Patent Publication Number: US-6990888-B2

Title: Mechanism for switching between closed and open center hydraulic systems

Description:
CROSS-REFERENCE 
   This patent application claims the benefit of domestic priority of U.S. Provisional Application Ser. No. 60/490,160, filed Jul. 25, 2003, and entitled “Mechanism For Switching Between Closed Center And Open Center Hydraulic Systems”. 

   BACKGROUND OF THE INVENTION 
   This invention is directed generally to a mechanism for operating a hydraulic tool. More particularly, the present invention is directed to a mechanism employing a novel adjustment assembly for a hydraulic tool which allows the tool to be used with either a constant pressure hydraulic fluid system or a constant volume hydraulic fluid system without requiring disassembly or replacement of any parts in the tool. 
   Hydraulic tools generally operate using one of two basic types of hydraulic systems. The hydraulic systems which are used to operate such tools include the constant volume system and the constant pressure system. 
   In the constant volume system, the hydraulic fluid, such as oil, must be free to flow back to the power source in an off or neutral position. The constant volume system uses an on-off control valve arrangement which has an open-center spool to allow the hydraulic fluid to flow through the valve and back to the source when the valve is in its off or neutral position. As such, the terms “constant volume” and “open-center” are used interchangeably with respect to this type of system. In the open-center system, a positive displacement pump is used which continuously pumps hydraulic fluid through the system. 
   In the constant pressure system, the hydraulic pump operates only intermittently to achieve and maintain a desired pressure. A control valve associated with a constant pressure system employs a closed center spool to prevent fluid flow therethrough in the off or neutral position in order to maintain a desired system pressure. As such, the terms “constant pressure” and “closed-center” are used interchangeably. In the closed-center system, the system operates until a predetermined pressure is sensed whereupon the pump “destrokes” and the pressure compensated pump apparatus then operates to pump just enough to maintain the desired pressure. Various pumps or systems of this type are well known in the art. 
   Hydraulically driven tools are used in many applications in the field, for example, by utility companies for making crimp connections on power lines or by municipalities and park districts for operating pruning devices for tree management and maintaining landscaping. It should be understood that while the present invention is shown in connection with both a crimping device and a pruning device, the present invention will find applications in a variety of hydraulically operated tools. 
   Many of the foregoing users of such tools frequently employ both constant pressure type and constant volume type hydraulic power sources. For example, various equipment such as central hydraulic power sources or trucks which are used in the field, may be equipped with one or the other type of hydraulic power source. Typically, it is undesirable or economically restrictive to maintain both types of power sources in each field location. Without being able to know which type of hydraulic power source will be used in any particular field application, many users of such hydraulic tools found it necessary or desirable to maintain duplicate sets of tools in order to operate with either type of system. Providing duplicate sets of tools, however, represents a substantial capital investment as well as storage and maintenance costs even though it overcomes the problems associated with having only one type of hydraulic power system. Further, maintaining duplicate sets of tools requires additional space and additional training to make sure that the proper tool is used with the proper type of hydraulic system. Alternatively, one set of tools may be maintained in one type of hydraulic system selected for any given application. Some devices, such as trucks, however, are provided with only one type of hydraulic system and therefore this may not be a feasible solution. 
   Another way of solving the problems associated with the two different types of hydraulic power sources is to design tools with interchangeable components, such as two spool valves, one spool valve designed for open-center operation and the other spool valve designed for closed-center operations. The operator of the tool could then select and install the proper spool to match the hydraulic power source. This, however, would require that duplicate spools be available for use with each tool, again requiring additional inventory and storage costs as well as space requirements. Moreover, providing interchangeable spool valves would require the operator to expend the time necessary to effect the change over and also have sufficient training and skills to properly disassembly and reassembly the valve portion of each tool. 
   Assuming that the problems associated with inventory and storage costs and space requirements and operator skill and training are overcome, the dual valve spools require additional time at the job site for disassembly and reassembly of the valves. Another problem arises in that the frequent removal and replacement of the valve spools will also unnecessarily disturb the hydraulic system and seals and produce increased tool wear and the opportunity for the introduction of dirt and debris into the hydraulic system. Because these tools are intended for field applications, the introduction of such dirt and debris and disturbance of a hydraulic system is an important concern. 
   The invention disclosed in U.S. Pat. No. 3,882,883 proposed a first solution to the foregoing problems. The &#39;883 patent discloses a valve assembly having a spool which may be rotated 180° to shift from a normally open operating mode to a normally closed operating mode. However, this valve design requires that a linkage rod be removed before the spool may be rotated. Thus, there is still the possibility of the linkage rod being improperly removed and improperly reassembled as well as possibly being lost, damaged during the removal or reassembly, or the introduction of contaminants into the system. 
   The invention disclosed in U.S. Pat. No. 4,548,229 proposed a second solution to the foregoing problems. The &#39;229 patent discloses a valve assembly for accommodating both open-center and closed-center modes of operation for use with an impact wrench. This valve assembly, however, is suitable only for use with rotating tools, because the valve assembly itself is designed to shunt hydraulic fluid back to the source when the tool is in the off or neutral state, and the open-center mode of operation. This tool is provided with a specifically designed valve cylinder or sleeve which surrounds the valve spool. The sleeve is configured for open-center operation when in a first orientation and for closed-center operation when it is rotated to a second orientation approximately 180° of rotation from its first orientation. This valve is designed to permit constant flow of hydraulic fluid through the tool when the valve is in its on position in both open-center and closed center modes of operation. The valve is designed to cut off the hydraulic fluid flow at the valve itself in the closed center mode of operation when the valve is in its closed or neutral position. In other words, in both open-center and closed-center modes, when the valve is in its off or neutral position, the valve does not permit flow of fluid past the valve and there is no fluid flow to the tool. However, such a valve arrangement will not work with a reciprocating type of hydraulic tool wherein it is necessary to alternately direct flow to opposite sides of a reciprocating piston. The crimping device and the pruner disclosed herein in order to illustrate the present invention are two such types of tools which utilize a reciprocating piston, rather than a rotating rotor as used in the tools such as the impact wrench of the above-mentioned &#39;229 patent. 
   The invention disclosed in U.S. Pat. No. 5,442,992 proposed a third solution to the foregoing problems. The &#39;992 patent, which was assigned to the assignee of the present invention, shows a control system designed for use with either an open-center system or a closed-center system. The system of the &#39;992 patent has a rotatable selector which assists in configuring the control system for use with either the open-center or closed-center system. 
   To overcome the disadvantages of the above-mentioned prior art, a hydraulic control mechanism was invented and disclosed in U.S. Pat. No. 5,778,755, which was assigned to the assignee of the present invention. The &#39;755 patent discloses a hydraulic control mechanism which is attached to a hydraulically operated tool to provide a desired hydraulically powered function. The present invention allows the hydraulic control mechanism to be used with either an open-center hydraulic system or a closed-center hydraulic power system. The adjustment assembly, which utilized screws, provided a structure which could be configured to force open shuttle spool valves in the control mechanism in a neutral condition for use with an open-center power supply. The adjustment assembly can also be configured to be disengaged from the shuttle spool valves in a neutral condition for use with a closed-center hydraulic power supply. Operation of the adjustment assembly is made using standard tools and without disassembly of the control mechanism. 
