Abstract:
A phaser valve for a piston and cylinder assembly helps to keep a pair of pistons in phase with each other. The phaser valve includes a poppet which is slidable between opposing seats of a valve housing. The poppet includes a head and a stem, both the head and the stem having concave portions which facilitate the flow of hydraulic fluid across the valve. The poppet head has two generally conical ends, so that the head makes a fluid-tight seal with a seat when the head is urged towards either seat. Due to its construction, the phaser valve may be built to fit within a very small space, while still allowing a substantial flow of fluid when the head is not in abutment with one of the seats.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
       [0001]    This application is a 35 USC 371 national stage filing of international application no. PCT/US2010/24858, filed Feb. 22, 2010, which claims priority to U.S. provisional application 61/154,523, filed on Feb. 23, 2009 in the USPTO. All of these applications are incorporated by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to hydraulic cylinders which are used to operate heavy equipment, such as lifts, cranes, and the like. 
         [0003]    In particular, the invention comprises a novel valve having a geometry designed to synchronize the position of two or more hydraulic cylinders operating together at the end of either or both of their directions of stroke. 
         [0004]    More particularly, the invention provides an improved valve structure which enables the above-described valve to accomplish its objective, and wherein the valve occupies a very small space. 
         [0005]    If two cylinders are operating in parallel, differences in leakage′ across piston seals, differences in cylinder or rod diameters, or other reasons, can cause differences in cylinder position to accrue over time. The rods and ends of these parallel cylinders are often fixed to the same structures. Should the cylinders become “out of phase” with respect to one another (with one cylinder fully extending or fully retracting before the other), undesirable deformation stresses such as twisting can result if the cylinders are driving the same structure. 
         [0006]    An example of the use of parallel hydraulic cylinders, in which the cylinders are attached to the same structure, is a device in which the cylinders are attached to the main boom of a crane. 
         [0007]    An example of two cylinders working in parallel, but attached to different structures, is outrigger cylinders. Hydraulic outriggers are often provided on mobile lifts, cranes, and construction equipment to stabilize the vehicle on which the lift, crane, or construction tool (shovel or bucket) is mounted. 
         [0008]    Outriggers generally serve two purposes. The first is to eliminate the vehicle suspension, and possibly pneumatic tires, in the support of the crane. Outriggers will generally lift the vehicle tires off of the ground such that the vehicle has a rigid support. It is undesirable for a crane platform or the like to move or bounce. 
         [0009]    The second purpose is to widen the vehicle&#39;s contact with the ground so that it becomes more difficult to extend the center of gravity of the vehicle over the most lateral contact point with the ground. Such extension of the center of gravity will result in tipping of the vehicle. Using a crane as an example, the wider the contact with the ground, the larger the “load envelope” within which the fully loaded crane can safely operate. 
         [0010]    Should outrigger cylinders become out of phase due to accrual of differences in piston leakage, one outrigger will strike the ground before the other, and the result will be tilting of the vehicle, a condition that the outriggers are provided to prevent. 
         [0011]    The present invention provides a solution for the problems described above, by providing an improved phaser valve which assists in synchronizing two or more cylinders. 
       SUMMARY OF THE INVENTION 
       [0012]    The cylinder phaser valves of this invention are to be installed in the pistons of cylinders that are working together. 
         [0013]    Normally, the blind end and the rod end of a cylinder are prevented from fluid communication by seals around the periphery of the piston. Pressurized fluid introduced into either the blind end or the rod end of the cylinder will then cause translational movement of the piston within the cylinder bore. The cylinder may then perform useful work. 
         [0014]    Differential pressure across the piston is responsible for movement of the rod of the cylinder. Any communication between the blind and rod ends will tend to lessen or eliminate this pressure differential. 
