Reciprocating actuator

Abstract: An actuator is driven in a first direction by equal working fluid pressures acting on different piston areas acting in opposite directions, and is driven in a second direction by applying equal pressures on both sides of one of the piston areas. Working fluid from an inlet port (71), acting in a working fluid chamber (27), on one side of the piston moves a valve plate (36), closing vent ports (33) and opening working fluid discharge ports (32), thereby permitting the working fluid to push the piston (21) in a first direction. Working fluid contained in the working fluid chamber (21) is exhausted from working fluid discharge ports (32) to the outside of the system through a working fluid discharge tube (52). When the movement of piston (21) reaches its limit, a valve plate (36) is engaged by a spring (41), thereby causing simultaneous closing of the working fluid discharge ports (32) and the opening of the vent ports (33). As a result of the opening of the vent ports (33), working fluid pressures in the working fluid chamber (27) and a second working fluid chamber (26) are equalised. The effective piston area facing the second chamber is greater than the effective area facing the first chamber so the direction of piston movement is reversed. The working fluid discharge pipe (52) is fluid-tightly fitted through the piston (21) so as to provide a discharge path of working fluid from the system.

Description: The present invention relates to a reciprocating actuator which may for example be pneumatically operated with a directional control valve that minimises the length of the actuator by eliminating the need for an auxiliary cylinder in the actuator assembly. A conventional reciprocating type actuator, such as the one disclosed in Japanese Utility Model Publication No. 37829/1986, has a configuration comprising a piston slidably fitted in a cylinder with a directional control valve mechanism attached to the piston. Valve mechanism switching, at each end of reciprocating stroke, transfers a working fluid alternately between two working fluid chambers formed in the cylinder and separated from each other by the piston. A conventional reciprocating actuator must have an auxiliary cylinder below the main cylinder into which the piston is retracted. The auxiliary cylinder has an axial length sufficient for the full stroke of the piston so that when the piston moves upward, discharge of working fluid above the piston is accomplished regardless of the position of the piston. A reciprocating actuator having a directional control valve mechanism attached to the piston, such as the above example, requires that the actuator assembly have sufficient length in the axial direction, as determined by the length of the discharge auxiliary cylinder, to enable discharge. The reader will be further enlightened as to the state of the art by reference to FR-A-1 342 507 with respect to which claim 1 is characterised. It is an object of the present invention to provide a reciprocating actuator of reduced length by using a directional control valve that eliminates the need for an auxiliary cylinder at one end of the actuator. Accordingly the present invention provides a reciprocating actuator according to claim 1. The piston may accommodate three exhaust valves and three vent valves. Working fluid in the exhaust chamber is released through the exhaust valve and hence the exhaust port as the piston commences its travel towards a cap end of the actuator. At the end of the stroke towards the cap end of the actuator, the exhaust and vent ports in the directional control valve mechanism assume alternate states, allowing working fluid to fill the exhaust chamber thus reversing the direction of stroke from the cap end of the actuator. Reciprocating movement of the plunger attached to the piston continues as long as working fluid is introduced at an inlet port of the actuator. As with the prior art actuators with valving mechanisms in the piston, the piston is propelled towards the cap by the application of fluid at high pressure in the inlet chamber while venting the exhaust chamber through the exhaust passages such that the exhausts chamber is at a lower pressure. The piston is propelled in the opposite direction because equal pressures are applied to both piston faces, however, one piston face defining the inlet chamber has a smaller area than the other. When the upward movement of the piston reaches its limit, a valve plate is engaged by a spring, thereby causing a simultaneous closing of the exhaust valves and the opening of the vent valves. Other objects, features and advantages of the present invention will become apparent from the following exemplary description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. Fig. 1 is a cross section showing an embodiment of a reciprocating actuator according to the present invention. Fig. 2 is a cross section of the reciprocating actuator taken along line II-II in Fig. 1. Referring to Fig. 1, a reciprocating actuator has a cylinder 13 screwed to a body 11, with an O-ring 12 between them. A cap 15 is screwed onto one end of the cylinder 13, with a seal 14 between them. A plunger 17 is vertically slidable in a bore of the body 11 and connected to a piston 21 by a hollow piston-rod 18. Piston 21 comprises packing 22 fitted in cylinder 13 and interposed between an upper washer 23 and a lower washer 24. Packing 22 and upper washer 23 and lower washer 24 are clamped between the rod 18 and a plug 25 screwed into the end of the rod 18. Piston 21 divides the space inside cylinder 13, into an upper working fluid chamber 26 and a lower working fluid chamber 27. A directional control valve 31 is attached to piston 21. Directional control valve 31 switches at both ends of reciprocating movement for the supply and discharge of working fluid between the upper and lower working fluid chambers 26 and 27. Directional control valve 31 comprises three coupler members 34 mounted in respective ports 32 working fluid discharge formed in piston 21 (see figure 2). Working fluid discharge ports 32 serve as exhaust ports for working fluid. Coupler 34 connects together an upper valve plate 35 and a lower valve plate 36. The upper valve plate 35 faces upper seats 37 of working fluid discharge ports 32 and lower valve plate 36 faces lower valve seats 38 of a set of three vent ports 33, in the piston 21, also shown in figure 2. When the piston is at the top of cylinder 13, the upper valve plate 35 contacts an upper spring 41, and is thus urged into contact with coupler 34, thereby urging lower valve plate 36 out of contact with working fluid discharge ports 32. At the opposite end of the stroke, the lower valve plate 36 contacts a lower spring 42 fitted around rod 18 which urges lower valve plate 36 upwards towards the cap 15 end of the actuator, thereby moving valve plate 36 into sealing contact with working fluid discharge ports 32. Returning to Fig. 1 an axial discharge tube 52 is mounted in cap 15 of the cylinder at the end opposite to the rod 18 and extends along substantially the complete length of cylinder 13. An O-ring 51 and a protruding part 53 of discharge tube 52 are fixed through a stopper 54 to an exhaust cylinder 55 screwed to cap 15. Exhaust cylinder 55 includes exhaust ports 56 to allow exhaust of working fluid from upper discharge tube 52. Plug 25 of piston 21 is slidably mounted on discharge tube 52 by a O-ring 61. Communicating axial bores 62, 63 and 64 are formed in plug 25, rod 18 and plunger 17 respectively. Working fluid discharge ports 32 of directional control valve 31 communicate with the interior of discharge tube 52 via radial apertures 65 bored in plug 25 and the bores 62, 63 and 64. An air inlet port 71 in body 11 communicates with lower working fluid chamber 27 via a counterbore 72 at the upper end of body 11. Air can thus be supplied through a hole 73 bored in washer 43 into lower working fluid chamber 27. A threaded hole 74 in body 11 connects a pump discharge port 75 to an external pump mechanism. Explanation of the external pump is omitted herein. In operation, as shown in Fig. 1, when piston 21 is in its uppermost position, toward the cap end of the actuator, upper valve seats 37 of working fluid discharge ports 32 are closed by upper valve plate 35. At the same time, lower valve seats 38 of vent ports 33 are opened by the action of couplers 34 acting on lower valve plate 36. This allows air pressure supplied from inlet port 71 into lower working fluid chamber 27 to be applied also to upper working fluid chamber 26 through vent ports 33, thereby applying virtually the same air pressure on both the top and the bottom of piston 21. The areas of the surfaces of piston 21 which receive the pressure are different, i.e., the upper surface is greater than the lower surface. The lower surface area being less than the upper end area by the amount of the sectional area of plunger 17, causes piston 21 to move downward. At the end of the downward stroke the lower spring 42 urges upper valve plate 35 and lower valve plate 36 in the upward direction, causing upper valve seats 37 of working fluid discharge ports 32 to open and lower valve seats 38 to close. As a result, pressure of the air supplied from inlet port 71 into lower working fluid chamber 27 works only upon the lower surface of piston 21, thus urging piston 21 to move in the upward direction. At that time, air contained in upper working fluid chamber 26 is discharged through working fluid discharge ports 32, radial holes 65, inner bores 62, 63 and 64 of the piston rod and plunger into the working fluid discharge tube 52 and thus to exhaust port 56 of exhaust cylinder 55 to the outside of the system. At the end of the upward stroke of piston 21, upper valve plate 35 abuts and compresses upper spring 41 to the point where the upper valve plate 35 and lower valve plate 36 are pushed downward in relation to the upward movement of single-rod piston 21. At this point, upper valve seats 37 of working fluid discharge ports 32 close, and lower valve seats 38 of vent ports 33 open, causing the piston 21 to reverse direction to the downward stroke.