   While the hydraulic control mechanism disclosed in the &#39;755 patent has been well-received in the marketplace, there have also been some disadvantages associated therewith. For example, the adjustment of the screws was not convenient due to the location of the screws relative to a handle of the tool. Additionally, the components required for this method of adjustment occasionally led to fracture of the shuttle dump spools and external leakage. The number of parts required and costs to manufacture or purchase these parts, also resulted in higher manufacturing costs than desired. 
   Thus, there is a need for a mechanism for operating a hydraulic tool which overcomes the disadvantages associated with the prior art systems. The present invention provides such a mechanism. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   A primary object of the invention is to provide a mechanism for a tool which provides for easier operation of the tool in an open-center or closed-center hydraulic system than other such tools of the prior art. 
   An object of the invention is to provide a tool which is configured to operate in either an open-center or closed-center hydraulic system where the parts for adjusting the tool between the open-center and closed-centers are conveniently placed for a user of the tool. 
   Another object of the invention is to provide a tool which is configured to operate in either an open-center or closed-center hydraulic system where the parts required for adjusting the tool between the open-center and closed-centers are low in number and cost. 
   Another object of the invention is to provide a configuration for a tool which can operate between both an open-center hydraulic system and a closed-center hydraulic system, but which minimizes or eliminates external leakage of the hydraulic fluid. 
   Yet another object of the invention is to provide a novel hydraulic fluid flow mechanism for use with a hydraulic tool which allows the tool to be converted for use with a constant volume system to a constant pressure system and vice-versa, without the disassembly or removal of any parts from the tool. 
   Still another object of the invention is to provide a novel hydraulic fluid flow mechanism for use with a hydraulic tool which can be quickly and easily converted for operation with either a constant volume system or a constant pressure system as a power source using available common tools and skills. 
   Another object of the invention is to provide a novel hydraulic fluid flow mechanism based on a generally available and understood hydraulic tool thereby providing a hydraulic tool which can be used with either a constant volume system or a constant pressure system without requiring additional training or the maintenance of such a hydraulic tool. 
   Briefly, and in accordance with the foregoing, a mechanism is provided for use with a hydraulic control mechanism of a hydraulic tool. The hydraulic control mechanism is attached to the hydraulically operated tool to provide a desired hydraulically powered function. The mechanism of the present invention allows the hydraulic control mechanism to be used with either a constant volume hydraulic system or a constant pressure hydraulic system. The mechanism provides a valve chamber and a valve member positioned within the valve chamber. The valve chamber communicates with both a central passageway and a cross passageway of the hydraulic control mechanism. The valve chamber defines a valve seat proximate to one of the central and cross passageways. The valve member is displaceable within the valve chamber and is configured such that, depending on the position of the valve member within the valve chamber, the hydraulic mechanism can be used with either a constant volume hydraulic system or a constant pressure hydraulic system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention which are believed to be novel are described in detail hereinbelow. The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference numerals identify like elements in which: 
       FIG. 1  is a partial fragmentary, cross-sectional view of a hydraulic crimping tool which incorporates features in accordance with a first embodiment of the invention, which is configured for use with a constant volume or “open-center” hydraulic power system in which a reciprocal piston and a crimping ram attached thereto are in a retracted position with the system in a neutral condition; 
       FIG. 2  is an enlarged, partial fragmentary, cross-sectional view of an adjustment assembly of the hydraulic crimping tool illustrated in  FIG. 1 ; 
       FIG. 3  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the crimping tool as shown in  FIG. 1  in the trigger activated condition in which the crimping ram is advanced by hydraulic forces acting on the reciprocal piston of the control mechanism; 
       FIG. 4  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the crimping tool as shown in  FIG. 1  in the trigger deactivated condition in which the crimping ram is retracted by hydraulic forces acting on the reciprocal piston of the control mechanism; 
       FIG. 5  is an enlarged, partial fragmentary, cross-sectional view of the adjustment assembly of the hydraulic crimping tool illustrated in  FIGS. 3 and 4 ; 
       FIG. 6  is an enlarged, partial fragmentary, cross-sectional view of the crimping tool as shown in  FIGS. 1–5  which has been configured for operation with a constant pressure or “closed-center” hydraulic power system in the trigger deactivated condition in which the piston and crimping ram are in a retracted position; 
       FIG. 7  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the crimping tool as shown in  FIG. 6  in the trigger activated condition in which the crimping ram is advanced by hydraulic forces acting on the reciprocal piston of the control mechanism; 
       FIG. 8  is an enlarged, partial fragmentary, cross-sectional view of the adjustment assembly of the hydraulic crimping tool illustrated in  FIGS. 6 and 7 ; 
       FIG. 9  is a partial fragmentary, cross-sectional view of a hydraulic utility pruner tool which incorporates features in accordance with a second embodiment of the claimed invention which is configured for use with a constant volume or “open-center” hydraulic power system in which a reciprocal piston and an extension rod attached thereto are in an extended or advanced position with the system in a neutral condition; 
       FIG. 10  is an enlarged, partial fragmentary, cross-sectional view of an adjustment assembly of the hydraulic utility pruner tool illustrated in  FIG. 9 ; 
       FIG. 11  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the utility pruner tool as shown in  FIG. 9  in the trigger activated condition in which the extension rod is retracted by hydraulic forces acting on the reciprocal piston of the control mechanism; 
       FIG. 12  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the utility pruner tool as shown in  FIG. 9  in the trigger deactivated condition in which the extension rod is extended or advanced by hydraulic forces acting on the reciprocal piston of the control mechanism; 
       FIG. 13  is an enlarged, partial fragmentary, cross-sectional view of the adjustment assembly of the hydraulic utility pruner tool illustrated in  FIGS. 11 and 12 ; 
       FIG. 14  is an enlarged, partial fragmentary, cross-sectional view of the utility pruner tool as shown in  FIGS. 9–13  which has been configured for operation with a constant pressure or “closed-center” hydraulic power system in the trigger activated condition in which the piston and extension rod are in a retracted position; 
       FIG. 15  is an enlarged, partial fragmentary, cross-sectional view showing the control mechanism of the utility pruner tool as shown in  FIG. 14  in the trigger deactivated condition in which the extension rod is extended or advanced by hydraulic forces acting on the reciprocal piston of the control mechanism; and 
       FIG. 16  is an enlarged, partial fragmentary, cross-sectional view of the adjustment assembly of the hydraulic utility pruner tool illustrated in  FIGS. 14 and 15 . 
   

   DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
   While this invention may be susceptible to embodiment in different forms, there is shown in the drawings and will be described herein in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated. 
   A first embodiment of the invention in which a crimping tool  100  is shown to have a novel control mechanism  102 , which incorporates features of the invention, is illustrated in  FIGS. 1–8  with reference numerals being in the one hundreds. A second embodiment of the invention in which a utility pruner tool  300  is shown to have a novel control mechanism  302 , which incorporates features of the invention, is illustrated in  FIGS. 9–16  with reference numerals being in the three hundreds. Like reference numerals in the first and second embodiments denote like elements. 
