Patent Publication Number: US-2007101861-A1

Title: Two-speed cylinder

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims priority to U.S. Provisional Application No. 60/728,520 filed Oct. 20, 2005 and also U.S. Provisional Application No. 60/733,411 filed Nov. 4, 2005, both of which applications are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to cylinder assemblies and, in one particular embodiment, to a dual-acting hydraulic cylinder.  
      2. Technical Considerations  
      Fluid powered linear actuators are commonly referred to in the mechanical arts as “cylinders”. These known cylinders are generally either pneumatically or hydraulically actuated. One common type of cylinder is a conventional rod cylinder in which a cylindrical rod moves into and out of the cylinder casing or cylinder tube due to fluid pressure on the end of the rod. Another common type of cylinder is a piston cylinder. A piston cylinder is similar to a rod cylinder but the end of the rod has a larger surface area “piston” attached thereto to increase the surface area upon which the fluid pressure acts.  
      As will be appreciated by one of ordinary skill in the art, the force that a cylinder can exert on a load is based primarily on two factors. These are (1) the pressure of the fluid utilized in the cylinder and (2) the surface area across which the pressure is exerted. A rod cylinder utilizes the surface area of the end of the rod to do work. A piston cylinder utilizes the larger surface area of the piston to do work. If two cylinders of identical tube size, rod size (diameter and length), and fluid pressure are compared, the piston cylinder will be able to exert more force than the rod cylinder due to the fluid pressure acting over the larger surface area of the piston. However, given equal flow rates of the actuating fluid into the cylinders, the rod cylinder will travel more quickly than the piston cylinder. That is, the rod of the rod cylinder will extend more rapidly out of the tube than the rod of the piston cylinder.  
      One of the challenges of fluid power has been to get a piston cylinder to act as quickly as a rod cylinder when greater force is not needed. Two conventional ways to achieve this goal are: (1) to use a pump with variable volume displacement to increase fluid flow when high pressure (larger force) is not needed or (2) regenerating fluid from the rod side of the piston into the opposite side of the piston. Both of these methods have drawbacks. The use of variable displacement pumps increases the cost and size of the system. Regeneration requires the use of a complex combination of valves to divert the fluid from one side of the piston to the other or to a reservoir. Regeneration also typically requires higher fluid flow rates through the cylinder ports and associated piping.  
      Therefore, it would be advantageous to provide a cylinder that combines the operational characteristics of both a piston cylinder and a rod cylinder. That is, a cylinder that provides a large initial force (characteristic of a piston cylinder) to move a load followed by a lower force but faster speed (characteristic of a rod cylinder) once the load is in motion.  
     SUMMARY OF THE INVENTION  
      A two-speed cylinder of the invention comprises a cylinder tube having an open end and a closed end, with at least one cylinder port located in the tube. A bearing sleeve is movably mounted in the tube. The bearing sleeve comprises a body and a bearing sleeve head. The bearing sleeve body has at least one port. The cylinder also comprises a rod having a first end and a second end, with the second end of the rod configured to slidably engage the bearing sleeve when the bearing sleeve is inside the tube.  
      A dual-acting cylinder of the invention comprises a cylinder tube having an open end and a closed end. The cylinder tube includes an extension port spaced from a retraction port. A bearing sleeve is movably mounted in the tube. A rod is provided having a first end and a second end, with the second end of the rod configured to slidably engage the bearing sleeve. The bearing sleeve comprises at least one sleeve port and at least one retraction port.  
      A cylinder of the invention comprises a cylinder tube having at least one cylinder port and a bearing sleeve movably carried on the cylinder tube. The bearing sleeve comprises a body portion and at least one sleeve port. A rod is provided having a first end and a second end, with the second end of the rod movably carried in the bearing sleeve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Additional advantages and details of the invention are explained in greater detail below with reference to the exemplary embodiments illustrated in the accompanying schematic figures, in which like reference numbers identify like parts throughout.  
