Fluid pressure actuator and actuator system

Abstract: A fluid pressure-operated actuator comprises a cylinder body (12) containing a piston (11) slidable therein, the piston defining respective chambers (16, 17) at opposite sides thereof. A rod (13) projects from one side of the piston and extends through an end wall (14) of the cylinder and a plunger (20) is slidable on the rod and also extends through the end wall. The arrangement is such that the application of fluid pressure simultaneously to the chambers (16, 17) results in movement of the piston lengthwise of the rod, the piston movement including a first part during which the plunger applies force to the rod in addition to that arising on the piston, and a second part during which the force from the plunger is absent.

Description: This invention relates to a fluid pressure-operated actuator system having an actuator including a piston movable in a cylinder under the action of pressure fluid introduced into the cylinder, the piston actuating a working device by way of a piston rod extending through an end wall of the cylinder. The use of such an actuator to move an initially stationary working device can require a high initial force to initiate the movement. Where an actuator system is constrained by an upper limiting value of available fluid pressure, it is commonly the practice to increase the effective piston areas of the actuator in order to provide the necessary initial force. Such a solution, however, results not only in an increase in the mass of the actuator but, by increasing the volume of fluid required to be displaced at each stroke, increases.the mass of a pump which provides the required fluid pressure. In many applications, in particular in aircraft systems, such increases of mass are to be avoided. UK Patent 708582 discloses an actuator system utilizing an actuator of similar form but arranged to maintain a single piston generated operating force throughout substantially the whole of the actuator stroke and then to utilize a second piston in opposition to reduce the force adjacent the termination of the stroke. It is an object of the invention to provide a fluid pressure-operated actuator system of compact dimensions which can impart an increased initial force to a working device. In accordance with the present invention there is provided a fluid pressure actuator system comprising an actuator having a body defining therein a cylinder, a piston slidable in said cylinder and defining therewith first and second chambers at opposite sides respectively of the piston, a rod projecting from one side of the piston and extending within said first chamber and through an end wall of the cylinder, said piston presenting a larger effective area to said second chamber than to said first chamber, and a plunger slidable on said rod and extending through said end wall, the system further including valve means for controlling fluid flow to the cylinder chambers, the valve means being arranged so that, in one operative state thereof, it causes fluid to be directed to both of said chambers simultaneously to cause movement of the piston in the lengthwise direction of the rod, the piston movement including a first part during which the plunger applies force to the rod in addition to that arising on the piston, and a second part during which the force from the plunger is absent. Preferably said valve means when in its first operative state, interconnects said first and second cylinder chambers so that fluid may be transferred between the chambers in a regenerative action. Desirably said valve means in another operative position, connects the chamber at the larger area side of the piston to a low pressure region. Conveniently said valve means is a spool valve. Preferably, the plunger has an effective area less than the smaller effective piston area and the plunger is conveniently not in sealing engagement with the cylinder internal wall. Typically, the rod carries an abutment against which the plunger engages during the first part of the piston movement and the plunger conveniently carries abutment means arranged to engage a stop, conveniently formed by a surface of the cylinder body, which defines the extent of the first piston movement part. The invention will now be described, by way of example, with reference to the single accompanying drawing which is a cross-sectional representation of one form of a piston actuator system of the invention. As shown in the drawing, an actuator of the system designated generally by the reference 10 comprises a piston 11 slidable in a cylinder body 12 and having a rod 13 which extends through an end wall 14 of the cylinder 12. The effective area of the left-hand end of the piston, as viewed in the drawing, is the full cross-sectional area thereof, whilst the effective area of the right-hand end is the smaller area surrounding the piston rod. A supply pressure P and a return low pressure R are connectable to the cylinder 12 by way of a selector valve designated generally by the reference 15 and preferably in the form of a spool valve. In the illustrated operative position of the valve 15 the supply pressure P is applied to chambers 16, 17 respectively at opposite sides of the piston 11, the effect being to urge the latter rightwards as a result of the aforesaid difference in effective areas of opposite ends of the piston. As the piston moves to the right, fluid is displaced from the chamber 16 to the chamber 17 in a regenerative action, whereby the net flow of pressurised fluid from a source (not shown), such as a pump, is reduced. Leftward movement of the piston 11 is effected by operation of the valve 15 which, in its alternative position, applies the supply pressure P to the chamber 16 only and connects the chamber 17 to the return pressure R. Since the volume of the chamber 16 over one stroke of the piston 11 is substantially less than that of chamber 17, the net fluid flow over one piston cycle remains reduced. In order to effect a high initial force during rightward movement of the piston 11, the actuator 10 includes a plunger 20 through which the rod 13 is slidable, being sealed against the internal surface of the plunger by a first seal 20A. The plunger 20 is itself slidable within an opening in the end wall 14, being sealed against the internal surface of the opening by a second seal 20B. A shoulder 21 on the rod 13 acts as an abutment for the plunger 20 when the piston 11 is at the leftward extent of its travel, and the plunger 20 has a flange 22 which limits its rightward movement by engagement with an adjacent surface of the cylinder body. As is apparent in the drawing the flange 22 does not contact the wall of the cylinder and moreover the flange 22 presents a greater area to the piston 11 than to the wall 14. In use, in the operating condition of the valve 15 shown in the accompanying drawing the pressure P is applied to both chambers 16, 17, i.e. both to the lefthand end of the piston 11 and also to the plunger 20. Thus, given that the larger effective area of the plunger 20 is presented to the piston 11, during initial movement of the piston 11 from its fully leftward position the plunger 20 imparts an additional force to the rod 13 via the shoulder 21 until the flange 22 engages the opposing surface of the wall 14. The force applied to the rod, therefore, during the initial phase of operation is greater than that resulting from the piston 11 acting alone. Following engagement of the flange 22 against the end wall 14, a force less than the initially applied force is applied to the rod from the piston 11 only. When the valve 15 is placed in its alternative condition, the chamber 17 is connected to the low pressure R and the piston is returned to the left by high pressure in the chamber 16. It can be seen that the effective area of the plunger 20 exposed to the pressure in the chamber 16 and urging the plunger to the right is less than the area of the piston 11 exposed to the pressure in the chamber 16 and thus leftward movement of the piston is not prevented by the plunger 20, the plunger 20 being returned to its illustrated inward position by the piston 11 acting on the plunger through the rod 13 and the shoulder 21.

