Abstract:
A linear actuator includes an actuator wall, and the actuator wall includes a first wall layer having an inner surface that defines an actuator chamber. The actuator chamber is configured to accommodate an actuator fluid. The first wall layer is also subjected to a pre-load such that the first wall layer is compressively pre-stressed. The actuator wall further includes a second wall layer disposed outwardly from the first wall layer. The linear actuator further includes a piston supported within the actuator chamber, and the piston is movable in response to the actuator fluid entering and exiting the actuator chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit to U.S. Provisional Application No. 61/347,677 filed May 24, 2010. 
     
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to linear actuators, particularly hydraulic actuators having multiple-layered walls. 
       BACKGROUND OF THE INVENTION 
       [0004]    High-pressure hydraulic actuators typically operate at pressures in the range of 500-700 bar (˜7250-10150 psi). Ultra high-pressure hydraulic actuators operate at pressures greater than those of the above range. Considering these pressures, the walls of these actuators are subjected to high hoop stress. To resist this stress, actuator walls are typically thick (e.g., 1 inch or more) and comprise high-strength materials (e.g., a high-strength steel). However, the highest stress occurs at the inner surface of the actuator wall, and the stress decreases from the inner surface to the outer surface. As such, most actuator walls make inefficient use of material because high-strength materials are not needed in portions of the wall away from the inner surface. 
         [0005]    Furthermore, some materials, such as some corrosion-resistant materials, cannot be considered for use in high-pressure hydraulic actuators due to their relatively low strength and the high stress near the inner surface of the actuator wall. However, the use of such materials could address drawbacks of actuators comprising high-strength materials, such as actuator corrosion. 
         [0006]    Further still, in order to provide the high-strength and thick sections described above, actuator walls are typically manufactured by machining solid billet. Unfortunately, such a process wastes a large amount of material by cutting the billet to provide an internal actuator chamber. This causes relatively high manufacturing times and material costs, both of which are ultimately reflected in the cost of the final product. 
         [0007]    Considering the above drawbacks, an improved actuator wall structure and a method for its manufacture are needed. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect, the present invention provides a linear actuator comprising an actuator wall having a first end and a second end. The actuator wall includes a first wall layer that has an inner surface that partially defines an actuator chamber, and the actuator chamber is configured to accommodate an actuator fluid. The first wall layer is also subjected to a pre-load such that the first wall layer is compressively pre-stressed. The actuator wall further includes a second wall layer disposed outwardly from the first wall layer. The linear actuator further comprises a first actuator cap supported at the first end of the actuator wall and a second actuator cap supported at the second end of the actuator wall. The first and second actuator caps partially define the actuator chamber. The linear actuator further includes a piston supported within the actuator chamber, and the piston is movable in response to the actuator fluid entering and exiting the actuator chamber. A rod is supported by the piston so as to move with the piston, and the rod extends through the second actuator cap as the piston moves. 
         [0009]    In another aspect, the present invention provides a linear actuator wall comprising a first wall layer having an inner surface defining an actuator chamber. The first wall layer comprises steel and is subjected to a pre-load such that the first wall layer is compressively pre-stressed. The linear actuator wall further comprises a second wall layer disposed radially outwardly from the first wall layer. The second wall layer comprises aluminum. 
         [0010]    In yet another aspect, the present invention provides a method of manufacturing a linear actuator comprising the steps of: forming an actuator wall by: a) providing a first wall layer having an inner surface defining an actuator chamber, the actuator chamber being configured to accommodate an actuator fluid; b) providing a second wall layer; c) positioning the first wall layer within the second wall layer such that the first wall layer is subjected to a pre-load that compressively pre-stresses the first wall layer; and movably positioning a piston within the actuator chamber. 
         [0011]    The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0013]      FIG. 1  is a longitudinal section view of a linear actuator wall according to the present invention; 
           [0014]      FIG. 2  is a longitudinal section view of a linear actuator including the actuator wall of  FIG. 1 , a piston, a rod, and end caps; and 
           [0015]      FIG. 3  is an exemplary stress chart of the actuator wall of  FIG. 1  compared to a previous actuator wall structure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    A linear actuator (e.g., a hydraulic actuator) according to the present invention includes an actuator wall having multiple layers. This multi-layered wall construction permits specific materials (e.g., high-strength materials, corrosion-resistant materials) to be used in specific areas where they are particularly useful (e.g., high-stress areas, corrosion-prone areas). The multi-layered construction also permits one or more of the layers to be pre-loaded such that the actuator wall is subjected to a lower maximum operating stress compared to previous actuator walls. Furthermore, the material of each layer and the magnitude of the pre-load can be specified based on other application-specific considerations and advantages. These aspects are described in further detail below. 
