Frequently Asked Questions

QUESTION 1: Can you give me the general characteristics and properties of commonly used seals materials?

This elastomer is a copolymer of ethylene and propylene and is sometimes compounded with a third monomer (EPT). Good to excellent compression set resistance is obtained by the addition of peroxide cures during vulcanisation. Ethylene propylene materials have excellent resistance to phosphate esters such as Skydrol, Fyrquel, Pydraul, water and steam, acids, alkali, salt solutions, ketones, alcohol's, glycol's, and silicone oils and greases. EPR has very poor resistance to petroleum oils and diester-base lubricants. Ethylene propylene is a close contender to Buna-N and butyl in the important sealing properties, except that it does not have the petroleum oil and fuel resistance of Buna-N, nor the low-gas-permeability quality of butyl.

NITRILE OR BUNA-N RUBBER: More nitrile seals are used than all the other elastomers combined, since nitrile is the most versatile material. Nitriles are a copolymer of acrylonitrile and butadiene. As the acrylonitrile content of nitriles increases, the oil and fuel resistance increases while the low-temperature flexibility decreases. Nitrile-based elastomers are usually specified by military MS and AN O-rings when used in oil and fuel applications, but because nitrile compounds vary widely within such a large overall temperature range, particular attention should be paid to specifying physical properties. Materials can be formulated to perform satisfactorily over the temperature range 65 to +300 degrees F, so it is necessary to make sure that the particular nitrile chosen meets the temperature requirements of the application. The nitrile materials are recommended for general-purpose sealing of alkaline and salt solutions, petroleum oils and fluids, vegetable and diester oils, silicone greases and oils, ethylene glycol-base fluids, alcohol's, gasoline's and water. They are not suited for use with strong oxidising agents; chlorinated solvents such as carbon tetrachloride or trichlorethylene, nitrated hydro-carbons such as nitrobenzene or aniline; keytones such as methyl ethyl ketone (MEK) and acetone; and aromatic hydrocarbons. Ozone will will usually attack nitrile materials, but resistance can be greatly improved by the addition of antioxidant compounds.

FLUOROCARBON RUBBER (FKM): Fluorocarbon elastomers have been compounded to meet a wide range of chemical and physical requirements. Under the trade name Viton®, Fluorel, and Kel-F and fluorocarbons seals have been employed where other materials cannot survive severe chemical conditions. The working temperature range of FKM is between -20 and +400 degrees F (-29 and +204 degrees C) and limited temperature spikes of 600 degrees F have been incurred. New compoundings have greatly improved the compression set of fluorocarbon O-ring seals.

SILICONE RUBBER (Si): Silicone elastomers are compounded from dimethy silicone polymers, and thus will deteriorate if used with silicone oils and greases. Various additives have extended the functional temperature range of silicone rubber beyond any other elastomer. Flexibility below -175 degrees F (-114 degrees C) and service above 700 degrees F (371 degrees C) for short periods of time have been demonstrated. High production cost have normally limited the use of silicone seals to applications requiring extreme temperature resistance. Production moulding of silicone seals involves high-temperature secondary cure which results in greater than normal shrinkage. The finished O-ring seal is usually undersized when produced in standard moulds. The designer should be aware of this size difference when designing glands for silicone O-rings. Silicone elastomer have poor resistance to keytone solvents such as MEK and acetone, and poor resistance to most petroleum fluids. They have very poor physical properties that make them unattractive for dynamic applications. Silicone seals are recommended for extreme temperature use with ozone, oxygen, high-aniline point oils, and chlorinated diphenyls.

POLYURETHANE (AU, EU): Polyurethane elastomers are compounds of polyethers and diisocryanates. These materials have excellent physical properties of abrasion resistance and tensile strength, which make them outstanding for dynamic applications. They have excellent resistance to weather, ozone, and oxygen, good resistance to hydrocarbon fuels, petroleum oils, and aliphatic solvents and fair resistance to aromatic hydrocarbons. Acids, keytones, and chlorinated hydrocarbons attack and deteriorate polyurethane. Because polyurethane is available in castable liquids, injection-mouldable pellets and millable gums, it is a very useful material for specialised sealing problems.

