| In
contrast to static
seals, dynamic
seals exist where there is relative
motion between the mating
surfaces being sealed. In most instances,
the dimensional variations inherent in dynamic
seals make them more difficult to design and
more expensive to construct than static seals.
Nevertheless, dynamic O-ring seals are indispensable
to a wide variety of applications. Here’s
a closer look at the major types of dynamic seals: RECIPROCATING
SEALS
Reciprocating
seals involve relative reciprocating motion along
the shaft axis
between the inner and outer elements. In reciprocating
seal applications, the O-ring slides or rocks back
and forth within its gland with
the reciprocating motion. Reciprocating
seals are most often seen in cylinders and
linear actuators. Some examples of reciprocating
O-ring seals are shown in Figures 113
and 114. Gland design measurements
for industrial reciprocating O-ring seals can be
found in Table 48. Gland
dimensions can be found in Table 49. FLOATING
PNEUMATIC PISTON SEALS
Floating pneumatic piston seals are reciprocating
in nature, but the way in which the seals are effected
is unique. Normal reciprocating designs rely on the
O-ring being stretched over a piston and then squeezed
radially (on the inside diameter, or I.D., and
the outside diameter, or O.D.). In
floating O-ring designs, however, there is no radial squeeze on
the seal’s cross-section.
The O-ring’s O.D. is larger than the cylinder
bore diameter. Peripheral squeeze is applied to
the O.D. as the O-ring is installed into the bore.
Incoming air pressure forces the O-ring against
the groove wall,
and a seal is effected as shown in Figure
115. Floating
designs offer a number of advantages, including
greatly reduced breakout
friction and longer seal life. Floating pneumatic
piston seals are suited for applications in which
the air pressure does not exceed 200 psi (or
in hydraulic designs where a small amount of leakage
is permissible). Floating O-rings are NOT suitable
as rod seals. Gland design measurements for floating
pneumatic piston O-ring seals can be found in Table
50. Gland dimensions can be found
in Table 51. ROTARY
SEALS
Rotary
seals involve motion between a shaft and a housing.
Typical rotary seals include motor shafts and wheels
on a fixed axle. Installation of a rotary O-ring
seal is shown in Figure 116. R.L.
Hudson & Company recommends lip type shaft
seals for most rotary applications. There are applications,
however, where an O-ring will provide an effective
rotary seal. O-ring
seals are NOT recommended for rotary applications
under the following conditions: • Pressures
exceeding 800 psi.
• Temperatures
lower than -40° C (-40° F) or higher than
107° C (225° F).
• Surface
speeds exceeding 600 feet per minute (fpm). Note:
Feet per minute = .2618 X shaft diameter (inches)
X rpm When
an elastomer is
stretched and heated, it will contract. This is
called the Gough-Joule
effect. This is an important design consideration
in a rotary application because if an O-ring is
installed in a stretched condition, frictional heat
will cause the O-ring to contract onto the shaft.
This may cause the O-ring to seize the rotating
shaft so that the dynamic interface
becomes the O-ring O.D. and the groove I.D. The
contraction will also cause more frictional heat,
further exacerbating the situation and causing
premature failure of the O-ring. We
designed our rotary O-ring seals so that the free
O-ring I.D. is larger than the shaft onto which
it fits. The gland I.D. is smaller than the free
O-ring O.D. so that when it is placed into the
gland, the O-ring is peripherally squeezed, and
the I.D. is reduced so that a positive interference
exists between the O-ring I.D. and the shaft. Because
the O-ring is not in a stretched condition, it
will not build up heat, seize the shaft, and rotate
in the groove. Rotary
seals (such as the one shown in Figure
117) do not dissipate heat as well
as reciprocating seals do, so provisions must be
made to keep heat
build-up to a minimum. • The
housing I.D. should not be used as a bearing surface.
• Bearings
should be provided to ensure that the shaft
runout does not exceed .002" TIR.
• The
O-ring groove should be located away from the bearing
and close to the lubricating fluid.
• The
housing length should be 8 to 10 times the O-ring
cross-section to provide for better heat transfer. To
prevent extrusion of
the O-ring, we recommend the clearance
gap (extrusion gap) to be no more than .005" per
side. If pressures greater than 800 psi are encountered,
it is recommended that an 80 durometer O-ring
be used. The
minimum hardness for
the section of shaft that comes into contact with
the O-rings is Rockwell C30. To prevent excessive
wear, scratches, nicks, and handling damage, a
hardness of Rockwell C45 is recommended. A shaft
finish of 10-20 micro-inches is recommended, and
plunge grinding with no machine lead is the preferred
finishing method. The shaft ends should be chamfered with
a 15/30° chamfer to prevent installation damage. Gland
design measurements for rotary O-ring seals can
be found in Table 52.
Gland dimensions can be found in Table
53. OSCILLATING
SEALS
Oscillating
seals are commonly used in faucet valves. In
oscillating applications, the shaft or housing rotates
back and forth through a limited number of turns
around the axis of the shaft. An oscillating O-ring
seal is shown in Figure 118. Because
the surface speed in oscillating seals is so slow,
reciprocating design charts are used. The
gland dimensions for the above seals can be calculated
with our online O-Ring
Calculator. |
