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.


Gen­er­al requirements

The lead-in cham­fers on rods, shafts and bores must be provided for the dif­fer­ent seals as shown in the illus­tra­tion for each install­a­tion situation.

Plastic and elast­omer seal­ing and wiper/scraper ele­ments must not be pushed over sharp edges, thread crests, cross holes, grooves or rough sur­faces. This aspect must be con­sidered in the design phase or dur­ing construction.

If this is not the case, these points must be covered before installation.


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.