In any high pressure seal, the tendency for the fluid pressure to extrude the seal into the clearance gap is a major cause of failure. In seals for reciprocating applications, friction between the seal and the wall against which it moves may either augment this effect or oppose it. These circumstances are usually called pump condition and motor condition respectively, since they are found in piston seals on these machines.

Seal performance is likely to be better in motor condition. Often, it is possible at the design stage to arrange for seals to be working in this condition. If so, this should certainly be done, but remember that if it results in the seal having to operate within a bore, this cannot usually be given such a high finish, or be made to such close tolerance, as an external surface.

The condition of lubrication between the seal and the surface against which it slides vary from boundary to thick film lubrication, depending on the viscosity of the fluid, sliding speed and the pressure. With film lubrication normally arising from a combination of viscous fluid, high speed and, perhaps also, low pressure, the amount of fluid left behind a seal as it moves may be considerable. In the pump condition this may be termed leakage, but on the motoring stroke a similar effect may occur if the shaft remains wetted by the fluid, in spite of the fact that this then means that fluid is being moved from a low to a high pressure, that is, negative leakage. It should be noted that negative leakage may result in air ingress, and thus attempts to obtain zero leakage may result in aeration of the fluid to be sealed.

Fluid and equipment should be maintained properly even before the installation of the fluids. All fluids should be stored inside sealed containers, or under a roof and on their sides to minimise entry of water and dirt. Caps and drums tops should be cleaned thoroughly before opening. Only clean hoses and containers should be utilised to transfer fluids from cans, drums or bulk storage areas to hydraulic reservoirs. These hoses should be equipped with proper filtration devices. The reservoir to which the fluid is to be placed should also contain a mesh screen on its filler pipe.

The installation of the fluid itself should be done properly and correctly and to clean equipment. Use only the type and grade hydraulic fluid recommended by the hydraulic equipment manufacturer or by the fluid supplier. Different types of hydraulic fluids should not be mixed. Before installing the hydraulic fluid, the system should be flushed and cleaned exactly as recommended by the manufacturer. All hydraulic filters and air breathers should be checked before filling. The hydraulic system itself should be thoroughly checked to eliminate leaks or entry points for contaminants. The equipment should them be filled to the proper level. Once the system is filled proper maintenance and monitoring should be carried out throughout the life of both the fluid and equipment in question.

The fluid should be replaced at recommended drain intervals and the system should be drained when it is warm and has just been circulated so that the maximum amount of contaminant is removed Proper maintenance of the hydraulic system includes replacing filters and air breathers at recommended intervals and training operators to service the equipment.

Whether replacing hydraulic system components or in the course of general maintenance, cleanliness should be maintained at all times. The use of quick-disconnect fittings is useful in eliminating contaminate entry. Fittings should be wiped clean before use and should be covered when not in use.

Proper fluid filtration should be provided to keep contamination levels down. Selection of appropriate filters and their strategic placement in proper parts of the system is vital in a well maintained system. Filter size should be checked with manufacturer before installation. Some filters employed today are so effective there is a chance they will filter out certain additives if proper selection is not maintained. Filtration and contamination are important aspects of maintenance and are well-documented.

The quality of the fluid in the reservoir and the system is also critical. Insufficient fluid can limit complete immersion of some components and can allow the air to be drawn into the system. Drawing air into the system creates "spongy" cylinder action and can set up pump cavitation conditions which result in high or catastrophic wear. Therefore, it is important to prevent fluid leakage and to make sure the fluid level is always full.

The hydraulic fluid itself should be monitored on a periodic basis. Table 1 gives some basic monitoring requirements for the basis types of hydraulic fluids discussed. Manufacturer’s recommendations should be thoroughly followed and a program should be developed to adhere to these recommendations. Fluid should be drained and replaced as required. Continued use of a fluid that has outlasted its life in the long run causes costly down time due to equipment outages.

Since hydraulic fluids are increasing in cost, and reclamation procedures have improved greatly, more and more hydraulic oil users are reclaiming their own hydraulic fluids. If reclaimed fluid is to be utilised, care must be taken that the fluid has been reclaimed properly and its quality has been put back to its original state.