         [0015]    The valve of the present invention is configured in such a manner that as the piston of the cylinder approaches one end of its travel, the valve is opened to allow communication between the blind end and the rod end. Therefore, while the valve is opened, translational motion of the piston within the cylinder bore is stopped or slowed, as oil is discharged from the pressurized end of the cylinder, across the piston, and out of the depressurized end of the cylinder. 
         [0016]    Thus, when one cylinder reaches the end of its stroke, the cylinder phaser valve in its piston is opened, and another cylinder or cylinders operating in parallel is (are) allowed to synchronize its (their) position(s), or “catch up” with the first cylinder. 
         [0017]    The cylinder phaser valve of the present invention is therefore effective only during a limited portion of the operating cycle of the piston and cylinder assembly. That is, the phaser valve of the present invention does not allow communication of the rod end of the cylinder with the blind end under most circumstances, but allows such communication only when a piston is approaching one end of its path of travel. 
         [0018]    The valve of the present invention includes a poppet head and stem which slide axially within a cartridge. Both the poppet head and stem have concave surfaces which allow fluid to flow around them (when the poppet head is not forming a seal against the seat defined by the cartridge body or the seat defined by a stem guide). This structure enables the valve to be constructed in a small size, while still allowing sufficient fluid to flow through when desired. 
         [0019]    The phaser valve also functions as a cushion valve, slowing the speed of travel of the piston rod at the end of its stroke, and thereby minimizing the impact of the piston on the end of the cylinder bore. The latter results in less vibration and noise, and is likely to increase the lifespan of the cylinder assembly. 
         [0020]    The present invention therefore has the primary object of providing a valve which enables the synchronization of two or more cylinder assemblies. 
         [0021]    The invention has the further object of minimizing the size of the valve described above, while still permitting a desired flow of fluid when the valve is opened. 
         [0022]    The invention has the further object of reducing the size of a phaser valve used in synchronizing two or more cylinders. 
         [0023]    The invention has the further object of reducing vibration and noise in a cylinder assembly. 
         [0024]    The invention has the further object of prolonging the useful life of a cylinder assembly. 
         [0025]    Other objects and advantages of the present invention will be apparent to those skilled in the art, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF. THE DRAWINGS 
         [0026]      FIGS. 1   a - 1   c  provide a cross-sectional view ( FIG. 1   b ) and two end views ( FIGS. 1   a  and  1   c ) of a cartridge body and valve stem guide, together with a poppet head and stem, as used in the present invention. 
           [0027]      FIG. 2  provides a cross-sectional view of the poppet head and stem in the cartridge body and valve stem guide of the present invention, the poppet head being positioned so that it is in sealing contact with the valve stem guide. 
           [0028]      FIG. 3  provides another cross-sectional view of the poppet head and stem in the cartridge body and valve stem guide of the present invention, wherein the poppet head is positioned so that it is in sealing contact with the opposite seat, relative to  FIG. 2 , of the cartridge body. 
           [0029]      FIG. 4  provides a cross-sectional view similar to that of  FIG. 3 , in which the poppet head is positioned between the sealing surfaces of the valve stem guide and the cartridge body. 
           [0030]      FIGS. 5   a - 5   c  provide an elevational view ( FIG. 5   b ) and two end views ( FIGS. 5   a  and  5   c ), taken from mutually opposite directions, of the poppet head and stem structure used in the present invention. 
           [0031]      FIGS. 5   d  and  5   e  provide perspective views, taken from different directions, of the poppet head and stem of the present invention. 
           [0032]      FIGS. 6   a - 6   c  provide an elevational view ( FIG. 6   b ) and two end views ( FIGS. 6   a  and  6   c ), taken from mutually opposite directions, of a poppet head and double stem structure used in the present invention. 
           [0033]      FIG. 7   a  provides a cross-sectional view of the valve stem guide used in the present invention. 
           [0034]      FIG. 7   b  provides an end view of the valve stem guide of  FIG. 7   a.    
           [0035]      FIG. 8   a  provides a side elevational view of the cartridge body of the present invention. 
