Sealing Technology and Material Choices in Hydraulic Systems

Sealing is the key to keeping a hydraulic system running well.Any oil leak from the cylinder or piston,or any dirt getting inside,will shorten the life of the whole system and make it less efficient.

To stop oil from leaking out and dirt from getting in,the industry has created many different seals,techniques,and methods.Each has its own strengths.In some tough jobs,one seal alone may not be enough,so engineers use a full sealing system instead.

Choosing and Using Sealing Systems

A sealing system is usually made of several special seals that work together to give great overall performance.A high-pressure cylinder sealing system normally has four parts:a wiper,a rod seal (or main seal),a buffer seal (or secondary seal),and a guide ring.A piston sealing system is usually just a main seal plus a guide ring.

The four materials used most often in hydraulic seals today are polyurethane(PU),nitrile rubber(NBR),fluoro-rubber(FKM),and PTFE.

How to Pick the Right Material

The choice of material depends on the working conditions.Different chemicals react

differently with each material,and some can handle higher pressure or temperature.The material also has to resist being squeezed out of shape.So the right pick always depends on the exact job.

Here are the most common sealing materials and what they are good at.

1.Polyurethane(PU)

Polyurethane is a strong plastic material with many urethane groups in its chemical structure.It is a type of thermoplastic elastomer.It acts partly like hard plastic and partly like rubber,filling the gap between the two.

Its properties come from three main ingredients:polyol,diisocyanate,and chain extender. The type and amount of each,plus how they react,decide the final performance. Polyurethane usually gives:

·High mechanical strength

·High stretch strength

·Very good wear resistance

·Excellent flexibility

·Stiffness that can be adjusted over a wide range

·Wide hardness range while still staying elastic

·Good resistance to ozone and aging

·Excellent resistance to wear and tearing

·Good resistance to oil and gasoline

Temperature range:-30 to 80C.Special high-performance types can handle up to 110℃ for long periods in mineral oil.

2.Nitrile Rubber(NBR)

NBR is made from butadiene and acrylonitrile.The amount of acrylonitrile (ACN)changes its properties a lot:

·Elasticity

·Low-temperature toughness

·How easily gas passes through

·Compression set

·Resistance to swelling in mineral oil,grease,and fuel

Low-ACN NBR is very flexible at low temperatures(down to about -45°C)but only medium resistance to oil and fuel.High-ACN NBR has the best oil and fuel resistance but may only

stay flexible down to-3℃.As ACN goes up,elasticity drops and compression set gets worse.

NBR is good at:

·Resisting swelling in aliphatic hydrocarbons,grease,HFA/HFB/HFC fire-resistant hydraulic oils,vegetable and animal oils,light fuel,and diesel

·Handling hot water up to 100℃(like in plumbing),mild acids and alkalis

·Medium resistance to high-aromatic fuels

It swells a lot in aromatic hydrocarbons,chlorinated hydrocarbons,HFD fire-resistant oils, esters,polar solvents,and glycol brake fluid.

Temperature range:-40 to 100℃(short time up to 130℃).Special mixes can go down to -55C at low temperatures.Above the limit the material gets hard.

3.Fluoro-Rubber (FKM)

FKM is made by combining vinylidene fluoride (VF)with different amounts of

hexafluoropropylene(HFP),tetrafluoroethylene (TFE),and other ingredients.The mix and fluorine content(65%to 71%)decide how well it resists chemicals and low temperatures.It can be cured with diamines,bisphenols,or organic peroxides.

FKM is known for:

·Excellent high-temperature resistance

·Outstanding resistance to oil,gasoline,hydraulic oil,and hydrocarbon solvents

·Good fire resistance

·Very low gas permeability

·High swelling in polar solvents,ketones,Skydrol-type fire-resistant hydraulic oils,and brake fluid

Temperature range:about -20 to 200℃(short time up to 230℃).Special grades can go from-50 to 200C.

4.Polytetrafluoroethylene(PTFE)

PTFE is made from tetrafluoroethylene.This non-elastic material is special because:

·Its surface is very smooth and stable

·It is non-toxic up to 200℃·Extremely low friction against almost any other surface-static and dynamic friction are almost the same

·Excellent electrical insulation (almost unaffected by frequency,temperature,or weather)

·Better chemical resistance than any other plastic or rubber

·Only attacked by liquid alkali metals and a few fluorine compounds at high temperature

Temperature range:-200℃ to 260℃.Even at very low temperatures it still has some

flexibility,so it is used in many extreme cold applications.Because PTFE is not very elastic and can creep over time,most hydraulic seals combine it with a spring or rubber part to keep the lip tight.

Common Seal Designs in Hydraulic Cylinders

Here are the most common seal types used in hydraulic cylinders.

1.Piston Seals

·Seal between the piston and the cylinder tube-very important for the cylinder to work properly

·Most common design is a lip seal,but O-rings or T-seals are also used

·Must give a tight seal while keeping friction low

·Made from different materials depending on the job

·Needs system pressure to push the lip tight

2.Wipers(Dust Seals)

·Strong scraping action to keep mud,water,dust,and dirt out

·Wipe the oil film back into the system when the rod retracts

·Protect the main seals and make them last longer

·Usually made from high-wear polyurethane

·Often used as grease seals on linkage pins too

3.Rod Seals

·Stop oil from leaking out of the system

·Must work well at both low and high pressure

·Need excellent extrusion resistance and wear resistance

·Should bring the oil film back into the system

·Usually handle pressures up to 31.5 MPa

4.Buffer Seals

·Handle sudden high-pressure shocks

·Protect the rod seal from pressure spikes

·Can release trapped pressure between seals,which makes the rod seal last longer and allows a wider gap without extrusion.Also very wear-resistant.

5.Guide Rings (Wear Bands)

·Stop metal parts inside the cylinder from touching each other

·Keep the piston rod and piston centered

·Help the seals last longer

6.O-Rings

·Most common for static(non-moving)seals

·Seal by being squeezed radially or axially

·Work in both directions

·Can be used as a force provider or as the main seal

·Self-sealing-no extra pressure or speed needed

Sealing technology is the heart of hydraulic system reliability,long life,and efficiency.From picking the right single material to designing a complete multi-part system,every choice must match the exact pressure,temperature,and working conditions.