How temperature affects hydraulic seal performance?

Temperature is among the most crucial variables that affect sealing life for hydraulic seals, directly impacting the hardness of the material and elasticity, as well as compression set and chemical stability. Temperatures that are high accelerate seal material degradation, decrease elasticity, and boost compression set, resulting in leakage. The lower temperatures cause seals to shrink and lose flexibility and cause an insufficient seal and a risk of damage to the extrusion. The selection of the correct elastomer for the range of operating temperatures for the system, typically NBR for -40°C to 100°C, FKM for -20°C up to 200°C, and PTFE for 200°C to 260°C, is essential to ensure leak-free, reliable performance.

What is the significance of temperature when selecting seals?

Systems that generate heat are characterized by mechanical friction, fluid compression, and pressure drops on orifices and valves. This heat, in conjunction with the surrounding environment, creates a temperature range that every seal within the system has to endure. Contrary to metal components, as well as polymer-based seals, are more susceptible to thermal cycling, as their sealing functions depend on the maintenance of a precise elasticity and the stability of their dimensions.

When engineers select seal materials without accounting for the full operating temperature range—including startup cold-soak conditions and peak operating temperatures—premature seal failure becomes almost inevitable, regardless of how well the groove or hardware is designed.

The effects of high temperatures on seals

Set of compression that is speedy

The term "compression set" refers to the seal's inability to return to its original shape following being compressed. Temperatures that are higher speed up the process, causing breakdown of the molecular structure of polymers that is cross-linked. When a seal has a permanent "set," it cannot maintain the pressure necessary to prevent leakage under pressure from the system.

Embrittlement and hardening

Long-term exposure to heat causes a variety of elastomers post-cure hardening. The material becomes more rigid and less pliable. A seal that has hardened is unable to adapt to surface imperfections and follows fluid motion, resulting in irregular sealing and eventually leakage pathways.

Chemical degradation and the oxidation process

Heating accelerates oxidation reactions in rubber compounds, and in particular, NBR (nitrile) seals that are particularly susceptible to breakdown at temperatures over their ceiling ratings. The signs of degradation are usually cracks on the surface, discoloration, or a chalky appearance on the seal's surface. These are clear indicators that the material has surpassed its thermal limit.

Lowered viscosity of fluids

Although it is not an immediate seal effect, the high temperature of high temperatures can thin the hydraulic fluid, which reduces the film that lubricates the seal and its mating surface. This causes friction and wear on seals that are dynamic, such as piston seals or rod seals, causing further thermal degrading that is already taking place in the material used to seal.

The effects of low temperatures on seals

Loss of elasticity

In the lower temperatures, elastomers are approaching the glass-to-glass transition point (Tg)--the temperature that the material transitions from a rubbery, flexible state to a rigid glassy one. At this point sealing seals cease to bounce back and retain the force of their seal, leading to instant leakage at the time of startup.

Shrinkage and less squeeze

The majority of seals contract as temperatures decrease. The shrinkage may decrease the interference (squeeze) between the sealing and the mating surfaces, which can result in gaps that allow for bypass of fluid, which is particularly troublesome in static seals, such as O-rings.

Extrusion risk in cold startups

In contrast to what you might think, cold seals are more prone to damage from extrusion. A stiffer, harder seal is unable to flow through gap extrusions the way that an appropriately conditioned seal will be, which makes it more vulnerable to tearing or slashing on gaps of clearance when system pressure is applied for the first time.

Seal material comparison based on temperature range

The thermal cycle A stressor that is not obvious

Beyond the limit of steady-state temperature, the continuous expansion and contraction when an entire system heats in operation and cools down during idle time causes cumulative fatigue in the seal material. Each cycle creates micro-stresses at the molecular level, slowly decreasing the seal's elastic memory even when either extreme is extreme. Systems that have frequent stop-start cycles or broad day-to-night fluctuations, like the outdoor construction equipment or agricultural machinery, must take into account the resistance to fatigue cycles in the material selection process, not only the temperature range.

Steps to take to prevent sealing failures caused by temperature

1. Map the entire thermal envelope. Record cold-start, steady-state, and peak transient temperatures at the seal's location, not just the temperature of the reservoir for fluid.
2. Match the material to the worst-case. Select seal compounds with the highest predicted temperature, with an adequate safety margin that is not limited to the operating temperature.
3. Consider fluid-seal compatibility jointly. High-temperature-rated seals can still fail prematurely if the hydraulic fluid's additive package is incompatible with the elastomer.
4. Check for warning indicators. Surface hardening, cracking, or a change of seal color in routine inspections usually precedes leakage by a couple of weeks or months.
5. A localized heat source account. Areas near pumps, valves, or restricting orifices are often hotter than the bulk fluid temperature. Seals in these zones might require more powerful materials, even in a moderate temperature system.

Which is the optimal operating temperature for seals on hydraulics?

The majority of standard hydraulic systems that use NBR seals are best suited between -20°C and 90°C. Beyond this range, seal life drops significantly unless a higher-temperature-rated material like FKM or PTFE is used.

Does a seal for hydraulics fail due to temperature alone, without pressure issues?

Yes. The process of compression set, hardening, and shrinkage that is caused through thermal exposure could cause leakage, even in the correct system with a rating of pressure and with no damage to mechanical components.

Do high or low temperatures cause greater seal breakdowns?

The higher temperature can cause greater cumulative degradation over time (compression set and inflammation), while low temperatures typically cause functional failure upon startup due to the loss of elasticity.

What is the effect of temperature on the viscosity of hydraulic fluid and seal performance?

Higher temperatures lower the viscosity of the fluid and cause the thinning of the protective film that lubricates the seal's edge, which creates friction and wear, thereby compounding the thermal degradation already taking place in the material of the seal.

What is the ideal seal material to handle large temperature variations?

FKM and silicone provide a strong performance across a wide temperature range, but silicone's lower abrasion resistance means it is not appropriate for dynamic sealing in comparison to static seals.