External contamination reduces the oil seal lifetime significantly. So, if there are dust, dirt, mud, water, humidity etc. in the environment, a rotary shaft seal with a dust-covered part should be preferred. In this case, a proper grease should be filled between dust lip and primary sealing lip before mounting. Thus, not only dust lip’s friction reduces, but also a buffered region is created between two lips for particulates reaching dust protection somehow. Determining which grease type should be used according to existence humidity or chemical steam with grease manufacturers is recommended in an external environment.
You can see a simple comparison of basic designs under dirty environments, in the related table. Additionally, especially in the external environment, this dust lip of classical lipped seal (KK-T) can do negative effect unexpectedly, if protective grease exists not enough or none, and shaft rotation is high. Lifetime of dust lips (thin rubber) is not very long generally. Correspondingly working conditions, dust lip is abraded easily more than the primary lip and comes to its size. In fact, dust lip diameter may be bigger than shaft diameter according to secretion. In this case, when grease is too little between the gap of two lips or none, it can not hold particulates such as small dust or filth, and these particulates pile up to gap because of not going back to the external environment. These particulates penetrate to primary seal lip more quickly and easily than the lipless product. This shortens seal lifetime of working against liquid oil. Increasing shaft tightness of dust lip doesn’t contribute positively to such cases. On the contrary, this shortens seal lifetime and increases friction torque, heat, force to shaft and abrasion.
So, it is suggested that dust lip seals should use with grease which fills the gap. Applications, which this suggestion is not possible, are more convenient usage of lipless seal or usage of lipped seal which has a limited gap between shaft and itself (means dust lip diameter is bigger than shaft dia.). These protective lips prevent big particulates from entering the gap, and also let small particulates getting in and out to the gap, prevent extreme dirtiness.
Special designs should be used like cassette or combination seal rather than classical designs, in situations of various and intense contamination.
|Estimated lifetime under low dirt, filth and so forth||-||○||++||++||++|
|Estimated lifetime under high dirt, filth and so forth||--||-||+||+||++|
|Wet protection against water, humidity, mud and so forth||--||-||○||+||++|
|Low friction torque and heating||+||○||-||-||--|
++ Very good
-- Too weak
Need to be extra careful about choosing oil seals, circumstances of corrosive effect in metal-case by the existence of water, humidity, acid or steam. In these applications, various methods are to cover seal’s corrosion side completely with rubber, to prefer stainless material for metallic parts, to choose stainless material like chrome for spring, to take spring in a sleeve to prevent touching water, to embed spring seal lip.
In this way, hydrophobic structured seals are preferred when water exists and penetrates to seal.