Hydraulic Seals By : A.K.SHRIVASTAVA :ADV[TECH.]TRAINING CENTRE :DOLVI:August 15, 2012
Introduction No hydraulic system can operate without proper seals. They hold in the fluid and keep out dirt, grime and water out. Seals appear simple but can be very complex. Seals are used in two applications: Static – Fixed applications Dynamic – Used on moving parts
O-Rings The most popular seal used in mobile hydraulics. Usually made of synthetic rubber. Used in both static and dynamic applications. Are designed for use in grooves where they are compressed between two surfaces. In dynamic use they must have a smooth surface to work against. O-Rings are not normally used where they must cross an opening, pass a corner or on rotating shafts due to wear problems.
O-Rings Are often used with a back- up ring to prevent squeezing out in the gap when used in high pressure applications. Back-up rings are made of fiber, leather, synthetic plastic, or rubber. In dynamic applications the O-ring Groove must be wider than the O-ring to allow for the O-ring to roll and lubricate itself.
Some Do’s & Don’t s Normally the O ring must be replaced if an assembly containing an O-ring is dismantled for repairs. O-rings must be stored in climate controlled environment [AC enclosure] to prevent hardening . Never hang an O-ring on a nail or peg , as it may develop a permanent kink at the point of suspension.
U and V Packings U and V packings are dynamic seals for rods, pump shafts, and gland nuts. Typically made of multiple rings of leather, synthetic or natural rubber, and plastics. Most often installed with the open side toward the area with the greatest pressure. Often though U and V seals are installed in the opposite direction because a special tool is required for their installation. This is acceptable if necessary.
Spring Loaded Lip Seals These seals are an improvement to the simple U and V packings. The rubber lip is ringed by a spring that gives the sealing lip some tension Often the seal has a metal case which is pressed into a housing bore. Often used in rotary shaft applications. Generally installed with the cup facing the greatest pressure. Double lip seals are also manufactured for applications with pressure on both sides.
Cup and Flange Packings Are dynamic seals made of leather, plastic, or synthetic rubber. Seals by the expansion of the lip or beveled edge of the packing under pressure. The higher the pressure the tighter the seal. Generally used on cylinder pistons and piston rods.
Cap Seals The most common on large high pressure cylinders Consists of a Teflon seal, rubber expander ring , and back-up washers. The rubber expander supplies constant force, pushing the Teflon against the cylinder wall while the back-up washers prevent the seal from being extruded into the clearance between the cylinder and the piston.
Mechanical Seals Are primarily dynamic seals Are usually made of steel and rubber They have a fixed outer part attached to the housing, an inner part attached to the revolving shaft and a spring that holds the two parts together.
Metallic Seals Usually used on pistons and piston rods and are a dynamic seal. Similar to piston rings on an engine except that their ends overlap to form a continuous seal. Have moderate friction and leakage Especially well suited for high temperature applications.
Metal Face Seals Used only in dynamic sealing Sealing is done at the highly polished surfaces of two identical steel sealing rings(1). The metal sealing rings are held in position by two identical rubber rings (2). They fit on a shoulder on each of the sealed and into an internal bore on both the fixed and rotating member (3). The tight fit of the rubber seals prevent the seals from rotating relative to the supporting member, making the polished surface the only friction point.
Compression Packing Sometimes referred to as Jam Packings. Made from plastic, asbestos, cloth, ru bber-laminated cotton, and flexible metals. Often used in the same ways as U or V packings. Generally suitable for low pressure systems and lubrication is very important due to the fact they will score the shaft if they become dry.
Consideration for Seal Selection Will seal resist calculated pressure Can seal withstand the expected operating temperatures Will seal wear too rapidly Will fluid harm or deteriorate the seal Does seal fit without dragging on moving parts Will seal score or damaged the polished components.
Seal Failures and Remedies A seal should prevent all leakage but this is not always practical. Often leakage is necessary to provide an oil film for lubrication. If after continued operation there are no obvious signs of pools or drips the seal is considered adequate. Seals are fragile and proper handling and installation are vital. To prevent future problems keep seals in their container, dry, and dirt free before installation. In general replace all seals that are disturbed during the repair of a system or component.
O-Ring Maintenance Cuts or nicks can easily damage o-rings. Heat, improper fluid, inadequate lubrication, and improper installation all contribute to o-ring failure.
Checking Seals for Leaks Before disassembly check out the cause of any leakage. This may save a return job by fixing the cause and not the result. Before cleaning the component find the path of the leakage. Often leakage is caused by other than seal failure. Leakage also results from worn gaskets, loose bolts, cracked housings, and loose lines. Inspect the outside sealing area to see if it is wet or dry. If wet, see whether the oil is running out or is a lubricating film.
Checking Shafts Shafts and bores should be smooth and free from abrasions. Deep scratches, rust, and nicks all damage seals Check shaft splines, keyways, burred ends to determine if the can cause a nick or cut seal.
Checking Bores Bores should be inspected for sharp edges, scores, nicks or gouges that can cause a path for oil flow. Coarsely machined bores can also create a spiral path of oil leakage.