           [0036]      FIG. 8   b  provides an end view of the cartridge body of  FIG. 8   a.    
           [0037]      FIG. 9  provides a simple hydraulic schematic diagram of a hydraulic cylinder circuit, as used in the present invention. 
           [0038]      FIG. 10   a  provides a cross-sectional view of a hydraulic cylinder assembly, showing the present invention incorporated within the piston of such assembly. 
           [0039]      FIG. 10   b  provides an expanded cross-sectional view of a portion of  FIG. 10   a , showing the present invention as incorporated into the piston of the cylinder assembly, the device of the present invention being oriented so that it is actuated when the piston rod approaches full extension. 
           [0040]      FIG. 11   a  provides a cross-sectional view of a hydraulic cylinder assembly with the present invention incorporated into the piston. 
           [0041]      FIG. 11   b  provides an expanded cross-sectional view of a portion of  FIG. 11   a , in which the device of the present invention is oriented so that it is actuated when the piston rod approaches full retraction. 
           [0042]      FIG. 12   a  provides a cross-sectional view of a hydraulic cylinder assembly having two valves, made according to the present invention, incorporated within the piston of the cylinder assembly. 
           [0043]      FIG. 12   b  provides an expanded cross-sectional view of a portion of  FIG. 12   a , showing the valves of the present invention within the piston of the cylinder assembly, each valve being oriented in opposite directions such that one will be actuated when the piston rod approaches full extension, while the other is actuated when the piston rod approaches full retraction. 
           [0044]      FIG. 13   a  provides a cross-sectional view of a hydraulic cylinder assembly with a device of the present invention incorporated within the piston of the cylinder assembly, the poppet head being shown at a position between the sealing surfaces of the valve stem guide and cartridge, respectively. 
           [0045]      FIG. 13   b  provides an expanded cross-sectional view of a portion of  FIG. 13   a , in which the device of the present invention is oriented so that it is actuated when the piston rod approaches full extension. 
           [0046]      FIG. 14   a  provides a cross-sectional view of a hydraulic cylinder assembly with the device of the present invention incorporated within the piston of the cylinder assembly, the poppet head being shown urged to a position in sealing contact with the valve stem guide. 
           [0047]      FIG. 14   b  provides an expanded cross-sectional view of a portion of  FIG. 14   a , the device of the present invention being oriented so that it is actuated when the piston rod approaches full extension. 
           [0048]      FIG. 15   a  provides a cross-sectional view of a hydraulic cylinder assembly having the device of the present invention incorporated within the piston of the cylinder assembly. 
           [0049]      FIG. 15   b  provides an expanded cross-sectional view of a portion of  FIG. 15   a , showing the device of the present invention in the piston of the cylinder assembly, the stem of the poppet being in contact with the end of the cylinder as the rod approaches maximum extension, and the poppet being therefore no longer in sealing contact with the valve stem guide, despite pressure being applied to the blind end. 
           [0050]      FIG. 16   a  provides a cross-sectional view of a hydraulic cylinder assembly having the device of the present invention incorporated within the piston of the cylinder assembly, the poppet head being urged to a position in sealing contact with the cartridge. 
           [0051]      FIG. 16   b  provides an expanded cross-sectional view of a portion of  FIG. 16   a , with the device of the present invention in the piston of the cylinder assembly. 
           [0052]      FIG. 17   a  provides a cross-sectional view of a hydraulic cylinder assembly having the device of the present invention incorporated within the piston of the cylinder assembly, wherein the poppet head is urged to a position in sealing contact with the cartridge. 
           [0053]      FIG. 17   b  provides an expanded cross-sectional view of a portion of  FIG. 17   a , with the device of the present invention in the piston of the cylinder. 
           [0054]      FIG. 18  provides a fragmentary cross-sectional view of a cylinder assembly having the device of the present invention installed, and showing a single-stemmed poppet head and a portion of a poppet retaining member typical of a valve-in-piston design. 
