Field of the Invention

[0001] The present invention is directed to a variable volume container suitable for use in a variety of fluid power systems, including without limitation, a re-circulating hydraulic system.

Description of the Related Art

[0002] Traditional re-circulating hydraulic systems, such as power steering systems for motor vehicles, include a fluid reservoir that provides fluid to a hydraulic pump via a low pressure supply hose. The hydraulic pump pressurizes the fluid and feeds it to an actuator, such • as a steering rack, through a high pressure hose assembly. The displaced fluid from the actuator returns to the reservoir via a low pressure return line.

[0003] In a re-circulating hydraulic system, the reservoir serves a variety of functions. It provides a serviceable means of charging the system with fresh fluid. It also holds excess fluid created from thermal changes in the volume of hydraulic fluid and provides a means of allowing any air to separate out of the fluid while resident in the reservoir. However, the use of a reservoir is often undesirable, since a reservoir occupies a relatively large amount of space and necessitates the use of a relatively large amount of fluid.

[0004] To overcome these limitations, re-circulating hydraulic systems have been developed that provide a variable volume container, such as an expandable hose, in fluid communication with the low pressure return line so as to define a volume buffer to accommodate increases and decreases in the volume of hydraulic fluid in the system. The variable volume container replaces the traditional reservoir.

[0005] Among other requirements, a variable volume container should readily expand to increase its volume without generating excessive back pressures in the fluid. The container construction selected should also exhibit a good memory, i.e. a tendency of the container to return to its original shape after expansion, to provide a more consistent fluid level when the hydraulic system is relatively cold. On the other hand, should the volume of hydraulic fluid in the system decrease, or fluid be drawn from the variable volume container in order to meet a temporary demand from the hydraulic pump, then the variable volume container may need collapse inwardly. Preferably, after total collapse of the variable volume container, at least one

passageway should remain to maintain a fluid passageway through the container. Such a passageway allows fluid to re-enter and expand the tube and also maintains a passageway through which air escaping from the air separator may pass. Elliptical or generally flat and elongated cross-sectional container profiles have traditionally been used in an attempt to satisfy these requirements.

[0006] Variable volume containers that are currently being used in re-circulating hydraulic systems suffer from a number of limitations, including a failure to return to their original shape after expansion and a tendency to completely collapse under vacuum without generating a fluid passageway through the container. Accordingly, a need exists for an improved variable volume container that overcomes the noted limitations of the prior art.

Summary of the Invention

[0007] A variable volume container is provided that includes a flexible tube having a wall that defines a variable volume chamber. In an embodiment, the tube wall includes a pair of generally curved end wall portions and a pair of generally flat intermediate wall portions separated from the end wall portions by a transition wall portion. The generally flat intermediate wall portions are spaced-apart in a neutral state. A method of making a variable volume container is also provided.

Brief Description of the Drawings

[0008] Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

[0009] FIG. 1 is a perspective view of a prior art variable volume container;

[0010] FIG. 2 is a cross-sectional view of the prior art variable volume container of FIG. 1 in a neutral state;

[0011] FIG. 3 is a cross-sectional view of the prior art variable volume container of FIG. 2 in an expanded state;

[0012] FIG. 4 is a perspective view of a variable volume container according to an embodiment of the present invention;

[0013] FIG. 5 is a cross-sectional view of the variable volume container of FIG. 4 in a neutral state;

[0014] FIG. 6 is a cross-sectional view of the variable volume container of FIG. 4 in an expanded state; and

[0015] FIG. 7 is a cross-sectional view of the variable volume container of FIG. 4 applied over a mandrel during manufacture.