   Crimping Tool  100  Having The Novel Control mechanism  102   
     FIGS. 1–5  show the control mechanism  102  of the crimping tool  100  employed with a constant volume or open-center hydraulic power system, whereas  FIGS. 6–8  show the control mechanism  102  of the crimping tool  100  employed with a constant pressure or closed-center hydraulic power system. Further,  FIG. 1  has been provided to show the entire crimping tool  100 , whereas  FIGS. 3 ,  4 ,  6  and  7  have been substantially enlarged to show only a portion of the crimping tool  100  which includes the control mechanism  102  of the crimping tool  100 .  FIGS. 2 ,  5  and  8  illustrate enlarged views of the control mechanism  102 , specifically illustrating an adjustment assembly  146  of the control mechanism  102 . 
   The hydraulic crimping tool  100  includes a crimping ram unit  104  having a head  106  and a hydraulic crimping ram  108 . The crimping ram unit  104  is attached to the control mechanism  102  to provide reciprocal movement of the ram  108  along the head  106 . Movement of the ram  108  relative to the head  106  provides crimping forces on a crimp connection (not shown) placed in a C-shaped aperture  110  defined therebetween. The control mechanism  102  regulates hydraulic forces to advance and retract the ram  108  to provide a desired crimping effect on the crimp connection. It should be understood that the control mechanism  102  may also be used with a variety of other hydraulic tools which require the ability to be used with either an open-center or a closed-center hydraulic power system. The present disclosure is illustrated by way of reference to the crimping tool  100  as shown herein but is not limited to the crimping tool  100 . 
   As shown in each of the  FIGS. 1 ,  3 ,  4 ,  6  and  7 , the control mechanism  102  includes a housing  112  defining a cavity  114  therein with a reciprocal piston or driving piston  116  retained in the cavity  114  for movement toward and away from the head  106 . The ram  108  is attached to a first side  118  of the piston  116  by cap screws  120 . 
   The piston  116  divides the cavity  114  into a retract chamber  122  and a drive chamber  124 . The retract chamber  122  is defined between the first side  118  of the piston  116  and the corresponding walls which define the cavity  114  in the housing  112 . The drive chamber  124  is similarly defined between a second side  126  of the piston  116  and the corresponding walls which define the cavity  114  in the housing  112 . 
   The control mechanism  102  includes a handle structure  128  containing a valve assembly  130 . The handle structure  128  is defined about a central axis  131 . An inlet passageway  132  and an outlet passageway  134  extend axially through the handle structure  128  for connection to a hydraulic power system (not shown) of a known construction. The inlet passageway  132  extends along one side of the central axis  131  while the outlet passageway  134  extends along another side of the central axis  131 . The inlet passageway  132  and the outlet passageway  134  can be connected to either the constant volume system or the constant pressure system. A central passageway  136  extends axially within the handle structure  128  along the central axis  131  and selectively connects either the inlet passageway  132  or the outlet passageway  134  via the valve assembly  130  with the retract chamber  122  as will be described in greater detail hereinbelow. A cross passageway  138  extends axially within the handle structure  128 , on the same side of the central axis  131  as the outlet passageway  134 , and selectively connects either the inlet passageway  132  or the outlet passageway  134  via the valve assembly  130  with the drive chamber  124  as will be described in greater detail hereinbelow. 
   The valve assembly  130  includes a spindle valve  140  which is axially displaceable within a spindle valve chamber  141  along a spindle axis  142 . The spindle axis  142  is perpendicular to the central axis  131  of the handle structure  128 . A trigger  144 , which is pivotally attached to the handle structure  128 , is gripped by an operator to displace the spindle valve  140  to selectively configure the inlet passageway  132 , outlet passageway  134 , central passageway  136  and cross passageway  138  in order to extend or retract the piston  116  as described herein. The spindle valve  140  has an annular groove  143  proximate to the trigger  144 . The annular groove  143  is connected to a first enlarged diameter portion  145 . A second enlarged diameter portion  147  is spaced from the first enlarged diameter portion  145  by a first reduced diameter portion  149 . A third enlarged diameter portion  151  is spaced from the second enlarged diameter portion  147  by a second reduced diameter portion  153 . The third enlarged diameter portion  151  extends to an opposite end of the spindle valve  140 . A passageway  180  extends through the spindle valve  140  and has a first opening or port  181  in the first enlarged diameter portion  145  and a second opening or port  183  in the second reduced diameter portion  153 . Further description of the operation of the valve assembly  130  and the movement of the piston  116  will be provided in greater detail hereinbelow. The structure and operation of such a spindle valve  140  is well known in the art as shown in U.S. Pat. No. 5,442,992 which is assigned to the assignee of the invention disclosed and claimed herein. Additionally, U.S. Pat. No. 5,442,992 is incorporated herein by reference. 
   The adjustment assembly  146  is provided in the handle structure  128  to allow the control mechanism  102  to be configured for either a constant volume or a constant pressure hydraulic power source. The adjustment assembly  146  is between the valve assembly  130  and the cavity  114 . The adjustment assembly  146  includes a valve chamber  148 , an adjustable valve member  150 , and a retaining ring  152 . 
   The valve chamber  148  is provided in the handle structure  128  on an opposite side of the trigger  144 . The valve chamber  148  is perpendicular to the central axis  131  of the handle structure  128  and always is in fluid communication with the cross passageway  138  and can be in fluid communication with the central passageway  136 , depending upon the positioning of the adjustable valve member  150  and the pressure within the central passageway  136 . The valve chamber  148  provides a valve seat  154  proximate to the central passageway  136 . 
   The adjustable valve member  150  is positioned within the valve chamber  148 . As best shown in  FIGS. 2 ,  5  and  8 , the adjustable valve member  150  includes a head  156 , a normally expanded spring  158 , an enlarged section  160 , and a knob  162 . The knob  162  is preferably provided proximate to an outer surface of the handle structure  128  such that a user of the hydraulic crimping tool  100  can easily operate the knob  162  by moving the knob  162  in either a first or second direction, preferably clockwise or counterclockwise. An outer end  164  of the knob  162  may have a slot  166  provided therein such that a user of the hydraulic crimping tool  100  can move the knob  162  by use of another tool, such as a screwdriver. 
   An outer end  168  of the enlarged section  160  is secured to an inner end  170  of the knob  162 . The enlarged section  160  has a diameter which is larger than a diameter of the knob  162 . Because the enlarged section  160  has a larger diameter than the knob  162 , a shoulder  172  is provided between the enlarged section  160  and the knob  162 . The diameter of the enlarged section  160  is preferably commensurate with a diameter of the valve chamber  148  such that any fluid provided within the valve chamber  148  cannot escape out of the valve chamber  148  and, thus, out of the hydraulic crimping tool  100 . 
   A first end of the normally expanded spring  158  is connected to an inner end  174  of the enlarged section  160 . A second end of the normally expanded spring  158  is connected to the head  156 . 
