Abstract:
A hydraulic fluid reservoir comprises a body defining a variable volume chamber having one end portion movable with the level of fluid in the chamber. A biasing member acting on a traction rod extending from the movable end portion restrains movement thereof under fluid pressure. The fluid pressure in the variable volume chamber advantageously counterbalances the force of reaction in the biasing member.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to the field of hydraulic circuits and, more particularly, to a variable volume reservoir.  
           [0003]    2. Description of the Prior Art  
           [0004]    Hydraulic circuits typically include a hydraulic reservoir of fixed volume, a pump for circulating the hydraulic fluid within a specific circuit, a filter and a cooler. The volume of the hydraulic reservoir is typically defined in accordance with the pumping rate of the pump. In general, the capacity of the reservoir is two to three times greater than the pumping rate of the pump and sometimes even more. This results in bulky reservoirs.  
           [0005]    Furthermore, the presence of air in hydraulic fluid is often problematic. For instance, the air may contaminate and oxidize the hydraulic fluid, cause pump cavitation problems, and may represent a risk of fire hazard.  
           [0006]    Accordingly, efforts have been made to isolate the reserve of fluid of a hydraulic system from the atmosphere and the surrounding medium. For instance, U.S. Pat. No. 3,099,189, issued on Jul. 30, 1963 to Blondiau, discloses a fluid reservoir having a hollow body for containing a fluid and an elastic diaphragm adapted to fit within the hollow body to exert a pressure on the fluid. The bottom surface of the diaphragm follows the fluid level, according to the demand from the hydraulic circuits connected to the reservoir.  
           [0007]    The AMSAA technical report No. 426 entitled “Hydraulic Design Guidebook Survivability And System Effectiveness” that was published by the Fluid Power Research Center Of the Oklahoma State University in August 1986 discloses a critical volume reservoir (CVR) comprising a cylindrical vessel and a piston that is axially slidable in the cylindrical vessel. The piston divides the interior space of the cylindrical vessel into first and second variable volume chambers. The first chamber is connected in fluid flow communication with a hydraulic system. The second chamber houses a compression spring acting on the piston to resist movement thereof under the pressure exerted thereon by the fluid in the first chamber. The force of reaction induced in the spring is directly transmitted from the piston to the top cover plate of the cylindrical vessel. The top cover plate must therefore be of sturdy construction. The fact that the spring is located within the cylindrical vessel also contributes to increasing the space occupied by the reservoir.  
           [0008]    Although the variable volume reservoirs disclosed in the above-mentioned documents permits isolating the hydraulic fluid from the atmosphere, it has been found that there is still a need for a new lightweight and compact reservoir that is adapted to feed a hydraulic fluid under pressure to a hydraulic system, without inducing additional mechanical stress in the structure of the reservoir.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore an aim of the present invention to provide a minimal volume reservoir for supplying hydraulic fluid to a hydraulic system in order to meet the particular needs thereof.  
           [0010]    It is also an aim of the present invention to isolate a hydraulic fluid from a potential source of contamination.  
           [0011]    It is a further aim of the present invention to provide a fluid reservoir that is relatively simple and economical to manufacture.  
           [0012]    It is a further aim of the present invention to provide a variable volume reservoir adapted to slightly pressurize a reserve of hydraulic fluid, while minimizing mechanical stress in the structure of the reservoir.  
           [0013]    Therefore, in accordance with the present invention, there is provided a reservoir for supplying hydraulic fluid to a hydraulic system to meet the needs thereof, comprising a body defining a variable volume chamber, a port for connecting said variable volume chamber to the hydraulic system, and a restrainer urging said variable volume chamber towards a collapsed position, said restrainer being arranged so that when the variable volume chamber expands under the fluid pressure of the hydraulic fluid against a biasing force of the restrainer, a force of reaction in the restrainer equal and opposite to the biasing force is transmitted to an outer surface of the body in a direction opposite to the fluid pressure exerted by the hydraulic fluid on an inner surface of the body opposite said inner surface, thereby allowing the force of reaction in the restrainer to be counterbalanced by the fluid pressure in the variable volume chamber.  
