Movable workpiece-clamping attachment for sewing machine
FIELD OF THE INVENTION 
     My present invention relates to a device, actuatable by compressed air or some other pressure fluid, for releasably clamping a workpiece moving along a predetermined path between two stations, i.e. a first station where the workpiece is engaged and a second station where it is released. 
     BACKGROUND OF THE INVENTION 
     Commonly owned U.S. Pat. Nos. 4,102,280 and 4,104,977 disclose and claim devices of this type designed to hold a workpiece, such as a stack of fabric layers to be stitched together along a linear seam, under a certain tension during transportation of that workpiece under a pressure foot of a sewing machine. The device comprises a trailing clamp reciprocable between a loading position and an unloading position, the clamp being entrained toward the unloading position by the engaged workpiece and being returned to its loading position by a counteracting force after releasing that workpiece. 
     According to U.S. Pat. No. 4,104,977, the counteracting force is provided by a weight; U.S. Pat. No. 4,102,280 discloses means for varying the counteracting force between a low value during forward motion and a high value during the return stroke, e.g. with the aid of a turbine driven by an air stream. 
     In systems of this type, wherein the workpiece is gripped by a moving clamp which is fluidically operated, the activating fluid must be supplied to the clamp-supporting carriage through a flexible conduit capable of following the movement of that carriage. Rubber hoses or the like used for this purpose have only a limited service life as they tend to fatigue upon repeated flexing and unflexing. Their elasticity varies with temperature and gives rise to unpredictable forces acting upon the clamp carriage, particularly when hoses are bent with a small radius of curvature. Finally, the loop formed by such a hose in certain carriage positions may interfere with the mobility of the operator and could also lead to entanglement with extraneous objects, particularly in a large plant comprising a multiplicity of closely juxtaposed sewing stations or the like. 
     Similar problems arise with fluid-actuated mobile clamps which are not entrained by the engaged workpiece but, on the contrary, are used to displace that workpiece over a predetermined path. 
     OBJECTS OF THE INVENTION 
     The general object of my present invention is to provide means for supplying an operating fluid under pressure to a mobile clamp in a predetermined loading position of its carriage without the use of flexible conduits between the source of pressure fluid and the carriage. 
     A related object is to provide means for holding such a clamp closed against a biasing force while the carriage, disconnected from the source of pressure fluid, moves from its loading position to a remote unloading position. 
     SUMMARY OF THE INVENTION 
     In accordance with an important feature of my invention, a clamp mounted on a carriage guided along a track between two stations is provided with fluidically actuatable operating means, such as a pneumatic jack, having an inlet channel which communicates with a source of pressure fluid in a loading position at the first of these stations so that the clamp is closed against a biasing force by the admitted pressure fluid. The inlet channel is provided with valve means settable in the loading position to maintain the pressure of the admitted fluid, the valve means including a control valve which is reversible in an unloading position at the second station to relieve the fluid pressure and allow the clamp to release. 
     Pursuant to another feature of my invention, a station at one end of a guide track for a clamp carriage is provided with a stationary socket which forms a discharge opening connected via a supply line to a source of pressure fluid, the clamp-operating means having an inlet channel receivable in the socket so that an entrance end of this channel confronts the aforementioned discharge opening. When the source is activated, as by the opening of a normally closed shut-off valve, a plunger located at the socket--preferably a piston fluidically operated from the same source--forces the entrance end of the inlet channel into fluidtight contact with the confronting discharge opening to pressurize the clamp-operating means. Advantageously, the channel entrance is a lateral aperture of a nipple extending in the direction of carriage motion, the nipple moving past the sealing ring at close distance therefrom as the carriage enters and leaves its loading position. Such a nipple can be brought into contact with the socket wall by a minor deflection transverse to its direction of motion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other features of my invention will now be described in detail with reference to the accompanying drawing in which: 
     FIG. 1 is a side-elevational view of a workpiece-gripping device according to my invention; 
     FIG. 2 is a plan view of the assembly shown in FIG. 1; and 
     FIG. 3 is a sectional detail view of the left-hand portion of FIG. 1, drawn to a larger scale. 
