This invention relates generally to the field of linear actuators, and more particularly to a deep submergence electro-mechanical linear actuator.
Hydraulic linear actuators are known for use in deep submergence applications, such as tube hatch actuators for submarines. Such actuators are mounted on the exterior of a submarine hull, and are exposed to large pressure variations during operation of the submarine, such as from atmospheric pressure to submergence pressures exceeding 1,000 psi. The ingress of seawater into such actuators is avoided because the operating pressure of the hydraulic working fluid is higher than the external seawater pressure. However, the hydraulic working fluid supply system for such actuators is complicated and costly to maintain, and there is a desire to replace such hydraulic actuators with electro-mechanical actuators.
Electro-mechanical actuators do not require a high pressure working fluid, and the internal pressure within the actuator housing may typically be near atmospheric pressure. Sealing against the ingress of sea water is imperative for ensuring the reliability of any actuator operating on the exterior of a submarine hull.
It is known to use a pressure compensated hydraulic seal for rotary actuators designed for deep submergence applications. Such seals require the pressurization of the interior region of the actuator with oil using a pressure compensation system that maintains the pressure within the actuator to a value that is consistently somewhat higher than the pressure of the surrounding seawater by means of a bellows, bladder, constant force spring, etc. The relatively higher pressure within the actuator interior ensures that there is no leakage of sea water into the actuator past the drive shaft gland. One possible sealing arrangement for linear actuators is to use the same pressure compensated seal design as is used for rotary actuators. However, a typical submarine tube hatch actuator must be large enough to generate 50,000-100,000 pounds of force and may have a ram cross-sectional diameter of about 6 inches. As the ram moves into and out of the actuator interior during operation, the open interior volume of the actuator (total volume minus volume displaced by the ram and other drive mechanism components) would vary greatly, and thus the volume of pressure-compensated fluid required for the sealing function would also vary greatly. The pressure compensating apparatus that would be required for hydraulic sealing of such an apparatus would need to accommodate this substantial variation of the interior volume of the actuator as the ram travels into or out of the housing, and as such, would be large, expensive and difficult to maintain.
An embodiment of the invention for a deep submergence linear actuator comprises a housing and a ram extending from within the housing through an open end of the housing. An electro-mechanical drive mechanism is disposed within the housing distal to the open end of the housing and is configured to drive the ram in selectively reversible linear motion relative to the housing. A first dynamic seal is disposed between an outside surface of the ram and an inside surface of the housing inboard of the open end of the housing. A second dynamic seal is disposed between the outside surface of the ram and the inside surface of the housing and is disposed between the first dynamic seal and the electro-mechanical drive mechanism, thereby forming an isolation region defined between the ram and the housing and between the first and second dynamic seals. In addition, a pressure compensated isolation fluid supply is in fluid communication with the isolation region for maintaining an isolation fluid in the isolation region at a pressure greater than a fluid pressure existing exterior to the housing.
The isolation region is maintained at a pressure higher than an exterior fluid pressure surrounding the actuator so that any leakage past the first dynamic seal will be in a direction out of the actuator, thereby avoiding any ingress of exterior fluid into the actuator. Leakage past the lower dynamic seal can be tolerated because the isolation fluid is compatible with components of the drive mechanism disposed within the interior of the actuator.
The invention is explained in the following description in view of the drawings that show:
A cross-sectional view of a submersible linear actuator 10 in accordance with one embodiment of the present invention is illustrated in
Subsections of the housing 12 are joined together and sealed to prevent the incursion of seawater with any style of joint 22 known in the art, such as with bolted/gasketed flange joints. The annular space between the outside surface of the ram 14 and the inside surface of the open end 16 of the housing 12 must be sealed with a dynamic seal to allow for the linear movement of the ram 14 relative to the housing while preventing the incursion of seawater into the interior region 24 of the housing 12.
Known mechanical seals such as labyrinth seals, knife edge seals and/or ring seals may be used to seal a linear actuator ram; however, for high pressure deep submergence applications, the inevitable leakage past such mechanical seals would shorten the life of the actuator and would suggest the use of leak-off plumbing and the resulting penetrations into the submarine hull.
To overcome the limitations of prior art designs, the present inventor has innovatively developed a deep submergence linear actuator seal configuration that delivers a pressure compensated isolation fluid to only an isolation region defined between an opposed pair of gland seals disposed along the ram 14 within the housing 12. By avoiding the pressurization of the entire actuator interior as is done in prior art designs, the problem of a variable interior fluid volume is solved while the positive sealing characteristics of a pressure compensated hydraulic seal are retained. Details of one embodiment of this invention are illustrated in
Advantageously, any possible leakage across the first dynamic seal 36 should be minimized because of the relatively low pressure differential across that seal, and it will be in the direction of isolation fluid flowing out of the actuator 10 rather than sea water flowing into the actuator 10. Any possible leakage across the relatively higher pressure second dynamic seal 38 should be tolerable provided that the isolation fluid 28 is selected to be benign to any component of the electro-mechanical drive mechanism 20 exposed within the interior region 24 of the actuator. In one embodiment, the isolation fluid 28 is the same type of fluid as is used for lubrication of the moving parts of the electro-mechanical drive mechanism 20.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/186,819 filed Jun. 13, 2009, and incorporated herein by reference.
Number | Date | Country | |
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61186819 | Jun 2009 | US |