Parker O-Ring Handbook.pdf

Dynamic O-Ring Sealing
5-14
5.15.1 Speed of Stroke
Investigations have disclosed that one of the primary causes of
spiral failure is by reciprocating speeds of less than one foot
per minute. It appears that at this slow speed, the sliding or
running seal friction created is very high and comparable to
break-out friction. Extreme twisting will occur on low or balanced
pressure components, such as hydraulic accumulators, in
a relatively few (about 200) cycles if the temperature is above
39°C (100°F). O-ring seals are not recommended, therefore,
for speeds less than one foot per minute when the pressure differential
is less than 27.6 Bar (400 psi). If the system pressure
is slowly lost, as through slow valve leaks, and a sealed piston
moves slowly through a cylinder a number of times, spiral
failure of the O-ring very probably will result. The obvious
remedy here is to provide good maintenance of the system so
that slow leaks are prevented, or make it an operational practice
to quickly exhaust the system after the day’s work.
5.15.2 Lack of Lubrication
The lack of lubrication on a surface exposed to the atmosphere
is one of the prime contributors to spiral failure. Excessive
wear will normally occur. However, twisting of the seal and
spiral failure can result if the unlubricated surface is actuated
through the seal with little or no pressure on the seal to hold
it and prevent it from rolling. This applies primarily to long
stroke (greater than 152.4 mm (6")) applications.
The remedy for this situation is to:
a. Use lubricating (or lubricated) wiper rings.
b. Apply a suitable grease, that will not evaporate, to the
exposed surface.
c. Use a fl uid that will not tend to evaporate or become
tacky at the operating temperature.
d. Lubricate metal surface prior to assembly.
e. Use a metal or surface plating that will produce less
friction.
5.15.3 Pressure Differential and Direction
As explained earlier, the direction of pressure and seal friction
should oppose each other. Spiral failure is more likely
to occur if the pressure and seal friction are both in the same
direction. In other words, seals in a pump are more likely to
spiral than are those in an actuator.
Normally an O-ring will not twist when the pressure differential
across the seal is greater than 27.6 Bar (400 psi)
during operation.
5.15.4 Squeeze
The aerospace industry has generally found that more than
0.043 mm (0.017 in.) of squeeze on the side of a 5.3 mm
(0.210") cross section (W) O-ring will make some long stroke
applications prone to spiral failure. It can be easily seen that
more rolling force is created on the cross section with an increase
in squeeze. Other factors are normally involved when
failure occurs with the standard squeezes recommended for
reciprocating seals.
Parker O-Ring Handbook
5.15.5 Shape of Groove and Split Groove
If a V-shaped groove is used, it is evident that the hydraulic
holding force is reduced because the area on the side of
the V-groove is less than at the bottom and side of a square
groove. V-grooves are much more prone to produce spiral
failures. This is especially true if any of the other factors are
out of balance. Split grooves give trouble if the hydraulic
holding force on the O-ring against both the side and the
bottom of the groove is not maintained. Great care should
be used when designing glands which have an opening in the
bottom in order to make sure the normal holding force will
be maintained (see Figure 5-16).
5.15.6 Temperature of Operation
When the temperature in and around a system is substantially
increased, the seals are more prone to fail. This is because
lubricants are more likely to evaporate, or lose their, “light
ends”, and/or lose some of their lubricity, the seal becomes
softer, the squeeze is increased due to the rubber expansion,
and the metal clearances may become greater.
5.15.7 Length of Stroke
As a general rule, the longer the stroke of a cylinder or rod,
the greater the eccentricity, bending, side load, and other factors
that contribute to wear and/or spiral failure. We do not
recommend an O-ring for service when the stroke is greater
than 304.8 mm (12") unless extra precautions are taken to
avoid trouble.
5.15.8 Surface Finish
When a cylinder or rod is actuated, side loads, bending, chips
or other foreign material, and non perfect machining, drilling
and fi nishing all in some way tend to contribute to scoring,
galling, marring, or scratching of the surface over which the
seal must slide (refer to metals and fl oating glands). When
this occurs, the roughness is unevenly distributed around the
circumference or periphery. Even though it may be very slight,
it creates an uneven friction condition and thus can contribute
to spiral failure and/or uneven, excessive wear.
5.15.9 Back-Up Rings
Back-up rings sometimes provide enough extra lubrication on
the return stroke to assist in the prevention of spiral failure.
For further information see the discussion on back-up rings
in Section VI.
5.16 Modifi cations for Special Applications
Normally, the gland dimensions given in Design Tables
5-1 and 5-2 are adequate and give trouble-free service. If
not applicable, the following modifi cations will help solve
specifi c problems:
• Small Amount of Leakage
• Early Stress-Aging
• Low Temperature Leakage
• Excessive Swells (above 20%)
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com