Rotary Seals - Dilanda.it

Radial Oil Seal
Lubrication
Adequate lubrication is of extreme importance as regards
the function and useful life of the seal. A film of liquid
must be formed between the sealing lip and the shaft in
order to minimize friction and the associated generation of
heat and wear and thus to avoid destruction of the lip
material. In cases where the sealed medium is oil or grease,
lubrication does not normally present a problem. However,
care must always be taken to ensure that the lubricant
comes in contact with the sealing lip. Components such as
gearwheels, oil thrower and taper roller bearings exert a
pumping action which either prevents the lubricant from
reaching the seals or causes a heavy flow of lubricant to be
directed towards the seal. In the former case, circulation
passages should be provided to ensure that the seal is
lubricated. In the latter case, the flow may cause a pressure
rise in excess of the permissible values. In machines where
the lip seal is not normally lubricated, grease or oil must be
supplied by other means. Prior to installation the seal must
be pre-lubricated with oil or grease. In some cases this
lubrication during the installation may be sufficient. On
seals with double lip the space between the lips should be
filled to about 50% with grease before installation. A wide
range of oils and lubricants are available on the market,
and these may have different effects on the elastomers.
Care should therefore be taken to ensure that the lubricant
used is not detrimental to the lip material. See the
resistance Table V.
Lubrication and leakage
Absolute tightness cannot possibly be achieved. The
medium to be sealed also lubricates the lip and affects
the service life of the seal. A complete dry running destroys
the sealing lip. The German standard DIN 3761 classifies the
tightness of lip seals into leakage classes 1 to 3. A so called
zero-leakage is also defined. Zero-leakage means a
function related film of moisture at the sealing edge to a
non-drip formation of medium over the back-face of the
seal. It is better to accept this “minimum leakage“ rather
than risking the lip to be damaged due to insufficient
lubrication. The permissible leakage in class 1 to 3 is max
1 g to 3 g per seal for a test time of 240 hours.
special design of the sealing lip, reduction of the spring
force or by employing a special grade of rubber. Our
technical department will be pleased to provide advice on
such matters. It should be noted that the frictional loss
during the “running in“ period is greater than shown in
the figure. The normal “running in“ period is a few hours.
After a long period of standing idle the starting friction
may be also relatively high.
Frictional loss, W
Figure 12
350
300
250
200
150
100
50
ø 200
ø 150
ø 125
ø 100
0
0 5 10 15 20 25
Peripheral speed, m/s
Frictional loss for TRA/CB type
seal of nitrile rubber
ø 80
ø 70
ø 60
ø 50
ø 40
ø 30
ø 20
ø 10
Frictional loss
The frictional loss is often of significant magnitude,
particularly when low powers are transmitted. The
frictional loss is affected by the following parameters:
seal design and material, spring force, speed, temperature,
medium, shaft design, and lubrication. Figure 12 shows the
frictional losses in watts caused by a seal without dust lip
when fitted in accordance with our technical instructions.
In certain cases the frictional loss can be reduced by a
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Edition April 2006