Understanding the Function of Seals in Hydraulic Cylinders

Hydraulic systems have many different components, all of which must work in unison so that the system functions flawlessly. Among these components are the many configurations of seals that ensure optimum pressure, protect the hydraulic cylinder’s parts, and perform other critical functions. 

Before we discuss hydraulic seals, let’s look at hydraulic system basics, including hydraulic cylinder components.

Hydraulic Systems and How They Work

Hydraulic systems typically consist of a piston-and-rod assembly in a cylinder. Hydraulic fluid is introduced under pressure into the hydraulic cylinder, exerting pressure on the piston and rod, thus creating linear force and motion. You find hydraulic cylinders in both large and small applications. Some examples include:

• Heavy equipment—The hydraulic cylinders on these pieces of equipment require a great deal of pressure to raise and lower forks and maintain them in position with a load, or to raise, lower, and rotate the blade of a tractor. 
• Dump trucks—the hydraulic systems that raise and tilt the bed of a dump truck to dump the load must be able to move tons of weight.
• Injection molding or extrusion—Hydraulic cylinders in injection molding require consistent pressure to inject molten plastic or metal into a mold or extrude it through a die.

Some industrial processes use a single hydraulic cylinder. Others require two or more, working together to move a heavy load. As an example, a bulldozer typically uses two cylinders to raise and lower the blade.

Single- and Double-Acting Systems

There are also both single- and double-acting hydraulic cylinders within a system. 

• Single-acting—In a single-acting system, the hydraulic fluid acts on one side of the system, creating the pressure that moves the piston and creates that linear movement. The piston returns by the weight of the load or a spring assembly. A floor jack in an automotive garage is an excellent example of a single-acting hydraulic system. Turn the dial, and the car's weight pushes the pack down. 
• Double-acting–Double-acting systems can introduce  fluid on both sides of the piston, allowing it to operate in both directions. This allows the operator to extend the rod and return it to its original position in a controlled manner. A lift that raises a lineman to work on an electrical pole is a good example of a double-acting system.

Why are Hydraulic Seals Important?

The hydraulic cylinder features a round, bored space in which the piston moves back and forth, creating the force. Under pressure, the hydraulic fluid will try to escape through the minute space between the piston and the cylinder wall (also called the bore). If this happens, you lose fluid and pressure and risk contaminants entering the system. 

The hydraulic seal is a ring that fits into a machined groove in the cylinder. The seal presses against the bore, creating a barrier that prevents fluid from escaping, thus maintaining the pressure in the hydraulic cylinder and allowing it to do its job.

Seals Work as a System in the Hydraulic Cylinder

Hydraulic seals include two broad categories. 

• Static seals–These seals are used in applications where the parts meet, but they don’t move against each other, such as cylinder barrel end-caps. The seal prevents the pressurized fluid from leaking through the space between the two components. 
• Dynamic seals–You find dynamic seals in applications involving movement between two components, such as the piston and the bore. 

While both types of seals are essential, the movement of components means dynamic seals receive the most significant forces, so they are more likely to wear out.

Piston seals

These components create the seal between the piston and the bore, preventing the escape of the hydraulic fluid. Piston seals are single-acting or double-acting, depending on whether the hydraulic cylinder is single-acting or double-acting. It’s vital in an application that calls for a single-acting piston to ensure the seal ring is facing the correct way, or fluid and pressure will pass over it. In a single-acting application, there also may be two seals. The primary seal is the most formidable and does most of the work. The secondary seal is a backup measure.

Guide rings

In addition to piston seals there are guide rings, also known as wear rings. The guide rings fit on the piston on either side of the piston seal. The purpose of the wear rings is to guide the piston as it travels back and forth on the cylinder, preventing metal-to-metal contact. Depending on the application, there might be sideways or transverse loads on the rod. The guide ring balances the force of this load and keeps the rod and the piston centered.