   The head  156  is sized to fit within the valve seat  154 , but may also be moved out of the valve seat  154  as will be described in greater detail herein. When the head  156  is seated in the valve seat  154 , the valve seat  156  prevents the central passageway  136  from being in fluid communication with the cross passageway  138  through the valve chamber  148 . If, however, the head  156  is not seated in the valve seat  154 , the central passageway  136  and the cross passageway  138  are in fluid communication through the valve chamber  148 . 
   The retaining ring  152  is provided within the valve chamber  148  and is positioned proximate to the outer surface of the handle structure  128 . The retaining ring  152  has an aperture  176  therethrough which defines an inner diameter formed by the wall of the aperture  176 . The inner diameter of the retaining ring  152  is larger than the diameter of the knob  162 , but is smaller than the diameter of the enlarged section  160 . Thus, the knob  162 , upon movement thereof, can move through the aperture  176  of the retaining ring  152 , but the enlarged section  160  is trapped within the valve chamber  148  as the shoulder  172  abuts against the retaining ring  152 , preventing the enlarged section  160  from moving beyond the retaining ring  152 . Therefore, the adjustable valve member  150  is secured within the valve chamber  148  by the retaining ring  152 . 
   The adjustment assembly  146  provides benefits for the control mechanism  102  in comparison to the control mechanisms of the prior art. The adjustment assembly  146  utilizes a minimum number of parts and minimal manufacturing costs. The adjustment assembly  146  further is conveniently located relative to the handle  128 . Thus, the adjustment assembly  146  of the control mechanism  102  provides an easy, reliable and efficient means for configuring the hydraulic crimping tool  100  for use with either a constant volume or a constant pressure system. 
   The tool  100  has central tube  186  which extends from the central passageway  136 , through the drive chamber  124  and into the piston  116 . The central tube  186  has an opening therethrough which is in fluid communication with the central passageway  136 . A central chamber  184  is provided in the ram  108  and is in fluid communication with the central tube  186 . A radial port  182  extends through the ram  108  and places the central chamber  184  and the retract chamber  126  into fluid communication with one another. 
   Operation of the hydraulic crimping tool  100  will now be discussed and attention is directed to  FIGS. 1–8 . Operation of the hydraulic crimping tool  100  will first be discussed where the hydraulic crimping tool  100  is employed in a constant volume or open-center hydraulic power system, as illustrated in  FIGS. 1–5 . Operation of the hydraulic crimping tool  100  will then be discussed where the hydraulic crimping tool  100  is employed in a constant pressure or closed-center hydraulic power system, as illustrated in  FIGS. 6–8 . 
   Attention is directed to  FIGS. 1–5  and the operation of the hydraulic crimping tool  100  where the hydraulic crimping tool  100  is employed in a constant volume or open-center hydraulic power system. In order to operate the hydraulic crimping tool  100  such that the hydraulic crimping tool  100  is employed in a constant volume or open-center hydraulic system, the user first rotates the knob  162  of the adjustable valve member  150  in a first direction, preferably counterclockwise, until the shoulder  172  of the enlarged section  160  contacts the retaining ring  152 , as illustrated in  FIGS. 1–5 . In this position, the spring  158  is expanded such that the head  156  is seated in the valve seat  154 , as best illustrated in  FIG. 5 . The knob  162  may extend out of the handle structure  128  in this position. 
   In order to provide crimping forces on a crimp connection placed in the C-shaped aperture  110 , the user activates the trigger  144  by moving the trigger  144  toward the handle structure  128 , as illustrated in  FIG. 3 . When the trigger  144  is in the position illustrated in  FIG. 3 , the spindle valve  140  places the inlet passageway  132  into fluid communication with the cross passageway  138 , via the passageway  180  through the spindle valve  140 , with the first opening  181  being in fluid communication with the inlet passageway  132  and the second opening  183  being in fluid communication with the cross passageway  138 . The spindle valve  140  also places the central passageway  136  into fluid communication with the outlet passageway  134  as fluid is allowed to travel into the spindle valve chamber  141  and around the reduced diameter section  149  of the spindle valve  140 . Thus, hydraulic fluid from the reservoir (not shown) of a hydraulic power system flows into the inlet passageway  132 , into the first port  181 , through the passageway  180  of the spindle valve  140 , out of the second port  183 , into the cross passageway  138 , and into the drive chamber  124 . The hydraulic fluid flowing through the central passageway  136  is prevented from flowing directly into the cross passageway  138  via the valve chamber  148  because the head  156  is seated within the valve seat  154  and the spring  158  is expanded, i.e., the force of the fluid within the central passageway  136  is not sufficient to force the spring  158  to contract such that the head  156  will be unseated from the valve seat  154 , allowing the hydraulic fluid flowing through the central passageway  136  to flow straight into the valve chamber  148  and back into the cross passageway  138 . 
   As the amount of fluid in the drive chamber  124  increases, the pressure within the drive chamber  124  also increases, such that the driving piston  116  is advanced axially through the cavity  114 . The advancement of the driving piston  116  through the cavity  114  axially advances the ram  108  thereby crimping a crimp connection placed in the C-shaped aperture  110 . Advancement of the driving piston  116  through the cavity  114  forces hydraulic fluid from the retract chamber  122  through the radial passageways  182 , into and through the central chamber  184 , into and through the central tube  186 , into the spindle valve chamber  141 , around the reduced diameter section  149  of the spindle valve  140 , into and through the outlet passageway  134 , and into the reservoir. 
   Once the crimping forces on the crimp connection are made, the user releases the trigger  144  such that it moves to the position illustrated in  FIG. 4 . As illustrated in  FIG. 4 , the release of the trigger  144  causes the inlet passageway  132  to not be in fluid communication with the cross passageway  138  through the passageway  180  of the spindle valve  140 . Rather, the inlet passageway  132  is placed into fluid communication with the central passageway  136  because of the positioning of the spindle valve  140  within the spindle valve chamber  141 , and the cross passageway  138  is placed into fluid communication with the outlet passageway  134  because of the positioning of the spindle valve  140  within the spindle valve chamber  141 . Thus, the hydraulic fluid from the reservoir flows into and through the inlet passageway  132 , into the spindle valve chamber  141 , around the reduced diameter section  149  of the spindle valve  140 , into and through the central passageway  136 , into and through the central tube  186 , into and through the central chamber  184 , into and through the radial passageways  182 , and into the retract chamber  122 . The hydraulic fluid flowing through the central passageway  132  is prevented from flowing directly into the cross passageway  134  via the valve chamber  148  because the head  156  is seated within the valve seat  154  and the spring  158  is expanded, i.e., the force of the fluid within the central passageway  136  is not sufficient to force the spring  158  to contract such that the head  156  will be unseated from the valve seat  154 , allowing the hydraulic fluid flowing through the central passageway  136  to flow straight into the valve chamber  148  and back into the cross passageway  138 . 
   As the amount of fluid in the retract chamber  122  increases, the pressure within the retract chamber  122  also increases, such that the driving piston  116  is axially retracted within the cavity  114 . The driving piston  116  retracting within the cavity  114  causes ram  108  to axially retract and the crimping forces on the crimp connection to be stopped. Retraction of the driving piston  116  within the cavity  114  causes the hydraulic fluid within the driving chamber  124  to flow out of the driving chamber  124 , into and through the cross passageway  138 , into the spindle valve chamber  141 , around the reduced diameter section  153  of the spindle valve  140 , into and through the outlet passageway  134 , and back into the reservoir. 