           [0055]      FIG. 19  provides a cross-sectional view of a cartridge and valve stem guide, and showing a double-stemmed poppet within the cartridge, according to the present invention. 
           [0056]      FIG. 20  provides a fragmentary cross-sectional view of a piston within a cylinder assembly, and showing a double-stemmed poppet and a poppet retaining member, according to the present invention. 
           [0057]      FIG. 21   a  provides a cross-sectional view of a cylinder assembly, in which there is shown a double-stemmed poppet within a valve assembly, according to the present invention. 
           [0058]      FIG. 21   b  provides an expanded cross-sectional view of a portion of  FIG. 21   a , showing more clearly a portion of the cross-section of the piston, and the double-stemmed poppet within a cross-section of a valve assembly. 
           [0059]      FIG. 22  provides a cross-sectional view of the cartridge of the present invention, showing a variant of the double-stemmed poppet, the poppet being positioned so that it is in sealing contact with the valve stem guide. 
           [0060]      FIG. 23  provides a cross-sectional view of the valve assembly of the present invention, showing a variant of the double-stemmed poppet within the valve assembly, the poppet being positioned so that it is in sealing contact with the cartridge. 
           [0061]      FIG. 24  provides a cross-sectional view of the valve assembly of the present invention, showing a variant of the double-stemmed poppet, the poppet being positioned between the sealing surfaces of the valve stem guide and the cartridge. 
           [0062]      FIG. 25  provides an end view of the poppet within a fragmentary cross-section of a cartridge (or, in a valve-in-body embodiment, a portion of a cross-section of a cavity of a cylinder piston), according to the present invention. 
           [0063]      FIG. 26  provides an end view of a valve stem within a fragmentary cross-section of a cartridge and a valve stem guide (or, in a valve-in-body embodiment, a portion of a cross-section of a cavity of a cylinder piston), according to the present invention. 
           [0064]      FIG. 27  provides cross-sectional views of two cylinder assemblies operating together, with their rods fixed to the same structure, plus expanded cross-sectional views of portions of each piston showing a valve of the invention in each, the upper cylinder in the figure having almost reached the end of its stroke such that the stem of its phaser valve has just come in contact with the end of the cylinder. 
           [0065]      FIG. 28  provides cross-sectional views of two cylinder assemblies operating together, with their rods fixed to the same structure, plus expanded cross-sectional views of portions of each piston showing a valve of the invention in each, the upper cylinder in the figure having almost reached the end of its stroke such that the stem of its phaser valve has come in contact with the end of the cylinder and its poppet has moved away from sealing contact with the stem guide. 
           [0066]      FIG. 29  provides cross-sectional views of two cylinder assemblies operating together, with their rods fixed to the same structure, plus expanded cross-sectional views of portions of each piston showing a valve of the invention in each, the upper cylinder in the figure having reached the end of its stroke, and the lower cylinder having almost reached the end of its stroke such that the stem of its phaser valve has come in contact with the end of the cylinder. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0067]    The phaser valve of the present invention comprises an assembly having three main parts (exclusive of seals): 
         [0068]    1. Poppet/stem 
         [0069]    2. Valve Stem Guide 
         [0070]    3. Cartridge Body Housing 
         [0071]    The poppet includes a poppet head  100  and a stem portion  3  (see, for example,  FIG. 1   b ). 
         [0072]    As shown in  FIGS. 5   a - 5   c , the poppet head  100  comprises two conical surfaces  1  and  2 , oriented 180 degrees apart. One surface  2  seals against a seat (indicated by reference numeral  13  of  FIG. 8   a ) in the cartridge body. The opposite surface  1  seals against a similar sealing surface (indicated by reference numeral  6   a  of  FIG. 7   a ) in the valve stem guide  4 . 
