The main purpose of the seal is to avoid the leakage of the hydraulic oil from the cylinder weather the piston is in moving condition or not.

Incorrect 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.

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.

The housing has been correctly machined. The parts haven’t been modified. The seal is correct for its housing
Previously replaced with an incorrect part. Assurance of the correct replacement seal. Throughout this catalogue there are recommendations to change seal designs where we believe we may stock a superior design of seal in specified applications. However, it must be pointed out that the recommended interchanges are superior in only those applications which have been specified, as the original seal may be suitable for use in a number of other applications as well.

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

·         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.

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 seal’s performance. At high speeds the oil film between the sealing surfaces and the seal itself may break down, causing the seal to run dry.

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. Rods therefore 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.

·         Standard Loaded Lip Seal
Loaded Lip Seals are very versatile and can be used as rod seals or piston seals.  They are considered to be multi-purpose hydraulic seals and are designed for significantly improved performance.  American Seal & Packing Loaded Lip Seals are molded from 92 Durometer Urethane with a 70 Durometer Nitrile energizer.  The energizer ensures constant pressure on the sealing lips.

               Temperature:  -65˚F to +300˚F
Pressure:  0 to 5000 psi
Fluids:  Petroleum based Hydraulic Fluids

·         Standard Loaded Lip Seal
The Standard Loaded Lip Seal is the most widely used design of Loaded Lip Seal.  By energizing the sealing lips with an endless O-ring, you are assured of uniform, positive lip contact and excellent low-pressure sealing.  The seal depth is equal to the radial width, which results in a square design.  Standard Loaded Lip Seals can be widely used to interchange with existing hydraulic packing and/or O-rings without changing dimensions.

·         Loaded Lip Seal Type “B”
The Loaded Lip Seal Type “B” features a back-beveled sealing lip.  This provides greater film breaking and increased unit loading at the sealing surface.  The seal depth is generally 1 ½ times the radial width to ensure hydraulic seal stability in the most rugged applications.  It is most commonly used as a rod seal.

·         Loaded Lip Seal-Deep Style
The Loaded Lip Seal-Deep Style incorporates the same unique design as the Sthard Loaded Lip Seal, plus its depth is 1 ½ times greater than the radial width.  This increases seal stability to withstand rugged, high shock, heavy-duty applications.  This seal will replace many kinds of Vee Packing.  When minimum drag is needed, use Standard Loaded Lip Seals or Loaded Lip Seals-Deep Style.

Asymmetric piston seal for standard applications. Design provides stable fit in the housing, ultimate sealing effect over a wide temperature range. Avoids extensive drag pressure. Back to back arrangement with guide rings in between or for double acting pistons or to separate different fluids.

Hydraulic, Single Acting
O-ring activated asymmetrical piston seal. Interference fit on inside diameter maintains stable fit in the housing. Design provides ultimate sealing effect. Especially suitable for short stroke applications (e.g. spindle- seals, coupling actuators). Pneumatic, Single Acting

Asymmetric piston seal, extremely wear resistant for use in lubricated or dry pneumatic applications. Special design of sealing lip allows retention of initial lubricating film. 
Hydraulic, Single/Double Acting
O-ring activated symmetric PTFE piston seal, low friction. For extreme low or high speed, suitable for positioning functions. For mobile hydraulics, machine tools, injection molding machines, heavy hydraulics.

Reference: -
•           www.wikipedia.com               


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