Abstract:
The pressure in the downstream side of a pneumatic cylinder&#39;s piston ( 1   g ) is allowed to exhaust. At a certain point, the downstream exhaust is blocked, causing the pressure to rise against the downstream side of the piston. A valve ( 5 ) opens when the downstream chamber has reached its maximum pressure. The output of valve ( 5 ) opens a second valve ( 3 ). Valve ( 3 ) rapidly exhausts the remaining air on the downstream side of the piston ( 1   p ). With no air on the downstream side of piston ( 1   p ), piston ( 1   p ) stops and does not bounce back. Changing the volume of inactive regions ( 1   m ) or ( 1   n ) sets the stopping point to coincide with the end of stroke of piston ( 1   p ). A check valve ( 4 ) and orifice ( 6   a ) allow the air in the pilot port in valve ( 3 ) to slowly bleed out, resetting valve ( 3 ) for the next cycle.

Description:
INTRODUCTION 
     Fluid powered, expandable chamber motors are used to apply a force along a straight line. These motors are usually known as either pneumatic or hydraulic cylinders. Pneumatic cylinders are powered by compressible fluids. Hydraulic cylinders are powered by incompressible fluids. Fluid powered cylinders are simple to make, easy to use, and relatively low in cost. Furthermore, pneumatic cylinders are safe in fire and explosive environments. 
     The characteristics of the fluid affect the dynamics of these cylinders. For example, the compressibility of air makes it hard to control a pneumatic cylinder&#39;s deceleration. The easiest solution is to apply no controls, and simply let the piston to run into the end of the cylinder. For many applications, where the speed of travel is relatively slow, this method of control may be acceptable. Unfortunately, many applications require higher speeds. The resulting high-speed impact between the piston, and the cylinder end, causes undo stresses. 
     DESCRIPTION OF PRIOR ART 
     Since the beginning of the 20 th  Century, many inventors have proposed many devices to cushion a cylinder&#39;s piston. The United States alone has issued well over 50 patents. There are too many previous patents to include all of them here. Nevertheless, a few previous patents shall be included. 
     U.S. Pat. No. 1,604,548 shows a pneumatic cylinder used to open a door. This early device used mechanical springs for shock absorption. 
     U.S. Pat. No. 2,755,775 allowed for the air pressure in the deceleration end to build up. A flexible cushion sleeve decreases in diameter when more air pressure is applied to it. The decreased diameter increases the clearance between the sleeve and the cylinder. More air can escape. Air pressure is released, minimizing bounce-back. 
     U.S. Pat. No. 3,805,672 has a raised boss on the piston. As the piston moves along the stroke, the air at the low pressure end of the cylinder exits through a bore at the end of the cylinder. Near the end of the stroke, a raised piston boss enters a bore in the end of the cylinder. This prevents air from exiting through the bore. A second passage allows air to continue to escape through a needle valve. The needle valve determines how quickly air can exit. The needle valve can be adjusted to adjust the cushioning rate. When the needle valve is fully open, the exhausting air flows freely. This give minimal, or no, cushion. A fully closed needle valve traps the remaining air. The trapped air can then keep the piston from getting to the end of stroke. 
     U.S. Pat. No. 3,933,080 uses two chambers. The first chamber is the same chamber as mentioned in U.S. Pat. No. 3,805,672. The second chamber is a chamber formed by the end of the boss and the bottom of the bore. According to this patent, the air on the low pressure side of the piston does not exit through the bore, but rather through a second hole. As the piston nears the end of the stroke, the piston boss again enters the cylinder end bore. Pressure builds in the second chamber. Building pressure in chamber  2 , decreases the size of the main exhaust path. This slows the speed of the air exiting from the first chamber. 
     Through a complex set of valves, and cross-bars, U.S. Pat. No. 4,523,511 gradually closes the exhaust valves of a cylinder as its piston approaches the end of stroke. 
     In U.S. Pat. No. 4,700,611 adds special cushioning chambers to each end of the cylinder. Compressed air fills these cushioning chambers. Near the end of stroke, the main piston impacts a mechanical cushioning pad. A mechanical cushioning pad pushes into the special cushioning chamber. This increases the air pressure in the cushioning chamber. It also opens valves in the special cushioning pad to allow air to escape. The combined affect cushions the piston. 