The function of a scraper ring is to prevent dirt particles from entering the components in a hydraulic or pneumatic circuit; cylinders or valves. This helps prevent contamination of the media which will damage the seal, the surface of the housing and the guide bush.

Bellows provide a complete protection for reciprocating shafts, but they are complicated to produce and can be easily damaged so their use is restricted to special applications. In most cases a scraper ring having one-lip made of wear resistant elastomer, is sufficient.It is important to protect the scraping edge from damage and to have adequate contact with the groove and piston rod diameter.

The lip of the scraper ring is designed to have a pre-load with the piston rod, and this has an influence on the break-out friction. Together the pre-load and the sharp scraper edge wipes off the oil film passing the seal.

Scraper rings are made from elastomeric or thermoplastic materials and some have metal case for stiffness. Special designs are available for applications where, compensation for large radial movements or a stiffer inner lip is required.It is recommended to use scraper rings which are designed to be a tight fit in the housing.

When in trouble there is a simple process to follow in correcting any type of seal failure.

D. Contact Economos UK Limited. to assist in analysis of failure and recommended solution.

SYSTEM CONTAMINATION is usually caused by external elements such as dirt, grit, mud, dust even ice and internal contamination from circulating metal chips, break-down products of fluid, hoses or other degradable system components. As most external contamination enters the system during rod retraction, the proper installation of a rod wiper/scraper is the best solution. Internal contamination is best prevented by a proper filtering of system fluid. Contamination is indicated by scored rod and cylinder bore surfaces, excessive seal wear and leakage.

SPIRAL FAILURE is often the result of a combination of factors such as basic seal geometry, long stroke and/or too soft an elastomer. The classic spiral failure usually is found in a simple O-ring type seal but will sometimes be evident in unsupported lip type seal as well. The use of T-Seals, harder durometer materials, and seals with rectangular cross-sections will usually solve this problem.

CHEMICAL BREAKDOWN of the seal material is most often the result of incorrect material selection in the first place, or subsequent change of system fluid. Misapplication or use of non compatible materials can lead to chemical attack on seal by fluid additives, hydrolysis and oxidation reduction of seal elements. Chemical breakdown can result in loss of seal lip interface, softening of seal durometer, excessive swelling or shrinkage.

IMPROPER INSTALLATION is a major cause of seal failure. The three broad areas to be watched during seal installation are; cleanliness, protecting the seal from nicks and cuts and proper lubrication. Other problem areas are over tightening of the seal gland where there is an adjustable gland follower or folding over a seal lip during installation. The solution to these problems is common sense and taking reasonable care during assembly.

EXTRUSION of the seal element is usually caused by excessive clearances in metal components, high axial loading, high pressure and use of too low a durometer seal material. Extrusion causes a loss in seal volume and stability. The prevention of extrusion usually requires a type of seal with built-in anti-extrusion rings.

HEAT DEGRADATION is to be suspected when the failed seal exhibits a hard, brittle appearance and/or shows a breaking away of parts of the seal lip or body. Heat degradation results in loss of sealing lip effectiveness through excessive compression set and/or loss of seal material. Causes of this condition may be use of incorrect seal material, high dynamic friction, excessive lip loading, no heel clearance and proximity to outside heat source. Correction of heat degradation problems may involve reducing seal lip interference, increasing lubrication, change of seal material. In borderline situations consider all upper temperature limits to be increased by 50 degrees F in dynamic reciprocating seals at the seal interface due to running friction.

SLIP-STICK is an expression of the differential between the static and dynamic coefficient of friction as it relates to start-up of a sliding mechanism. Slip-stick occurs when the seal "hangs-up" in the transition period between static and dynamic modes or there is a variation in the system fluid pressure, or shock loads cause the piston to jump back to the static mode. The most noticeable result of Slip-Stick is erratic or jerky movement of the outer rod. Slip-Stick often creates an audible noise and excessive heat and seal wear can occur.

QUESTION 6: Can you give me some information on the "Basic Theory" of how hydraulics work?

The basis for all hydraulic systems is expressed by Pascal’s law which states that the pressure exerted anywhere upon an enclosed liquid is transmitted undiminished, in all directions, to the interior of the container. This principle allows large forces to be generated with relatively little effort. As illustrated, a 5 pound force exerted against a 1 inch square area creates an internal pressure of 5 psi. This pressure, acting against the 10 square inch area develops 50 pounds of force.