       FIG. 1   a  is a side view of a cylinder rod of the invention;  
       FIGS. 1   b  and  1   c  are a side view and a plan view of a bearing sleeve of the invention;  
       FIG. 1   d  is a side view of a cylinder tube of the invention;  
       FIG. 1   e  is a side view of a cylinder gland of the invention;  
       FIG. 1   f  is a side view of a gland cap of the invention;  
       FIGS. 1   g  and  1   h  are a side view and a plan view of a rod bearing of the invention;  
       FIGS. 2   a  and  2   b  are a side view and a plan view of a cylinder of the invention at 0% extension;  
       FIGS. 3   a  and  3   b  are a side view and a plan view of the cylinder of  FIGS. 2   a - b  at 25% extension;  
       FIGS. 4   a  and  4   b  are a side view and a plan view of the cylinder of  FIGS. 2   a - b  at 50% extension;  
       FIGS. 5   a  and  5   b  are a side view and a plan view of the cylinder of  FIGS. 2   a - b  at 75% extension;  
       FIGS. 6   a  and  6   b  are a side view and a plan view of the cylinder of  FIGS. 2   a - b  at 100% extension;  
       FIG. 7  is a side view of various components of a double-acting cylinder of the invention;  
       FIG. 8  is a side, sectional view of a double-acting cylinder of the invention in a retracted position;  
       FIG. 9  is a side, sectional view of the double-acting cylinder of  FIG. 8  in an extended position;  
       FIGS. 10   a - 10   d  are side views of the components of another exemplary cylinder of the invention; and  
       FIGS. 11   a - 11   f  are side views of another exemplary cylinder of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As used herein, spatial or directional terms, such as “up”, “down”, “above”, “below”, “top”, “bottom”, “left”, “right”, and the like, relate to the invention as it is shown in the drawing figures. However, it is to be understood that the invention can assume various alternative orientations and, accordingly, such terms are not to be considered as limiting.  
      The individual components of an exemplary embodiment of the invention will first be described and then operation of an exemplary cylinder of the invention will be described.  
      As shown in  FIGS. 1   a - h  and  2   a - b , a cylinder  10  of the invention includes an outer casing or tube  12  having a first end (open end)  14  and a second end (closed end)  16 . The closed end  16  can be unitary with the rest of the tube  12  or can be closed by a removable cap, such as a conventional screw-type end cap. A cylinder port  18  is formed on the tube  12 , for example, adjacent the closed end  16  of the tube  12 . Thus, the tube  12  defines a hollow interior into which fluid can be introduced via the cylinder port  18 . As will be appreciated by one skilled in the art, the tube  12  can be of any desired material, such as but not limited to metal or any other materials commonly used in the art.  
      The cylinder  10  further includes a cylinder gland  22 . The cylinder gland  22  can be positioned at or near the open end  14  of the tube  12 , as shown in  FIGS. 2   a - b . The cylinder gland  22  has an outer end  24  and an inner end  26 . The cylinder gland  22  has a seal  28  configured to seal against the inside of the tube  12 . Additionally, the cylinder gland  22  includes a central passage  30  having a rod seal  32  configured to seal against the outside diameter of a rod, as will be explained in more detail below. The cylinder  10  of the invention can include a conventional gland cap  34  configured to retain the cylinder gland  22  inside the tube  12 . For example, the gland cap  34  can have threads that can engage other threads on the first end  14  of the tube  12 .  
      The cylinder  10  further includes a rod  38  having a first end  40  and a second end  42 . The rod  38  can be of any desired material, such as but not limited to metal. In the illustrated embodiment, the rod  38  has a first diameter for the majority of its length and a second, smaller diameter at or near the second end  42 . A rod bearing  44  can be connected to the second end  42  of the rod  38 . The rod bearing  44  can have a larger diameter than the first diameter of the rod  38  to prevent the rod from being pushed out of the tube  12 , as will be explained in more detail below.  