   When the driving piston  116  is retracting within the cavity  114 , the driving piston  116  will come to a fully retracted position, as illustrated in  FIG. 1 . Because the driving piston  116  cannot be retracted further, and because the hydraulic fluid continues to fill in the retract chamber  122  such that the pressure is increased within the retract chamber  122 , the back pressure provided within the central passageway  136  is such that it overcomes the strength of the spring  158  which holds the head  156  in the valve seat  154 . Thus, the spring  158 , at a predetermined pressure, contracts within the valve chamber  148  such that the head  156  becomes unseated from the valve seat  154 , as illustrated in  FIGS. 1 and 2 . Thus, in order to alleviate the pressure within the retract chamber  122 , the hydraulic fluid flows from the inlet passageway  132 , into the spindle valve chamber  141 , around the reduced diameter section  149  of the spindle valve  140 , into the central passageway  136 , into the valve chamber  148 , into the cross passageway  138 , back into the spindle valve chamber  141 , around the reduced diameter section  153  of the spindle valve  140 , into and through the outlet passageway  134 , and into the reservoir. This is the neutral position of a constant volume system which allows fluid to continuously flow from the inlet passageway  132  through the adjustment assembly  146  of the control mechanism  102 , and back through the outlet passageway  134 . In this position, the pressure in the retract chamber  122  and the drive chamber  124  is generally equalized such that the hydraulic fluid will continuously flow through the adjustment assembly  146  of the control mechanism  122  until the user again activates the trigger  144 . 
   Attention is directed to  FIGS. 6–8  and the operation of the hydraulic crimping tool  100  where the hydraulic crimping tool  100  is employed in a constant pressure or closed-center hydraulic power system. In order to operate the hydraulic crimping tool  100  such that the hydraulic crimping tool  100  is employed in a constant pressure or closed-center hydraulic system, the user first rotates the knob  162  of the adjustable valve member  150  in a second direction, preferably clockwise, until the head  156  is fully seated within the valve seat  154 . The head  156  is fully seated within the valve seat  154  when the normally expanded spring  158  is fully contracted, as best illustrated in  FIG. 8 . 
   In order to provide crimping forces on a crimp connection placed in the C-shaped aperture  110 , the user activates the trigger  144  by moving the trigger  144  toward the handle structure  128 , as illustrated in  FIG. 7 . When the trigger  144  is in the position illustrated in  FIG. 7 , the spindle valve  140  places the inlet passageway  132  into fluid communication with the cross passageway  138 , via the passageway  180 . The spindle valve  140  also places the central passageway  136  into fluid communication with the outlet passageway  134  as fluid is allowed to travel within the spindle valve chamber  141  around the reduced diameter section  149  of the spindle valve  140 . Thus, hydraulic fluid from the reservoir (not shown) of a hydraulic power system flows into the inlet passageway  132 , through the passageway  180  of the spindle valve  140 , into the cross passageway  138 , and into the drive chamber  124 . The hydraulic fluid flowing through the inlet passageway  132  is prevented from flowing directly from the valve chamber  148  into the outlet passageway  134  because the head  156  is fully seated within the valve seat  154 . 
   As the amount of fluid in the drive chamber  124  increases, the pressure within the drive chamber  124  also increases, such that the driving piston  116  is advanced through the cavity  114 . The advancement of the driving piston  116  through the cavity  114  causes the ram  108  to axially advance and the crimping forces on a crimp connection placed in the C-shaped aperture  110 . Advancement of the driving piston  116  through the cavity  114  forces hydraulic fluid from the retract chamber  122  through the radial passageways  182 , into and through the central chamber  184 , into and through the central tube  186 , into the spindle valve chamber  141 , around the reduced diameter section  149  of the spindle valve  140 , into and through the outlet passageway  134 , and into the reservoir. 
   Once the crimping forces on the crimp connection are made, the user releases the trigger  144  such that it moves to the position illustrated in  FIG. 6 . As illustrated in  FIG. 6 , the release of the trigger  144  causes the inlet passageway  132  to not be in fluid communication with the cross passageway  138  through the passageway  180  of the spindle valve  140 . Rather, the inlet passageway  132  is placed into fluid communication with the central passageway  136  because of the positioning of the spindle valve  140  within the spindle valve chamber  141 , and the cross passageway  138  is placed into fluid communication with the outlet passageway  134  because of the positioning of the spindle valve  140  within the spindle valve chamber  141 . Thus, the hydraulic fluid from the reservoir flows into and through the inlet passageway  132 , into the spindle valve chamber  141 , around the reduced diameter section  149  of the spindle valve  140 , into and through the central passageway  136 , into and through the central tube  186 , into and through the central chamber  184 , into and through the radial passageways  182 , and into the retract chamber  122 . The hydraulic fluid flowing through the inlet passageway  132  is prevented from flowing from the valve chamber  148  directly into the outlet passageway  134  because the head  156  is fully seated within the valve seat  154 . 
   As the amount of fluid in the retract chamber  122  increases, the pressure within the retract chamber  122  also increases, such that the driving piston  116  is retracted within the cavity  114 . The driving piston  116  retracting within the cavity  114  causes the ram  108  to axially retract and the crimping forces on the crimp connection to be stopped. Retraction of the driving piston  116  within the cavity  114  causes the hydraulic fluid within the driving chamber  124  to flow out of the driving chamber  124 , into and through the cross passageway  138 , into the spindle valve chamber  141 , around the reduced diameter section  153  of the spindle valve  140 , into and through the outlet passageway  134 , and back into the reservoir. 
   Once the driving piston  116  comes to a fully retracted position, because the hydraulic fluid continues to fill in the retract chamber  122 , pressure will continue to build within the cavity  114 . Because the head  156  is mechanically locked in the valve seat  154 , such that hydraulic fluid is not allowed to flow past the head  156 , pressure will continue to build until it reaches a predetermined value established by a relief valve (not shown) in the hydraulic circuit. Relief valves within hydraulic circuits are well-known in the art and, therefore, are not explained herein in detail. In systems with a positive displacement pump, the relief valve diverts flow back to the reservoir. On systems with a variable stroking pump, pressure will continue to build until it reaches the predetermined value established by the relief valve, whose control system then reduces the flow of hydraulic fluid to adequately maintain system pressure. 
   In the constant pressure system, the force of the fluid within the central passageway  136  is never sufficient to unseat the head  156  from the valve seat  154  as the spring  158  is already fully contracted. Thus, the hydraulic fluid will never flow directly or continuously from the central passageway  136 , into the valve chamber  148 , and back into the cross passageway  138 . 