     U.S. Pat. No. 5,423,243 adds a boss to the piston. At the cylinder end is a bore. For most of the piston stroke, the air exhausts through the bore. When the boss enters the bore, the remaining air becomes trapped. The air then goes through a secondary passage to one chamber of a spring adjusted relief valve. A tertiary passage connects the bore with a second chamber in the relief valve. The pressure difference between the first and second chambers opens the relief valve. Trapped air now exits through the relief valve. The amount of opening determines the exhaust flow rate, and the deceleration of the piston. 
     U.S. Pat. No. 5,517,898 uses a two-fold method to cushion the cylinder. The first step uses a set of sleeves to gradually restrict the exhausting air flow, as the cylinder approaches the end of travel. The second step has the cylinder piston depress a plunger to rapidly exhaust any remaining air in the cylinder chamber, near the end of stroke. The rapid exhaust is based on piston position. 
     U.S. Pat. No. 5,623,861 uses a special venting sleeve, with two pistons, three separate chambers, and two slowing orifices, to control the speed, and impact force of the piston. 
     U.S. Pat. No. 6,178,868 uses an external set of components that include accumulators to pressurize the exhaust air. The pressurized exhaust air provides a deceleration force to the piston. The exhausting air is directed to the accumulators based on electrical signals sent from position sensors, or from computers. The accumulators can be sized differently to allow for some adjustability in the rate of deceleration. 
     U.S. Pat. No. 6,536,327 uses a two part cushion system. The first part of this invention purposefully traps some air in the end of cylinder, in a special case version of U.S. Pat. No. 3,805,672. The second part of the cushion adds rubber pads to further absorb the impact forces. 
     U.S. Pat. No. 6,758,127 uses a variation of U.S. Pat. No. 3,805,672. Cushioning air from either end is forced to flow to a single throttling valve. This arrangement permits a more compact cylinder. 
     U.S. Pat. No. 7,395,749 uses a hollow piston rod to handle two tasks. First, a hollow rod is lighter than a solid rod, allowing for faster acceleration. For the second task, the hollow rod performs holds a secondary piston. The secondary piston acts to shut off exhaust air from escaping during the retract direction. The shut off piston traps air between the piston and the cylinder end. The trapped air cushions the piston as it reaches its end of travel. 
     Japanese Patent JP2002130213 first uses a relief valve to directly release pressure from the downstream side of the piston. In the later stages of cushioning, after the relief valve closes due to insufficient pressure, the air from the downstream side of the piston exhausts through a throttle groove. 
     In Japanese patent JP2003254303, a succession of holes open as the piston nears its end of stroke. This succession of holes provides for a multi-step means to slow the piston. 
     Japanese Patent JP2006046500 uses an add-on device to cushion a pneumatic cylinder. The slowdown rate of the cylinder is adjusted by changing the flow in a throttle (needle) valve. The stroke of the cylinder is adjusted by varying the length of a stroke adjustment bolt. 
     Japanese Patent JP2613150 uses an external pneumatic shock absorber to slow and stop a separate pneumatic cylinder. A pressure reducer takes the supply line air, and regulates the pressure to the pneumatic shock absorber, in order to provide a constant stopping force. 
     The prior art suggest various pneumatic circuits. The Parker Design Engineer&#39;s Handbook, Bulletin 0224-B1, provides an example of an air cushion composed of components external to cylinder. These external components allow the cylinder air to exhaust freely, until pressure builds in the pilot port of the exhaust valve. When the pilot pressure builds, the exhaust air then goes through a variable orifice. Slowing the velocity of the exhausting air, slows down the piston. 
     SUMMARY 
     When air is supplied to a pneumatic cylinder, the cylinder&#39;s piston moves along its stroke. As the piston approaches the end of its stroke, the air exhaust passage is blocked. Blocking the exhaust passage traps some air on the downstream side of the piston. The continued movement of the piston compresses this downstream air. The compressed air trapped in the downstream side of the piston brings the piston to a stop. Furthermore, the air pressure on the downstream side of the piston becomes greater than the air pressure on the upstream side of the piston. This inverted pressure difference reverses the piston&#39;s direction of travel, or makes the piston ‘bounce back’. Opening a valve in the downstream chamber, just before the piston stops, allows air pressure in the downstream chamber to rapidly decrease. The piston stops with no more downstream pressure to make it bounce back. Changing the inactive volume moves the piston&#39;s stopping point to coincide with the end of the stroke. 
    
