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Hydraulic Seals

Types and Definitions


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 ˝ 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 ˝ 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.

 

 

U-Cups

 

U-Cups derive their name from their shape and are self-sealing on both the inside diameter and outside diameter.  They are most often used for sealing rams or rods and are very popular due to the small gland space required.  They are especially adapted to low speed, low pressure environments for both pneumatic and hydraulic cylinders.  They provide an extremely smooth, steady movement because of their inherent low friction.

 

The seal is obtained by means of the flared, flexible lips coming in contact with the rod or cylinder wall.  Clearance is provided at the heel for minimum friction.  Through this design, the packing is preloaded when installed.  Sealing effectiveness is increased by internal pressure of the air or fluid (hydraulic seal) against the inner surface of the lips.  For continued sealing effectiveness and long seal life, the surface finish of the working surface of either rods or bores should be a maximum of 16 micro-inches.  It is not recommended that U-cups or any other single lip seal be used in any application where there is excessive side loading.

8400 Series U-Cups

 

The 8400 Series U-Cups has a champered lip and is available at American Seal & Packing in 80 Durometer Nitrile.

 

6226 Series U-Cups

 

 

The 6226 Series U-Cups has a flat sealing lip and is available at American Seal & Packing in 70 Durometer Nitrile.

 

U-Cup Installation Data

 

The lips of the U-Cup should always be toward the pressure force.

 

The U-Cup is a floating seal which means the height of the U-Cup should be between .015” to .090” shorter than the depth of the gland.  This added gland space allows the pressure to equalize on both lips, thus keeping the U-Cup from rolling.

 

The surface finish for a static seal surface should be 32 RMS maximum.  The dynamic surface should be in the range of 8 to 16 RMS.  Finishes rougher than 16 RMS could cause premature lip wear and finishes finer than 8 RMS can result in failure of the seal due to loss of lubrication.

 

 

Rod Wipers

 

The main function of the Rod Wiper is to keep the rod and cylinder clean and free from foreign matter; thus preventing premature failure.  The Rod Wiper is not a pressure sealing element, yet it avoids possible abrasive effects which may result if contaminates were to reach the sealing and rod bearing elements.

 

Style WD Rod Wipers

 

One-piece snap-in type rod wiper scrapers for heavy duty applications.  WD wipers are continuous (not split) and conform with wear and side motion for effective wiping action over the entire life span of the seal.  Part numbers listed below are molded from 90 durometer urethane for maximum strength and abrasion resistance.  American Seal & Packing also stocks selected sizes in 90 durometer fluoroelastomer.  Please inquire.
 

Style WAN Rod Wipers                     

Designed for light duty applications where style WD wipers are not required.  Interchangeable with corresponding dash numbers of MS28776 wiper rings.  Consult American Seal & Packing for AN6231 retrofit.  Part numbers listed below are molded from 90 durometer urethane for maximum strength and abrasion resistance.  American Seal & Packing also stocks selected sizes in 90 durometer fluoroelastomer.  Please inquire.

Style WH Rod Wipers                       

 

Designed for medium duty applications and can provide both wiping and sealing action in one component in low pressure systems.  The external lip scrapes foreign matter form the rod, and the other lips act as an effective pressure seal in low pressure applications.  This wiper can also be used as a secondary seal to trap slight leakage from the primary seal.  This creates a very dry rod system.  Molded from 90 Durometer Urethane for maximum strength abrasion resistance.

Style WK Rod Wipers     Double lip design provides the ultimate in wiper action to remove foreign material from the exposed rod while still maintaining an adequate lubricating film.  It protects both the rod and the primary seal. 

Metal Clad Wipers These rod wipers are molded form urethane and encased in a heavy duty metal case.  This gives them exceptional abrasion resistance and outstanding endurance under difficult operating conditions.  Highly effective in removing dirt and contaminants from the rod.  The dimensions for Metal Clad Wipers stocked by American Seal & Packing are listed below.  For availability of additional sizes not listed, please inquire.

Wear Rings American Seal & Packing Wear Rings are glass filled nylon.  The high compressive strength and load bearing capabilities of this material meet or exceed that of most metals used for wear rings.  The use of these non-metallic wear rings totally eliminates metal-to-metal contact and the subsequent galling and scoring of cylinder bores and rods.  These wear rings readily ingest harmful metal flakes, burrs and small chips.  They will not generate contamination as is common with metallic wear rings.  They are compatible with all common hydraulic oils, water emulsions, water glycol, and phosphate ester fluids.  Operation temperature to 350˚F.  Butt cut-standard stocking configuration.

 

T-Seals

 

The T-Seal is compact and provides an excellent option to bulkier types of seals in a double or single acting application.  It requires significantly less packing space than Vee Packings, V-Cups, or Piston Cups.  The result is a compact, efficient and less costly overall component.

 

The T-Seal is used in reciprocating hydraulic and pneumatic equipment and in high-pressure applications.  It is an excellent direct replacement for other packings )such as those in standard grooves).  It fits into the same simple groove as O-rings and eliminates rolling or spiraling, even under long stroke or dry rod conditions.  It does not wedge into clearances or pinch off under motion or pressure and has good low-pressure sealing capabilities.

 

Because the T-Seal is supplied with a back-up on both sides of the sealing area, it functions very well in double acting applications.  The back-ups are split for ease of installation.

 

Like other similar packings, the T-Seal is compressed to eliminate low-pressure leakage.  However, the back-up rings float between the T-Seal’s flanges and the rod or bore, minimizing friction and wear.  When pressure is applied, a bridge is formed before extrusion can occur.

 

The T-Seal’s back up utilizes hydraulic pressure to keep it in contact with the rod or cylinder wall.  This prevents extrusion from occurring.