         [0073]    The stem  3  of the poppet ( FIG. 5   b ) extends axially from one conical surface. The stem must be sufficiently long such that it extends outside the valve housing, or cartridge, during at least a portion of the travel of the poppet. 
         [0074]      FIGS. 5   d  and  5   e  provide perspective views of the poppet head and stem of the present invention. As is best shown in  FIG. 5   e , the poppet head has a forward or distal portion  220  which is generally conical. The poppet head also has a rearward or proximal portion  223  which is also generally conical. The poppet head includes a plurality of lands  224  which are separated by concave regions  221 , the lands and concave regions being located between the forward or distal portion  220  and the rearward or proximal portion  223 . The concave regions  221  are generally aligned with elongated concave surfaces  222  of the poppet stem. The concave surfaces  222  extend along substantially the entire length of the stem. The alignment of the concave surfaces  222  with the concave regions  221  facilitates the flow of fluid around the poppet, because such fluid is not required to make any more turns than necessary, thus minimizing the pressure drop. The stem also includes lands  225 . The lands of the stem abut, or nearly abut, the interior surface of the valve stem guide (not shown in  FIGS. 5   d - e ). 
         [0075]    The poppet head, in cross-section, is thus generally circular, except to the extent that the circle is “gouged” by the concave regions, as is illustrated, for example, in  FIGS. 5   c - e.    
         [0076]    The number of concave regions and surfaces, on the poppet head and the poppet stem, respectively, can be varied, but the preferred number is three such regions and three such surfaces. Thus, in the preferred embodiment, the stem has a generally triangular cross-section. 
         [0077]      FIG. 5   e  shows that the forward portion of the poppet head will make a suitable fluid seal when it abuts a circular opening comprising the valve seat. As long as the head engages the seat at a position which is forward relative to the concave region, the conical head will fully engage the opening, and no leakage of fluid will occur. Similarly,  FIG. 5   d  shows that the rearward portion of the poppet head will make a seal when it abuts the opposing valve seat. 
         [0078]    As shown in  FIGS. 7   a - b , the valve stem guide  4  has a central bore  5  through which the stem extends. The stem is thus guided within a generally cylindrical channel defined by the stem guide. The valve stem guide has an outer diameter such that it may be assembled within the cartridge body. A seal  7  is placed peripherally about the valve stem guide to prevent leakage of fluid between the valve stem guide and the cartridge. 
         [0079]    As shown in  FIGS. 8   a - b , the cartridge body  8  has a stepped central bore of several diameters to accommodate the poppet/stem and the valve stem guide. The valve stem guide is placed within the largest diameter  9  at the end of the cartridge, and abuts against a shoulder  10 . A smaller diameter  11  in the central region of the cartridge is designed to contain the poppet. A smaller through bore  12  has a seat  13  immediately adjacent to the central diameter, of a geometry necessary to interface with the poppet (in this case, conical). The cartridge has an outer seal  14  to prevent flow around the cartridge and the bore in the cylinder piston within which it resides. Therefore, the only flow path is through the central bore. 
         [0080]    The cartridge body has an external thread  15  that is meant to engage in an internal thread in its corresponding cavity. An internal wrenching surface  16  is provided within the first part of the central bore of the valve. It is recognized that a wide variety of options of wrenching means may be incorporated on the cartridge other than the internal hex shown. 
         [0081]    The conical seat  13  of the cartridge body  8  is spaced a distance from the seat of the valve stem guide. This distance is greater than the distance between the opposing sealing surfaces of the poppet. Thus, the poppet may occupy a position at either seat, or somewhere in between.  FIG. 2  shows the poppet head  100  at an upper seat  101  of cartridge body  8 , while  FIG. 3  shows the poppet head  100  at a lower seat  102  of the cartridge body.  FIG. 1   b  and  FIG. 4  show the poppet head  100  at an intermediate position.  FIGS. 1   a  and is show opposite end views of the structure of  FIG. 1   b.    
