Parker O-Ring Handbook.pdf

Dynamic O-Ring Sealing
5-12
Film Thickness ( δ2) 4
3
2
1
Seal — O-ring
Single Stroke (Not Pressurized)
Borderline
0 1 2 3 4 μ m 5
Original Film Thickness ( δ0) v = 0.2 m/s. v = 0.6 m/s.
v = 1.0 m/s. v = 1.4 m/s.
Figure 5-15: Change in Original Film Thickness as a Function
of Running Speed During a Single Stroke (Border Line Δ
Theoretical maximum Values)
This action has little effect upon the starting friction but brings
a noticeable improvement in running friction levels.
If slow pneumatic piston velocities are achieved by throttling
the pressurizing air, the risk of high stick-slip increases. Stickslip
is effected directly and negatively by long seal lips and
sharp seal edges. An extremely rough or fi ne polished metallic
running surface both cause equally higher stick-slip.
5.14.3 Wear
Friction causes wear. However, friction can be anticipated
and taken into consideration in the design stage. The wear
rate however is diffi cult to predict but directly governs the
lifetime of an O-ring and the frequency of maintenance.
Today’s high precision machinery tends in most cases to
eliminate hydrodynamic lubrication because of the increased
wipe-off effect. This means the seal always functions in semidry
condition and for this reason wear resistance depends on:
• properties of the compound;
• lubricating properties of the medium;
• running surface roughness;
• working conditions.
Wear in fl uid solutions can be divided into four groups:
• Scuff wear develops with metal-to-metal contact in the
semidry condition where both materials tend to form
mixed crystals. High Performance Lubricating (HPL)
oils help to prevent this contact because of their additives.
These additives have no infl uence in rubber/steel or rubber/metal
combinations.
• Fatigue wear becomes evident when particles are released
from the metal structure and is usually the result of
pulsating loads.
Parker O-Ring Handbook
• Corrosion wear manifests itself in the form of rust and
can normally be reduced by suitable oil additives. Seals
are not directly affected by the above types of wear.
However, in dynamic applications particularly these wear
conditions can cause the seal to fail through abrasion.
• Abrasive wear can affect both metallic and seal areas.
Metals are abraded by hard compounds or by hard foreign
matter in the medium. A rough metal surface normally
is the cause of elastomer abrasion.
The seal user normally has no profound knowledge of seal
wear characteristics. It is therefore recommended to consult
the manufacturer about details of all extreme application
conditions so that the correct seal can be offered.
5.14.4 Interdependence of
Friction Wear and an Effective Seal
In order to obtain a problem-free seal it is necessary to have
stability with regard to the clearance gap to avoid possible
extrusion. However, stability is diffi cult to achieve because
the relevant parameters often work conversely.
The fi rst consideration is the lubricating fi lm in the clearance
gap. To estimate friction, lifetime and leakage it is necessary
to know the width of the gap and how it varies under working
conditions. To keep friction as low as possible the lubricating
fi lm should be fairly substantial. This, however, can result in
leakage because the “thick” fi lm is wiped off the rod surface
during the return stroke. In the other extreme a lack of lubricating
fi lm causes problems due to high friction. The effectiveness of
a seal and friction therefore are inversely proportional.
Hardness, together with the width and length of a clearance
gap is very important. The hardness determines the elasticity
of the seal and assures that the seal gives way to the lubricating
fi lm under pressure. The instantaneous viscosity of
the fl uid also plays an important role in resisting the wiping
effect of the seal.
It is still not known which factors infl uence the lubricating
fi lm and which mechanisms act in the clearance gap. A soft
compound favors a thicker fi lm. Hard and soft compounds
behave differently at high velocities, harder compounds help
form a lubricating fi lm whereas a soft compound will hinder
this by strong adhesion to the running surface.
The lubricating fi lm is very important but only one of the factors
affecting seal friction. Other factors are, for example, the
seal compound, seal shape, pressure, velocity, and changes in
direction. Often many of these factors are diffi cult to measure
or reproduce.
It is therefore quite understandable that seal manufacturers
cannot give customers fi xed fi gures regarding friction and
wear for an individual seal. Information about seal lifetimes
only can be made when all parameters affecting the seal are
known and reproducible. General assumptions from a few
tests are not acceptable because laboratory tests never can
reproduce real working situations.
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com