      To this point, the components of the cylinder  10  of the invention will be well understood by one of ordinary skill in the art. However, in the practice of the invention, the cylinder  10  also includes a bearing sleeve  48  of the invention ( FIGS. 1   b  and  1   c ). The bearing sleeve  48  has an open end  50 , a closed end  52 , and a hollow interior. In one non-limiting embodiment, the closed end  52  can be closed by a removable sleeve cap  54 . Alternatively, the closed end  52  of the bearing sleeve  48  can be permanently closed. The bearing sleeve  48  includes a hollow body portion  56  of a first diameter and a head portion  58  of a second, greater diameter. The head portion  58  includes a cylinder seal  60  configured to seal the bearing sleeve  48  against the inner diameter of the tube  12  to prevent fluid flow past the cylinder seal  60 . The head portion  58  also includes a rod seal  62  configured to seal against the rod  38 , as will be described in more detail below. The body  56  and head  58  define an annular lip  64  at the rear of the head  58 . The bearing sleeve  48  also includes one or more sleeve ports. The illustrated bearing sleeve  48  has two spaced sleeve ports, e.g., a first or acceleration port  66  and a second or deceleration port  68 .  
      Operation of the illustrated cylinder  10  will now be described.  
      An assembled cylinder  10  in accordance with the invention is shown in  FIGS. 2   a  and  2   b .  FIGS. 2   a  and  2   b  illustrate the cylinder  10  in the fully retracted position, that is, with the rod  38  fully retracted into the tube  12 . As can be seen, the first end  40  of the rod  38  extends out of the first end  14  of the tube  12  and is slidably supported by the cylinder gland  22 . The second end  42  of the rod  38  with the rod bearing  44  attached extends into the hollow body  56  of the bearing sleeve  48 . Thus, the rod  38  is slidably engaged with the rod seal  32  of the cylinder gland  22  and the rod seal  62  of the bearing sleeve  48 .  
      When the rod  38  is to be extended, fluid, for example liquid or gas, is directed through the cylinder port  18  into the interior of the tube  12  between the tube end  16  and the head portion  58  of the bearing sleeve  48  around the bearing sleeve body  56 . As will be appreciated from  FIGS. 2   a - b  and  3   a - b , as the fluid enters the tube  12 , the fluid pressure against the inner surface of the bearing sleeve head  58  (i.e., against the annular lip  64 ) and the rear end of the sleeve body  56  pushes the bearing sleeve  48  to the right inside the tube  12 . This carries the rod  38  to the right also. Some of the fluid in the interior of the tube  12  also enters into the body  56  of the bearing sleeve  48  through the first and second sleeve ports  66 ,  68 . This is the beginning of the stroke that most resembles that of a conventional piston cylinder. That is, the increased area (lip  64 ) of the sleeve head  58  and the rear of the sleeve body  56  act like a piston surface and move the bearing sleeve  48  (carrying the rod  38 ) at a relatively high force but relatively low speed to the right.  
      As shown in  FIGS. 4   a  and  4   b , this “force stroke” continues until the front end  50  of the bearing sleeve  48  engages the inner end  26  of the cylinder gland  22 . This contact prevents any further movement of the bearing sleeve  48  to the right. At this position, fluid fully fills the tube  12  between the rear surface (lip  64 ) of the sleeve head  58  and the tube end  16 . As additional fluid is pumped into the tube  12  via the cylinder port  18 , the fluid enters the sleeve body  56  through the sleeve ports  66  and  68  and begins to exert a force on the second end  42  of the rod  38  (that is, on the end of the rod bearing  44 ) to begin pushing the rod  38  out of the bearing sleeve  48  and, thus, farther out of the tube  12 . The small gap between the outside of the rod bearing  44  and the inside of the sleeve body  56  provides a smooth acceleration of the rod  38  as it extends further out of the tube  12 . When the inner end of the rod  38  (i.e., the end of the rod bearing  44 ) moves past the acceleration port  66 , the rod acceleration will increase since the fluid no longer has to flow between the small gap between the rod bearing  44  and the inside of the tube  12 . This is the beginning of the “high speed” stroke which simulates the action of a conventional rod cylinder. That is, at this time, the rod extension speed increases but the force of the rod decreases.  
      As shown in  FIGS. 5   a  and  5   b , the rod  38  continues to move out of the tube  12  (i.e., out of the bearing sleeve  48 ) as more fluid enters the sleeve ports  66  and  68 . However, as the rod bearing  44  is pushed adjacent to the deceleration port  68  of the sleeve body  56 , fluid becomes trapped between the rod bearing  44  and the sleeve head  58 , causing the speed of the rod  38  to decrease as fluid is slowly squeezed out between the outside of the rod bearing  44  and the inside of the sleeve body  56 .  
