Parker O-Ring Handbook.pdf

5.16.1 Small Amount of Leakage
1. Examine the O-ring for signs of cutting during
installation.
2. Increase the squeeze on the cross-section of O-ring.
3. Reduce the groove length. A wide groove may cause
leakage because of pumping action of the O-ring. This
is especially possible when the piston is cycled rapidly.
4. Improve the surface fi nish of metal rubbing surface.
5. Check for eccentric machining of gland.
5.16.2 Early Stress-Aging
1. Redesign groove to reduce stretch of the O-ring.
2. Redesign groove to reduce squeeze of the O-ring.
3. Use a more heat-resistant rubber compound.
4. Make certain O-ring is not being twisted during dry
assembly.
5. Use larger O-ring to reduce stretch.
6. Make sure O-rings are not closer than six feet from an
electric motor (operating) during shelf storage. Ozone
causes rapid deterioration of most elastomers.
5.16.3 Low Temperature Leakage
1. Make certain that O-ring compound was designed for
operation at low temperatures.
2. Increase squeeze of the O-ring. Coeffi cient of contraction
of rubber is about 10 times that of steel and several
times greater than aluminum.
3. Spring load the O-ring (see Figure 3-1).
4. Make sure all gland surfaces are smooth enough (see
paragraph 5.3).
Note: Minute leakage is to be expected and is in fact, desirable,
when an O-ring is used as a reciprocating seal. An O-ring
that does not by-pass a little fl uid at each stroke is running
dry and high friction and rapid seal wear will result.
5.16.4 Excessive Swell (above 20%)
1. Replace O-ring with one made from a compound more
resistant to the fl uid being sealed.
2. Increase groove length. If the volume of the groove is
too small, increased friction and excessive stress may
cause premature failure of the O-ring (refer to discussions
of friction and spiral failure).
5.17 Gland Dimensions for
Reciprocating Hydraulic O-Ring Seals
For most reciprocating applications in which an O-ring is
sealing a liquid of any kind (the design is not limited to hydraulic
oils), the dimensions of either Design Table 5-1, the
military design, or Design Table 5-2, the industrial design,
would be suitable. Of the two, Parker Seal Group normally
recommends the Table 5-2 dimensions because these industrial
designs, in most cases, allow for the use of standard drill rod
Parker O-Ring Handbook
X X
Washer must be
tight against
shaft shoulder
Figure 5-18: Floating Gland
E
2
Snap
Ring
Clearance: X = 0.003 to 0.006 in.
E To prevent extrusion
2 refer to Figure 3-2
or Table 5-1 or 5-2
sizes and standard boring tools. The dimensions in this table
are actually in good agreement with early versions of the
aerospace table. The military dimensions cause less stretch
on the O-rings. The percent reduction is so slight, except in
the smallest sizes, that the effect cannot be signifi cant, while
the cost of the special machined rods and boring tools that
are required could be high.
In reciprocating applications for which neither table applies
because of a predetermined dimension that does not agree, the
following procedure may be used to fi nd gland dimensions.
1. For piston seals, select an O-ring having an OD near to or
preferably slightly larger than the cylinder bore diameter.
2. For rod seals, select on O-ring having an ID closest to
the rod diameter. It may be slightly larger or smaller,
but ID stretch should not exceed 5% as installed for
optimum design.
3. In all reciprocating seals, make sure minimum squeeze
recommendations are considered.
5.18 Floating Glands
Since it is impossible to bore, drill or tap perfect, true holes,
and to machine perfect parts providing perfect alignment,
the engineer should consider the fl oating gland. Eccentricity
(lack of concentricity) is allowable, but it does cause high
unit loads on small portions of bearing surfaces. In turn, this
causes minute scratches on the metal surface on which the
O-ring must rub (with the possible exception of very soft
bearing materials, such as babbitt).
In order to reduce or eliminate the high bearing loads, the
relatively inexpensive fl oating gland should be used whenever
possible. The object of this gland is to allow the piston or rod
bearing (containing the O-ring groove), to pivot, adjust, or fl oat
a small amount, offsetting misalignment. (See Figure 5-18.)
This gland design increases the life of the O-ring and eliminates
many of the spasmodic or unscheduled failures, as well
as reducing the maintenance cost.
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
X
Dynamic O-Ring Sealing
5-15