Patent Publication Number: US-6336390-B1

Title: Linear actuator with air cushion mechanism

Description:
TECHNICAL FIELD 
     The present invention relates to a linear actuator for causing two air cylinder mechanisms mounted in a pedestal to operate synchronously to cause a slide table on the pedestal to operate linearly and more specifically relates to a linear actuator having means for stopping the slide table at a stroke end in a cushioned manner. 
     PRIOR ART 
     As disclosed in Japanese Patent Application Laid-open No. 10-61611, for example, there is a known linear actuator having two air cylinder mechanisms mounted in a pedestal and causing the air cylinder mechanisms to operate synchronously to cause a slide table on the pedestal to reciprocate linearly. 
     In such a linear actuator, various cushioning mechanisms are attached for stopping the slide table at a stroke end in a cushioned manner. For example, a damper elastically biased by a spring is provided to a side face of the pedestal and a contact member provided to a side face of the slide table is brought into contact with the damper at the stroke end. 
     However, the cushioning mechanism provided to any known linear actuator mechanically absorbs a shock, has a simple structure, and its operation is reliable, but cannot be used for some uses because a sound of the shock is produced or the cushioning mechanism projects from a side face. 
     On the other hand, in a normal air cylinder device, a cushioning mechanism of an air cushion type is used in which air is temporarily sealed in pressure chambers on an exhaust side in operation of pistons to increase pressure of the pressure chambers and to decelerate the pistons by the exhaust pressure, thereby causing the pistons to stop at the stroke ends in the cushioned manner. 
     However, because a long cushion ring is provided on at least one side of the piston and a long cushion chamber into which the cushion ring is fitted is provided in the pressure chamber in the above cushioning mechanism of the air cushion type, a length in an axial direction of a cylinder increases and a size of the linear actuator is increased if the cushioning mechanism is applied to the linear actuator. Furthermore, because the linear actuator has two air cylinder mechanisms, the size of the linear actuator is further increased if the air cushion is provided to each the air cylinder mechanism. 
     DISCLOSURE OF THE INVENTION 
     It is a technical object of the present invention to provide a linear actuator having a small and rational design structure including a cushioning mechanism of an air cushion type. 
     To achieve the above object, a linear actuator of the invention comprises two air cylinder mechanisms which are arranged in parallel with each other and operate synchronously, a pair of ports common to both the air cylinder mechanisms, and at least one air cushion mechanism common to both the air cylinder mechanisms, wherein the air cushion mechanism has an exhaust hole which is provided to a position adjacent to at least one of the ports and which communicates with the pressure chambers at positions closer to chamber ends than the ports, a flow rate restricting mechanism which is connected between the exhaust hole and the port and which restricts a flow rate of exhaust discharged from the pressure chambers, and cushion packing which is mounted to an outer peripheral face of one of the pistons and which gets over the port on an exhaust side immediately before the piston reaches the stroke end to cause the compressed air in the pressure chambers to be discharged only from the exhaust hole. 
     In the linear actuator of the invention having the above structure, if the compressed air is supplied to or discharged from the pressure chambers of the respective air cylinder mechanisms through the pair of ports, the pistons of both the air cylinder mechanisms operate synchronously and a slide table reciprocates linearly on a pedestal. 
     Stopping of the slide table in a cushioned manner when the slide table reaches the stroke end is carried out by synchronously decelerating the pistons of the two air cylinder mechanisms by the common air cushion mechanism. In other words, in sliding of the pistons of the respective air cylinder mechanisms, the compressed air in the respective pressure chambers on the exhaust side is discharged at first mainly through the port. When the piston approaches the stroke end and the cushion packing gets over the exhaust-side port, the port is separated from the pressure chambers and the compressed air in the pressure chambers is discharged only from the exhaust hole through the flow rate restricting mechanism in a restricted manner. As a result, the pressure in the pressure chambers is increased by control of the flow rate by the flow rate restricting mechanism and the increased pressure functions as piston back pressure to decelerate the pistons while causing the pistons to reach the stroke ends. 
     As described above, because the linear actuator has the cushioning mechanism of the air cushion type, the linear actuator does not produce a sound of a collision and is quiet unlike a mechanical cushioning mechanism. The linear actuator does not produce dust and can be used in a clean room and the like. If the mechanical cushioning mechanism is provided to only one side of the slide table like in prior art, the slide table is supported on the one side when it stops and therefore, an axis of the slide table is likely to incline. In the present invention, however, cushioning effect acts on the pistons of the respective air cylinder mechanisms coaxially with a direction in which thrust of the pistons is produced and the air cylinder mechanisms are synchronously decelerated. Therefore, inclination of the slide table is not generated. Furthermore, not only because the one air cushion mechanism common to the two air cylinder mechanisms is provided but also because the air cushion mechanism does not require a long cushion ring and a long cushion chamber into which the cushion ring is fitted unlike the prior art, it is possible to obtain the linear actuator with a very small and rational design structure. 