      As shown in  FIGS. 6   a  and  6   b , when the rod  38  is fully extended, the front end of the rod bearing  44  contacts an annular retaining surface in the bearing sleeve  48  to prevent the rod  38  from extending any farther out of the tube  12 .  
      The rod  38  can be retracted by allowing the fluid inside the tube  12  to flow out of the cylinder port  18  to permit the rod  38  and bearing sleeve  48  to move to the left, back to the fully retracted position shown in  FIGS. 2   a - b.    
      While the above describes one embodiment of the invention, it is to be understood that the invention is not limited to this particular embodiment. For example, an additional cylinder port can be provided at or near the open end  14  of the tube  12  to provide a double-acting cylinder. That is, when the rod  38  is to be retracted, fluid can be pumped into the cylinder port at the front of the tube  12  to push against the outer face of the sleeve head  58  and cause the bearing sleeve  48  to move to the left.  
      In another non-limiting embodiment, rather than having the acceleration and deceleration ports  66  and  68  as described above, the bearing sleeve  48  could have a single port to allow fluid to flow from the inside of the tube  12  into the interior of the bearing sleeve  48  so as to act upon the rod  38 . For example, this single port could be located on the closed end  52  of the bearing sleeve  48  or anywhere on the sleeve body  56 .  
      A double-acting cylinder  200  of the invention is shown in  FIGS. 7-9 . The cylinder  200  includes a tube  202  having a first end (open end)  204  and a second end (closed end)  206 . As discussed above, the closed end  206  can be unitary with the rest of the tube  202  or can be closed by a removable cap, such as a conventional screw-type end cap. The tube  202  includes at least one extension port  208  and at least one retraction port  210 . Thus, the tube  202  defines a hollow interior into which fluid can either be introduced or removed via the extension port  208  and/or retraction port  210 . As will be appreciated by one skilled in the art, the tube  202  can be of any desired material, such as but not limited to metal or any other materials commonly used in the cylinder art.  
      The cylinder  200  further includes a cylinder gland  214 . The cylinder gland  214  can be positioned at or near the open end  204  of the tube  202 . The cylinder gland  214  includes a central passage  216  which can have a rod seal (not shown) configured to seal against the outside diameter of a rod, as described above. The cylinder gland  214  can also include a cylinder seal (not shown) configured to seal against the inside of the tube  202 , in similar manner as described above.  
      The cylinder  200  further includes a rod  220  having a first end (outer end)  222  and a second end (inner end)  224 . The rod  220  can be of any desired material, such as but not limited to metal. In the illustrated embodiment, the rod  220  has a first diameter for the majority of its length and a second, smaller diameter at or near the second end  224 . In the illustrated embodiment, the second end  224  includes a threaded extension  226 .  
      The cylinder  200  further includes a rod bearing (rod piston)  230  configured to engage the rod  220 . In the illustrated embodiment, the rod bearing  230  is annular in shape and has a threaded central passage  232  extending at least partly therethrough. The rod bearing  230  further includes a seal  234 . In one non-limiting embodiment, the rod bearing  230  may include a chamfered or tapered end region  236 .  
      The cylinder  200  further includes a bearing sleeve  238 . The bearing sleeve  238  has an open end  240  and a closed end  242 . In one non-limiting embodiment, the closed end  242  can be closed by a removable sleeve cap (not shown). Alternatively, the closed end  242  can be permanently closed. The bearing sleeve  238  includes a hollow body portion  244  of a first diameter and a head portion  246  of a second, greater diameter. The head portion  246  includes a cylinder seal  248  configured to seal the bearing sleeve  238  against the inner diameter of the tube  202  to prevent fluid flow past the cylinder seal  248 . The head portion  246  also includes a rod passage  249 . The body  244  and head  246  define an annular lip  250  at the rear of the head  246 . The bearing sleeve  238  also includes at least one sleeve port  252  providing fluid access into and out of the interior of the bearing sleeve  238 . Additionally, the bearing sleeve  238  also includes one or more retraction channels. In the illustrated embodiment, the bearing sleeve  238  includes a first retraction channel  254  and a second retraction channel  256  extending through the head portion  246 . The retraction channels  254  and  256  provide fluid communication between the interior of the bearing sleeve  238  and the exterior thereof. It is to be understood that the sleeve port(s)  252  and/or retraction channel(s)  254 ,  256  are not limited to the positions shown in the attached exemplary drawings but could be located anywhere on the bearing sleeve  238  to achieve the results described below. For example, the retraction channel could simply be a gap between the rod  220  and the inner diameter of the rod passage  249  or any similar arrangement.  