In a basic hydraulic circuit, the force is exerted by a cylinder bore size and the pump pressure. (There is no force generated unless there is resistance to the movement of the piston). With 1000 psi pump pressure exerted against a 12 square inch piston area (approximately 4" dia.), a force of 12,000 pounds is developed by the cylinder. The speed at which the piston will move is dependent upon the flow rate (gpm) from the pump and the cylinder area. Hence, if pump delivery is 1 gallon per minute (231 cu.in./min.).

The simplest hydraulic circuit consists of a reservoir, pump, relief valve, 3-way directional control valve, single acting cylinder, connectors and lines. This system is used where the cylinder piston is returned by mechanical force. With the control valve in neutral, pump flow passes through the valve and back to the reservoir. With the valve shifted, oil is directed to the piston side of the cylinder, causing the piston to move, extending the rod. If the valve is returned to neutral, the oil is trapped in the cylinder, holding it in a fixed position, while pump flow is returned to the reservoir. Shifting the valve in the opposite direction permits the oil to pass through the valve back to the reservoir. The relief valve limits the system pressure to a pre-set amount. Relief valves are commonly incorporated into the directional valve.

A static seal is one used in an application that has no movement between mating surfaces, discounting such infinitesimal motion as may be caused by pressure and temperature variations, expansion and contraction, normal wear, and shock and vibration. Examples include seals for flanges, plugs, and tubing.

If the static seal is an installation activated seal, the seal and its mating parts must be in contact at a pressure level greater that the pressure of the fluid to be sealed. This pressure level often is obtained mechanically.

Other static seal installations are termed pressure-activated. Here the initial sealing pressure is quite low and is caused by the connecting squeeze of the joint. If that were the only pressure available, the seal would leak quickly.

However, as system pressure increases, it deforms and reshapes the seal so that sealing pressure increases to withstand the greater containment requirements. O-rings, V-rings, and other shapes are typical examples.

Reciprocating motion between two members adds a major dimension to the sealing problem. Friction, and its accompanying wear, joins the list of factors which the design engineer must contend with; temperature, fluid compatibility, pressure.

The seal used for sealing a reciprocating member must meet static sealing requirements at its contact area with the stationary member, and also seal effectively at its contact area on the reciprocating member.

Curiously, the first two properties are at odds with each other in a dynamic sealing situation. A good compression forces holding the seal against a reciprocating member accelerate seal wear and shorten service life. Therefore, every dynamic seal design is a compromise to produce an acceptable balance between these two desirable properties. The great variety of seal shapes and materials allows the designer to select the degree of compromise for a particular application while, at the same time, satisfying the other requirements on the list.

ANSWER: The first rotary shaft seal units were made from leather - the hat shaped until being clamped between metal pressings to form the outer casing.

Not only were those heavy and expensive to make, but they were true ‘brute force’ seals relying on heavy, wide contact between lip and shaft backed up by a powerful coiled garter spring to prevent oil leakage. If it was not for the fact that leather allowed a certain amount of lubricant to find its way to the interface, the lips would have burned up at much lower speeds than they did. Even so, the peripheral speeds which could be handled were relatively low and heavy loading resulted in high shaft wear.

The introduction of synthetic rubber in 1938 did not improve matters much since the seal design followed the principles of leather.

Seal manufacturers began to research the function of oil seals and it was shown that a seal acted like a bearing - an oil film being generated between lip and shaft. It was demonstrated that so long as a certain oil film thickness was not exceeded the meniscus which formed at the air side of the interface would not break and leakage was avoided. Relatively light loads were required to achieve this situation and so friction was greatly reduced with lower running temperatures and much longer seal life.

For the first time, seals could be designed which would work successfully without trial and error. The heavy metal cased construction has long since been replaced by a bonded seal using a single metal pressing with an outer layer of rubber to give a better fluid-tight joint with the machine housing.