     According to a concrete embodiment of the invention, the flow rate restricting mechanism includes a throttle hole for restricting a flow rate of exhaust flowing from the exhaust hole toward the port and a check valve for restricting a flow of exhaust from the exhaust hole toward the port and for allowing a flow of compressed air in a reverse direction. 
     In this case, it is preferable that a valve chamber communicating with the exhaust hole and the ports is formed in the pedestal and the flow rate restricting mechanism is mounted into the valve chamber by disposing a valve member having the throttle hole in the valve chamber through a lip seal forming the check valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of a linear actuator with an air cushion mechanism according to the present invention. 
     FIG. 2 is a side view of the linear actuator in FIG.  1 . 
     FIG. 3 is a sectional view taken along a line A—A in FIG.  2 . 
     FIG. 4 is a sectional view taken along a line B—B in FIG.  2 . 
     FIG. 5 is a sectional view showing an operating position different from that in FIG.  4 . 
     FIG. 6 is an enlarged view of an essential portion of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the present invention will be described in detail by reference to the drawings. A linear actuator of the embodiment shown in FIGS. 1 to  5  has a pedestal  1  in a form of a flat and short prism, a linear guide  2  provided to an upper face of the pedestal  1 , a slide table  3  provided to the upper face of the pedestal  1  for sliding along the linear guide  2 , first and second two air cylinder mechanisms  4 A and  4 B which are mounted in parallel inside the pedestal  1 , drive the slide table  3 , and operate synchronously, and air cushion mechanisms  5   a  and  5   b  for stopping the air cylinder mechanisms  4 A and  4 B at stroke ends in a cushioned manner. 
     The linear guide  2  has a rectangular guide block  10  fixed to a central portion of the upper face of the pedestal  1 , the slide table  3  is mounted astride the guide block  10 , a plurality of balls  13  are respectively mounted for rolling between grooves  11  on opposite side faces of the guide block  10  and grooves  12  on inner faces of opposite guide walls  3   a  of the slide table  3 , and the slide table  3  reciprocates linearly along the guide block  10  in response to rolling of the balls  13 . 
     Balls  13  are also housed in ball holes  14  formed in parallel to the grooves  11  at positions near the opposite side end portions of the guide block  10  and the balls  13  in the grooves  11  and the balls  13  in the ball holes  14  are arranged to form annular lines. In sliding of the slide table  3 , the balls  13  roll to circulate along the grooves  11  and the ball holes  14 . 
     As is clear from FIGS. 4 and 5, the two air cylinder mechanisms  4 A and  4 B are mounted in parallel inside the flat pedestal  1  and have substantially the same structures except that structures of the pistons  21 A and  21 B are slightly different from each other as described below. In the following description, the pistons  21 A and  21 B will be represented by a common reference numeral “ 21 ” when they need not be distinguished from each other. 
     In other words, two cylinder bores  20 ,  20  extending in an axial direction are provided in parallel to each other on left and right within the pedestal  1 , the pistons  21  are housed for sliding in the respective cylinder bores  20 , and piston rods  22  connected to the pistons  21  are provided such that tip ends of the piston rods  22  project from one ends of the cylinder bores  20 . End portions of the respective cylinder bores  20  on head sides are closed with head covers  24  and rod covers  25  are mounted to the end portions on rod sides. The piston rods  22  pass through the rod covers  25  such that the piston rods  22  can slide airtightly through sealing members. 
     Thus, on opposite sides of the pistons  21 , head-side pressure chambersg  26   a  are formed between the pistons  21  and the head covers  24  and rod-side pressure chambers  26   b  are formed between the pistons  21  and the rod covers  25 . Corresponding pressure chambers of the two air cylinder mechanisms  4 A and  4 B, i.e., the head-side pressure chambers  26   a ,  26   a  and the rod-side pressure chambers  26   b ,  26   b  respectively communicate with each other through through holes  27   a  and  27   b  formed in the pedestal  1 . 
     A pair of ports  30   a  and  30   b  for supplying compressed supplied air to the pair of pressure chambers  26   a  and  26   b  of the first air cylinder mechanism  4 A are provided to the side face of the pedestal  1  on the first air cylinder mechanism  4 A side. The ports  30   a  and  30   b  are common to the two air cylinder mechanisms  4 A and  4 B. By supplying compressed air alternately to the head-side pressure chamber  26   a  and the rod-side pressure chamber  26   b  of the first air cylinder mechanism  4 A from the ports  30   a  and  30   b  through the through holes  30   c , the pistons  21 A and  21 B of the two air cylinder mechanisms  4 A and  4 B whose pressure chambers communicate with each other slide synchronously with each other. 