    
     
       DRAWINGS 
       Figures 
         FIG. 1  is a high level logic schematic of the cushioning cartridge, with a typical single rod, double acting, pneumatic cylinder. 
         FIG. 2  is a cross-sectional view of the pneumatic cylinder showing the inactive volume spacers. 
         FIG. 3  is a cross-sectional view of the pneumatic cylinder with the cushioning cartridge 
         FIG. 4  is a 3D view of the sides and the pressure end of the cushioning cartridge. 
         FIG. 5  is a 3D view of the sides and the exhaust end of the cushioning cartridge 
         FIG. 6  is an end view of the pressure end of the cushioning cartridge. This view is used to define the cross-sectional views of  FIG. 7  and  FIG. 8 . 
         FIG. 7  is a cross-sectional view of the cushioning cartridge showing the pressure relief valve. 
         FIG. 8  is a cross-sectional view of the cushioning cartridge showing the main exhaust passages. 
         FIG. 9  is a 3D view of the cylinder, with the main cylinder tube removed for clarity, showing the inactive volume spacers. 
         FIG. 10  is a 3D view of the cylinder with one embodiment of a cushion with components external to the cylinder. 
     
    
    
     REFERENCE NUMERALS 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  1 
                 pneumatic cylinder 
               
               
                   
                  1a 
                 cap end chamber 
               
               
                   
                  1b 
                 head end chamber 
               
               
                   
                  1c 
                 cylinder rod 
               
               
                   
                  1d 
                 head bore passage 
               
               
                   
                  1e 
                 cap bore passage 
               
               
                   
                  1f 
                 head end piston assembly boss 
               
               
                   
                  1g 
                 piston flange 
               
               
                   
                  1h 
                 cap end piston assembly boss 
               
               
                   
                  1j 
                 cap port passage 
               
               
                   
                  1k 
                 head port passage 
               
               
                   
                  1m 
                 head end inactive region 
               
               
                   
                  1n 
                 cap end inactive region 
               
               
                   
                  1p 
                 piston 
               
               
                   
                  1q 
                 spacer pocket 
               
               
                   
                  1r 
                 space pocket 
               
               
                   
                  1s 
                 spacer pocket 
               
               
                   
                  1t 
                 spacer pocket 
               
               
                   
                  1u 
                 cushion pocket 
               
               
                   
                  1v 
                 cushion pocket 
               
               
                   
                  1w 
                 end cap 
               
               
                   
                  1y 
                 main tube 
               
               
                   
                  1z 
                 head cap 
               
               
                   
                  2 
                 cushioning cartridge 
               
               
                   
                  2′ 
                 second article of cushioning cartridge 
               
               
                   
                  2a 
                 pressure feed passage 
               
               
                   
                  2b 
                 pilot passage 
               
               
                   
                  2c 
                 check valve exhaust passage 
               
               
                   
                  2d 
                 valve inlet passage 
               
               
                   
                  2e 
                 interconnect passage to spool 
               
               
                   
                  2f 
                 spool inlet cavity 
               
               
                   
                  2g 
                 spool outlet cavity 
               
               
                   
                  2h 
                 interconnect passage from spool 
               
               
                   
                  2j 
                 cushion exit passage 
               
               
                   
                  2k 
                 tool insert passage 
               
               
                   
                  2m 
                 o-ring groove 
               
               
                   
                  3 
                 main exhaust valve 
               
               
                   
                  4 
                 check valve 
               
               
                   
                  5 
                 pressure relief valve 
               
               
                   
                  6 
                 restricting orifice plug 
               
               
                   
                  6a 
                 orifice 
               
               
                   
                  7 
                 bolt 
               
               
                   
                  8 
                 inactive volume spacer 
               
               
                   
                 11 
                 exit manifold 
               
               
                   
                 21 
                 pressure end cap 
               
               
                   
                 22 
                 main housing 
               
               
                   
                 23 
                 exhaust end cap 
               
               
                   
                 31 
                 spool 
               
               
                   
                 31a 
                 spool upstream hole set 
               
               
                   
                 31b 
                 spool downstream hole set 
               
               
                   
                 31c 
                 spool partition 
               
               
                   
                 31d 
                 spool chamber 
               
               
                   
                 31e 
                 outer wall 
               
               
                   
                 31f 
                 spool open end 
               
               
                   
                 31g 
                 spool closed end 
               
               
                   
                 32 
                 spool spring 
               
               
                   
                 33 
                 o-ring 
               
               
                   
                 41 
                 check valve ball 
               
               
                   
                 42 
                 check valve spring 
               
               
                   