         [0017]    Referring to  FIGS. 1 and 2 , the linear actuator  10  includes an actuator wall  12  that is described in further detail below. Other components of the linear actuator  10  supported by the actuator wall  12  will first be briefly described. 
         [0018]    Generally, the linear actuator  10  includes a piston  14  and a rod  16  disposed within a chamber  18  partially defined by the actuator wall  12 . The piston  14  moves within the actuator chamber  18  as an actuator fluid (e.g., hydraulic oil) enters and exits the actuator chamber  18 , and the rod  16  extends out of the chamber  18  by various amounts as the piston  14  moves. The actuator wall  12  also supports a first actuator cap  20  at a first end and a second actuator cap  22  at a second end, both of which also partially define the actuator chamber  18 . The first actuator cap  20  includes an actuator fluid passageway  24  in fluid communication with the actuator chamber  18  for delivering and receiving the actuator fluid. The second actuator cap  22  includes a rod passageway  26  through which the rod  16  passes as the piston  14  moves. The actuator caps  20  and  22  and the piston  14  may support seals  28  (e.g., polymer o-rings) to prevent the actuator fluid from leaking from the actuator wall  12  or between the opposite sides of the piston  14 . 
         [0019]    Still referring to  FIGS. 1 and 2  and as briefly described above, the actuator wall  12  has a multi-layered construction. Specifically, the actuator wall  12  includes a first wall layer  30  that has a generally open-cylindrical or tubular shape. That is, the first wall layer  30  has an inner surface  32  that defines the actuator chamber  18 . The first wall layer  30  also has open first and second ends disposed proximate the first and second actuator caps  20  and  22 , respectively. The first and second ends are preferably spaced apart such that the first wall layer  30  extends over the entire stroke of the piston  14 . The first wall layer  30  also has an outer surface  34  opposite the inner surface  32 . 
         [0020]    The first wall layer  30  may comprise any of a variety of materials depending on, for example, application-specific considerations. For example, the first wall layer  30  may comprise a high-strength material, such as 0.25 inch thick 4140 chromium-molybdenum steel, to resist high stress near the inner surface  32  imparted by the pressurized actuator fluid. As another example, the first wall layer  30  may comprise a corrosion-resistant material, such as stainless steel, in applications where corrosion is a concern. As yet another example, the first wall layer  30  may comprise a relatively inexpensive material, such as 1045 carbon steel, to reduce costs if operating pressures are relatively low. As yet another example, the first wall layer  30  may comprise bronze to provide a bushing-like interface for engaging the piston  14 . Other appropriate materials may also be used without departing from the scope of the invention. 
         [0021]    The actuator wall  12  further includes a second wall layer  36  disposed radially outwardly from the first wall layer  30 . Like the first wall layer  30 , the second wall layer  36  has a generally open-cylindrical or tubular shape. That is, the second wall layer  36  includes an inner surface  38  that engages the outer surface  34  of the first wall layer  30  along the entire length of the first wall layer  30 . The second wall layer  36  also has an outer surface  40  opposite the inner surface  38 . The second wall layer  36  also includes first and second ends  42  and  44  that preferably extend past those of the first wall layer  30  and are spaced apart such that the second wall layer  36  extends over the entire stroke of the piston  14 . Unlike the first wall layer  30 , however, the first and second ends  42  and  44  of the second wall layer  36  may threadably engage the first and second actuator caps  20  and  22 , respectively. The first end  42  of the second wall layer  36  also includes an actuator fluid opening  46  in fluid communication with the actuator fluid passageway  24  of the first actuator cap  20 . 
