Are polyurethane hydraulic seals better than rubber seals?

Polyurethane seals typically outperform rubber seals when it comes to the areas of abrasion resistance and extrusion resistance as well as load-bearing capability, which makes them the best option for high-pressure, hydraulic systems that are high-cycle. However, sealing with rubber (particularly NBR and FKM) provides better flexibility at low temperatures and a faster recovery of elastic and is less expensive, which makes them the ideal option for systems with lower pressure as well as cold-climate or designs that are budget-conscious. The two materials are not necessarily "better"—the best choice is based on the pressure, temperature cycle speed, and compatibility with fluids.

The choice of hydraulic seals can be one of the choices that appear small on a list of materials but has an enormous impact on system reliability. The choice between rubber and polyurethane is usually the first decision to cross, but making the wrong choice could lead to premature seal failure, expensive downtime, or spending on a product that doesn't meet the requirements. This guide explains how these two types of materials differ in the aspects that are crucial in the real-world hydraulic system.

Understanding the two families of materials

What makes a seal "rubber"

For hydraulics, "rubber" is a general term used to refer to elastomers such as NBR (NBR) and fluorocarbon (FKM/Viton) or hydrogenated NBR (HNBR). These materials are admired for their elasticity, rapid recovery following compression, and the long-running track record of their use in the field of fluid power systems. NBR is, in particular, the most commonly used O-ring material and seal in hydraulics for years since it is able to balance costs along with oil compatibility and temperature range quite well.

What are the characteristics of a sealable polyurethane?

Polyurethane (PU) is technically an elastomer. However, its molecular structure provides it with distinct performance characteristics. It's a thermoset plastic known for its exceptional mechanical strength—high tension strength and excellent tear resistance—as well as outstanding abrasion resistance. Seals made of polyurethane are used for piston seals, rod seals, and wiper seals on cylinders subject to extreme pressure, a high load on the side, or in areas that are contaminated.

Comparison of head-to-head

Abrasion and wear resistance

This is why polyurethane has an advantage. Its abrasion resistance is greater than the majority of rubber compounds. This is important specifically for rod seals and wiper seals that are exposed to dirt dust and debris in construction equipment or mobile devices. For high-cycle applications this can lead to a longer seal life and fewer unexpected cylinder rebuilds.

Extrusion resistance and pressure

The higher toughness and tensile strength lets it withstand extrusion into the clearance between mating components much better than conventional rubber compounds. This is what makes PU the ideal choice for high-pressure systems (above approximately 3,000-4,000 pounds) or in situations with greater-than-ideal machining tolerances. Extrusion is a typical failure mode for rubber seals.

The temperature range and cold flexibility

The rubber compounds, particularly NBR and specialized nitriles with low temperatures, tend to be more flexible at lower temperatures than regular polyurethane. Between -20°F and 40°F, some polyurethane formulations get stiffer and lose their sealing power while some compounds are still able to perform. When it comes to systems that operate in colder climates or without heating outdoor equipment This is a significant aspect that is directly connected to the broader design and construction of cold-climate hydraulic systems.

At the extreme temperature, FKM rubber compounds often outperform conventional polyurethane and can tolerate continuously exposed temperatures of up to 400°F in certain formulations, whereas the majority of polyurethanes can only be used for around 200-250°F before their mechanical properties begin to degrade.

Fluid and chemical compatibility

The rubber compounds have a long history of evidence-based compatibility information across mineral oil, water-glycol liquids, and a range of synthetic and biodegradable fluids. It is typically compatible with mineral hydraulic fluids; however, it may be particularly sensitive to moisture content, some synthetic esters, and high-humidity environments in which hydrolysis may alter the quality of the material as time passes. If the system you choose to use is water-based or biodegradable hydraulic fluid, compatibility must be checked prior to converting to polyurethane.

Set of compression and elastic recovery

Rubber, in particular with a well-formulated NBR as well as FKM compounds, is able to recover its form faster after compression than polyurethane, which can be beneficial in dynamic seals that are frequently cycled even at low pressures. The compression set resistance of polyurethane is generally high, but it can be affected more by the formulation as well as its hardness (Shore A/D ratings) than similar rubber compounds.

Cost

The rubber seals are usually more affordable to produce and procure, particularly in the standard sizes of O-rings and sizes, which makes them the most popular option when the performance requirements do not need polyurethane's added durability. Polyurethane seals can be more costly upfront, but they often provide an overall lower cost of ownership for demanding applications by extending intervals of service.

Which material is the winner?

Polyurethane is generally the best option for:

• Seals for wipers and rod seals on construction and mobile equipment exposed to abrasive and dirt-laden contamination
• High-pressure cylinder applications in which the risk of extrusion is increased
• Systems with side-loading and off-axis Cylinder forces
• Applications that prioritize seal longevity over initial cost

Rubber is generally the best choice for:

• Cold-climate or outdoor equipment operating in sub-zero temperatures
• Systems that use water-based, water-glycol, or biodegradable substances
• Seals with lower pressure, high frequency, and low frequency in situations where speedy recovery is important.
• Standard O-rings and cost-sensitive designs as well as standard applications
• High-temperature systems in which FKM's heat tolerance is needed

Making the final decision

The most effective seal specification choices aren't based on "which material is better." The decision is made based on the operating envelope, which includes maximum system pressure and ambient and fluid temperature ranges as well as the exposure to contamination, kind of fluid, and frequency of cycles. A cylinder mounted on an excavator's boom operating in cold temperatures with mineral oil is a different seal material that is ideal for the hydraulic press that is operating at 5,000 psi inside an industrial facility controlled by climate. Making sure the seal material is in line with the full operational picture instead of settling for the material that is most familiar to you is what prolongs the life of the service and decreases failures in the field.

It's also important to know that a lot of modern design cylinders use both of these materials: polyurethane to form the rod seal, or wiper, for the external environment and are paired with O-rings made of rubber for back-up or static seals in less demanding locations. This type of hybrid design often provides superior performance overall compared to committing to a single material in all seals.

1. Are polyurethane seals more dependable than rubber seals?

In the majority of abrasive, high-pressure, or high-cycle processes Yes, polyurethane's higher abrasion and extrusion resistance generally means it has an extended service life than conventional rubber compounds in the circumstances.

2. Polyurethane seals can be used in cold-weather applications.

Polyurethane that is used in the standard formulation can be stiffened and lose sealing properties when temperatures are sub-zero, which is why cold-climate systems typically use special low-temperature rubber products or cold-rated polyurethane formulas.

3. Are polyurethane seals compatible using biodegradable hydraulic fluids?

Compatibility is dependent on the formulation and biodegradability as well as water-based liquids, which can trigger degradation of polyurethane due to hydrolysis, and therefore fluid compatibility should always be confirmed by comparing the seal's specifications of the manufacturer.

4. Why are seals made of rubber being used when polyurethane is more effective in certain areas?

Seals made of rubber remain popular due to their lower cost and recover their elasticity more quickly when compressed and work best in temperatures that are cold as well as certain fluid conditions where polyurethane is not as well-suited.

5. Are rubber seals and polyurethane incorporated into the same cylinder?

Yes, most models of cylinders combine a polyurethane primary seal (or wiper) with O-rings to provide permanent or backup sealing, which combines the abrasion resistance of PU and the efficiency of cost and performance of rubber.