                 51 
                 pressure relief stem 
               
               
                   
                 52 
                 pressure relief spring 
               
               
                   
                 53 
                 pressure relief washer 
               
               
                   
                 54 
                 pressure relief pressure adjuster 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 ,  FIG. 2  and  FIG. 3 , the body of a typical, single rod, double acting, pneumatic cylinder  1  consists of three main components: the head cap  1   z , the main tube  1   y , and the end cap  1   w . Inside the cylinder is an internal moving element. The internal moving element is usually known as a piston  1   p . Piston  1   p  consists of a piston flange  1   g , a piston rod  1   c , two bosses  1   f ,  1   h , and miscellaneous fasteners and seals, which are not shown. Rod  1   c  goes through a hole, not shown, in cap  1   z . A head end chamber  1   b  is the internal section of cylinder  1  between flange  1   g  and cap  1   z . Chamber  1   b  communicates with port B via head bore passage  1   d , and head port passage  1   k . Cap end chamber  1   a  is the internal section of cylinder  1  between flange  1   g  and cap  1   w . Chamber  1   a  communicates with port A via cap bore passage  1   e , and cap port passage  1   j.    
     Piston  1   p  is free to travel inside cylinder  1 , from one end to the other end. The distance that piston  1   p  can travel is known as the stroke. When piston  1   p  is located at the head end of cylinder  1 , an inactive region  1   m  forms between flange  1   g  and cap  1   z . Region  1   m  is called inactive, because it never completely empties of air. In this embodiment, region  1   m  consists of spacer pockets  1   s ,  1   t , cushion valve pocket  1   u , and any gaps, not shown, that exist between cap  1   z , and flange  1   g . Pockets  1   s ,  1   t , and  1   v  are recessed into cap  1   z . A similar inactive region  1   n  forms between flange  1   g , and cap  1   w , when piston  1   p  is located at the cap end of cylinder  1 . Region  1   n  consists of spacer pockets  1   r ,  1   q , cushion valve pocket  1   v , and any gaps that exist between flange  1   g , and cap  1   w . Pockets  1   q ,  1   r , and  1   v  are recessed into cap  1   w.    
     There are two identical cushioning cartridges,  2 , and  2 ′, which thread into pockets  1   u  and  1   v , respectively, as shown in  FIG. 3 . The threads are not shown in the drawings. Per  FIG. 9 , when cartridge  2  or  2 ′ is installed into cylinder  1 , cap  23  faces outward from cylinder  1 , and cap  21  faces into the inside of cylinder  1 . 
     Referring to  FIG. 4 , this embodiment shows that cartridge  2  is cylindrical. Cartridge  2  consists of three parts: a pressure end cap  21 , an outlet end cap  23 , and a main housing  22 . The fasteners holding cap  21 , cap  23 , and housing  22  together are not shown. Per  FIG. 6 , cap  21  has three holes, or passages, which run through it, labeled  2   a ,  2   c , and  2   d . The pressure feed passage  2   a  is offset from the center axis of the cushioning cartridge. The check valve exhaust passage  2   c  is on the other side of the main center axis from the pressure feed hole  2   a . The valve inlet passage  2   d , lies between passage  2   a  and passage  2   c , and is offset to the side of passage  2   a  and passage  2   c.    
     Referring to  FIG. 1  and  FIG. 7 , passage  2   a  runs from the outside end of cap  21  to a pressure relief valve  5 . In this embodiment, valve  5  consists of several parts: a pressure relief stem  51 , a pressure relief spring  52 , a pressure relief washer  53 , and a pressure relief pressure adjuster  54 . Stem  51  is coaxial with passage  2   a . Spring  52  forces stem  51  against cap  21 . The other end of spring  52  rests against washer  53 . Washer  53  in turn rests against adjuster  54 . Adjuster  54  is threaded into housing  22 . The threads are not shown. A tool, not shown, is inserted through a tool insert passage  2   k , to move adjuster  54  back and forth, parallel to the arrow labeled L. Moving adjuster  54  back and forth adjusts the tension in spring  52 . Adjusting the tension in spring  52  adjusts the pressure needed in passage  2   a  to unseat stem  51  from cap  21 . When stem  51  is pushed against the cap  21 , seals, not shown, between stem  51 , and cap  21  prevent air from flowing to a pilot passage  2   b . Passage  2   b  leads to a cushion exhaust valve  3 , and a check valve  4 . 
     Valve  3  consists of several parts. Referring to  FIG. 7  and  FIG. 8 , the core of valve  3  is a spool  31 . Spool  31  is a hollow cylinder that is open at one end  31   f  and closed at the other end  31   g . Two sets of holes,  31   a  and  31   b , pass through the outer wall  31   e  of spool  31 . Both sets of holes  31   a  and  31   b  consist of a pattern of holes that are located radially about spool  31 . A spool spring  32  extends into the open end  31   f  of spool  31 . One end of spring  32  presses against an inside partition  31   c  of spool  31 . The other end of spring  32  presses against the inside surface of the cap  23 . Four o-rings  33  are set into grooves  2   m  in housing  22 . When spring  32  is fully extended, it pushes spool  31  into a spool stop, which is not shown, that keeps spool  31  from fully moving into passage  2   b.    
