Conventional pressure vessel penetration
Designers of submersible pressure vessels may avoid having a sealed shaft penetrate the wall or hull of a vessel, fearing seal failure in the crushing pressure of deep-sea environments. Conventional lip or face seals slide on the surface of the shaft and must contend with friction torque, all the greater under conditions of extreme pressure, even when anti-friction materials such as TFE are used. Moreover, the sliding surface must be very smooth and isolated from sand or grit that could cause rapid wear and damage sealing integrity.
Even so, it may be expedient to move an external control plane, for instance, by an internal actuator through such a sealed shaft for reasons of simplicity, low cost, and avoidance of exterior bulk.
Even so, it may be expedient to move an external control plane, for instance, by an internal actuator through such a sealed shaft for reasons of simplicity, low cost, and avoidance of exterior bulk.
The lamiflex bearing seal
The LAMIFLEX Bearing-Seal was developed by Randolph Research to deal with this kind of situation: very high pressure sealing with limited angles of shaft motion (not continuous), while maintaining inherent safety, small size, reasonable cost and requiring no lubrication. Failure to provide the expected sealing effect could be catastrophic, but safety is an inherent characteristic--there are no sliding surfaces of any kind. Rather, the LAMIFLEX Bearing-Seal is naturally a hermetic seal in its construction, relying solely upon movement within a thickness of rubber (i.e.,elastomeric shear) for all angular motion. Further, little torque is required to overcome friction, but rather a small spring torque develops proportional to the angle of motion.
The sectional view above and to the left shows a hull (light gray) penetrated by a shaft (light yellow). A LAMIFLEX Bearing-Seal surrounds the shaft with its load faces white) sealed by Static O-rings (black) and squeezed between a cylindrical receptacle in the hull and a flanged part of the shaft. The bearing-seal has an elastomer-metal composite construction, i.e., a circular stack of thin flat metal laminations or layers normal to the vertical axis that are separated and bonded together by thin layers of rubber. The thicknesses of the metal and rubber layers (white and black respectively) are greatly exaggerated for clarity; they are each typically only about .002" thick, and there are many of them, up to several hundred.
The large arrows on top represent hydrostatic fluid pressure (up to multiple tons per square inch) pushing down on the shaft and its flange and thereby compressing the bearing-seal against the hull receptacle, while also pressing into the annular opening surrounding the periphery of the bearing-seal. Because of the unitary bonded construction, there is no path for lateral fluid flow between the periphery and aperture--it is a hermetic seal. At the same time, it can be seen that limited angular rotation of the shaft is made possible by the accumulated circumferential shearing movements of all of the rubber layers, with an opposing spring torque resulting from their accompanying shear stress.
In the figure, the shaft is shown to be maintained in a centered position relative to the housing by a needle bearing below the bearing-seal. A low-friction reinforced TFE bushing or liner (red) surrounding the shaft flange could also be provided as shown. The bearing-seal may also include some means of lateral support against columnar deflection or buckling, if needed. One simple way is shown: a central thick metal layer extends slightly beyond the outside diameter of the laminations. Having a polished OD surface, this central washer can slide within the cylindrical housing also lined with a low-friction material, which is quite adequate for the relatively light forces that might occur. Such sliding surfaces could be shielded from a silt or mud environment with an interposed rubber boot, for instance. The bearing-seal end faces may be keyed or tabbed against torsional slippage relative to the shaft or the wall when they are unpressurized, ensuring that the spring action of the bearing will return it to the neutral position when un-torqued.
In addition, though failure of the bearing-seal is very unlikely, as shown in the illustration below, backup protection can be provided by a (normally unpressurized) O-ring seal in a groove encircling the shaft to block flow of the high-pressure fluid.
Despite high external pressures, comfortable safety factors can be designed in. Without concentrated stress points, loads are spread over the large annular area of the bearing. And since the rubber layers are very thin relative to their width, they cannot be squeezed out from between the metal layers even with normal compression loads of 10,000 psi or more. Meanwhile, the overall axial compression is very small, typically only a few thousandths of an inch.
For example, a small configuration with OD = 2.06 in. and ID = 1.00 in. was tested with 10,000 psi hydrostatic pressure, producing an average compressive stress of 13,100 psi in the bearing-seal (10000(2.06^2/(2.06^2-1^2)). The laminations of 100,000 psi tensile cartridge brass and natural rubber were both .002" thick. Torsional stiffness was 9 lb./in. at the end of an 8" lever arm, easily moveable by hand through +/-15 degrees. This design was oscillated at +/- 15 degrees for over a million cycles without failure. Many more cycles are available with normally smaller angles or greater laminate stack height. Larger or smaller dimensions, higher pressures or other parameters can be chosen according to known principles, taking into account the overall torsional stiffness, normal forces, columnar effects, differential pressures, fatigue life, and the extent of angular movement desired.
The flat laminates shown above represent the simplest type of LAMIFLEX Bearing-Seal -- other cross-sections that are conical or chevron-shaped (rather than flat) may be used to provide an inherent radial-centering and self-supporting action. Spherical and other types are also possible for specialized applications, such as a "fishtail" mode of propulsion involving a LAMIFLEX Bearing-Seal having semi-cylindrical or spherical surfaces.