   Utility Pruner Tool  300  Having The Novel Control Mechanism  302   
     FIGS. 9–13  show the control mechanism  302  of the utility pruner tool  300  employed with a constant volume or open-center hydraulic power system, whereas  FIGS. 14–16  show the control mechanism  302  of the utility pruner tool  300  employed with a constant pressure or closed-center hydraulic power system.  FIGS. 9 ,  11 ,  12 ,  14  and  15  have been substantially enlarged to show only a portion of the utility pruner tool  300  which includes the control mechanism  302  of the utility pruner tool  300 .  FIGS. 10 ,  13  and  16  illustrate enlarged views of the control mechanism  302 , specifically illustrating an adjustment assembly  346  of the control mechanism  302 . 
   The hydraulic utility pruner tool  300  includes an extension rod assembly unit  305  having an extension rod  307  which is operatively associated with cutting blades (not shown) of the hydraulic utility pruner tool  300 . The extension rod assembly unit  305  is attached to the control mechanism  302  to provide reciprocal movement of the extension rod  307 . Movement of the extension rod  307  provides for the opening and closing of the cutting blades. The control mechanism  302  regulates hydraulic forces to advance and retract the extension rod  307  to provide a desired cutting effect on items positioned between the cutting blades. It should be understood that the control mechanism  302  may also be used with a variety of other hydraulic tools which require the ability to be used with either an open-center or a closed-center hydraulic power system. The present disclosure is illustrated by way of reference to the utility pruner tool  300  as shown herein but is not limited to the utility pruner tool  300 . 
   As shown in each of  FIGS. 9 ,  11 ,  12 ,  14  and  15 , the control mechanism  302  includes a housing  312  defining a cavity  314  therein with a reciprocal piston or driving piston  316  retained in the cavity  314  for movement toward and away from the cutting blades. The extension rod  307  is attached to a first side  318  of the piston  316  by suitable means, but the extension rod  307  is preferably integrally formed with the piston  316 . 
   The piston  316  divides the cavity  314  into a retract chamber  322  and a drive chamber  324 . The retract chamber  322  is defined between the first side  318  of the piston  316  and the corresponding walls which devine the cavity  314  in the housing  312 . The drive chamber  324  is similarly defined between a second side  326  of the piston  316  and corresponding walls which define the cavity  314  in the housing  312 . 
   The control mechanism  302  includes a handle structure  328  containing a valve assembly  330 . The handle structure  328  is defined about a central axis  331 . An inlet passageway  332  and an outlet passageway  334  extend axially through the handle structure  328  for connection to a hydraulic power system (not shown) of a known construction. The inlet passageway  332  extends along one side of the central axis  331  while the outlet passageway  334  extends along another side of the central axis  331 . The inlet passageway  332  and the outlet passageway  334  can be connected to either the constant volume or constant pressure system. A central passageway  336  extends axially within the handle structure  328  along the central axis  331  and selectively connects either the inlet passageway  332  or the outlet passageway  334  via the valve assembly  330  with the retract chamber  322  as will be described in greater detail hereinbelow. A cross passageway  338  extends axially within the handle structure  328 , on the same side of the central axis  331  as the inlet passageway  334 , and selectively connects either the inlet passageway  332  or the outlet passageway  334  via the valve assembly  330  with the drive chamber  324  as will be described in greater detail hereinbelow. 
   The valve assembly  330  includes a spindle valve  340  which is axially displaceable within a spindle valve chamber  341  along a spindle axis  342 . The spindle axis  342  is perpendicular to the central axis  331  of the handle structure  328 . A trigger  344 , which is pivotally attached to the handle structure  328 , is gripped by an operator to displace the spindle valve  340  to selectively configure the inlet passageway  332 , outlet passageway  334 , central passageway  336  and cross passageway  338  in order to extend or retract the piston  316  as described herein. The spindle valve  340  has an annular groove  343  proximate to the trigger  344 . The annular groove  343  is connected to a first enlarged diameter portion  345 . A second enlarged diameter portion  347  is spaced from the first enlarged diameter portion  345  by a reduced diameter portion  349 . A third enlarged diameter portion  351  is spaced from the second enlarged diameter portion  347  by a second reduced diameter portion  353 . The third enlarged diameter portion  351  extends to an opposite end of the spindle valve  340 . A passageway  380  extends through the spindle valve  340  and has a first opening or port  381  in the first enlarged diameter portion  345  and a second opening or port  383  in the second reduced diameter portion  353 . Further description of the operation of the valve assembly  330  and the movement of the piston  316  will be provided in greater detail hereinbelow. The structure and operation of such a spindle valve  340  is well known in the art as shown in U.S. Pat. No. 5,442,992 which is assigned to the assignee of the invention disclosed and claimed herein. Additionally, U.S. Pat. No. 5,442,992 is incorporated herein by reference. An adjustment assembly  346  is provided in the handle structure  328  to allow the control mechanism  302  to be configured for either a constant volume or a constant pressure hydraulic power source. The adjustment assembly  346  is between the valve assembly  330  and the cavity  314 . The adjustment assembly  346  includes a valve chamber  348 , an adjustable valve member  350 , and a retaining ring  352 . 
   The valve chamber  348  is provided in the handle structure  328  on the same side as is the trigger  344 . The valve chamber  348  is perpendicular to the central axis  131  of the handle structure  328  and is always in fluid communication with the central passageway  336  and can be in fluid communication with the cross passageway  338 , depending upon the positioning of the adjustable valve member  350  and the pressure within the cross passageway  338 . The valve chamber  348  provides a valve seat  354  proximate to the cross passageway  338 . 
   The adjustable valve member  350  is positioned within the valve chamber  348 . As best shown in  FIGS. 10 ,  13  and  16 , the adjustable valve member  350  includes a valve  356 , a normally expanded spring  358 , an enlarged section  360 , and a knob  362 . The knob  362  is preferably provided proximate to an outer surface of the handle structure  328  such that a user of the hydraulic utility pruner tool  300  can easily operate the knob  362  by moving the knob  362  in either a first or second direction, preferably clockwise, or counterclockwise. An outer end  364  of the knob  362  may have a slot  366  provided therein such that a user of the hydraulic utility pruner tool  300  can move the knob  362  by use of another tool, such as a screwdriver. 
   An outer end  368  of the enlarged section  360  is secured to an inner end  370  of the knob  362 . The enlarged section  360  has a diameter which is larger than a diameter of the knob  362 . Because the enlarged section  360  has a larger diameter than the knob  362 , a shoulder  372  is provided between the enlarged section  360  and the knob  362 . The diameter of the enlarged section  360  is preferably commensurate with a diameter of the valve chamber  348  such that any fluid provided within the valve chamber  348  cannot escape out of the valve chamber  348  and, thus, out of the hydraulic utility pruner tool  300 . 
   A first end of the normally expanded spring  358  is connected to an inner end  374  of the enlarged section  360 . A second end of the normally expanded spring  358  is connected to the valve  356 . 
   The valve  356  is sized to fit within the valve seat  354 , but may also be moved out of the valve seat  354  as will be described in greater detail herein. When the valve  356  is seated in the valve seat  354 , the valve  356  prevents the cross passageway  338  from being in fluid communication with the central passageway  336  through the valve chamber  348 . If, however, the valve  356  is not seated in the valve seat  354 , the cross passageway  338  and the central passageway  336  are in fluid communication through the valve chamber  348 . 