           [0014]    In accordance with a further general aspect of the present invention, there is provided a reservoir for use in a hydraulic circuit, comprising a body defining a variable volume chamber, a port for operatively connecting the variable volume chamber to the hydraulic circuit, said variable volume chamber having a part movable with the level of fluid in said chamber, a device opposing movement of said part under fluid pressure, said device including a traction rod connected to said part, and a biasing member acting on said traction rod to urge said part towards a collapsed position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which:  
         [0016]    [0016]FIG. 1 is an elevation view, partly in section, of a variable volume reservoir, in accordance with a first embodiment of the present invention; and  
         [0017]    [0017]FIG. 2 is an elevation view, partly in section, of a variable volume reservoir, in accordance with a second embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 1 illustrates a variable volume reservoir  10  suited for supplying hydraulic fluid, such as oil, to mobile or stationary hydraulic systems where hauling excessive quantities of fluid is uneconomical, cumbersome or only poor in design. As will be seen hereinafter, one further advantage of using a variable volume reservoir is that the volume of the reservoir varies directly with the variation in fluid level of the reservoir, thereby preventing air from being trapped in the reservoir over the reserve of hydraulic fluid. This permits isolating the reserve of fluid from air, thereby avoiding potential particulate and chemical contamination of the fluid. The absence of air in the reservoir also reduces the risk of fire.  
         [0019]    The variable volume reservoir  10  is designed to contain only the minimal volume of fluid required to meet the particular requirements of a specific hydraulic system.  
         [0020]    The variable volume reservoir  10  is of compact construction and generally comprises a closed cylindrical body  12 , a piston  14  that is axially slidable in the cylindrical body  12 , a traction rod  16  extending from the piston  14  outwardly of the cylindrical body  12 , and a compression spring  18  acting on the traction rod  16  to bias the piston  14  towards a collapsed position, as illustrated in full lines in FIG. 1.  
         [0021]    The cylindrical body  12  includes a cylindrical sidewall  20  closed at an upper end thereof by a top cover plate  22  and at a bottom end thereof by a bottom cover plate  24 . The piston  14 , the surrounding sidewall  20  and the bottom cover plate  24  define a variable volume chamber for the hydraulic fluid. According to a preferred embodiment of the present invention, the top and bottom cover plates  22  and  24  are removably fastened to the cylindrical sidewall  20  by means of a number of threaded fasteners  26 .  
         [0022]    An air bleed valve  28  is provided on the piston  14  for allowing air contained in the hydraulic fluid to flow from the variable volume chamber to the opposite side of the piston  14 . The air collected in the space between the piston  14  and the top cover plate  22  is vented to the atmosphere through an air filter/breather  30  provided on the top cover plate  22 .  
         [0023]    The traction rod  16  has an upper threaded end threadably engaged with a nut  32  in order to structurally connect the rod  16  to the piston  14 . An annular stop  34  is mounted about the rod  16  and maintained thereat by a nut  36  threadably engaged with a lower threaded end of the rod  16 . The rod  16  extends outwardly of the cylindrical body  12  through a central passage  38  defined in the bottom cover plate  24 .  
         [0024]    The spring  18  is mounted about the traction rod  16  and has a first end abutted against an undersurface  40  of the bottom cover plate  24  about the central passage  38  and a second end abutted against the stop  34 . The spring  18  acts as a restrainer by exerting a biasing force on the stop  34  and, thus, the rod  16 , in a direction normal and away from the piston  14 . The corresponding force of reaction in the spring  18 , which is equal but opposite to the biasing force, is transmitted to the bottom cover plate  24 . This arrangement is advantageous in that the force of reaction is in opposition to the pressure exerted by the hydraulic fluid on the inner surface of the bottom cover plate  24 . The fluid pressure thus, counterbalances the force of reaction. In this way, no additional stress is induced by the spring  18  in the structure forming the cylindrical body  12 . Accordingly, thinner and less sturdy parts can be used in the construction of the cylindrical body  12 .  