     In this embodiment, valve  4  is directly across passage  2   b  from spool  31 . Valve  4  fits inside passage  2   c  of cap  21 . Valve  4  consists of two parts: a check ball  41 , and a check valve spring  42 . Ball  41  is prevented from fully entering passage  2   b  by a stop that is not shown. Spring  42  holds ball  41  in place. The other end of spring  42  butts against a restricting orifice plug  6 . The restricting orifice plug  6  has a hole  6   a  with a predetermined sized hole drilled in it. Hole  6   a  is sized so as to allow fluid in passage  2   b  to very slowly bleed into its associated pressure chamber  1   a , or  1   b.    
     Passage  2   d  runs completely through cap  21 , and approximately halfway through housing  22 . Passage  2   e  connects passage  2   d  with a cavity  2   f  that rings spool  31 . When spool  31  is seated in its stop, the location of cavity  2   f  aligns with hole set  31   b.    
     Cap  23  has two passages. Passage  2   k  is coaxial with passage  2   a . A second passage, a main exhaust port  2   j , is located opposite of the center-line of housing  22  from passage  2   d . Port C is the outer end of passage  2   j . Passage  2   j  runs completely through cap  23 , and part way into housing  22 . Passage  2   h  radially connects passage  2   j  to a second cavity  2   g  that rings spool  31 . 
     The logic schematic for cartridge  2  is shown in  FIG. 1 , inside the dashed lined box. Passage  2   a  connects inactive region  1   m  to valve  5 . The output of valve  5  travels through passage  2   b  to a pilot port in valve  3 . Passage  2   b  also feeds valve  4 . Opposite valve  4  is plug  6 . The output of plug  6  returns to passage  2   a . Passage  2   d  connects region  1   m  to a port in valve  3 . The output of valve  3  connects to port C. Similarly, cartridge  2 ′ interfaces with region  1   n.    
     Operation 
     Referring back to  FIG. 1 , and  FIG. 2 , to extend rod  1   c , compressed air is supplied to port A. Air flows through passages  1   e , and  1   j , and into chamber  1   a . Simultaneously, air exhausts from the downstream chamber  1   b  through passages  1   d ,  1   k , and out port B. As pressure increases in chamber  1   a , and decreases in chamber  1   b , piston  1   p  moves to the left. As piston  1   p  nears its end of stroke, boss if enters into passage  1   d . O-rings, not shown, in passage  1   d  engage boss  1   f . This engagement prevents additional air from leaving chamber  1   b  through passage  1   d . As piston  1   p  continues to move to the left, the pressure inside chamber  1   b  increases, as the air remaining in chamber  1   b  absorbs the inertial energy of piston  1   p . At some point in time, the kinetic energy, and the velocity of piston  1   p  will be zero. At this point, the pressure in chamber  1   b  is at its maximum value. Since the air pressure in chamber  1   b  now exceeds the pressure in chamber  1   a , piston  1   p  begins to move in the opposite direction, or bounce back. As piston  1   p  bounces back, the volume of chamber  1   b  increases, and the pressure in chamber  1   b  decreases. In the ideal situation, setting the tension in spring  52  to open at the maximum pressure, would cause the pressure in chamber  1   b  to immediately dissipate. Piston  1   p  would be stopped, and would have no pressure to make piston  1   p  reverse direction. However, to account for delays in exhausting the air, the tension in spring  52  is set to a pressure ‘just before’ the maximum pressure is reached. In practice, the tension in spring  52  is empirically determined. 
     Referring again to  FIG. 7 , and  FIG. 8 , once the pressure in chamber  1   b  reaches its predetermined value, stem  51  is pushed away from cap  21 . Valve  5  opens. When valve  5  opens, air flows into passage  2   b . Spool  31  moves, opening valve  3 . Prior to spool  31  moving, pressurized air from chamber  1   b  flows through passages  2   d ,  2   e  and cavity  2   f  into spool chamber  31   d . When spool  31  moves, spool hole set  31   b  moves away from cavity  2   f  and aligns with cavity  2   g . Spool  31  hole set  31   a  now aligns with cavity  2   f . Pressurized air from chamber  1   b  now exits through chamber  31   d , cavity  2   g , passages  2   h ,  2   j  and out port C. As pressure in chamber  1   b  decreases, air pressure in passage  2   b  overpowers spring  42 , unseating ball  41 . Valve  4  opens. Air in passage  2   b  now exits through valve  4  to chamber  1   b . Passage  6   a  is sized in order to delay the loss of pressure from passage  2   b . The delay in depleting air from passage  2   b  keeps spool  31  open longer. More air can escape from chamber  1   b.    
     To retract piston  1   p , air is redirected to port B. Chamber  1   a  becomes the downstream chamber, and cartridge  2 ′ cushions piston  1   p , as piston  1   p  reaches its retracted end of stroke. 
     Important Notes. 
     A few additional comments regarding the operation must be mentioned. 
     