     A common junction plate  32  is mounted to tip ends of the piston rods  22  of the two air cylinder mechanisms  4 A and  4 B, the junction plate  32  is connected to the slide table  3 , and the slide table  3  is driven by the two air cylinder mechanisms  4 A and  4 B thorough the junction plate  32 . 
     The air cushion mechanisms  5   a  and  5   b  are common to the two air cylinder mechanisms  4 A and  4 B. By attaching the air cushion mechanisms  5   a  and  5   b  to the first air cylinder mechanism  4 A, air cushioning operation is also generated in the second air cylinder mechanism  4 B by a chain reaction. In other words, the air cushion mechanisms  5   a  and  5   b  have exhaust holes  34  which are provided to positions adjacent to the pair of ports  30   a  and  30   b  and which open into the respective pressure chambers  26   a  and  26   b  at positions closer to chamber ends than the through holes  30   c ,  30   c  and flow rate restricting mechanisms  35  which are connected between the exhaust holes  34  and the ports  30   a  and  30   b  and which restrict flow rates of exhaust discharged from the pressure chambers  26   a  and  26   b.    
     As can be seen from FIG. 6, the flow rate restricting mechanism  35  is formed by connecting a throttle hole  37  for restricting the exhaust flow rate and a check valve  38  for intercepting a flow of exhaust that does not pass through the throttle hole  37  in parallel. The flow rate restricting mechanism  25  is housed in a valve chamber  39  formed in the side face of the pedestal  1 . In other words, the valve chambers  39  that communicate with the exhaust holes  34  and both the ports  30   a  and  30   b  are formed in the side face of the pedestal  1 . A valve member  40  in a form of a cylindrical column is housed in the valve chamber  39 , the throttle hole  37  is formed in the valve member  40 , and a lip seal forming the check valve  38  is disposed between an outer peripheral face of the valve member  40  and an inner peripheral face of the valve chamber  39 . A reference numeral  41  in the drawings designates channels for connecting the valve chambers  39  and the ports  30   a  and  30   b.    
     The throttle holes  37  are formed to connect the exhaust holes  34  and the ports  30   a  and  30   b . An area of an opening of the throttle hole  37  can be adjusted by a needle  37   a  provided to the valve member  40 . However, the throttle hole  37  is not limited to such a variable throttle type but may be a fixed throttle type without the needle  37   a.    
     On the other hand, the check valves  38  are for intercepting exhaust other than that flowing from the pressure chambers  26   a  and  26   b  into the ports  30   a  and  30   b  through the throttle holes  37  in a cushioning stroke on a stroke end side of the pistons  21  and for allowing compressed air from the ports  30   a  and  30   b  to freely flow into the pressure chambers  26   a  and  26   b  at the start of driving of the pistons  21 . 
     Two pieces of packing  43   a  and  43   b  are mounted to an outer peripheral face of the piston  21 A of the first air cylinder mechanism  4 A. The pieces of packing  43   a  and  43   b  of course have a function as piston packing for separating the two pressure chambers  26   a  and  26   b  on opposite sides of the piston  21 A and also have a function as cushion packing. Immediately before the piston  21 A reaches the stroke end, the packing  43   a  or  43   b  on a front side in a moving direction gets over the through hole  30   c  of the port  30   a  or  30   b  in an exhausting state such that compressed air in the pressure chamber  26   a  or  26   b  is discharged only through the exhaust hole  34 . At this time, the packing  43   b  or  43   a  on a rear side in the moving direction of the piston  21 A does not get over the through hole  30   c  of the exhaust-side port  30   b  or  30   a  and stops before the through hole  30   c  when the piston  21 A reaches the stroke end. 
     Only one piece of packing  43  is mounted to an outer peripheral face of the piston  21 B of the second air cylinder mechanism  4 B and the packing  43  functions as piston packing. 
     In the linear actuator having the above structure, when compressed air is supplied alternately to the pressure chambers  26   a  and  26   b  of the two air cylinder mechanisms  4 A and  4 B from the two ports  30   a  and  30   b , the pistons  21 A and  21 B of both the air cylinder mechanisms  4 A and  4 B operate synchronously and the slide table  3  moves along the linear guide  2  through the piston rods  22 ,  22  and the junction plate  32 . At this time, stopping of the slide table  3  at the stroke end in the cushioned manner is carried out by synchronously decelerating and stopping the pistons  21 A and  21 B of the two air cylinder mechanisms  4 A and  4 B at the stroke ends by the common air cushion mechanisms  5   a  and  5   b . This point will be described about a case of stopping the pistons  21 A and  21 B at the headside stroke ends in the cushioned manner by the air cushion mechanism  5   a  by using FIGS. 4 and 5. 