       FIG. 8  shows the components described above in a first assembled (retracted) position. As can be seen from  FIG. 8 , the bearing sleeve  238  is slidably positioned inside the tube  202 , with the cylinder seal  248  slidably engaging the inner diameter of the tube  202 . The rod  220  extends through the cylinder gland  214  and the rod passage  249  of the bearing sleeve  238 . The second end  224  of the rod  220  engages the rod bearing  230 . In the illustrated embodiment, the threaded extension  226  threadably engages the central passage  232  of the rod bearing  230 . However, it is to be understood that this is simply one exemplary configuration. As will be appreciated by one of ordinary skill in the art, the rod bearing  230  could be attached to or formed on the rod  220  in any conventional manner, such as but not limited to welding or screws. Additionally, the rod bearing  230  could be a unitary portion of the second end  224  of the rod  220 . That is, the rod bearing  230  could simply be a larger surface area portion at the second end  224  of the rod  220 . Alternatively, no rod bearing  230  could be present  
      Operation of the cylinder  200  will now be described.  
       FIG. 8  illustrates the cylinder  200  in the fully retracted position, that is, with the rod  220  fully retracted into the tube  202 . The first end  222  of the rod  220  extends out of the first end  204  of the tube  202  and is slidably supported by the cylinder gland  214 . The second end  224  of the rod  220  with the rod bearing  230  attached extends into the hollow body  244  of the bearing sleeve  238 .  
      When the rod  220  is to be extended, fluid, for example liquid or gas, is directed through the extension port  208  into the interior of the tube  202  between the closed end  206  of the tube  202  and the head portion  246  of the bearing sleeve  238 . As the fluid enters the interior of the tube  202 , the fluid pressure against the inner surface of the bearing sleeve head  246  (i.e., against the annular lip  250 ) and the rear end of the sleeve body  244  pushes the bearing sleeve  238  to the right inside the tube  202 . This carries the rod  220  to the right also. Some of the fluid in the interior of the tube  202  also enters into the bearing sleeve  238  through the sleeve port  252 . This is the beginning of the stroke that most resembles that of a conventional piston cylinder. That is, the increased area (lip  250 ) of the sleeve head  246  and the rear of the sleeve body  244  act like a piston surface and move the rod  220  (carried by the bearing sleeve  238 ) at a relatively high force but relatively low speed out of the tube  202 . This “force stroke” continues until the front end  240  of the bearing sleeve  238  engages the inner end of the cylinder gland  214 . This contact prevents any further movement of the bearing sleeve  238  to the right. At this position, fluid fully fills the interior of the tube  202  between the rear surface (lip  250 ) of the sleeve head  246  and the closed end  206  of the tube  202 . As additional fluid is pumped into the tube  202  via the extension port  208 , the fluid enters the sleeve body  244  through the sleeve port  252 . This fluid flows through the small gap formed between the rod bearing  230  and the inner diameter of the body  244  of the bearing sleeve  238 . The fluid begins to exert a force on the end of the rod bearing  230  to begin to push the rod bearing  230  (and thus the rod  220 ) to the right and out of the bearing sleeve  238 . The small gap formed between the rod bearing  230  and the inside of the sleeve body  244  provides a smooth acceleration of the rod  220  as it extends out of the tube  202 . When the end of the rod bearing  230  moves past the sleeve port  252 , the rod acceleration will increase since the fluid no longer has to flow through the small gap between the rod bearing  230  and the inside diameter of the body  244 . This is the beginning of the “high speed” stroke, which simulates the action of a conventional rod cylinder. That is, at this time, the rod extension speed increases but the force of the rod decreases.  
