Incorrect hydraulic seal selection is a major cause of seal failure. The following points should be kept in mind when selecting a seal, for either a new application or as a replacement seal:

●Select a seal that is capable of operating within your desired temperature range.

●Select a seal that will operate comfortably at your desired pressure and speed.

●Select a seal that will operate in your system fluid. All of our standard seals are designed  for operation in mineral based hydraulic fluids. If you are not using a mineral based hydraulic fluid, please consult our technical staff for a recommendation regarding your application.

●Keep in mind that designs which incorporate anti-extrusion devices will allow larger metal to metal clearances. These types of seals lend themselves to use with steel heads and pistons using separate or incorporated nylon wear rings. This will aid in keeping the machining times to a minimum, and hence reduce overall costs.

●At higher pressures, it is generally desirable to select a seal with a larger radial cross-section. You must however keep in mind any relevant space restrictions or fitting problems, such as blind glands.

●Asymmetrical seal designs are generally superior in performance to symmetrical ones. However, when interchanging seals you must be careful to select an appropriate design for your application.

REPLACEMENT SEALS

As several different manufacturers may produce seals to fit the same standard groove size, it is not uncommon for seals of the same nominal size & design to vary greatly, both appearance and in actual dimensions. It must be stressed that all dimensions specified will refer to the housing, not the actual seal itself. When replacing old seals it is far better to measure the actual metal parts,than to attempt determining the seal's dimensions. This is particularly helpful when trying to determine if the seal is metric or imperial.

Seal Installation Care should be taken when fitting and assembling the selected seals and the following general points should always be kept in mind. Ensure all metal parts are clean and free from contamination, burrs and sharp edges. Clean all seal housings.  Where seals pass over threads or ports, some form of protective sleeve should be used. Check that the correct seal is being used. When fitting single acting seals, ensure that the pressure side of the seal is installed correctly.  All parts, including the seal, should be well lubricated with either the operating fluid or compatible fitting grease during installation

NORMAL OPERATING CONDITIONS

PRESSURE

The prime consideration of pressure and how it affects a specific seal is centered on both the extrusion resistance of the seal material, and the extrusion gap which the seal must bridge without extruding. If the extrusion gap is too large at a given pressure, the seal will flow into it, gradually nibbling away the heel of the seal. As the pressure increases either the extrusion gap must be reduced, increasing machining costs, or an alternative seal design must be looked at. By fitting a seal with pressure activated anti-extrusion devices such as in Figure, a larger extrusion gap may be maintained.

TEMPERATURE

The effects of temperature within a hydraulic system are many and varied. If the temperature is too low the seal will become brittle, and if the temperature is too high the seal will take a compression set more readily. Higher temperatures generally make the seal more susceptible to extrusion, and also affect the viscosity of the sealing media, increasing friction and reducing seal life.

SPEED 

Speed relates to the friction generated at the sealing surface and as such affects a seals performance. At high speeds the oil film between the sealing surfaces and the seal itself may break down, causing the seal to run dry.

SURFACE FINISH

The working life of a hydraulic seal is greatly influenced by the properties of the sealing surfaces against which it operates. Surface values can only be determined by a comparison of average surface roughness (Ra). The aim of all surface finishes is to provide a surface which causes the least wear to the seal. Rod Seals, which seal against moving surfaces, can be damaged by fine abrasive particles which may adhere to a rough surface.Therefore Rods should have a low surface roughness value, and a surface similar to hard chrome.The ideal surface roughness for rods lies between 0.16µm (6.4µin) and 0.40µm (16µin) Ra. Piston seals seal against the inner surface of a cylinder. They are not affected to the same extent by abrasive particles entering from the atmosphere, and can therefore have a rougher surface. The ideal surface roughness for bores lies between 0.25µm (10µin) and 0.63µm (25µin) Ra. The surface finish for seal housings, where the seal is in static operation, should be about 16µm (63µin) Ra. How These Properties Affect Each Other From many years of experience with sealing hydraulic equipment, we know that it is necessary to link the three main operating conditions of pressure, temperature and speed to achieve optimum seal performance. After carefully considering each product, we have been able to specify a maximum pressure and speed with a temperature range within which the seal will operate safely. If either the pressure or the speed is significantly lower than the stated maximums, it may be possible for the other features value to exceed its stated maximum without damaging the seal. However, it must be noted also that pressure is closely linked with extrusion, and those pressures stated within this catalogue assume that specified extrusion gaps have been conformed with. Please consult our technical department for these values. The operating pressure of the system should also be taken into account when evaluating surface values. At higher operating pressures, the oil film between the seal and the sealing surface is thinner, resulting in greater friction. Under such conditions a surface roughness approaching the lower values given should be chosen. More info:fdscn.

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