         [0082]      FIG. 9  shows a simple hydraulic circuit for controlling the motion of a piston within a cylinder. Through the use of a directional control valve  17 , the blind end  18  of the cylinder may be pressurized, while the rod end  19  is vented to a hydraulic reservoir  20  at low pressure. This will cause the piston of the cylinder assembly to extend. Conversely, the rod end may be pressurized, while the blind end is vented to a reservoir. This will cause the piston of the cylinder assembly to retract. The hydraulic circuit of  FIG. 9  comprises a means for directing fluid alternately into opposite sides of the piston. The phaser valve of the present invention is essentially a control means which diverts a portion of the hydraulic fluid such that the movement of the piston is slowed, so as to bring the pistons into phase. 
         [0083]      FIGS. 10   a - b  show the invention as assembled in the piston of a hydraulic cylinder assembly. It is understood that the orientation of the valve within the piston can be in the other direction to cause the pistons to get back into phase at the opposite extent of travel (as shown in  FIGS. 11   a - b ). It is also understood that two valves, oriented in opposite directions, may be utilized to provide synchronization at the termination of each direction of travel (as shown in  FIGS. 12   a - b ). 
         [0084]    In  FIGS. 13   a - b , neither the blind end nor the rod end of a hydraulic cylinder is pressurized. As there is no differential pressure across the piston, it will remain stationary. The poppet will not tend to be urged to either seat in the absence of any other force, such as the force of gravity. 
         [0085]    In  FIGS. 14   a - b , the blind end  21  of a hydraulic cylinder is pressurized. The differential pressure acting on the blind end and rod end piston surfaces will cause the piston to move toward the rod end  22 . 
         [0086]    This pressure differential will also urge the poppet away from the conical seat on the cartridge body  23  (closest to the blind end) and toward the seat of the valve stem guide  24 . The interaction of the tapered seat and the conical surface of the poppet will form a circular metal to metal seal. Therefore, no communication will exist between the blind end and the rod end of the cylinder. 
         [0087]    As shown in  FIGS. 15   a - b , when the piston approaches the end of travel at the rod end (full extension), the stem  25  will make contact with the end of the cylinder cavity  26 . Any further travel will move the poppet off of its seat  27  on the stem guide, and will allow fluid communication through the piston, from the blind end to the rod end.  FIG. 15   b  shows the condition in which the poppet head has just moved off of its seat. 
         [0088]    Travel of the piston will then slow or stall. 
         [0089]    This will allow a parallel cylinder to continue to extend, at least at a faster rate, and synchronize its position with the first cylinder. 
         [0090]    Conversely, should the directional control valve be shifted in the opposite direction,  FIGS. 16   a - b  demonstrate movement of the poppet toward the seat on the cartridge. 
         [0091]      FIGS. 17   a - b  illustrate the condition wherein pressurized hydraulic fluid is directed to the rod end  28  of the cylinder. Fluid in the blind end  29  of the cylinder will now be directed by the directional control valve to the hydraulic reservoir at low pressure. The piston will be moved towards the blind end. 
         [0092]    The poppet of the phaser valve will be urged from the region of high pressure at the conical seat of the valve stem guide  30  (adjacent to the rod end) towards the region of lower pressure at the conical seat of the cartridge body  31 . The interface of the poppet head and the seat will again form a seal against the flow of hydraulic fluid, and prevent communication between the rod end and the blind end of the cylinder. 
         [0093]    As noted above, the poppet head has two conical surfaces opposed to each other. The included angle of the poppet as shown is 90 degrees. However, this angle can be varied to optimize for flow conditions. A shallower angle will better allow for laminar flow at higher flow rates, other things being equal. A shallower angle has a better tendency for the poppet head and seat to self-align. However, a steeper angle has better opening characteristics, with better gain of flow area for unit travel of the poppet. 