   The retaining ring  352  is provided within the valve chamber  348  and is positioned proximate to the outer surface of the handle structure  328 . The retaining ring  352  has an aperture  376  therethrough which defines an inner diameter formed by the wall of the aperture  376 . The inner diameter of the retaining ring  352  is larger than the diameter of the knob  362 , but is smaller than the diameter of the enlarged section  360 . Thus, the knob  362 , upon movement thereof, can move through the aperture  376  of the retaining ring  352 , but the enlarged section  360  is trapped within the valve chamber  348  as the shoulder  372  abuts against the retaining ring  352 , preventing the enlarged section  360  from moving beyond the retaining ring  352 . Therefore, the adjustable valve member  350  is secured within the valve chamber  348  by the retaining ring  352 . 
   The adjustment assembly  346  provides benefits for the control mechanism  302  in comparison to the control mechanism of the prior art. The adjustment assembly  346  utilizes a minimum number of parts and minimal manufacturing costs. The adjustment assembly  346  further is conveniently located relative to the handle  328 . Thus, the adjustment assembly  346  of the control mechanism  302  provides an easy, reliable and efficient means for configuring the hydraulic utility pruner tool  300  for use with either a constant volume or a constant pressure system. 
   The tool  300  has a central tube  386  which extends from the central passageway  336 , through the drive chamber  324  and into the piston  316 . The central tube  386  has an opening threthrough which is in fluid communication with the central passageway  336 . A central chamber  384  is provided in the ram  308  and is in fluid communication with the central tube  386 . A radial port  382  extends through the ram  308  and places the central chamber  384  and the retract chamber  326  into fluid communication with one another. 
   Operation of the hydraulic utility pruner tool  300  will now be discussed and attention is directed to  FIGS. 9–16 . Operation of the hydraulic utility pruner tool  300  will first be discussed where the hydraulic utility pruner tool  300  is employed in a constant volume or open-center hydraulic power system, as illustrated in  FIGS. 9–13 . Operation of the hydraulic utility pruner tool  300  will then be discussed where the hydraulic utility pruner tool  300  is employed in a constant pressure or closed-center hydraulic power system, as illustrated in  FIGS. 14–16 . 
   Attention is directed to  FIGS. 9–13  and the operation of the hydraulic utility pruner tool  300  where the hydraulic utility pruner tool  300  is employed in a constant volume or open-center hydraulic power system. In order to operate the hydraulic utility pruner tool  300  such that the hydraulic utility pruner tool  300  is employed in a constant volume or open-center hydraulic system, the user first rotates the knob  362  of the adjustable valve member  350  in a first direction, preferably counterclockwise, until the shoulder  372  of the enlarged section  360  contacts the retaining ring  352 , as illustrated in  FIGS. 9–13 . In this position, the spring  358  is expanded such that the valve  356  is seated in the valve seat  354 , as best illustrated in  FIG. 13 . The knob  362  may extend out of the handle portion  328  in this position. 
   In order to close the cutting blades, the user activates the trigger  344  by moving the trigger  344  toward the handle structure  328 , as illustrated in  FIG. 11 . When the trigger  344  is in the position illustrated in  FIG. 11 , the inlet passageway  332  is not in fluid communication with the cross passageway  338 . Rather, the inlet passageway  332  is placed into fluid communication with the central passageway  336  because of the positioning of the spindle valve  340  within the spindle valve chamber  341 , and the cross passageway  338  is placed into fluid communication with the outlet passageway  334  through the passageway  380  of the spindle valve  340 , because of the positioning of the spindle valve  340  within the spindle valve chamber  341 . Thus, the hydraulic fluid from the reservoir flows into and through the inlet passageway  332 , into the spindle valve chamber  341 , around the reduced diameter portion  349  of the spindle valve  340 , into and through the central passageway  336 , into and through the central tube  386 , into and through the central chamber  384 , into and through the radial passageways  382 , and into the retract chamber  322 . 
   As the amount of fluid in the retract chamber  322  increases, the pressure within the retract chamber  322  also increases, such that the driving piston  316  is caused to retract within the cavity  314 . The driving piston  316  retracting within the cavity  314  causes the extension rod  307  to retract which, in turn, causes the cutting blades to close, such that the article to be cut by the cutting blades is cut. Retraction of the driving piston  316  within the cavity  314  causes the hydraulic fluid within the driving chamber  324  to flow out of the driving chamber  324 , into and through the cross passageway  338 , into the second port  383 , through the passageway  380  of the spindle valve  340 , out of the first port  381 , into and through the outlet passageway  334 , and back into the reservoir. The hydraulic fluid flowing through the cross passageway  338  to the outlet passageway  334  is prevented from flowing directly into the central passageway  336  to the inlet passageway  332  via the valve chamber  348  because the valve  356  is seated within the valve seat  354  and the spring  358  is expanded, i.e., the force of the fluid within the cross passageway  338  is not strong enough to force the spring  358  to contact such that the valve  356  will be unseated from the valve seat  354 , allowing the hydraulic fluid flowing through the cross passageway  338  to flow straight into the valve chamber  354  and back into the central passageway  336  and the inlet passageway  332 . 
   Once a cut is made with the cutting blades, the user deactivates or releases the trigger  344  by moving the trigger  344  away from the handle structure  328 , as illustrated in  FIG. 12 . When the trigger  344  is in the position illustrated in  FIG. 12 , the spindle valve  340  places the inlet passageway  332  into fluid communication with the cross passageway  338  as fluid is allowed to travel into the spindle valve chamber  341  and around the reduced diameter portion  353  of the spindle valve  340 . The spindle valve  340  also places the outlet passageway  334  into fluid communication with the central passageway  336  as fluid is allowed to travel into the spindle valve chamber  341  and around the reduced diameter portion  349  of the spindle valve  340 . 
   Thus, hydraulic fluid from the reservoir (not shown) of a hydraulic power system flows into the inlet passageway  332 , into the spindle valve chamber  341 , around the reduced diameter portion  353  of the spindle valve  340 , into and through the cross passageway  338 , and into the drive chamber  324 . The hydraulic fluid flowing through the cross passageway  338  is prevented from flowing directly into the central passageway  336  via the valve chamber  348  because the valve  356  is seated within the valve seat  354  and the spring  358  is expanded, i.e., the force of the fluid within the cross passageway  338  is not sufficient to force the spring  358  to contract such that the valve  356  will be unseated from the valve seat  354 , allowing the hydraulic fluid flowing through the cross passageway  336  to flow straight into the valve chamber  348  and back into the central passageway  336 . 
   As the amount of fluid in the drive chamber  324  increases, the pressure within the drive chamber  324  also increases, such that the driving piston  316  is caused to advance through the cavity  314 . The advancement of the driving piston  316  through the cavity  314  causes the extension rod  307  to advance such that the cutting blades are opened. Advancement of the driving piston  316  through the cavity  314  forces hydraulic fluid from the retract chamber  322  through the radial passageways  382 , into and through the central chamber  384 , into and through the central tube  386 , into the spindle valve chamber  341 , around the reduced diameter portion  349  of the spindle valve  340 , into and through the outlet passageway  334 , and into the reservoir. 