         [0025]    The spring  18  is received in a tubular guide  42  depending centrally downwardly from the bottom cover plate  24 . The tubular guide  42  prevents the spring  18  from buckling. Consequently, the small fluid volume contained inside the tubular guide will minimize the thermal fluid contraction-expansion effects. A port and instrumentation block  44  is provided on the tubular guide  42 . The port and instrumentation block  42  may comprise a pressure gauge  46 , a temperature switch or sensor  48 , a fluid pre-fill dry disconnect fitting and inlet and outlet ports (not shown) adapted to be respectively connected in fluid flow communication with the return and distribution lines of a hydraulic fluid circuit (not shown). The hydraulic fluid flowing in the return line of the circuit is first received in the tubular guide  42  through the inlet port defined therein. When the tubular guide  42  is full of fluid and the spring  18  completely submerged in the hydraulic fluid, the piston  14  is urged by the fluid to a position away from the bottom cover plate  24  (as illustrated in broken lines in FIG. 1) against the biasing force of the spring  18 . The spring  18  is advantageously protected against oxidation by the hydraulic fluid. The piston  14  moves with the level of fluid in the cylindrical body  12 , while maintaining the hydraulic fluid under pressure, thereby allowing supplying pressurized hydraulic fluid to a pump operatively connected to the distribution line of the hydraulic circuit. This helps in preventing pump cavitations.  
         [0026]    As shown in FIG. 1, a drain plug  50  is threadably engaged in a hole defined in the base of the tubular guide  42 .  
         [0027]    The level of fluid in the cylindrical body  12  may be ascertained by visual inspection of a fluid level indicating magnet  52  that is axially slidable in a transparent tube  54  provided on an outer surface of the sidewall  20 . The piston  14  is, at least partly, made of a magnetic material to ensure conjoint movement of the magnet  52  and the piston  14 .  
         [0028]    High and low level switches  56  and  58  can be mounted on the cylindrical body  12  to send a control signal to a control system of the hydraulic system.  
         [0029]    In the following description that pertains to the reservoir of FIG. 2, components that are identical in function and identical or similar in structure to corresponding components of the reservoir of FIG. 1 bear the same reference numeral as in FIG. 1, but are tagged with the suffix “′”, whereas components that are new to the reservoir of FIG. 2 are identified by new reference numerals in the hundreds.  
         [0030]    The second embodiment essentially differs from the first embodiment in that the cylindrical body  12 ′ is provided in the form of a pair of end plates  22 ′ and  24 ′ flexibly connected to each other by a bellows  110 . The bellows  110  is made of a flexible impermeable material that is chemically inert to the hydraulic fluid. The end plates  22 ′ and  24 ′ and the bellows  110  define a variable volume chamber  112  for the hydraulic fluid. As illustrated in FIG. 2, the top end plate  22 ′ moves with the level of fluid in the variable volume chamber  112  against the biasing force of the compression spring  18 ′. The compression spring  18 ′ extends between a stop  114  extending inwardly from an upper end of the tubular guide  42 ′ and the stop  34 ′ provided at the lower end of the traction rod  16 ′. A hole  116  is defined in the upper end of the tubular guide  42 ′ for allowing the hydraulic fluid to pass from the tubular guide  42 ′ into the variable volume chamber  112 .  
         [0031]    The air bleed valve  28 ′ is mounted on the top end plate  22 ′ for venting air contained in the hydraulic fluid to the atmosphere.  
         [0032]    While the invention has been described by reference to preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. For instance, an extension spring could be used in lieu of a compression spring as described hereinbefore. Furthermore, other types of biasing members could be used to urge the variable volume chamber towards a collapsed position. It is also understood that the reservoirs illustrated in FIGS. 1 and 2 can be used in any desired orientation.