Parker O-Ring Handbook.pdf

It can be clearly seen from Figure 5-6 that surfaces produced
by roller burnishing have no sharp peaks which can cause
damage to a seal. Further, the valleys form potential lubrication
reservoirs which improve the dynamic behavior of a seal.
Surface fi nish values obtained in a single test are possibly
not typical. For this reason several readings should be taken.
When several results are to be compared, the length of the
test surface must be stated — for different trace lengths,
results are not comparable because they result from other
profi le heights.
5.4 Temperature Effects On Dynamic Seals
High Temperatures — It should be remembered that the
higher the temperature (above 38°C) (100°F) in and around
a reciprocating gland, the more critical the application becomes.
The higher the interface temperature, the greater the
tendency of the lighter fractions of the oil to evaporate from
an exposed surface. Lack of lubrication will cause greatly
accelerated seal wear. If the temperature is high enough, the
tacky residue (resins) which remains after oil evaporation
will char and create a hard, abrasive surface which, if not
removed, will quickly abrade away the seal until leakage or
complete seal failure occurs.
Low temperatures — Low temperature environments are
most troublesome, especially if the seal has been operating
at a high temperature for some time. This is because the
elastomer in the seal will take a compression set at high
temperature. When the seal is then subjected to low temperature,
there may be insuffi cient elastic memory to overcome
the relatively high coeffi cient of shrinkage (10 times that of
steel) at low temperatures.
1. R t
2. R t
3. R t
4. R t
5. R t
6. R t
7. R t
8. R t
R p
R p
R p
R p
R p
R p
R p
R p
Rt Rp Ra tp (%)
µm µm µm 0.25 0.50 0.75 Rt
1 0.5 0.5 50 50 50
1 0.5 0.5 50 50 75
1 0.5 0.5 50 50 75
1 0.75 0.28 12.5 25 37.5
1 0.25 0.28 62.5 75 87.5
1 0.785 0.188 3.5 14 35
1 0.215 0.188 65 86 96.5
1 0.5 0.39 43 50 57
Table 5-2: Diagramatic Representation of Surface Profi les
Parker O-Ring Handbook
Once unseated from a spot on a given metal surface, the seal
must be reseated by internal seal resilience or system pressure.
Therefore, it is much easier to seal a hydraulic system that goes
from zero-pressure to high-pressure almost instantaneously.
Low-pressure fuel, pneumatic, oil, and similar fl uid systems
are prone to leak if an O-ring is used as a dynamic seal at
-54°C (-65°F) because there is insuffi cient pressure to keep
the O-ring tightly seated during and immediately after motion
of the gland. Remember that the -54°C (-65°F) compound is
fl exible and capable of acceptable seal performance at -54°C
(-65°F) but may not be resilient below -43°C (-45°F).
5.5 Side Loads
Side loads on a piston or rod can cause the clearance in the
gland to be on one side only. If excess clearance is created by
side-loading, extrusion will result. If adequate squeeze has not
been applied, leakage will result. The higher unit load on the
opposite side causes uneven friction on the seal, and if high
enough, the rod or barrel will be galled or scored.
5.6 Direction of Pressure
The placement of a groove can be determined from the direction
of the system pressure in relation to the direction of
the moving friction force. If the friction of the moving metal
surface across the O-ring is in the same direction as the direction
of pressure, the O-ring will tend to be dragged into the
gap more readily and thus extrude at only 30 to 40% of the
pressure normally necessary to cause extrusion. By placing
the groove in the opposite metal part, any friction will work
against pressure. Snubbing cylinders, in which the motion
and force create the pressure, are the usual culprits.
5.7 Shock Loads and Pressures
Shock pressures, such as those created by the sudden stopping
of a rapidly descending hydraulic hoist cylinder on
which there is a heavy load, are often far in excess of the
pressure for which the seal and the system were designed.
The same could be said about the whip of a gun barrel, of
a tank on rough roads, or a truck tailgate and others if they
a) Cold Formed Surface
b) Machined Surface
Figure 5-6: Surfaces Produced by Roller Burnishing (a) and by
Normal Machining (b)
R p1
R p2
R t1
R t2
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
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Dynamic O-Ring Sealing