Hydraulic seals prevent leakage and loss of fluids from systems. When seals shrink or harden, they can crack and may lose elasticity, leading to seal failure. This can be caused by:
The use of incompatible hydraulic fluids can cause swelling, and/or shrinkage of the seal or chemical attack that can lead to the failure of lip seals as well as O rings.
The most commonly used material is acrylonitrile or nitrile butadiene rubber (NBR). Relatively inexpensive, NBR exhibits excellent resistance to petroleum-based hydraulic fluids for the temperature range -50°C to 120°C (-60°F to 250°F), but is not resistant to weathering. NBR can be used at temperatures up to 150°C (300°F) but service life will be shortened.
The second most common material is fluorocarbon rubber (FPM/FKM), commonly known under the DuPontTM trade name VITON® *. It can be used over a wider temperature range of -40°C to 200°C (-40°F to 400°F). There are two main classes of Viton materials, type A and type G. Type A is more common and lower cost than type G, but type G offers improved compatibility with most fluids and resistance to both weathering and ozone.
Other materials are used for specialised applications such as thermoplastic polyurethanes (TPU) with bio-hydraulic fluids.
To evaluate the interaction of hydraulic fluids and the sealing materials, it is important to understand the differences between hydraulic fluid base oils.
The majority of base oils used for hydraulic fluids need additives to enhance specific properties. These additives also introduce an additional set of potential interactions with the seal material.
It is possible to predict the swelling or shrinkage behavior of a seal material with reasonably accurate results under normal operating temperature conditions.
Given the wide range of variables, it is wise to seek guidance from the manufacturers when selecting seal materials for either a new build or maintenance replacement.
It is particularly important to consider the likely impact of any change in hydraulic fluid on the seals already in use before proceeding with a replacement.
For API Group I based fluids operating at temperatures below 100°C (212°F), NBR seals are the most cost-effective choice. Fluoroelastomers (FKM) increase the maximum operating temperature to 200°C (400°F), which is well above the recommended range for ordinary fluids, whilst only slightly reducing low temperature performance. All the other materials exist to fill application niches not covered by NBR or FKM materials.
For applications currently using NBR or FKM seals that are not exhibiting unusual failure modes or unexpectedly short service lives, it is unlikely that changing to a higher performance – and usually more expensive – material will reduce operating or maintenance costs enough to offset the cost of the premium seals.
For new designs that operate with the same fluids in the same temperature ranges as existing systems, the same seal materials will probably work in the new system too. For new systems that don’t duplicate existing designs, either NBR or FKM is a logical elastomer to start with, unless there is something very unusual about the system or known issues with the fluid that will be used.
For more information on hydraulic fluids, elastomer or seal performance, contact your Lubrizol representative.
* Viton is a registered trademark of the E.I. du Pont de Nemours and Company
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