Patent Application
20060086751
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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.
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.
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.
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.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:
Referring now to the drawings, a variable volume container 20 according to the prior art is shown in
The cross-sectional view of container 20 shown in
To overcome the limitations of the prior art container 20, a variable volume container 30 is provided in
As also shown in
Referring still to
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
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. In 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.
Referring to
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.
In order to quantify the effect of the container geometry of the present invention, the container geometry of
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.