   When the driving piston  316  is advancing within the cavity  314 , the driving piston  316  will come to a fully advanced position, as illustrated in  FIG. 9 . Because the driving piston  316  cannot be further advanced, and because the hydraulic fluid continues to fill in the drive chamber  324  such that the pressure is increased within the drive chamber  324 , the back pressure provided within the cross passageway  338  is such that it overcomes the strength of the spring  358  which holds the valve  356  in the valve seat  354 . Thus, the spring  358 , at a predetermined pressure, contracts within the valve chamber  348  such that the valve  356  becomes unseated from the valve seat  354 , as illustrated in  FIGS. 9 and 10 . Thus, in order to alleviate the pressure within the drive chamber  324 , the hydraulic fluid flows from the inlet passageway  332 , into the spindle valve chamber  341 , around the reduced diameter portion  353  of the spindle valve  340 , into the cross passageway  338 , into the valve chamber  348 , into the central passageway  336 , back into the spindle valve chamber  341 , around the reduced diameter portion  349  of the spindle valve  340 , into and through the outlet passageway  334 , and into the reservoir. This is the neutral position of a constant volume system which allows fluid to continuously flow from the inlet passageway  332 , through the adjustment assembly  346  of the control mechanism  302 , and back through the outlet passageway  334 . In this position, the pressure in the drive chamber  324  and the retract chamber  322  is generally equalized such that the hydraulic fluid will continuously flow through the adjustment assembly  346  of the control mechanism  322  until the user again activates the trigger  344 . 
   Attention is directed to  FIGS. 14–16  and the operation of the hydraulic utility pruner tool  300  where the hydraulic utility pruner tool  300  is employed in a constant pressure or closed-center hydraulic power system. In order to operate the hydraulic utility pruning tool  300  such that the hydraulic utility pruner tool  300  is employed in a constant pressure or closed-center hydraulic system, the user first rotates the knob  362  of the adjustable valve member  350  in a second direction, preferably clockwise, such that the knob  362  of the adjustable valve member  350  turns into the valve chamber  348  until the valve  356  is fully seated within the valve seat  354 . The valve  356  is fully seated within the valve seat  354  when the normally expanded spring  358  is fully contracted or solid, as best illustrated in  FIG. 16 . 
   In order to cut an article placed between the cutting blades, the user activates the trigger  344  by moving the trigger  344  toward the handle structure  328 , as illustrated in  FIG. 15 . When the trigger  344  is in the position illustrated in  FIG. 15 , the inlet passageway  332  is not in fluid communication with the cross passageway  338 . Rather, the inlet passageway  332  is placed into fluid communication with the central passageway  336  because of the positioning of the spindle valve  340  within the spindle valve chamber  341 , and the cross passageway  338  is placed into fluid communication with the outlet passageway  334  through the passageway  380  of the spindle valve  340 , because of the positioning of the spindle valve  340  within the spindle valve chamber  341 . Thus, the hydraulic fluid from the reservoir flows into and through the inlet passageway  332 , into the spindle valve chamber  341 , around the reduced diameter section  349  of the spindle valve  340 , into and through the central passageway  336 , into and through the central tube  386 , into and through the central chamber  384 , into and through the radial passageways  382 , and into the retract chamber  322 . 
   As the amount of fluid in the retract chamber  322  increases, the pressure within the retract chamber  322  also increases, such that the driving piston  316  is retracted axially within the cavity  314 . The driving piston  316  retracting within the cavity  314  axially retracts the extension rod  307  which, in turn, causes the cutting blades to close, such that the article to be cut by the cutting blades is cut. Retraction of the driving piston  316  within the cavity  314  causes the hydraulic fluid within the driving chamber  324  to flow out of the driving chamber  324 , into and through the cross passageway  338 , into the second port  383 , through the passageway  380  of the spindle valve  340 , out of the first port  381 , into and through the outlet passageway  334 , and back into the reservoir. The hydraulic fluid flowing through the cross passageway  338  to the outlet passageway  334  is prevented from flowing directly into the central passageway  336  to the inlet passageway  332  via the valve chamber  348  because the valve  356  is fully seated within the valve seat  354  as the spring  358  is fully contracted or solid. 
   Once a cut is made with the cutting blades, the user deactivates or releases the trigger  344  by moving the trigger  344  away from the handle structure  328 , as illustrated in  FIG. 14 . When the trigger  344  is in the position illustrated in  FIG. 14 , the spindle valve  340  places the inlet passageway  332  into fluid communication with the cross passageway  338  as fluid is allowed to travel into the spindle valve chamber  341  and around the reduced diameter section  353  of the spindle valve  340 . The spindle valve  340  also places the outlet passageway  334  into fluid communication with the central passageway  336  as fluid is allowed to travel into the spindle valve chamber  341  and around the reduced diameter section  349  of the spindle valve  340 . 
   Thus, hydraulic fluid from the reservoir (not shown) of a hydraulic power system flows into the inlet passageway  332 , into the spindle valve chamber  341 , around the reduced diameter portion  353  of the spindle valve  340 , into and through the cross passageway  338 , and into the drive chamber  324 . The hydraulic fluid flowing through the cross passageway  338  is prevented from flowing directly into the central passageway  336  via the valve chamber  348  because the valve  356  is fully seated within the valve seat  354  as the spring  358  is fully contracted. 
   As the amount of fluid in the drive chamber  324  increases, the pressure within the drive chamber  324  also increases, such that the driving piston  316  is caused to advance through the cavity  314 . The advancement of the driving piston  316  through the cavity  314  advances the extension rod  307  such that the cutting blades are opened. Advancement of the driving piston  316  through the cavity  314  forces hydraulic fluid from the retract chamber  322  through the radial passageways  382 , into and through the central chamber  384 , into and through the central tube  386 , into the spindle valve chamber  341 , around the reduced diameter portion  349  of the spindle valve  340 , into and through the outlet passageway  334 , and into the reservoir. 
   Once the driving piston  316  comes to a fully advanced position, because the hydraulic fluid continues to fill in the drive chamber  324 , pressure will continue to build within the cavity  314 . Because the valve  356  is mechanically locked in the valve seat  354 , such that hydraulic fluid is not allowed to flow past the valve  356 , pressure will continue to build until it reaches a predetermined value established by a relief valve (not shown) in the hydraulic circuit. Relief valves within hydraulic circuits are well-known in the art and, therefore, are not explained herein in detail. In systems with a positive displacement pump, the relief valve diverts flow back to the reservoir. On systems with a variable stroking pump, pressure will continue to build until it reaches the predetermined value established by the relief valve, whose control system then reduces the flow of hydraulic fluid to adequately maintain system pressure. 
   In the constant pressure system, the force of the fluid within the cross passageway  338  is never sufficient to unseat the valve  356  from the valve seat  354  as the spring  358  is already fully contracted. Thus, the hydraulic fluid will never flow directly or continuously from the cross passageway  338 , into the valve chamber  348 , and back into the central passageway  336 . 
   While preferred embodiments of the invention are shown and described, it is envisioned that those skilled in the art may devise various modifications without departing from the spirit and scope of the foregoing description and the appended claims.