      As shown in  FIG. 9 , the rod  220  continues to move to the right as fluid is pumped through the extension port  208  into the interior of the tube  202  and from there flows through the sleeve port  252  into the interior of the sleeve body  244 . The rod bearing  230  continues to move to the right until it contacts the annular inner surface of the sleeve head  246 . The rod  220  can be hydraulically locked in any desired position by closing the extension port  208  to cease fluid flow into the tube  202 .  
      When the rod  220  is to be retracted, the extension port  208  can be opened to allow fluid to flow out of the interior of the tube  202  and allow the rod  220  to move to the left inside the tube  202 , such as by the weight of the load. This is similar to the embodiment described above. However, in the embodiment illustrated in  FIGS. 8 and 9 , the rod  220  could also be retracted under fluid pressure. For example, to retract the rod  220 , the extension port  208  can be opened to allow fluid flow out of the interior of the tube  202  and fluid can be injected into the interior of the tube  202  through the retraction port  210 . As will be appreciated from  FIGS. 8 and 9 , as fluid enters the retraction port  210 , the fluid pushes against the front end  240  of the bearing sleeve  238 , which pushes the bearing sleeve  238  to the left and begins pushing the rod  220  back into the interior of the tube  202 . This movement can continue until the second end  242  of the bearing sleeve  238  is at or adjacent the second end  206  of the tube  202  and fluid fills the interior of the tube  202  between the front end  240  of the bearing sleeve head  246  and the rear end of the cylinder gland  214 .  
      As more fluid is pumped into the interior of the tube  202  through the retraction port  210 , this fluid flows through the retraction channels  254  and  256  into the interior of the bearing sleeve  238 . As can be appreciated from  FIG. 9 , as the fluid flows through the retraction channels  254  and  256 , the fluid exerts pressure on the front end of the rod bearing  230  and pushes the rod bearing  230  to the left. As the rod bearing  230  moves to the left, the rod bearing  230  pushes fluid out of the interior of the sleeve body  244 , through the sleeve port  252 , and into the interior of the tube  202  where it can flow out of the extension port  208 . This movement of the rod bearing  230  can continue until the rod bearing  230  is at or adjacent the second end  242  of the sleeve body  244 , thus retracting the rod  220  fully into the tube  202 .  
      Another cylinder  300  is shown in  FIGS. 10   a - 10   c . The cylinder  300  includes a tube  302  (similar to  202  described above). The tube  302  includes at least one extension port  308  and at least one retraction port  310 . The cylinder  300  further includes a rod  320  having a first end (outer end)  322  and a second end (inner end)  324 . The rod  320  has a piston head  326  (rod bearing) attached to the inner end  324  of the rod  320 .  
      The cylinder  300  further includes a bearing sleeve  330  having a first end  332  and a second end  334 . The rod  320  slides through a bore in the first end  332  of the bearing sleeve  330 . The bearing sleeve  330  has a hollow body portion  336  with a first head portion  338  adjacent the first end  332  and a second head portion  340  adjacent the second end  334 . The first and/or second head portions  338 ,  340  can include wear bands or other conventional sealing devices to seal the bearing sleeve  330  against the inner diameter of the tube  302 . The second head portion  340  has a tapered end region  342  to allow fluid to flow behind the bearing sleeve  330 , as will be described in more detail below. The bearing sleeve  330  also includes at least one sleeve port  344  providing fluid access into and out of the interior of the bearing sleeve  330 . In the illustrated embodiment, the sleeve port  344  is formed through the second end (end wall) of the bearing sleeve  330 . Additionally, the bearing sleeve  330  includes one or more retraction channels. In the illustrated embodiment, the bearing sleeve  330  includes a first retraction channel  346  and a second retraction channel  348  extending through the first head portion  338 . The retraction channels  346  and  348  provide fluid communication between the interior of the bearing sleeve  330  and the interior of the tube  302 .  