Detailed Description

[0016] Referring now to the drawings, a variable volume container 20 according to the prior art is shown in FIGS. 1-3. Container 20 includes an elongated tubular member 22 having opposing side walls 24 and opposing end walls 26 that define a chamber 28. Tubular member 22 is made from an elastic material, such as rubber, enabling it to flex radially outwardly and inwardly depending on the pressure or vacuum, respectively, within chamber 28. [0017] The cross-sectional view of container 20 shown in FIG. 2 illustrates container 20 in a neutral state without any pressure or vacuum applied to tubular member 22. In contrast, the cross-sectional view of container 20 shown in FIG. 3 illustrates chamber 28 under pressure, which forces tubular member 22 to expand radially outwardly and the volume of chamber 28 to increase. Under pressure, tubular member 22 is subjected to strain. While a significant portion of tubular member 22 may be subjected to strain under pressure, areas of tubular member subjected to particularly high levels of strain are denoted by arrows 29 adjacent the opposing end walls 26. It is these areas of relatively high strain that have a tendency to damage the elastic material in tubular member 22. This damage results in, among other things, a loss of elasticity that compromises the memory of tubular member 22. The relatively high strain 29 may also form cracks in the end walls 26, which can result in failure of container 20 under pressure. [0018] To overcome the limitations of the prior art container 20, a variable volume container 30 is provided in FIGS. 4-6 according to an embodiment of the present invention. In the illustrated embodiment, container 30 includes a flexible tube 32 having a wall 34 that defines a variable volume chamber 36. As shown in FIG. 5, tube wall 34 includes a pair of generally curved end wall portions 38 and a pair of generally flat intermediate wall portions 40 separated from end wall portions 38 by a transition wall portion 42. The generally flat intermediate wall portions 40 are spaced-apart in a neutral state when no pressure or vacuum is applied to flexible tube 32 and may become curved when pressure is applied to container 30 that causes

intermediate wall portions 40 to bow outwardly without unduly straining the flexible tube material.

[0019] As also shown in FIG. 5, the height H ₂ of variable volume container 30 proximate intermediate wall portions 40 is less than the height Hi ₁ H ₃ of variable volume container 30 proximate end wall portions 38. This feature gives the tube wall 34 and chamber 36 a generally dog-bone shaped profile in the neutral state. When so configured, the variable volume chamber 36 includes a pair of generally bulbous end cavities 44 and a generally rectangular intermediate cavity 46 connecting the two end cavities 44 in the neutral state. The end cavities 44 remain open even when a vacuum is applied to flexible tube 32 that causes intermediate wall portions 40 to be drawn together closing intermediate cavity 46. While the height Hi ₁ H ₃ of variable volume container 30 proximate end wall portions 38 is shown as being substantially equal, container 30 is not necessarily limited to the illustrated construction.

[0020] Referring still to FIG. 5, end wall portions 38 are defined by a first radius Ri and transition wall portions 42 are defined by a second radius R ₂ . The flexible tube wall 34 includes an inner surface 48 that defines the variable volume chamber 36. In an embodiment, the first radius Ri is the radius of inner surface 48 included within the end wall portions 38 of flexible tube wall 34, and the second radius R ₂ is the radius of inner surface 48 included within the transition wall portions 42 of flexible tube wall 34. When so configured, the first radius Ri is generally less that the second radius R ₂ . In the illustrated embodiment, for example, the length of the second radius R ₂ is approximately twice the length of the first radius R ₁ ; however, the length of the radii are not intended to be limited thereto.

[0021] The radiused transition wall portions 42 cooperate with the curved end wall portions 38 to maximize the overall elastic strain on flexible tube 32 during expansion, while decreasing localized elastic strain in the tube wall. The transition wall portions 38 also cooperate with the intermediate wall portions to increase the surface area of flexible tube 32 that is subjected to elastic strain during expansion. As a result, the elastic strain is more evenly distributed over the cross-section of flexible tube 32, particularly when compared to the prior art. As shown in FIG. 6, for example, relatively high levels of elastic strain is dispersed across an outer surface 50 of flexible tube 32 (denoted by arrows 52 in) proximate transition wall portions 42, instead of being concentrated at the inner surface of end wall portions 38 like the prior art container 20. Among other benefits, a more uniform distribution of elastic strain reduces or even eliminates

concentrated areas of relatively high strain, e.g. the areas of strain denoted by arrows 29 in FIG. 3, which may form cracks in flexible tube 32 or compromise the elasticity of the flexible tube material.