         [0094]    Although the invention is described as a cartridge, it is recognized that the same function can be accomplished with a “valve in body” approach, shown in  FIG. 18 . In such an approach, the working parts of the valve would be placed directly within a specially machined complementary cavity in the cylinder piston. In some circumstances, the “valve in body” approach may be preferred. The cartridge body housing would not be required, and would be replaced by a retaining member  32 . This retaining member is not necessarily threaded, but may utilize a flange, snap-ring, or other means to retain the valve parts within the cavity. In this example, one valve seat  33  is machined directly in a cavity within the piston bore, while the other is machined on the retaining member  34 . 
         [0095]    The invention may be used in a broad range of cylinder sizes. However, the design features make it particularly suited for small cylinders, particularly if a “valve in body” approach is taken. These features allow it to be placed within a smaller envelope than common hydraulic cartridge valves. 
         [0096]    It is understood that the poppet can be made in other geometries, such as spherical. By using the “valve in body” approach, the dimensions of the components can be customized for that particular piston. It is also understood that the poppet can be fashioned with a stem on each surface, as shown in  FIGS. 6   a - c , and the valve configured accordingly, such that there will be phasing at the piston limit in each direction. A poppet so fashioned may be used either with the cartridge design (as shown in  FIGS. 19 and 21   a - b ) or with the valve in body design (as shown in  FIG. 20 ). It is understood that a poppet/stem assembly of this configuration must be customized to traverse the thickness of the particular piston in which it is used. 
         [0097]    The double-stem poppet can be used to provide more axial stability of the poppet, and can be utilized for bidirectional phasing.  FIGS. 22-24  show the poppet and stem assembly in a range of positions, from one extreme in sealing contact with the valve stem guide (shown in  FIG. 22 ), to the other extreme in sealing contact with the cartridge (shown in  FIG. 23 ), and in an intermediate position (shown in  FIG. 24 ). In  FIG. 23  it is apparent that the second stem never protrudes from the cartridge body, and therefore cannot be used to manually shift the poppet off of the seat. It is used here to provide additional guidance to the poppet for better alignment of the poppet and the seat. 
         [0098]    Good alignment is desirable for a poppet with a conical sealing surface, as angular offset of the poppet and seat may result in incomplete contact between the two and leakage. This can be problematic should the end of the cylinder piston strike the poppet in a non-axial manner, due to tolerances in perpendicularity of the cartridge cavity, or tolerances in flatness and/or perpendicularity of the cylinder end. The double stem, by virtue of an increased length of bearing contact, is better suited for wider tolerances in this regard. The use of the double stem also allows for less strict tolerances in the central bore of the stem guide. Thus, the increase in cost of such a poppet may be more than offset by a decrease in manufacturing cost in these other areas. It is recognized that a spherical poppet does not have this angular alignment issue, however this geometry can be more difficult to machine and inspect. 
         [0099]    In  FIG. 25 , the poppet head is shown with three fluid flow channels  35  around its periphery to improve flow, and lessen pressure drop, although a smaller or larger number of fluid channels can be employed. This allows for a smaller overall diameter of the cartridge than if the flow area was provided only by clearance around the poppet&#39;s outside diameter. These channels must fall short of the sealing diameter of the poppet and seat interface so as to avoid leaking of the valve in the closed positions. The three raised portions  36  that remain are guide lands that interface with the bore  37  in the cartridge (or piston bore in a valve-in-body design) to guide the poppet axially between opposing seats. Similar flow channels can be fashioned in a poppet of a spherical configuration, with resulting guide lands. 
         [0100]    The poppet is coaxial with the valve stem. The valve stem  38  shown in  FIG. 26  is triangular in cross-section, and, when placed in the bore  39  of the valve stem guide, results in three distinct flow areas  40 . The flats of the triangle may be radiused with a negative bias  41 , as shown, to improve flow area and reduce pressure drop. 