Claims: A fluid pressure-operated actuator system comprising an actuator (10) having a body (12) defining therein a cylinder, a piston (11) slidable in said cylinder and defining therewith first and second chambers (16, 17) at opposite sides respectively of the piston (11), a rod (13) projecting from one side of the piston and extending within said first chamber (16) and through an end wall (14) of the cylinder, said piston (11) presenting a larger effective area to said second chamber (17) than to said first chamber (16) and, a plunger (20) slidable on said rod (13) and extending through said end wall (14), the system being characterised by valve means (15) for controlling fluid flow to the cylinder chambers (16, 17), the valve means being arranged so that, in one operative state thereof, it causes fluid to be directed to both of said chambers simultaneously to cause movement of the piston (11) in the lengthwise direction of the rod (13), the piston movement including a first part during which the plunger (20) applies force to the rod in addition to that arising on the piston, and a second part during which the force from the plunger is absent. A system as claimed in Claim 1, characterised in that said valve means (15), when in its first operative state, interconnects said first and second cylinder chambers (16, 17) so that fluid may be transferred between the chambers in a regenerative action. A system as claimed in Claim 1 or Claim 2, characterised in that said valve means (15) in another operative position, connects the chamber (17) at the larger area side of the piston (11) to a low pressure region (R). A system as claimed in any one of Claims 1 to 3, characterised in that said valve means (15) is a spool valve. A system as claimed in any one of the preceding claims, characterised in that the plunger (20) has an effective area less than the smaller effective area of said piston (11). A system as claimed in any one of the preceding claims, characterised in that said plunger (20) is not in sealing engagement with the cylinder internal wall. A system as claimed in any one of the preceding claims, characterised in that said rod (13) carries an abutment (21) against which the plunger (20) engages during said first part of the piston movement so as to apply said additional force to said rod. A system as claimed in any one of the preceding claims, characterised in that said plunger (20) carries abutment means (22) arranged to engage a stop which defines the extent of the first piston movement part. A system as claimed in Claim 8, characterised in that said plunger abutment means (22) is a generally radial flange on the plunger and is arranged to engage a surface of the cylinder body (12) forming the stop. A system as claimed in any one of the preceding claims, characterised in that said plunger (20) is sealingly engaged around the rod (13) and within the cylinder end wall (14).

Inventors: DAVIES STEPHEN HARLOW, ROLLASON ALAN TERRY, DAVIES, STEPHEN HARLOW, ROLLASON, ALAN TERRY
Applicants: LUCAS INDUSTRIES LTD, LUCAS INDUSTRIES LIMITED
Application year: 1997
Main classification: F15B15
IPC codes: F15B15, F15B11