Hydraulic Seals Piston Seals - Seal & Design

Hydraulic SealsPiston Seals

Piston sealsProfile overviewProfileDescriptionMPVPolyurethane slide ring, nitrile rubber energizer; suitable for heavy duty applicationsDPVPolyurethane slide ring, nitrile rubber energizer; fits wide, shallow inch size housings;suitable for heavy duty applicationsLPVPolyether based polyurethane slide ring,nitrile rubber energizer; suitable for light tomedium duty applicationsCPVPolyurethane slide ring, nitrile rubber energizer; fits wide, shallow inch size housings;suitable for light to medium dutyapplicationsGHPTFE slide ring, nitrile rubber energizer; lowbreakaway frictionAPRPTFE slide ring with incorporated rubberX-ring, nitrile rubber energizerLCPPTFE slide ring supported by polyamideanti-extrusion rings, nitrile rubber energizer; very good gap extrusion resistance;suitable for high pressuresLTPSealing ring made of nitrile rubber, supported by polyamide anti-extrusion rings;good gap extrusion resistance; suitable forhigh pressureswww.SealAndDesign.com

Profile overviewProfileDescriptionCUTStep cut polyamide slide ring, nitrile rubberenergizer; suitable for high pressures2SCPStep cut polyamide slide ring, oval nitrilerubber energizer; fits wide, shallow inch sizehousings; suitable for extreme pressuresMD-LOne-piece nitrile rubber sealing ring integrated polyester elastomer support ringsand POM guide rings; common in older cylinder designsUNPPolyurethane U-cup profile; single-acting;may be used in double-acting cylinderswhen facing in opposite directionsRod seals that can be used as piston sealsPTBThese rod seal profiles are designed withsimilar inside and outside sealing geometryand, therefore, can also be used as pistonseals.STDDZwww.SealAndDesign.com

Piston sealsBasicsFig. 1Piston seals († fig. 1) maintain sealing contact in sliding motion between the piston andthe cylinder bore. Differential pressures actingon the piston to extend or retract the pistonrod can be in excess of 400 bar (5 800 psi).The pressure acting on the piston sealincreases contact forces between the pistonseal and cylinder surface. Therefore, the surface properties of the sealing surfaces arecritical to proper seal performanceTypical piston seal arrangement for double-actingcylindersSlide ringPiston seals are typically classified intosingle-acting (pressure acting on one sideonly) and double-acting (pressure acting onboth sides) seals.MaterialsDepending on the profile and the requiredcharacteristics of its components, a piston sealcan consist of one or several materials. Commonmaterials used for piston seals are thermo plastic polyurethane (TPU), polytetrafluor ethylene (PTFE), polyamide (PA), and nitrilerubber (NBR).EnergizerGap extrusionExternal forces acting on the piston rod,reacted by the fluid inside the cylinder, canresult in abrupt pressure peaks. These peakscan be far in excess of the system operatingpressure and may press a piston seal into thegap between the piston and bore, therebycausing damage to the seal and adverselyaffecting seal performance and cylinder operation. Seal materials must be carefully chosento avoid gap extrusion.This risk of gap extrusion can also be minimizedby using seals with anti-extrusion rings.Piston guidanceGuide rings avoid sliding metal-to-metal contact between the piston and cylinder bore andaccommodate the radial loads of forces actingon the cylinder assembly. Although pistonseals are designed to accommodate slightradial motion between the piston and bore,effective guide ring function to accurately centre the piston within the bore is important forpiston seal performance.www.SealAndDesign.com

BasicsPiston driftWhen the piston rod is at rest and held in pos ition by fluid, any amount of flow passing thepiston can result in an unintended movementof the piston rod and cause drift. Although piston seal leakage is a possible source of drift,internal valve leakage, external system leakage and flow between the piston and rod staticconnections should also be carefully checked.In some applications, a minimal amount offlow passing the piston seal within specifiedlimits is permitted. This tolerance allows theuse of piston seals of low friction designs andmaterials or split slide rings for simpleinstallation.Sealing between piston and rodThe piston can be welded to the rod († fig. 2)if the disassembly of the cylinder can be doneby removing the rod end attachment. The piston can also be fixed to the rod end by a locknut († fig. 3), which enables removing thepiston from the rod during complete disassembly of the cylinder. When using a lock nut,static sealing is required between the pistonand the rod end.Fig. 2Piston welded to the rod end tap2Fig. 3Piston fixed to the rod by a lock nut and with a static sealand two back-up rings between piston and rod endwww.SealAndDesign.com

Piston sealsDouble-acting piston sealsPiston seals with polyurethane slideringsDouble-acting cylinders are the most widelyused cylinder types. They operate with pressure on both sides and, therefore, requiredouble-acting seal arrangements († fig. 1,page 48).Double-acting piston seals have a sym metrical profile (cross section) and identicalsealing functions in both directions. Typically,double-acting piston seals consists of a slidering and an energizer. The deformation of therubber energizer when installed provides adequate force to keep the slide ring indynamic sealing contact with the cylinderbore, while sealing statically against the sealhousing groove.A double-acting cylinder typically has thesame fluid on both sides of the piston. Therefore, a relatively thick lubrication film can bepermitted between the piston seal and the c ylinder bore to minimize friction and wear.The transportation of fluid occurring duringdynamic operation is, however, small andinsignificant in most applications.SKF supplies double-acting piston seals inmany different profiles and in a wide range ofseries and sizes, which make them appropriatefor a wide variety of operating conditions andapplications.Piston seals with thermoplastic polyurethane(TPU) slide rings have a nitrile rubber (NBR)energizer. The wear-resistant slide ring hasa profiled dynamic sealing surface optimizedto reduce friction and resist gap extrusion.Notches in the slide ring edges ensure rapidpressurization of the seal to react to abruptchanges in pressure. These profiles can usually be installed without special equipment andare resistant to damage during installationand cylinder assembly.MPV profilesMPV profiles († fig. 4) have slide rings madeof hard X-ECOPUR PS (TPU) developed forpressures up to 400 bar (5 800 psi). They aresuitable for high fluid temperatures and inheavy duty applications, such as earthmovingequipment. MPV profiles are available in metric sizes and some fit seal housings in accordance with ISO 7425-1.Fig. 4MPVwww.SealAndDesign.com