     In other words, as shown in FIG. 4, when the compressed air is supplied from the rod-side port  30   b  to the rod-side pressure chambers  26   b  of the air cylinder mechanisms  4 A and  4 B, the pistons  21 A and  21 B move rightward in FIG. 4 toward head sides. At this time, the compressed air in the head-side pressure chambers  26   a  that are on the exhaust side is discharged through the through hole  30   c  of the head-side port  30   a  and the exhaust hole  34 . However, when the piston  21 A approaches the stroke end and packing  43   a  on the front side in the moving direction gets over the through hole  30   c  of the exhaustside port  30   a  as shown in FIG. 5, the port  30   a  is separated from the pressure chambers  26   a  and the compressed air in the pressure chambers  26   a  is discharged only from the exhaust hole  34  of the air cushion mechanism  5   a  through the flow rate restricting mechanism  35  in a restricted manner. Therefore, pressure in the pressure chambers  26   ab  increases as a result of control of the flow rate by the flow rate restricting mechanism  35  and the increased pressure functions as piston back pressure to decelerate the two pistons  21 A and  21 B while causing the pistons  21 A and  21 B to reach the stroke ends. 
     In a case opposite to the above case, the pistons  21 A and  21 B move leftward in the drawing toward the rod sides from the head-side stroke ends when the compressed air is supplied to the head-side port  30   a . At this time, the through hole  30   c  of the port  30   a  is closed between the two pieces of packing  43   a  and  43   b  on the piston  21 A. However, because the compressed air from the port  30   a  pushes the check valve  38  of the flow rate restricting mechanism  35  open to freely flow into the pressure chambers  26   a , the pistons  21 A and  21 B can be actuated at a predetermined speed. As shown in FIG. 4, when the packing  43   a  on the rear side in the moving direction of the piston  21 A gets over the through hole  30   c  of the port  30   a , the compressed air directly flows into the pressure chamber  26   a  through the port  30   a . Therefore, the piston  21  continues to move. 
     When the pistons  21 A and  21 B reach the rod-side stroke ends, the rod-side air cushion mechanism  5   b  functions. In other words, when the piston  21 A approaches the stroke end, the packing  43   b  on the front side in the moving direction switches a path of exhaust flowing out of the rod-side pressure chambers  26   b  from a path through which the exhaust is directly discharged from the port  30   b  through the through hole  30   c  to a path through which the exhaust is discharged in a restricted manner through the exhaust hole  34  of the air cushion mechanism  5   b  and the flow rate restricting mechanism  35 . As a result, the two pistons  21 A and  21 B stop at the rod-side stroke ends in the cushioned manner while being decelerated. 
     As described above, because the linear actuator has the cushioning mechanisms of the air cushion type, the linear actuator does not produce a sound of a collision and is quiet unlike a mechanical cushioning mechanism. The linear actuator does not produce dust and can be used in a clean room and the like. If the mechanical cushioning mechanism is provided to only one side of the slide table  3  like in prior art, the slide table  3  is supported on the one side when it stops and therefore, an axis of the slide table  3  is likely to incline. In the present invention, however, cushioning effect acts on the pistons  21 ,  21  of the respective air cylinder mechanisms  4 A and  4 B coaxially with a direction in which thrust of the pistons  21 ,  21  is produced and the air cylinder mechanisms  4 A and  4 B are synchronously decelerated. Therefore, inclination of the slide table  3  is not generated. Furthermore, not only because the air cushion mechanisms  5   a  and  5   b  common to the two air cylinder mechanisms  4 A and  4 B are provided but also because the air cushion mechanisms  5   a  and  5   b  do not require long cushion rings and long cushion chambers into which the cushion rings are fitted unlike in the prior art, it is possible to obtain the linear actuator with a very small and rational design structure. 
     Although the two air cushion mechanisms  5   a  and  5   b  are provided in positions of the opposite stroke ends so as to stop the pistons  21  at normal and reverse opposite stroke ends in the cushioned manner in the above embodiment, either one of the air cushion mechanisms  5   a  and  5   b  may be provided to stop the pistons  21  at one stroke ends in the cushioned manner. 
     As described above, according to the invention, it is possible to obtain the linear actuator with the small and rational design structure having the cushioning mechanisms of the air cushion type.