         
         a. The volume of regions  1   m , and  1   n , affects the stopping ability of cartridges  2  and  2 ′. Changing the volume changes the rate at which the pressures in chamber  1   b  increases. For example, a larger volume will build deceleration pressures more slowly. Piston  1   p  can move farther, before it reaches its bounce-back position. The ideal volume will place the position of bounce back at the end of stroke. To achieve this ideal position, the volume of the inactive regions can be machined to a predetermined value, depending on the expected load, speeds, and air supply pressures that will be used. However, precisely machining the inactive region does not allow for flexibility in changing the volume of regions  1   m  and  1   n , to account for changes in the expected loads, speeds, and air supply pressures. As an alternative, pockets  1   q ,  1   r ,  1   s , and  1   t  can be machined into caps  1   z , and  1   w . Arc-segment shaped spacers  8 , of varying thicknesses are secured into pockets  1   q ,  1   r ,  1   s , and  1   t  with bolts  7 . Varying the number and thicknesses of spacers  8 , changes the volume of regions  1   m , and  1   n . This gives the ability to adjust the location of the bounce back point for piston  1   p.    
         b. The air pressure needed to decelerate piston  1   p  will be several times greater than the pressure needed to accelerate piston  1   p . Therefore valve  5  will not open during acceleration. 
         c. The length of boss  1   f , affects when the pressure in  1   b  begins to increase. A longer boss  1   f , will begin to cushion piston  1   p  sooner. 
       
    
     Embodiments 
     The described embodiment is for an easily replaceable cushioning cartridge  2 . However, the above mentioned detailed description is just one embodiment. The central idea for cushioning piston  1   p  is the method diagramed in the logic schematic found in  FIG. 1 . The main components, valve  3 , valve  4 , valve  5 , and restricting orifice plug  6  can just as easily be installed as separate items inside, or outside of, cylinder  1 .  FIG. 10  gives one possible embodiment of an external component arrangement. In addition to the already discussed components, the air supply enters the cylinder through either port A or port B. After activating the cushioning stage, the exhaust air is routed through an exit manifold  11  to either cartridge  2  or  2 ′. 
     Additional embodiments can take the form of replacing some of the components described with off-the-shelf or custom designed sub-assemblies. For example, items  61 ,  41 , &amp;  42  can be made into a single check valve. Items  51 ,  52 ,  53 , and  54  can be made as a single relief valve. Additionally, valve  5  can be replaced with an air-piloted relief valve to give a tighter break-free range. Valve  3  can be replaced with a suitably designed poppet, or other type of valve. Orifice  6   a , can be placed upstream of the check valve  4 . Furthermore, orifice  6   a  can be replaced with a variable orifice, needle valve. 
     Another embodiment uses an external accumulator  9  to replace spacers  8  in order to adjust the effective inactive region. Either an appropriately sized accumulator may be used, or an accumulator with an adjustable internal volume may be used. 
     Finally cartridge  2  is not limited to a pneumatic cylinder. Cartridge  2  can also be used to depressurize a hydraulic or pneumatic fluid chamber. The relative amount of unloading can be adjusted by changing the spring constant of spring  32 . A lower spring constant will give a higher percentage of unloading.