 

The T-Seal resists spiral failure.  When piston or rod movement does occur, even under very low pressures, the design allows the back-up rings to lock the sealing element in place.

 

At present, American Seal & Packing stocks 70 Durometer Nitrile T-Seals with nylon back-ups.  Call for availability of other compounds.

 

Vee Packings

American Seal & Packing Vee Packings (or V-Packings) are multiple-lip (chevron) seals comprised of a group of endless rings and are popular for sealing rods and pistons.  They can be used for low or high pressure applications and with sing or double acting cylinders.

 

American Seal & Packing presently stocks Female and Male Adapters (top and bottom) in fabric reinforced nitrile and V-Rings (center rings) in fabric reinforced nitrile and in homogeneous 90 Durometer nitrile*.  In general, fabric Vees are for high pressure, and homogeneous Vees are for low pressure.  The selection of fabric or homogeneous compounds should be made on the basis of operating pressure, temperature, and the type of gaseous or fluid medium with which the packing must function.  (*Call for availability of other compounds)

 

 

 

Installation

 

Properly installed Vee Packings will generally outperform other lip-type seals.  They are always installed in sets.  Each set contains a number of V-Rings (based upon the pressure) plus male and female adapters.

 

The V-Rings in each set are referred to as pressure rings.  Refer to the table below to determine the number of V-Rings per set based on solid rings.  This table applies to the majority of installations.  There will be exceptions, depending upon special operating conditions, where the table guidelines do not apply.

 

WV Packings

While similar to Vee Packings, the WV style provides sealing properties that are largely independent of stack packing pressure.  The WV is designed so that it will provide a multiple lip hydraulic seal in a fixed gland depth packing.  Because of the design of the WV packing, te lips of each ring are free to flex when in a nested stack so each lip automatically reacts to fluid or air pressure, providing a better seal in a given packing space.  There is no compression adjustment required during the seal life.

 

Since the WV Packing is a multiple lip hydraulic seal, it is recommended over a single lip seal for applications where side loading is a factor.  The multiple lip also provides extra stability.  If one seal fails, there will be another seal to serve as back-up.

 

There is an interference built into both the I.D. lips and the O.D. lips so that there will be sealing capabilities at low pressure.

 

American Seal & Packing stocks Center Ring and Flat Back WV packings in homogenous Buna 90 Durometer and fabric reinforced Buna.

 

 

 

WV Packing Installation Data

 

Since WV packings are designed to be free floating, it is not recommended that the rings be installed in a compressed state.  Thus should be between .014” and .060” of area in the gland depth so that the rings are allowed to float.  The amount of this free floating is determined by the ring cross section and the number of rings being installed.  (See chart below).

 

For designs of medium to high pressure, it is recommended that fabric reinforced o-rings be utilized.  The adjacent table is a guide to pressure rating of multiple rings.  A homogeneous o-ring may be also used for sealing at low pressure.  The homogeneous o-ring should not be included in calculating pressure rating.

Piston Cups

Piston Cups are primarily used on piston heads, plungers or rams in hydraulic or pneumatic service.  Its mechanical sealing surface is at its periphery.

 

Piston Cups are commonly used because of economical cost, excellent service life, efficient assemble, universal acceptance, and simple accessories.

 

Homogeneous Piston Cups are recommended to handle low pressures (under 150 psi) in air, water, hydraulic fluids, and lubricating oils.  Homogeneous Piston Cups stocked by American Seal & Packing are 80 Durometer Nitrile.  They are also available in other homogeneous materials such as EPR, fluorelastomer, and silicone.

 

Fabric reinforced Piston Cups prevent extrusion in medium pressure installations (up to 1000 psi) or where diametrical clearances are greater.

 

 

Piston Cup Installation Data

 

Figure 1 shows a Piston Cups packing (P) attached directly to a ram ® without the use of a separate backing plate.

 

This is a proper method, as long as the rules of good design are observed.  The cylinder should be correctly finished to close tolerances and the ram of a size and material that can be finished to leave only a minimum working clearance between it and the cylinder wall.  The cylinder and ram must be of suitable material so that no scoring of the cylinder will take place.

 

Due to the nature of the work being done by a ram of this type, wear is inevitable and usually occurs to a great extent on one side.  When this wear has progressed to a certain degree, the Piston Cup packing will not be properly supported.

 

To insure lip contact, Piston Cups generally are designed with the heel .010”-.015” under nominal O.D., and the lip diameter is .020”-.030” over nominal O.D.  This insures heel clearance and full lip contact. 

 

Heel clearance is needed so full pressure is not carried by this narrow area, or it wears excessively.  Clearance between the piston and cylinder wall must be minimized to prevent extrusion at the cup’s heel.

 

The seal between the bottom of the cup and the piston head is a compression seal, created by bolting or otherwise fastening the cup between the piston head (or backing plate) and follower plate.  Do not over-tighten the follower nut especially when using homogenous Piston Cups.  Over-tightening will cause heel to extrude cause premature failure.

 

Piston Cup packings are installed with the sealing lips toward the pressure medium, thus the mechanical seals between the piston and the cylinder wall for which the cup is designed is created by the pressure applied against the cup.  In the dynamic seal area indicated in Figure 2, make sure there is enough compression, with no gaps, to insure a seal.  It is also important to be sure that the  heel of the Piston Cup is thick enough to create compression between the areas of B and C.

 

Figure 2 illustrates two Piston Cup packings (A and A) which are installed back to back against a common backing plate C in a double-acting cylinder.  B serves as the follower plate to compress the static seal.  A good solid plate is provided which is a piston head.  Packings should not be installed without a backing plate between them.