Instead of a single rubber-laminated bearing-seal, two can be employed to provide double-sealing. As seen below, there are two conical LAMIFLEX Bearing-Seals, facing one another with the conforming central shaft flange sandwiched between them. The additional top bearing-seal mirrors the features of the bottom unit and is loaded by a piston that sees (and is O-ring-sealed against) the hydrostatic environment. The piston is supported within the cylindrical bore of the housing and has an aperture for extension of the shaft.
Whereas the single bearing-seal sees the high pressure on its periphery, the top bearing-seal will instead
experience the high pressure in its aperture. Adding the top bearing-seal puts it in a serial primary sealing
relation to the bottom bearing-seal, preventing fluid pressure from acting directly upon the bottom unit. If the top sealing function were to fail for any reason, the bottom bearing-seal would become the primary seal and it would function in the same manner and with the same loads as the single bearing-seal.
The large arrows on top represent hydrostatic fluid pressure (up to multiple tons per square inch) pushing down on the shaft and its flange and thereby compressing the bearing-seal against the hull receptacle, while also pressing into the annular opening surrounding the periphery of the bearing-seal. Because of the unitary bonded construction, there is no path for lateral fluid flow between the periphery and aperture--it is a hermetic seal. At the same time, it can be seen that limited angular rotation of the shaft is made possible by the accumulated circumferential shearing movements of all of the rubber layers, with an opposing spring torque resulting from their accompanying shear stress.
In the figure, the shaft is shown to be maintained in a centered position relative to the housing by a needle bearing below the bearing-seal. A low-friction reinforced TFE bushing or liner (red) surrounding the shaft flange could also be provided as shown. The bearing-seal may also include some means of lateral support against columnar deflection or buckling, if needed. One simple way is shown: a central thick metal layer extends slightly beyond the outside diameter of the laminations. Having a polished OD surface, this central washer can slide within the cylindrical housing also lined with a low-friction material, which is quite adequate for the relatively light forces that might occur. Such sliding surfaces could be shielded from a silt or mud environment with an interposed rubber boot, for instance. The bearing-seal end faces may be keyed or tabbed against torsional slippage relative to the shaft or the wall when they are unpressurized, ensuring that the spring action of the bearing will return it to the neutral position when un-torqued.
In addition, though failure of the bearing-seal is very unlikely, as shown in the illustration below, backup protection can be provided by a (normally unpressurized) O-ring seal in a groove encircling the shaft to block flow of the high-pressure fluid.
Despite high external pressures, comfortable safety factors can be designed in. Without concentrated stress points, loads are spread over the large annular area of the bearing. And since the rubber layers are very thin relative to their width, they cannot be squeezed out from between the metal layers even with normal compression loads of 10,000 psi or more. Meanwhile, the overall axial compression is very small, typically only a few thousandths of an inch.
For example, a small configuration with OD = 2.06 in. and ID = 1.00 in. was tested with 10,000 psi hydrostatic pressure, producing an average compressive stress of 13,100 psi in the bearing-seal (10000(2.06^2/(2.06^2-1^2)). The laminations of 100,000 psi tensile cartridge brass and natural rubber were both .002" thick. Torsional stiffness was 9 lb./in. at the end of an 8" lever arm, easily moveable by hand through +/-15 degrees. This design was oscillated at +/- 15 degrees for over a million cycles without failure. Many more cycles are available with normally smaller angles or greater laminate stack height. Larger or smaller dimensions, higher pressures or other parameters can be chosen according to known principles, taking into account the overall torsional stiffness, normal forces, columnar effects, differential pressures, fatigue life, and the extent of angular movement desired.
The flat laminates shown above represent the simplest type of LAMIFLEX Bearing-Seal -- other cross-sections that are conical or chevron-shaped (rather than flat) may be used to provide an inherent radial-centering and self-supporting action. Spherical and other types are also possible for specialized applications, such as a "fishtail" mode of propulsion involving a LAMIFLEX Bearing-Seal having semi-cylindrical or spherical surfaces.
Instead of a single rubber-laminated bearing-seal, two can be employed to provide double-sealing. As seen below, there are two conical LAMIFLEX Bearing-Seals, facing one another with the conforming central shaft flange sandwiched between them. The additional top bearing-seal mirrors the features of the bottom unit and is loaded by a piston that sees (and is O-ring-sealed against) the hydrostatic environment. The piston is supported within the cylindrical bore of the housing and has an aperture for extension of the shaft.
Whereas the single bearing-seal sees the high pressure on its periphery, the top bearing-seal will instead
experience the high pressure in its aperture. Adding the top bearing-seal puts it in a serial primary sealing
relation to the bottom bearing-seal, preventing fluid pressure from acting directly upon the bottom unit. If the top sealing function were to fail for any reason, the bottom bearing-seal would become the primary seal and it would function in the same manner and with the same loads as the single bearing-seal.
Lamiflex bearing-seals are robust
We ran a pressure test on a very small bearing-seal, only 17 mm OD, about the size of a dime, with a 6 mm hole. It was oscillated +/- 15 degrees at 16,000 psi for many hours. In the video below, the pressure gauge is almost maxed-out, indicating that the internal seawater pressure is about 16,000 psi. Start the video to tweak the lever arm, and you can see the resilient action of the small bearing-seal, even under all that pressure. It's free and springy! Pressure does NOT affect it.
And the You Tube video below shows a larger bearing seal at 10,000 psi while linked up to a crank shaft at a speed of 600 cycles per minute.