FLUID POWER HANDBOOK
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HYDRAULIC ONE
SEALS
of the arguments often used against hydraulic systems is that dreaded word, leakage. But with
proper sealing, leakage won’t be a problem with your system. Sealing is a vital factor in the proper function of hydraulic applications. Leakage from the cylinder or across the piston, along with the ingress of unwanted contaminants, can not only decrease the lifespan of the components, but it will affect the efficiency of the entire application.
MATERIAL OPTIONS Proper material choices will be determined by the seal’s environment. Different types of chemicals react differently to different fluids, while some materials have higher pressure and temperature limits. They also must be able to withstand extrusion, so materials are very application-specific.
Polyurethane
Polyurethane is an organic material whose chemical composition is characterized by a large number of urethane groups. Urethanes belong to the thermoplastic elastomers (TPE) family and close the gap between thermoplastic and elastomeric materials regarding hardness, deforming behavior and consistency. Within certain temperature limits, polyurethane possesses the elastic characteristics of rubber combined with the advantages of a rigid plastic. The composition of the material is determined by three components: polyol, diisocyanate and a chain extender. The type and amount of these materials used, and the reaction conditions, are decisive in determining the properties of the resulting polyurethane material. In general, polyurethanes possess the following properties: … … … … … …
high mechanical, tensile strength good abrasion resistance modulus of elasticity is variable wide range of hardness values, while retaining good elasticity good resistance to ozone and oxygen outstanding resistance to abrasion and tear
Acrylonitrile-Butadiene-Rubber (NBR)
NBR is a polymer of butadiene and acrylonitrile. The acrylonitrile (ACN) component affects the following properties of the NBR: … elasticity … cold flexibility … gas permeability … compression set … swelling resistance in mineral oils, greases and fuels An NBR material with low ACN content has very good cold flexibility (down to approximately –45° C) and moderate resistance to oil and fuel. In contrast, a material with very high ACN content with optimum resistance to oil and fuels, may have a cold temperature flexibility only down to –3° C. With rising ACN content, the elasticity and the gas permeability decrease and the compression set becomes worse. NBR provides: … good resistance to swelling in aliphatic hydrocarbons; greases; fire retardant hydraulic fluids of Groups HFA, HFB and HFC … good resistance to hot water at temperatures up to 100° C (sanitary fittings), inorganic acids and bases at concentrations, and temperatures which are not too high … high swell in aromatic hydrocarbons, chlorinated hydrocarbons, flame retardant hydraulic fluids of the Group HFD, esters Temperature range for use (depending on the composition of the blend): –40 to 100° C and for short periods up to 130° C (the material hardens at higher temperatures). For special blends, the cold flexibility extends down to –55° C.
Fluoro-Rubber (FKM)
Copolymers, terpolymers or tetrapolymers with various compositions and with fluorine contents from 65 to 71%, which have varying resistance to surrounding media and varying cold flexibility.
Temperature range for use: –30 to 80° C; high performance types (compounds) up to 110° C in mineral oils (long-term exposure temperature). T E S Y O F P R E C I S I O N A S S O C I AT E S I M AG E C O U R
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FLUID POWER WORLD
7 • 2020
www.fluidpowerworld.com
