Parker O-Ring Handbook.pdf

For the same conditions, friction at high temperature (= low viscosity)
is high because the lubricating fi lm is often interrupted.
The most important factors can be seen in Figure 5-13. Here
friction is shown as a function of pressure and velocity. Figure
5-13 is valid only for a specifi c seal in a particular application.
For other seals and applications the interdependence varies.
The stick-slip effect also is related to the friction at the sealing
face. The friction, or better expressed the difference between
break-out and running friction, plays an important role in
evaluation and selection of a suitable elastomer.
Break-out friction occurs when the three following conditions
are present:
• When the break-out friction is higher than the running
friction a running velocity Vµ min (see Figure 5-11);
• The running velocity is Vµ min;
• The power is transmitted through the elastic body of
the “compressible” oil.
To assist in the explanation of the term stick-slip, please refer
to Figure 5-14. To accelerate a mass m from zero to maximum
velocity, the break-out friction µH must be overcome by F1. The
spring element is loaded with F1 and with increasing velocity the
friction value µH reduces to µG and the force to F2. The potential
energy stored in the spring accelerates the mass even further.
When the stored energy is used, the mass is decelerated by the
increasing friction in direction µH. This requires once again an
increase in force level of F1, and the procedure repeats again.
Frictional Force F (kN)
1.5
1.0
0.5
3
Velocity V (m/mm)
2
1
0
100
200
Parker O-Ring Handbook
Pressure P(bar)
Running velocity is a product of seal friction, the piston
mass and the load. Of all these factors, only friction can be
infl uenced and makes for a better relationship between sealing
surface fi nish, lubricating fi lm and surface fi nish. Certain
improvements can be made making the system stiffer, this
means the smallest possible oil volume under pressure on
the hydraulic side.
Radial oscillation of the piston will occur when the lubricating
fi lm breaks down. Conversely oils with strong fi lm building
properties do not break down under the same working conditions
using the same seals.
5.14.2 Pneumatic Seals
In principle the same conditions apply here as for the hydraulic
seal, except that the effects of certain extreme conditions
are more serious. This is particularly the case when lubrication
is poor, as found when lubricated air is not available.
Lubricated air gives more or less the same results as in a
hydraulic application.
When lubricating grease is not continually replaced, it can
eventually be removed by a seal lip. The effectiveness of lubrication
with grease depends on the thickness of the original
fi lm and the running velocity of the seal (Figure 5-15).
The lower the velocity the thinner will become the lubricating
fi lm. With an O-ring seal the loss of grease can lead to total
breakdown of the hydrodynamic lubricating fi lm after only
a few slow strokes.
Breakdown of the lubricating fi lm after long operation also
results in contact between the seal and the metal surfaces. This
makes the seal move in the mixed friction range, the increase in
friction causes high wear. The lubricating fi lm therefore must
be protected by rounding of the seal wiper edges and complete
wiping of grease from the running surface must be prevented.
Figure 5-13: Frictional Force is Dependent Upon Pressure and
Velocity – Compact Rod Seal 90° Shore A Figure 5-14: System Diagram for Stick-Slip Effect
Coefficient of Friction μ
μ
μH
μG
Vμmin. Velocity (V)
F m
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
Dynamic O-Ring Sealing
5-11