Abstract:
A pneumatic or hydraulic cylinder ( 10 ) having a bore ( 12 ) cooperating with a piston ( 18 ). Extending from the piston ( 18 ) is a piston rod ( 37 ). There is further provided a pawl ( 26 ) which is engageable with the piston ( 18 ) to retain the piston ( 18 ) at a desired location. The pawl ( 26 ) is moved from engagement with the piston ( 18 ) upon fluid under pressure being delivered to one end of the cylinder ( 10 ).

Description:
TECHNICAL FIELD 
     The present invention relates to pneumatic and hydraulic cylinders. 
     BACKGROUND OF THE INVENTION 
     Hydraulic and pneumatic cylinders suffer from the disadvantage that should there be any leakage, the cylinder will either extend or retract depending on its preloaded condition. 
     OBJECT OF THE INVENTION 
     It is the object of the present invention to overcome or substantially ameliorate the above disadvantage. 
     SUMMARY OF THE INVENTION 
     There is disclosed herein a hydraulic or pneumatic cylinder including: 
     a bore, the bore generally surrounding a chamber having opposite ends; 
     a piston in said bore and co-operating therewith to divide said chamber into a first and a second sub-chamber; 
     a piston rod fixed to and extending from said piston and extending beyond one of said ends; 
     means closing said opposite ends; 
     releasable locking means to retain said piston at a desired location within said bore, said locking means being released to permit movement of said piston upon delivery of a fluid under pressure to one of said chambers; and 
     duct means to provide for fluid flow from said second sub-chamber to said first sub-chamber when said locking means is engaged with said piston to retain said piston at said desired location. 
     Preferably, the locking means includes a pawl mounted in said body, and a recess in said piston, said recess being alignable with said pawl so as to be engaged therewith to retain the piston at said desired location, said pawl being movable between a first position at which it is engaged within said recess and a second position spaced from said recess to provide for movement of said piston. 
     Preferably, the cylinder includes a spring urging the pawl into engagement with said recess. 
     Preferably, the delivery of fluid under pressure to said first sub-chamber causes movement of said pawl from the first position to the second position thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred form of the present invention will now be described by way of example with reference to the accompanying drawing which schematically depicts in section side elevation a pneumatic or hydraulic cylinder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the accompanying drawing there is schematically depicted a hydraulic or pneumatic cylinder  10 . The cylinder  10  has a body  11  providing a bore  12  generally surrounding a chamber  13 . The chamber  13  has opposite ends  14  and  15  which are closed by plug members  16  and  17 . 
     Mounted within the body  11  and slidably engaging the cylindrical surface of the bore  12  is a piston  18 . The piston  18  divides the chamber  13  into a first sub-chamber  19  and a second sub-chamber  20 . The piston  18  has a tapered leading surface  21  which is of frusto-conical configuration. The piston is further provided with an annular recess  22  as well as a wear ring  23  and seal  24 . 
     The body  11  includes a housing  25  providing a bore  39  which slidably receiving and guiding a pawl  26 . The pawl  26  has a leading projection  27  which is aligned with an aperture  28  in the body  11 . A spring  29  urges the pawl  26  to a position at which the projection  27  is located within the chamber  13 . 
     Located between the pawl  26  and an end cap  30  is a locking chamber  31  within which the spring  29  is located. Extending from the chamber  31  is a duct  32  extending to an aperture  33 . The aperture  33  communicates with the sub-chamber  20  when the piston  18  is located adjacent the end  15 . 
     The piston  18  is sized so that there is an annular space  34  permitting fluid to pass from the sub-chamber  19  to a position at which pressure can be applied to the pawl  26 . 
     Extending from the body  11  is a first fluid coupling  36  which provides for movement of fluid relative to the sub-chamber  19  while a second fluid coupling  35  provides for movement of fluid relative to the second sub-chamber  20 . 
     Attached to and extending from the piston  18  is a piston rod  37  which exits via the plug member  16  which sealingly engages the longitudinal peripheral surface of the piston rod  37 . 
     In operation of the above described hydraulic cylinder  10 , fluid under pressure is delivered to the sub-chamber  19 . Fluid passing the passage  34  and entering the chamber  40  causes retraction of the pawl  26  to a position at which the projection  27  is no longer engaged within the recess  22 . Pressure within the sub-chamber  19  causes movement of the piston  18  toward the end  14 . Fluid is allowed to leave the sub-chamber  20  via the fluid coupling  35 . When the pawl  26  moves to its retracted position, fluid within the chamber  31  is allowed to return to the chamber  13  via the duct  32 . When the piston  18  is spaced from the end  17  and fluid under pressure is delivered to the sub-chamber  20 , the piston  18  moves towards the end  15  and engages the projection  27 . The tapered surface  21  engages the projection  27  and causes the pawl  26  to move to a position at which the piston  18  can pass. Once the piston  18  is in the position depicted, the pawl  26  moves to a position at which the projection  27  is engaged within the recess  22 . Fluid from the sub-chamber  20  passes through the duct  32  to urge the pawl  26  to engage within the recess  22 . This movement is enhanced by means of the spring  29 . 
     The above mentioned duct  32  also enables the cylinder  10  to be used in a phasing circuit. In relation to the cylinder  10  being used in a phasing circuit, the pawl  26  is provided with a single acting seal  38  which permits the flow of fluid only from the chamber  31  past the pawl  26  to enter the sub-chamber  19 . When the cylinder  10  is used in a phasing circuit and the piston  18  located adjacent the end  15 , fluid can flow via the duct  32  past the seal  38  to exit via the coupling  36  from where it would pass to the next cylinder. Reverse flow is prevented by the seal  38 . 
     The above preferred embodiment ensures that the piston  18  is retained adjacent the end  15  until fluid under pressure is delivered to the sub-chamber  19 .