Claims: A reciprocating actuator comprising a double acting piston (21) reciprocally displaceable within a cylinder (13) to define an inlet chamber (27) and an exhaust chamber (26), a fluid supply to supply fluid to the inlet chamber (27) and a fluid discharge tube (52) extending axially through the exhaust chamber (26) to sealingly communicate an exhaust port (56) remote from the inlet chamber (27), said piston (21) including a directional control valve mechanism including a coupler (34) and adapted to open and close an eccentrically located working fluid discharge port (32) in the piston (21) and an eccentrically located vent port (33), in the piston (21), said discharge port (32) communicating the exhaust chamber (26) to the fluid discharge tube (52), said vent port (33) said vent port (33) communicating the inlet chamber (27) to the exhaust chamber (26) characterised in that the control valve mechanism includes an upper valve plate (35) to open and close the working fluid discharge port (32) by sealing and unsealing against an upper valve seat (37), a lower valve plate (36) arranged to open and close the vent port (33) by sealing and unsealing against a lower valve seat (38), said coupler (34) extending through the vent port (33) to couple the lower and upper valve plates (35, 36) so that when one port is opened the other is closed, and, the fluid discharge tube (52) being accommodated in coaxially communicating bores (62, 63, 64) which extend through the piston (21) into a connecting rod (18) and a plunger (17). An actuator according to claim 1 wherein the lower valve seat (38) is provided in a lower washer (24). An actuator according to claim 2 wherein the means to change the open or closed state of the valve mechanism comprises an inlet chamber spring (42) and an outlet chamber spring (41) one of which impinges on the lower or upper valve plates (35, 36) respectively as the piston reaches the end of each stroke.

Inventors: SAITA SEIJI C O K K YAMADA COR, SAITA, SEIJI, C/O K. K. YAMADA CORPORATION
Applicants: YAMADA CORP, KABUSHIKI KAISHA YAMADA CORPORATION
Application year: 1990
Main classification: F04B9
IPC codes: F04B9, F15B11, F03C1, F01L23, F15B15