[0022] Flexible tube 32 may be made from various elastic materials including, without limitation, acrylonitrile and chlorinated-polyethylene based rubber compounds. Elastic materials having good recovery are especially suited for use in flexible tube 32, given their ability to resist cracking and their tendency to return to their original shape after numerous expansion and contraction cycles. The thickness of the flexible tube material is generally optimized to, among other things, minimize the pressure required to fully expand container 30, prevent expansion of container 30 due to the weight of fluid within the container, maintain the elastic strain within an acceptable range during expansion of flexible tube 32, and achieve the required burst strength of container 30. hi an embodiment of the invention, for example, the thickness of flexible tube 32 is approximately 2.9 mm (0.114 in); however, the ultimate thickness used in a given implementation of the invention depends on, among other factors, the size of variable volume chamber 36, the desired burst pressure of container 30, and the physical properties of the materials used in container 30.

[0023] Referring to FIG. 7, a method of making a variable volume container according to an embodiment of the present invention will be discussed, hi the illustrated embodiment, variable volume container 30 is made using a generally dog-bone shaped mandrel 54 that includes a pair of bulbous end portions 56 having a first and second thickness Tl, T2, a generally flat, intermediate portion 58 between the two end portions 56 and having a third thickness T3 less than the first and second thickness Tl, T2, and a transition portion 60 connecting the bulbous end portions 56 to the intermediate portion 58. Each thickness T1-T3 is generally proportional to the heights H1-H3, respectively. A length of flexible tube material is applied over mandrel 54 forming a variable volume container 30 having a variable volume chamber 36 corresponding in approximate size and shape to mandrel 54 over which the flexible tube material is applied. [0024] Additional layers of material may applied over the flexible tube, such as, for example, an optional braided reinforcement layer 62 or a flexible polymer cover 64, prior to or after applying flexible tube 32 over mandrel 54. The overall characteristics of container 30 under pressure or vacuum are substantially similar for container designs that include the additional layers. An additional step of curing the variable volume container 30 after the length

of flexible tube material and optional reinforcing or cover layers are applied over mandrel 54 may be required for thermoset elastomer materials and other materials that do not necessarily retain their shape after removal from mandrel 54. The variable volume container 30 is removed from mandrel 54 prior to use.

[0025] In order to quantify the effect of the container geometry of the present invention, the container geometry of FIGS. 4-6 was compared by means of finite element analysis with the prior art container geometry shown in FIGS. 1-3. For comparison, both of the container profiles analyzed used the same flexible tube material. A pressure in chambers 28 and 46 similar to a pressure found in a typical re-circulating hydraulic system was simulated on both container profiles. Comparing FIGS. 3 and 6, the variable volume container 30 of the present invention exhibits less elastic strain (denoted by arrows 66) adjacent opposing end walls portions 38 than the prior art container 20 exhibits adjacent end walls 26. Another benefit of container 30 is that the maximum elastic strain 52 in flexible tube 32 is dispersed across outer surface 50 of container proximate transition wall portions 42, instead of being concentrated in end wall portions 38 of the prior art container 20. Moreover, the maximum elastic strain 52 at a given point in outer surface 50 of flexible tube 32 is significantly less than the maximum elastic strain 29 in the prior art container 20, which decreases the risk of cracks or other damage in the flexible tube material. However, the aggregate elastic strain in variable volume container 30 is greater than the aggregate elastic strain in the prior art container 20, which improves the memory of container 30.

[0026] The present invention has been particularly shown and described with reference to the foregoing embodiments, which are merely illustrative of the best modes for carrying out the invention. It should be understood by those skilled in the art that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention without departing from the spirit and scope of the invention as defined in the following claims. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby. This description of the invention should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the

foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.