Parker O-Ring Handbook.pdf

Dynamic O-Ring Sealing
5-10
Related to the working conditions:
• Working pressure;
• Velocity of movement;
• Type of material and surface fi nish of surfaces;
• Working tolerances;
• Axial loads and wear bands on pistons.
These factors cannot be quantifi ed because they overlap and
act cumulatively.
At the beginning of a stroke the seal goes through three
friction phases. Initially the seal is in direct contact with the
sealing face with few lubricated fi elds, e.g., µ = 0.3. Then
follows a wider area of mixed friction where the coeffi cient
of friction can drop as low as 0.06 to 0.08 according to the
proportion of lubrication/non-lubricated areas (Figure 5-11).
Finally, pure hydrodynamic friction which does not allow
direct contact between the seal and the running surfaces is
rarely reached.
As complete lubrication (= fl ooding) occurs, loss of fl uid
from a system increases.
Friction depends on a compound's sliding properties. Hardness
and deformation of the seal infl uence the seal pressure.
Specifi c seal pressure is in general related to, but not strictly
proportional, to the system pressure.
The working pressure controls the width of clearance gaps
and thereby the thickness of the lubricating fi lm. The result
depends on the geometry of the seal. Friction caused by Orings
increases with increasing pressure. Lip seals are more
sensitive to pressure, friction increases quicker than with seals
without a lip. This shows that the geometry of a seal directly
affects the amount of friction.
Friction is proportional to the working pressure and therefore
it is necessary to keep seal friction low, especially at
low pressures.
Unfortunately, reduction of the sealing force also results in an
increased tendency to leakage. This relationship can be modifi
ed within certain limits by selection of the seal geometry.
Coefficient of Friction μ
Vμmin. Figure 5-11: Stribeck Diagram
Stribeck diagram
Break-out friction
Mixed friction
Hydro-dynamic friction
Velocity V
Parker O-Ring Handbook
Normally the decision must be made between lower friction
and high leakage.
Additionally, an unstable seal geometry due to swelling in
the medium plays a role. Swelling means increase sealing
force and increased friction.
When the medium is mineral oil it would seem that suffi cient
lubrication is assured. However, the seal geometry once again
plays a role when, for example, a wiper seal scrapes a shaft
dry. Leakage at a wiper seal will not occur until the seal wears.
On the other hand lubrication can cause leakage amounting
to the thick lubricating fi lm with every stroke.
The optimum condition is a relatively thin lubricating fi lm
with suffi cient adhesive properties.
The dynamic piston actually causes less friction with increasing
velocity. In absolute terms there are very large discrepancies
according to the thickness of the lubricating fi lm. The
reduction of friction with increasing velocity stems from
the hydrodynamic properties of the lubricating fl uid. This is
also true for harder compounds. At low pressures the friction
varies to the piston speed. At high pressures friction is seen
to be more or less constant.
Friction is directly infl uenced by the seal diameter because the
wear-area is greater. The greater the metal surface roughness,
the more the contact surface consists of metallic “islands”
and therefore again mixed friction occurs.
As in many other areas break-out friction of elastomers
is signifi cantly higher than running friction. Apart from
compound type and seal geometry, tendency to adhesion,
deformation, the down-time and the surface fi nish play a role
in increasing break-out friction. The longer the down-time,
the more lubrication is squeezed from between the seal and
the running surface resulting in a non-lubricated vacuum. In
this condition the level of starting friction approaches that
for dry friction and is up to 10 times that found in running
friction (Figures 5-12 and 5-11).
Coefficient of Friction
1.2
0.8
0.4
Level of Starting Friction
Dependant Upon Time and Compound
10 sec. 1 min. 1 hr. 1 day 1 wk. 1 mo.
Downtime
Compounds: a) Polyurethane b) NBR
Figure 5-12: Level of Starting Friction Dependant Upon Time
and Compound
a)
b)
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com