         [0101]    The dimensions of the triangle are ideally made larger than the circumscribed circle of the valve guide bore so that the points of the triangle are replaced with three distinct guide lands. The stem within the bore provides the primary means of axial guidance of the poppet in the bore. 
         [0102]    The flow channels in the poppet are the same in number, and are aligned, with the flow areas around the stem, so as not to induce any tendency to rotate the poppet. 
         [0103]    The cross-section of the stem is shown to be a triangle. It is recognized that other cross-sections can be employed, such as a rectangular section, square section, or other polygonal section. The triangle shown is believed to be the preferred cross-sectional geometry of the stem. It provides the largest ratio of flow channel area per unit of stem cross sectional area, when compared to other regular polygons. Also, should there be a lack of perpendicularity of the bore in the piston with respect to the cylinder wall that the stem is designed to strike, the triangular cross-section will accommodate this better than, say, a thin rectangular cross-section. 
         [0104]    The poppet is ideally made of a hard material, such as hardened steel. The valve stem guide ideally should be made of a hard material as well so as to resist any wear that may be induced by the stem, particularly if the stem is struck in an non-axial manner. However, there should be a hardness mismatch between the poppet and stem guide, such that the poppet may slightly deform the seat in operation so that the mating surfaces conform to each other to limit leakage. A similar mismatch is desirable between the poppet and the opposing sealing surface on the cartridge. 
         [0105]    The operation of the present invention can be summarized as follows, with reference to  FIGS. 27-29 . 
         [0106]    As stated previously, the invention is designed to allow two or more cylinders working together to resynchronize or re-phase their positions at the end of their strokes.  FIG. 27  shows two cylinders working together that, due to the various reasons mentioned previously, have gotten “out of phase” with each other. In other words, the rod of the upper cylinder  201  is extended further than the rod of the lower cylinder  200 . The relative positions are exaggerated for clarity. In this example, the stem  202  of the phaser of the upper cylinder has just come into contact with the end of the cylinder. The stem  203  of the lower cylinder is still a distance away from its corresponding end. 
         [0107]    In  FIG. 28 , assuming that the blind ends of both cylinders are being pressurized, and the rod ends are being discharged to a hydraulic reservoir at low pressure, the piston of the upper cylinder  201  has advanced a small distance when compared to  FIG. 27 . The poppet of the phaser of the upper cylinder has been moved away from sealing contact with its valve stem guide. This will allow the flow of hydraulic fluid from the high pressure region in the blind end, to the lower pressure region in the rod end. This will tend to slow or stall movement of the upper piston. The lower piston will continue to move as before. 
         [0108]      FIG. 29  shows the result of continued application of pressurized hydraulic fluid to the blind ends of both cylinders. The upper cylinder  201  has slowly advanced to full extension. During this time, the lower cylinder  200  has advanced at a faster rate such that the stem  203  of its phaser is now in contact with the end of the cylinder wall. The relative positions of the ends of each rod are much closer. 
         [0109]    Note that the basic source of synchronization of the pistons in the cylinders is the fact that pressurized fluid is conveyed, simultaneously, into the blind ends, or the rod ends, of both cylinders. The phaser valves of the present invention serve to smooth the “rough edges” of each stroke. That is, both pistons are driven by the same application of pressure, but the valves of the present invention prevent one piston from “getting ahead” of the other, at or near the end of the stroke, by slowing that piston until the other piston has reached approximately the same position. 
         [0110]    Also, the structure of the valves implies that the first valve to reach the cylinder wall is the one which is slowed. Thus, either piston could be slowed, since both pistons are equipped with the same phaser valve of the present invention. The invention thus automatically slows the first piston to reach the cylinder wall. 
         [0111]    The invention can be modified in various ways. The exact geometry of the poppet head and poppet stem can be varied, as can the number of concave regions and surfaces. These and other modifications, which will be apparent to those skilled in the art, should be considered within the spirit and scope of the following claims.