Double-acting piston sealsDPV profilesDPV profiles († fig. 5) have slide rings madeof hard X-ECOPUR PS (TPU) developed forpressures up to 400 bar (5 800 psi). They aresuitable for high fluid temperatures and forheavy duty applications, such as earthmovingequipment. DPV profiles are available in inchsizes where wider, shallower seal housings areused.LPV profilesLPV profiles († fig. 6) have a polyether basepolyurethane (EU) slide ring that providesresistance to hydrolysis (attack from moisture)and good low temperature flexibility. The O-ringenergizer provides a cost-effective sealingsolution. These profiles are developed forpressures up to 250 bar (3 625 psi) and suitable for light to medium duty applications,such as agriculture and material handlingapplications. They are available in metric sizes.Fig. 5DPVFig. 6CPV profilesCPV profiles († fig. 7) have a polyester basepolyurethane (AU) slide ring. These profilesare developed for pressures up to 345 bar(5 000 psi) and suitable for light to mediumduty applications. They are available in inchsizes where wider, shallow seal housings areused.LPVFig. 7CPVwww.SealAndDesign.com

Piston sealsPiston seals with PTFE slide ringsPTFE slide rings may be preferred in applications with demands for low breakaway frictionand when it comes to chemical and thermalresistance. Notches in the slide ring edgesensure rapid pressurization of the seal to reactto abrupt changes in pressure. PTFE is hardand non-elastic when compared with polyurethane and rubber materials. Therefore,PTFE slide rings are combined with an energizer that provides the necessary sealing forceand the static sealing function in the sealhousing.Fig. 8GH profilesGH profiles († fig. 8) have a nitrile rubber(NBR) energizer, which is an O-ring as standard. Square ring expanders are available onrequest. These profiles are suitable for pressures up to 400 bar (5 800 psi) and are available in metric and inch sizes. Some metricsizes fit seal housings in accordance withISO 7425-1.GHFig. 9APR profilesAPR profiles († fig. 9) have a nitrile rubber(NBR) O-ring energizer. The PTFE slide ringincorporates a NBR X-ring to improve sealingperformance. They are suitable for pressuresup to 350 bar (5 075 psi) and are available inmetric and inch sizes. Some metric sizes fitseal housings in accordance with ISO 7425-1.APRwww.SealAndDesign.com

Double-acting piston sealsPiston seals incorporatinganti-extrusion ringsFig. 10These SKF piston seals incorporate patentedlocking anti-extrusion rings († fig. 10) madeof polyamide (PA). They are split for simpleinstallation. Their snap-in design makes iteasy to identify the correct mounting direction, holds them in place when mounted andprevents damage during assembly.Piston seals incorporating anti-extrusionrings have an improved high pressure performance and minimize the risk of gap extrusion at abrupt pressure peaks.Patented locking anti-extrusion ringsWrong mountingdirectionCorrect mountingdirection andposition2Fig. 11LCP profilesLCP († fig. 11) profiles have a PTFE slide ringsupported by harder PA anti-extrusion rings(† fig. 10) and a nitrile rubber (NBR) energizer.These capped T-seals are suitable for pressures up to 690 bar (10 000 psi) and availablein various industry common metric and inchsizes. Some metric sizes fit seal housings inaccordance with ISO 5597.LCPFig. 12LTP profilesLTP profiles († fig. 12) have a nitrile rubber(NBR) sealing ring supported by harder PA antiextrusion rings on both sides. The sealing ringserves as a slide ring and also seals staticallyagainst the seal groove. These T-seals aresuitable for pressures up to 345 bar (5 000 psi)and are available in inch sizes.LTPwww.SealAndDesign.com

Piston sealsPiston seals with rigid split slide ringsThese SKF piston seals have a rigid split slidering made of glass fibre reinforced polyamide(GFR PA) and a nitrile rubber (NBR) energizer.The rigid slide ring has high resistance to wearand gap extrusion and provides low friction,even under high pressure. The split slide ringdesign facilitates the mounting process intoa closed seal housing.Fig. 13CUT profilesCUT profiles († fig. 13) have a step cut slidering and a rectangular energizer. They aresuitable for pressures up to 500 bar (7 250 psi).They can be used for short pressure pulses upto 1 000 bar (14 500 psi) with proper systemdesign. For additional information, contactSKF. These profiles are available in metricsizes and some fit seal housings in accordancewith ISO 7425-1.CUTFig. 14SCPSCP profiles († fig. 14) have a step cut slidering and an oval energizer. They are suitablefor pressures up to 690 bar (10 000 psi). SCPprofiles are available in inch sizes where wider,shallower seal housings are used.SCPwww.SealAndDesign.com