydraulic systems on modern heavy equipment such as excavators, skid steers and tractors.

The cylinders use principles of fluid mechanics to translate hydrostatic pressure into linear force to power heavy equipment machinery and their attachments.

Typically, a hydraulic cylinder is filled with hydraulic fluid or oil. Inside the cylinder, a piston attached to a piston rod is moved back and forth inside the cylinder as fluid is pumped into and out of the cylinder at either side of the piston.

In turn, the moving piston causes the rod to extend and retract, powering the motion of arms, attachments and other mechanical systems on the machine.

Key to the performance of hydraulic cylinders are hydraulic seals, which are fitted around the piston, rod and other contact points inside the cylinder. 

Hydraulic seals keep fluid from leaking out of the cylinder and ensure that excessive hydraulic fluid does not transverse the piston.

Hydraulic Cylinder Seal Types

There are several common hydraulic cylinder seal types:

  • Piston seals fit around the piston. They help minimize clearance between the piston and the internal wall of the cylinder, preventing hydraulic fluid from bypassing the piston. 
  • Rod seals are installed in the head of the cylinder. They prevent hydraulic fluid from leaking out of the cylinder and help lubricate the piston rod. 
  • Buffer seals act as a buffer between the inside of the cylinder and the rod seal. They help dampen pressure fluctuations in the hydraulic fluid, reducing stress on the rod seal.
  • Wiper seals are the most external seal attached to the cylinder head. They help prevent the ingress of dirt and debris into the cylinder and wipe excess lubrication from the piston rod.
  • Guide rings are fitted to the inside of the cylinder head and around the piston. Their purpose is to prevent metal-to-metal contact of the hydraulic cylinder components and guide the moving components of the cylinder.

Maintaining the integrity of seals in your machines’ hydraulic systems is an important maintenance task for heavy equipment fleet operators. Seals that are missing, damaged or improperly installed can cause a loss of power and lead to breakdowns.

What Happens When a Hydraulic Seal Isn’t Installed Properly

It is important to have the seals in your machine’s hydraulic system replaced by a trained professional. Seals that aren’t installed properly can lead to hydraulic fluid leaks, reduced power and rod drift.

Hydraulic cylinder seals are usually made of soft materials that are susceptible to damage. New seals need to be handled and installed carefully, and they must remain sterile.

Seals that become chipped, torn, dirty or cracked should not be used. Using seals that have become damaged during installation is a common cause for hydraulic fluid leaks.

Consequences of a Hydraulic Leak

Hydraulic leaks can be categorized as external or internal. External leaks occur when hydraulic fluid leaks out of the hydraulic system, whereas internal leaks occur when hydraulic fluid leaks between the different parts of the hydraulic system.

The most common cause of internal hydraulic leaks is a damaged or degraded piston seal. Piston seals are responsible for keeping hydraulic fluid from leaking across the piston.

Internal hydraulic leaks do not cause fluid to leak out of your machine, but they do cause a loss of power. Internal hydraulic leaks may also cause rod drift which affects your machine’s ability to hold hydraulic attachments in a static position.

External hydraulic leaks, on the other hand, are typically caused by a damaged or worn rod seal. When the rod seal becomes degraded from debris or heat, it loses its function and hydraulic fluid begins to leak out of the system.

Installing New Hydraulic Cylinder Seals

When seals become degraded or damaged, they need to be replaced. Depending on the type of machine, you may be able to replace the seals yourself. If you don’t feel confident tinkering with your machine’s hydraulic system, then it’s best to enlist the help of a trained specialist.

Selecting the Right Type of Hydraulic Cylinder Seal

When replacing the hydraulic cylinder seals on your heavy equipment, it’s important to choose the correct seal type. Utilizing a seal that does not fit properly or is not made of the right material will increase the probability of leaks and may allow contaminants to enter the hydraulic cylinder.

For best results, only use the size and type of hydraulic cylinder seal identified in your machine’s service manual. Here are some other factors to consider when choosing seals for your machine:

  • Size: Hydraulic cylinder seal sizes are very precise. When choosing a new seal, make sure you choose a seal that is compatible with your particular machine.
  • Material: Certain materials are better suited to heavy duty applications and intense temperatures. Common seal materials include rubber, PTFE and thermoplastic elastomers.
  • Design: Various seals have different functions and particular designs. For example, a wiper seal has a special lip shape, while piston rings have a flat design. Only choose seals with designs compatible to your specific machine and cylinder.

Attaching a New Hydraulic Cylinder Seal

If you have the required tools and experience, installing new seals in a hydraulic cylinder is a straightforward task.

To attach the new seals, start by removing the old seals from the cylinder head and piston. If the seals are stuck, you can pry them loose with a flathead screwdriver or small file. Be careful not to score or scratch the cylinder’s components during this process.

Once the old seals have been removed, check the cylinder head and piston for any debris. If necessary, clean the piston and cylinder head with a damp rag. Be sure all moisture has evaporated from the parts before proceeding.

Next, begin inserting the new seals at the designated slots along the piston and in the cylinder head. Be careful that any seals with a lip, like the wiper seal, are oriented in the right direction. Installing seals in the wrong direction will allow dirt and debris to enter the cylinder.

Once the new seals are securely in place, look them over to make sure they haven’t sustained damage during installation. Any seals that have become chipped or torn cannot be used.

Reattach the piston and cylinder head to the cylinder to complete installation.

How Often Hydraulic Cylinder Seals Need to Be Replaced

Ideally, you should only need to replace your machine’s hydraulic cylinder seals once every three years. If you are keeping your hydraulic fluid clean from contaminants, high quality seals can last even longer.

When seals become damaged prematurely, it’s typically the result of some other problem with your hydraulic system. Contaminated hydraulic fluid, bent piston rods or damaged cylinder heads can cause your machine’s seals to fail early.

Benefits of a Seal Kit

Seal kits are a convenient way to shop for new hydraulic cylinder seals for your machine. Seal kits are typically made for particular machines and cylinder types, so they take the guesswork out of choosing replacement seals for your heavy equipment.

The EquipmentShare Shop is a great source for high quality heavy equipment seal kits. Click below to find the right seal kit for your needs. For more help selecting hydraulic cylinder seals and seal kits, reach out to one of our parts experts and get personalized assistance.


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