       FIG. 10   c  shows the components described above in a first assembled (retracted) position. Operation of the cylinder  300  is similar to that described above for the cylinder  200  except that the fluid enters the bearing sleeve  330  through the sleeve port  344  on the end of the bearing sleeve  330 , rather than a sleeve port on the side of the bearing sleeve  330 .  
      An alternative bearing sleeve  350  is shown in  FIG. 10   d . This bearing sleeve  350  has a body  352  of substantially uniform diameter having a first end  354  and a second end  356 . A seal or gasket can be provided at or near the first end  354 . The second end  356  can include a tapered end region  360 . The bearing sleeve  350  includes a sleeve port or fluid inlet  362  formed in the second end  356  of the bearing sleeve  350 . The bearing sleeve  350  is of a simpler design than of the bearing sleeve  330  and, thus, should be less expensive to manufacture.  
      In addition to providing a cylinder of the invention as described in any of the embodiments above, the present invention also includes the concept of converting or retrofitting a conventional cylinder to be a two-speed cylinder incorporating features of the invention. For example, a conventional cylinder having an outer tube and a piston slidably moveable within the tube can be converted to a two-speed cylinder of the invention by incorporating a bearing sleeve of the invention into the tube. For example, the original rod can be utilized or the rod can be modified to attach a piston head to the inner end of the rod, if desired. The conventional cylinder can be modified by adding a bearing sleeve, such as any of the bearing sleeves described above, into the conventional cylinder tube.  
      Another cylinder  364  of the invention is shown in  FIGS. 11   a - 11   f . Cylinder  364  includes a tube  366  that can include an extension port  368  and a breather port  370 . A rod  372  is moveably mounted in a bearing sleeve  374  in a similar manner as the cylinder of  300  described above. However, in this embodiment, the bearing sleeve  374  includes a sleeve extension port  376  and a sleeve retraction port  378  formed on the sidewall of the bearing sleeve  374 . The bearing sleeve  374  also includes a piston sleeve bearing  380  having a first piston sleeve orifice  382  and a second piston sleeve orifice  384 . However, unlike the previous embodiments, the cylinder  364  includes a check valve assembly which, in the illustrated embodiment, has a first check valve  386  in fluid communication with the second piston sleeve orifice  384 . A second check valve  388  is located on a piston rod bearing  373  and is in fluid communication with a channel extending through the piston rod  372 . A third check valve  390  is located on a bearing sleeve head  392 . The check valve assembly prevents vacuum locks during retraction of the rod  372 .  
      Operation of the cylinder  364  will now be described with particular reference to  FIGS. 11   a - f .  FIG. 11   a  shows the cylinder  364  in the retracted position. That is, the rod  372  and bearing sleeve  374  are to the left of the cylinder  364 . To extend the rod  372 , fluid is introduced through the extension port  368 , which causes the bearing sleeve  374  to begin moving to the right. Eventually, the bearing sleeve  374  will reach the right-most position, as shown in  FIG. 11   b . As fluid continues to enter the extension port  368 , the fluid is directed through the piston sleeve orifice and into the interior of the bearing sleeve  374  to move the rod bearing  373  to the right. The rod  372  continues to move to the right until it reaches the fully extended position shown in  FIG. 11   c . The rod  372  can be held in this extended position by fluid lock. When it is desired to retract the rod  372 , the fluid lock is removed and the weight on the outer end of the rod  372  causes the rod  372  to move to the left, as shown in  FIG. 11   d . During this retraction, fluid on the inside of the bearing sleeve  374  flows out of the sleeve extension port  376 , through the first check valve  386 , and out the extension port  368 . In the event that the rate of fluid flow causes a vacuum to develop in the region of the bearing sleeve  374  in front of the rod bearing  373 , the third check valve  390  allows fluid flow from the area between the outside diameter of the bearing sleeve  374  and the inside diameter of the tube  366  to flow into this region to prevent a vacuum lock and provide for smooth operation of the cylinder  364 . As shown in  FIG. 11   d , eventually the rod bearing  373  will move to close off the sleeve extension port  376 . At this point, fluid flows through the rod bearing orifice and out of the sleeve retraction port  378 , through the first check valve  386 , and out the extension port  368  to provide a cushioning effect for the rod  372  to move completely to the left (retracted) position.  
      It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.