It will be appreciated that any damage to the shaft where the seal runs will cause leakage because the optimum oil film thickness will be exceeded locally. Even a slight scratch can cause leakage so it is of the utmost importance to provide the right finish on the shaft. To maintain the oil film thickness within its required limits means that the seal lip must follow any shaft movement. This becomes difficult when the shaft is subject to eccentric running or vibration at high speeds. An application where such conditions exist is the rear end of an automotive crankshaft.

Some years ago, fluid seal manufacturers developed what was termed hydrodynamic or positive action features to help the seal cope with such conditions. These features consisted of raised ribs on the air side of the sealing lip which ‘bedded-in’ to form tiny projections to catch any oil leaking from the seal and return it to the oil film under the lip. Such features introduced great reliability to rotary shaft seals for difficult automotive applications hitherto impossible to achieve.

Although initially the positive action features were unidirectional a fact not of any consequence in many applications, duo-positive features soon became available making it possible to offer a standard range of seals of this type, which could be offered for any application without knowledge of the direction of shaft rotation.

Question 10: On a hydraulic cylinder, what should I look for and what area should I be concerned about for its proper performance?

EXCESSIVE CLEARANCES - When excess gland clearance exists, either on the inside or outside diameter of the gland lip or between the piston and cylinder wall, the seal will be subjected to deformation by extrusion.

Hydraulic pressure will cause the seal to extrude through the excess clearances. Constant pressure fluctuations will result in loss of sealing material through the clearance causing eventual seal failure. Prevent this problem by adhering to recommended clearances. NOTE: Care should be taken to properly centre or align assemblies so that clearance is equal all around.

SCORED RODS, STUFFING BORES OR CYLINDER WALLS - Severe scoring can prevent sealing lips from conforming to the entire surface to be sealed, resulting in leakage. Rings may never offer a positive seal if scores are excessively deep, but even if some service is obtained, optimum seal life cannot be attained. The solution is to refinish the scored rod, stuffing box bore or cylinder wall.

FOREIGN MATERIAL IN HYDRAULIC SYSTEM - Foreign matter in the hydraulic system (dirt, metal shavings, etc.) can cause many problems with seals, resulting in weepage and premature failure along with associated equipment damage. Proper filtration and preventative system maintenance solves this condition.

FAILURE TO MAKE INITIAL SEAL - This is caused by damage to sealing lips during installation. Care should be taken during insertion, and the use of sharp tools should he avoided. To facilitate installation, seals may be lubricated with a compatible medium.

EXCESSIVE FLUID TEMPERATURE - Built in interference and/or pressure on the packing is needed to maintain a positive seal throughout the pressure range. Excessive fluid temperature can cause seal to lose memory, resulting in fluid leakage, proper seal compound selection and maintenance of nominal fluid temperature will prevent seal failure.

CHEMICAL ATTACK OF SEAL - Chemical attack to seal is sometimes experienced when using exotic fluids as hydraulic mediums. Proper compound selection will prevent seal failure.

EXCESSIVE WEARING OF RINGS IN V-PACKING - Premature or excessive wear, particularly on the dynamic side of V-packing usually indicates that the seal has not been properly seated in the stuffing box or the piston. This results in the seal set rolling in the box short service life. Proper seating and/or adjustment will prevent this condition.

WEAR ON ONE SIDE OF SEAL - Conditions exist due to worn bearings or wear rings or rods that cause misalignment or improper guidance, resulting in excessive wear on one side of the seal. Correction is attained by rebushing the assembly for proper guidance.

ANSWER: Yes, using our TURBO SEAL Instant Seal Manufacturing Machine, we can ship seals in a hurry! An expediting fee is required for same day shipments. For further information on our rapid seal production click here.

ANSWER: To see how we make seals in our facility in Atlanta, Georgia click here.

ANSWER: With our MEGA TRUBO SEALS MACHINE we can make inch sizes from 1/64" to 60" and metrics from 1MM to 1500MM. To see our new MEGA TURBO SEALS MACHINE and for more information about our large diameter seals click here.

QUESTION 14: How do you I know what delivery services are available for a product?

ANSWER: Look for these color coded images below throughout EPM's site. These will indicate the service(s) that are available for that particular seal or gasket product.

Indicates SealsFAST™ Service is available.	Indicates Standard Service is available.	Indicates Traditional Service is available.