         [0022]    The second wall layer  36  may comprise any of a variety of materials depending on, for example, application-specific considerations and/or the material of the first wall layer  30 . For example, to provide a relatively inexpensive support layer for the first wall layer  30 , particularly if the first wall layer  30  comprises 4140 chromium-molybdenum steel, the second wall layer  36  may comprise 0.5 inch thick aluminum. As another example, the second wall layer  36  may comprise a medium-strength material, such as 1045 steel, particularly if the first wall layer  30  comprises stainless steel. Other appropriate materials may also be used without departing from the scope of the invention. 
         [0023]    Referring now to  FIG. 3 , the first and second wall layers  30  and  36  are sized to provide an interference or press fit at the interface between the layers. That is, the diameter of the inner surface  38  of the second wall layer  36  is slightly smaller than the diameter of the outer surface  34  of the first wall layer  30 . This size difference applies a pre-load to both of the wall layers  30  and  36 . Specifically, the press fit applies a pre-load that compressively pre-stresses the first wall layer  30  (i.e., the pre-load forces the first wall layer  30  radially inwardly). This compressive pre-stress is shown at line segment  50  in  FIG. 3 . Conversely, the press fit applies a pre-load that tensively pre-stresses the second wall layer  36  (i.e., the pre-load forces the second wall layer  36  radially outwardly). This tensile pre-stress is shown at line segment  52  in  FIG. 3 . A portion of the second wall layer  36  may also be subjected to a compressive pre-stress due to another press fit as described in further detail below. 
         [0024]    In operation (i.e., when the actuator chamber  18  is pressurized by actuator fluid), both wall layers  30  and  36  are subjected to tensile stress. This tensile stress is shown at line segments  54  and  56 , respectively, in  FIG. 3 . However, the maximum stress experienced by the first wall layer  30  is relatively low compared to the maximum stress experienced by a previous actuator wall (shown at line segment  58 ) due to the compressive pre-stress. Furthermore, the tensile stress experienced by the second wall layer  36  is similar to the stress experienced by both the first wall layer  30  and the middle portion of a previous actuator wall. 
         [0025]    Those skilled in the art will appreciate that the operating stress experienced by the first wall layer  30  may be further decreased by using a tighter interference fit. However, such a fit would also increase the pre-stress experienced by the first wall layer  30 . Conversely, the operating stress experienced by the first wall layer  30  may be increased and the pre-stress experienced by the first wall layer  30  may be decreased by using a looser interference fit. 
         [0026]    The previous paragraphs and the stress graph shown at line segments  54  and  56  in  FIG. 3  illustrate several advantages of the actuator wall  12 . For example and as described above, the material for each layer may be selected based on the maximum stress experienced by each wall layer instead of the overall maximum stress experienced by the actuator wall. As another example, the pre-stress experienced by the wall layers can provide a lower maximum stress and more uniform stress across the thickness of the wall for a given operating pressure compared to previous actuator walls. As such, the actuator wall  12  can include multiple layers of relatively low-strength materials for a given operating pressure, or the actuator wall  12  can include multiple layers with high-strength materials and operate at a higher pressure compared to previous actuator walls. As yet another example and as shown in  FIG. 3 , the second wall layer  36  may experience a higher maximum operating stress than the first wall layer  30 . Such a phenomenon permits the first wall layer  30  to comprise a relatively low-strength material that provides other advantages (e.g., stainless steel) if the second wall layer  36 , and any additional layers beyond the first wall layer  30 , in total, is/are stiff and strong enough to support the first wall layer  30 . 
         [0027]    Returning now to  FIGS. 1 and 2 , the actuator wall  12  includes a third wall layer  60  that further reduces the stress experienced by the wall layers. However, in some embodiments, the actuator wall  12  may include only first and second wall layers  30  and  36 . Further still, in other embodiments, the actuator wall  12  may include four or more layers, although manufacturing costs generally increase as the number of wall layers increases. 
         [0028]    Like the first wall layer  30 , the third wall layer  60  has a generally open-cylindrical or tubular shape and an inner surface  62  that engages the outer surface  40  of the second wall layer  36  along the entire length of the third wall layer  60 . The third wall layer  60  also has open first and second ends disposed proximate the first and second ends  42  and  44  of the second wall layer  36 , respectively. However, first and second ends of the third wall layer  60  are closer together than those of the second wall layer  36  and thereby define a shorter layer than the second wall layer  36 . That is, the third wall layer  60  is relatively short and may only extend over the stroke of the piston  14 . 
