Gene­ral design infor­ma­ti­ons

Fric­tion

IDG mate­ri­als are pri­ma­ri­ly based on com­pounds which are struc­tu­red on a poly­te­tra­fluo­roethy­le­ne (PTFE) matrix and the­re­fo­re have very low sta­tic and brea­k­out fric­tion.

This is becau­se pure PTFE has a low coef­fi­ci­ent of fric­tion which decrea­ses fur­ther under load.

Other fac­tors affect the coef­fi­ci­ent of fric­tion. They inclu­de the mating sur­face mate­ri­al, its sur­face rough­ness and hard­ness, the sli­ding speed and the tem­pe­ra­tu­re and lub­ri­ca­ti­on.

IDG PTFE based mate­ri­als are cha­rac­te­ri­zed by chat­ter-free sli­ding cha­rac­te­ris­tics.


Influ­ence of the mating sur­face

The sur­face rough­ness and accu­ra­cy of form of the mating sur­face have a very strong influ­ence on the deve­lop­ment of leaka­ge and the ser­vice life. Sur­faces with the hig­hest pos­si­ble mate­ri­al ratio should be the aim. This is achie­ved by ultra­fi­ne grin­ding, honing and lap­ping. Rough edges and hol­low edges are also smoot­hed. This is extre­me­ly important on hard mating sur­faces in par­ti­cu­lar.

A sur­face smoot­hed by rol­ling or calen­dering can beco­me uneven and wavy, which is very detri­men­tal to seals made of high-per­for­mance plastic com­pounds.

For rota­ry and oscil­la­ting moti­on, the coun­ter sur­face must be plun­ge-ground wit­hout any machi­ne lead.


Sur­face rough­ness, mate­ri­al ratio

Sur­face rough­ness accord­ing to DIN EN ISO 4287
The seal and mating sur­face are always con­si­de­red as a pair. The func­tio­n­al relia­bi­li­ty and life of a seal is the­re­fo­re great­ly depen­dent on the sur­face finish of the mating sur­face.

Scrat­ches, groo­ves, shrinkage cavi­ties and con­centric or spi­ral machi­ning threads or groo­ves are unac­cep­ta­ble. Hig­her requi­re­ments must be laid down for the mating sur­faces in dyna­mic than in sta­tic app­li­ca­ti­ons.

The para­me­ters most com­mon­ly used to descri­be the fine sur­face tex­tu­re (Rz, Rt and Ra) are defi­ned in DIN EN ISO 4287. In some cases the­se para­me­ters are insuf­fi­ci­ent by them­sel­ves and the mate­ri­al ratio (Rmr) should be spe­ci­fied.

The mate­ri­al ratio (Rmr) is vital for eva­lua­ti­on of sur­face finis­hes, as this para­me­ter is deter­mi­ned by the pro­fi­le form, which in turn results from the spe­ci­fic machi­ning pro­cess.

Sur­face pro­files

Clo­sed pro­fi­le

Ra 0,1 | Rz 1,0 | Rmr 70 %
Open pro­fi­le

Ra 0,2 | Rz 1,0 | Rmr 15 %

The gra­phic shows that the para­me­ters Ra and Rz do not ful­ly descri­be the pro­fi­le and that on their basis an eva­lua­ti­on of the sur­face for its sui­ta­bi­li­ty for the sealing sys­tem is insuf­fi­ci­ent.


Pro­tec­tion from hydro­dy­na­mic drag

A hydro­dy­na­mic drag flow deve­lops on line­ar bea­rings with a nar­row gap bet­ween the sta­tio­na­ry casing and the moving sur­face due to line­ar moti­on in the flu­id. If the flow is ‘blo­cked’ by a seal, a pres­su­re rise which can be many times grea­ter than the sys­tem pres­su­re is gene­ra­ted, par­ti­cu­lar­ly with ‘long’ move­ments.

For this rea­son the space in front of the seal should be as lar­ge as pos­si­ble.

If this does not suf­fice, then it is necessa­ry to pro­vi­de a back­flow chan­nel (in the form of a spi­ral groo­ve) or to fit MANOY® bea­ring rings with an ang­le cut split or bea­ring strips. This pre­vents cri­ti­cal pres­su­re build-up upstream of the seal.


Smoot­hing, run-in

The mating sur­face is smoot­hed by the sealing and bea­ring ele­ments during the run­ning-in peri­od.

Howe­ver, short strokes in quick suc­ces­si­on and oscil­la­ting move­ments pro­mo­te the for­ma­ti­on of unwan­ted run-in groo­ves. Extra­ne­ous vibra­ti­on ori­gi­na­ting from a dif­fe­rent point on the con­struc­tion cau­ses the same effect. To eli­mi­na­te the for­ma­ti­on of unwan­ted run-in groo­ves as much as pos­si­ble, the mating sur­face should be hard.


Hard­ness of mating sur­face

In gene­ral, sealing ele­ment wear on hard mating sur­faces is less than on soft mate­ri­al sur­faces. The­re is a dan­ger of run-in groo­ves being for­med, par­ti­cu­lar­ly in rota­ry app­li­ca­ti­ons.

If the mating sur­faces are not hard enough, con­ta­mi­nant scrat­ches can also lead to dama­ge to the sealing and bea­ring ele­ments. Soo­ner or later this will cau­se leaka­ge.

The mini­mum hard­ness should be over 58 HRC for line­ar moti­on and over 62 HRC for a rota­ry app­li­ca­ti­on.


Bea­ring, cen­te­ring

Metal con­tact bet­ween pis­ton, rods and casing must be pre­ven­ted. Groo­ving due to star­ting up would dama­ge the seal, bea­ring and wiper/scraper and lead to leaka­ge.

The seal and the wiper/scraper are not sui­ta­ble for bea­ring func­tions. Sepa­ra­te bea­ring ele­ments with suf­fi­ci­ent bea­ring capa­ci­ty must be spe­ci­fied.


Lub­ri­ca­ti­on, dry run­ning

Near­ly all IDG PTFE-based mate­ri­als have a dry­run capa­bi­li­ty, but dry run­ning redu­ces the life of the seal/mating sur­face sys­tem. A clo­sed film of lub­ri­cant in the clearan­ce gap redu­ces the coef­fi­ci­ent of fric­tion.

The lowest coef­fi­ci­ent is obtai­ned with hydrau­lic or lub­ri­ca­ting oil. A film of lub­ri­cant also redu­ces heat genera­ti­on in the clearan­ce gap and extends the seal life.

Life­time lub­ri­ca­ti­on on instal­la­ti­on is nor­mal­ly suf­fi­ci­ent for app­li­ca­ti­ons such as pneu­ma­tic cylin­ders and gui­de rods.


Abra­si­ve par­ti­cles in the flu­id

Par­ti­cles are not gene­ral­ly the cau­se of leaka­ge but they do gene­ra­te signi­fi­cant wear on the seal/mating face sys­tem, lea­ding to pre­ma­tu­re fail­u­re of the seal. The wear on a sys­tem is deter­mi­ned not only by the size of the sus­pen­ded abra­si­ve par­ti­cles but also by their num­bers.

With a typi­cal lub­ri­cant film thic­k­ness of 0.5 μm, small par­ti­cles float through but lar­ger ones cau­se wear on seals and mating faces.

The num­ber of par­ti­cles should be mini­mi­zed by design mea­su­res (e.g. fil­ter sys­tem). The stan­dards gover­ning the qua­li­fi­ca­ti­on and quan­ti­fi­ca­ti­on of flu­id puri­ty are ISO 4406 and NAS 1638.