The Rod Sealing System

The remaining seals form a three-part system within the hydraulic cylinder assembly where the rod passes through as it creates linear force. The combination of these seals prevents pressure and lubricant from escaping past the rod and also keeps contaminants from entering the system.

Buffer seals

Buffer seals are the first line of defense as the rod converts the linear force. They even out the pressure inside the hydraulic cylinder and prevent large contaminants, such as metal flakes, from damaging the rod or the rod seals.

Rod seals

Next are rod seals. These seals conform snugly to the rod profile, creating a tight seal that prevents internal pressure from escaping and external contaminants from entering the cylinder.

Wiper seals

These seals are also known as scrapers. A single-action wiper seal wipes the rod of dirt and contaminants as it retracts back into the cylinder. A double-action wiper seal also prevents residual hydraulic fluid from leaking out of the system.

Hydraulic Seal Materials

The material used in your hydraulic seal application depends on several different factors. For example, for cylinders operating at high temperatures, seals made from materials like Viton are used to prevent melting. Conversely, cylinders in extremely cold conditions require seals made from materials like polyurethane to avoid hardening and cracking. You can locate the specifications for the seals in the documentation that came with your hydraulic system. If not, your selection should include these considerations:

• Pressure– What kind of pressure is needed to accomplish your goal? In a low-pressure, low-temperature application, a rubber seal might be completely appropriate. 
• Hardware design– Different materials and designs interact in different ways. 
• Velocity and temperature–Increased speed means more friction, which leads to materials breaking down more quickly. 
• Media and lubrication– What kind of hydraulic fluid does your system use? Some fluids can degrade seal material.
• Running conditions– Is the environment dirty, with much contamination and dust, or are things pretty clean?

If you need to get new hydraulic seals, make sure you have answers to these questions when you talk to an expert about replacements.

Friction and Heat, and Wear on Seals

Any discussion of hydraulic cylinders should also address friction and heat. When you rub your hands together, the resistance of one hand against another causes friction. Friction causes heat. When two components rub together, such as the hydraulic seal and the cylinder wall or bore in your system, it also causes heat, and heat accelerates wear.

Ideally, the seals and other hydraulic cylinder parts work in unison to minimize heat. If you encounter excessive heat in your hydraulic system, it could be because of one or a combination of the following reasons:

• Adverse working conditions–Is there a lot of dust and dirt in the area? Is there a lot of ambient heat?
• Contamination–One of the functions of hydraulic fluid is to absorb and carry away heat. Contamination can prevent fluid from flowing smoothly and consistently. Overheating quickly follows. 
• Insufficient guidance or hardware–Everything wears out over time, including guide rings. Transverse forces, or wear in other components, can cause overheating. 
• Quality materials–Always use the highest quality parts when servicing your hydraulic system.

Tips on Hydraulic System Maintenance

Like any industrial or manufacturing system, hydraulic cylinders require regular maintenance. Be sure to check the documentation that comes with your system. The following are the most common tasks that need to be done. 

• Check fluid levels–This is especially pertinent if you see leaks. 
• Burned fluid–Discoloration often indicates burned fluids and overheating in your hydraulic system. At the very least, you should change the fluids and look for other signs of overheating. 
• Inspect all hoses, lines, and fixtures. Replace anything that looks suspect. 
• Change out all filters–It’s essential to keep the system clean.
• Worn parts–Look for anything with excessive wear, including cracked external seals, hoses, corrosion, or rust. 
• Operating pressures–Check for proper operating pressures if your system allows it. 
• Records—Maintain maintenance records. If you encounter a problem and have to call a repair service, your records will help you see what has already been done.

Rely on the Experts

CRC Distributors is your source for hydraulic cylinder parts. CRC covers virtually every industry, including oil and gas, heavy equipment, marine, hydroelectric, and more. We also machine custom seals in-house. CRC Distribution is ISO 9001 certified. 

Visit our website today, or call us at (866) 916-3998.