         [0029]    The third wall layer  60  may comprise any of a variety of materials depending on, for example, application-specific considerations and/or the materials of the first and second wall layers  30  and  36 . For example, the third wall layer  60  may comprise 0.375 inch thick high-strength steel, particularly if the first wall layer  30  comprises steel and the second wall layer  36  comprises aluminum, to prevent fatigue failure of the second wall layer. Other appropriate materials may also be used without departing from the scope of the invention. 
         [0030]    Referring again to  FIG. 3 , like the first and second wall layers  30  and  36 , the second and third wall layers  36  and  60  are sized to provide a press fit at the interface between the layers. That is, the diameter of the inner surface  62  of the third wall layer  60  is slightly smaller than the diameter of the outer surface  40  of the second wall layer  36 . This size difference applies a pre-load to both of the wall layers  36  and  60 . Specifically, the press fit applies a pre-load that compressively pre-stresses the second wall layer  36  (i.e., the pre-load forces the second wall layer  36  radially inwardly). This press fit, together with the press fit between the first and second wall layers  30  and  36 , may subject one portion of the second wall layer  36  to a compressive pre-stress and another portion of the second wall layer  36  to a tensile pre-stress. It may also increase the compressive prestress on the first wall layer  30 . Conversely, the press fit between the second and third wall layers  36  and  60  applies a pre-load that tensively pre-stresses the third wall layer  60  (i.e., the pre-load forces the third wall layer  60  radially outwardly). This tensile pre-stress is shown at line segment  64  in  FIG. 3 . 
         [0031]    In operation, the third wall layer  60  is subjected to tensile stress. This tensile stress is shown at line segment  66  in  FIG. 3 . Furthermore, the stress experienced by the third wall layer  60  is similar to the stress experienced by both the first wall layer  30  and the second wall layer  36 , although it is slightly greater than the stress experienced by the outer portion of a standard actuator wall. As such, stress on the actuator wall  12  may be more uniform across the thickness of the wall compared to previous actuator walls. 
         [0032]    The linear actuator  10  is preferably manufactured as follows. First, three pieces of tube stock are cut to appropriate lengths for providing the first wall layer  30 , the second wall layer  36 , and the third wall layer  60 . The pieces of tube stock preferably have the nominal inner and outer diameters of the first wall layer  30 , the second wall layer  36 , and the third wall layer  60 , respectively. However, it is unlikely that the pieces of tube stock will be accurately sized for providing the desired interference and pre-load between the wall layers. As such, the pieces of tube stock are then ground or honed to provide these dimensions. Next, the first wall layer  30  is slid into the second wall layer  36  to provide the press fit there between, and the second wall layer  36  is slid into the third wall layer  60  to provide the press fit there between. The piston  14  and the rod  16  are then positioned within the actuator chamber  18 , and the actuator caps  20  and  22  are then connected to the actuator wall  12 . 
         [0033]    The steps of the above manufacturing method may be varied without departing from the scope of the invention. For example, high forces are needed to slide the wall layers relative to one another and thereby provide the press fits. As such, the press fits may be provided in other manners, such as heat shrinking. Furthermore, if the first wall layer  30  becomes worn during use, it may be removed and replaced by a new first wall layer  30 . 
         [0034]    The hoop stress experienced by the actuator wall may be more uniform across the thickness of the wall compared to previous actuator designs, and appropriate materials for each layer may be selected accordingly. Similarly, the maximum operating stress experienced by the actuator wall for a given operating pressure is less than that experienced by similarly-sized previous actuator designs. As such, a linear actuator according to the present invention can be operated at higher pressures compared to previous actuator designs. Furthermore, the multi-layered construction of the actuator wall permits specific materials (e.g., high-strength materials, corrosion-resistant materials) to be used in specific areas where they are particularly useful (e.g., high-stress areas, corrosion-prone areas). Particularly, in some cases the multi-layered construction permits relatively low-strength materials to be used for the inner wall layer. Further still, manufacturing methods for the actuator wall use tube stock instead of wasting a large amount of material by machining solid billet. 
         [0035]    A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. Therefore, the invention should not be limited to the embodiment described, but should be defined by the claims that follow.