Func­tion of a plastic seal

Basic struc­tu­re

Plastic seals nor­mal­ly con­sist of two com­pon­ents.

The sealing ring is gene­ral­ly made from a wear­re­sistant PTFE com­po­und. This mate­ri­al is so effi­ci­ent that the­se seals can meet the hig­hest pres­su­re, speed and tem­pe­ra­tu­re resis­tan­ce spe­ci­fi­ca­ti­ons. Sin­ce the maxi­mum values are each based on dif­fe­rent com­pounds, preli­mi­na­ry tri­als are essen­ti­al to achie­ve the objec­tive on spe­cial requi­re­ments.

An elastic ener­gi­zer is used to press the sealing edge or sealing sur­face firm­ly against the mating sur­face. This squee­ze gua­ran­tees con­ti­nu­al sealing. An elas­to­me­ric pre­cisi­on O‑ring or pro­fi­led steel spring is used to gene­ra­te this squee­ze. O‑rings also act as a sta­tic seal in the base of the groo­ve.

All the ser­vice con­di­ti­ons must be con­si­de­red when selec­ting the mate­ri­al for the sealing ring and ener­gi­zer.

Inner or outer sealing

All radi­al­ly acting seals are manu­fac­tu­red in inner or outer sealing ver­si­ons.

Outer sealing, e.g. pis­ton seal MANOY® sli­ding ring series 112
Inner sealing, e.g. rod seal MANOY® sli­ding ring series 120

Genera­ti­on of sealing force

MANOY® sli­ding rings and MANOY® U‑Cups are pres­su­re-activa­ted sealing sys­tems. The elastic pre­cisi­on O‑ring when com­pres­sed acts as a sta­tic seal in the base of the groo­ve and pres­ses the dyna­mic sealing ele­ment against the mating sur­face.

When the O‑ring is pres­su­ri­zed by the medi­um it func­tions like an incom­pres­si­ble high vis­cous flu­id by app­ly­ing its sealing force on the sur­roun­ding sur­faces.

Up to about 2 MPa the sealing effect is pro­du­ced almost ent­i­re­ly by the com­pres­si­ve force of the ener­gi­zer.

MANOY® sli­ding ring, outer sealing
MANOY® U‑Cup, inner sealing

Pro­fi­le design

The dif­fe­rent pro­fi­le designs of the sealing rings are lar­ge­ly deter­mi­ned by the type of moti­on, the direc­tion of pres­su­re app­li­ca­ti­on, the level of pres­su­re and the flu­id to be sea­led.

Pre­cisi­on O‑ring squee­ze

To ensu­re a good seal, the housings are desi­gned so that medi­um squee­ze is pro­du­ced by the pre­cisi­on O‑ring or a pro­fi­led spring ener­gi­zer.

A smal­ler groo­ve depth pro­du­ces hig­her com­pres­si­on and the­re­fo­re impro­ves the sealing effect at low pres­su­re (up to about 2 MPa) and in unpres­su­ri­zed ope­ra­ti­on.

A lar­ger groo­ve depth pro­du­ces smooth run­ning but to the detri­ment of seal tight­ness.

Clearan­ce gap

The mini­mum clearan­ce gap requi­red is depen­dent on the bea­ring accu­ra­cy and the other design cons­traints. Pres­su­re expan­si­on and ther­mal expan­si­on must also be taken into account.

The clearan­ce gap should be as small as pos­si­ble on the side not under pres­su­re. On the side under pres­su­re, a grea­ter clearan­ce gap can pre­vent par­ti­cles sti­cking.

At pres­su­res of over 40 MPa, mini­mum fits of H8/f8 for the bore/piston dia­me­ters and/or the casing/rod dia­me­ters should be main­tai­ned.

If the clearan­ce gap is too lar­ge on the side not under pres­su­re, the sealing ele­ment extru­des into the gap and is des­troy­ed.

Seal cross-sec­tion and total tole­ran­ce ran­ge

If it is impos­si­ble to accom­mo­da­te a recess for the housing, the spe­ci­fied upper limit for a dia­me­ter ran­ge may pos­si­b­ly be excee­ded.

It should be remem­be­red that the smal­ler the sealing cross-sec­tion selec­ted, the smal­ler the total seal-rele­vant tole­ran­ces must be.

The­re­fo­re the objec­tive should be a seal cross-sec­tion which is as lar­ge as pos­si­ble.


An abso­lute­ly tight seal can only be achie­ved with sta­tic seals and not with dyna­mic seals. Nor would this be desi­ra­ble, as com­ple­te dry­ness would cau­se high wear on the sealing ele­ments. The aim is to lea­ve a very thin, dry loo­king lub­ri­ca­ting film on the mating sur­face.