Parker O-Ring Handbook.pdf

5.15 Spiral Failure
A unique type of failure sometimes occurs on reciprocating
O-rings which is called spiral failure. This name was given
to this type of failure because when it occurs the seal looks
as if it had been cut about halfway through the O-ring cross
section in a spiral or corkscrew pattern. Oddly enough, the
O-ring usually seals satisfactorily until a complete break or
separation occurs at one place. Sometimes the seal is twisted
in two without evidence of the spiral pattern, but in general,
the same factors cause the break.
A properly used O-ring slides during all but a small fraction of
any reciprocating stroke. This type of seal does not normally
tend to roll or twist because:
1. The hydraulic pressure, acting through the O-ring,
produces a greater holding force within the groove
(friction on a larger area) than that produced by the sliding
surface (rod or cylinder wall) opposite the groove (see
Figure 5-16).
2. The smoother fi nish of the sliding surface, in relation to
the groove surface-fi nish, produces less friction.
3. Running friction is lower than break-out friction.
4. The torsional resistance of the O-ring tends to resist
twisting.
The conditions which cause spiral failure are those that simultaneously
cause segments of the ring to slide and others
to roll. A small amount of twisting is not detrimental but,
when excessive, torsional failure or spiral failure will occur.
True spiral failure occurs after the seal has been excessively
twisted, but not broken, and then subjected to relatively high
Fluid
Pressure
Shaft Small Area
Large
Area
Figure 5-16: Action of Fluid Pressure to Prevent Rolling of O-ring
O-Ring Square-Ring Delta-Ring
X-Ring
All are subject to torsional or spiral failure.
Figure 5-17: Ring Cross-Sections for Reciprocating Seals
Parker O-Ring Handbook
pressure. The twisted seal is forced into the sharp corner at
the clearance gap by the pressure which puts an additional
stress on this portion of the seal. Rapid stress-aging, or stress
above the elastic limit of the rubber, causes a rupture of the
O-ring to start adjacent to the clearance gap. Slight fl exing,
motion, or working of the O-ring apparently causes the rupture
to penetrate about half way through the cross section.
When the O-ring is removed from the gland, it returns to its
original shape and the rupture appears as a tight spiral around
the cross section.
Torsional or spiral failure is not limited to the O-ring or torus
type of seal. Square, delta, four-leaf clover, and other cross
sectional shapes (see Figure 5-17) are also prone to fail by
twisting if the proper conditions exist.
The design and operational factors which contribute to spiral
failure of a seal are listed below in the order of their relative
importance:
1. Speed of stroke
2. Lack of lubrication
3. Pressure differential and direction
4. Squeeze
5. Shape of groove or split grooves
6. Temperature of operation
7. Length of stroke
8. Surface fi nish of gland
9. Type of metal surface
10. Side loads
11. ID to W ratio of O-ring
12. Contamination or gummy deposits on metal surface
13. Type of metal rubbing surface
14. Breathing
15. Concentricity of mating metal parts
16. Stretch of O-ring (see rotary shaft seals)
17. Lack of back-up rings
18. Poor installation of O-rings
Only the very important or less obvious factors which contribute
to spiral failure will be discussed. Some of those which
have been discussed elsewhere will also be omitted here. It
should be remembered that before spiral failure can occur,
an O-ring must be twisted by one or more of the above interrelated
factors. Usually, several factors combine to produce
any failure that develops. Some of the other seal designs will
leak excessively when twisted. The O-ring usually seals until
complete failure occurs.
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
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