    
    
     SPECIFIC DESCRIPTION 
     In FIGS. 1 and 2 I have shown a work station 5 comprising a sewing machine 6 with the usual presser foot 8 and fabric feed 46 serving to advance a workpiece 7 past its needle 47 for the purpose of forming a seam. The workpiece 7 is an elongate stack of fabric layers to be stitched together, e.g. to form a sleeve for a trouser leg, as it moves from left to right below the presser foot 8 on a nonillustrated work table. During this movement, the tail end of the workpiece is gripped by jaws 2, 3 of a trailing or drag clamp comprising a pneumatic jack 1 for the operation of the jaws. The clamp 1-3 is supported on a carriage 10 with rollers 16 which are guided in a track 15 for reciprocation between work station 5 and another station 50 remote from sewing machine 6. 
     Carriage 10 is engaged by a cable 17 which passes around rollers 18 and is subjected to a unidirectional force F tending to pull the carriage together with clamp 1-3 toward station 50, thus holding the engaged workpiece 7 under tension. The cable preferably forms an endless loop, as described in commonly owned U.S. Pat. No. 4,102,280, and is biased by a turbine or similar driving unit allowing the force F to be varied between a lower level during forward motion and a higher level during the return stroke, as likewise described in that prior patent. In principle, however, force F could also be provided by a weight as shown in commonly owned U.S. Pat. No. 4,104,977. 
     Station 50 at the left-hand end of track 15 comprises a block 4 which forms a socket 22 accommodating an extension 10&#39; of carriage 10 when that carriage is in a loading position remote from sewing machine 6. Carriage extension 10&#39; is a flat nipple of substantially rectangular cross-section whose upper surface (as best seen in FIG. 3) is closely overlain in the loading position by the upper wall of socket 22, with interposition of a preferably elastic sealing ring 32 secured to that wall. Ring 32 surrounds the discharge end of a channel 31 that is connected via a coupling 49 to a supply line 19 to which air from a compressor 44 can be admitted by a solenoid valve 43, line 19 being vented through a port 45 in the position of valve 43 shown in FIG. 3. A branch 21 of line 19 extends to a cylinder 20 which is fixedly secured to the block 4 and has a piston 27 with a piston rod 25 projecting into socket 22 from below, opposite sealing ring 32, piston 27 being loaded by a spring 26 seeking to retract it from the socket. 
     The discharge opening of channel 31 confronts in the loading position an entrance aperture of an L-shaped conduit 9 forming part of an air-inlet channel which includes a further conduit 28 separated from conduit 9 by a spring-loaded check valve 11. Conduit 28 communicates, in the loading position illustrated in FIG. 3, with a port 29 by way of an L-shaped passage 28&#39; in the body of a control valve 12 having two positions A and B spaced 90° apart. Port 29 is connected through a tube 13 with the cylinder 33 of jack 1 whose single-acting piston 34 is held against rotation by a radial pin 40 traversing a longitudinal slot 39 in the cylinder wall. An axial bore within piston 34 receives the free end of a shaft 37 which is integral with clamp jaw 3 and rotatably journaled in the opposite end of cylinder 33, being held against axial displacement by a retaining ring or circlip 41. A restoring spring 35 in the bore of piston 34 biases the latter toward the left, resisting its displacement by compressed air admitted via valve 12 and line 13 into cylinder 33. The right-hand end of piston 34 forms a helicoidal ramp face 38 which coacts with a radial pin 36 of shaft 37 and, upon a rightward shift of the piston under air pressure, rotates the shaft together with jaw 3 through about 90° against the force of a torsion spring 42. The mobile jaw 3, upon being thus rotated, closely approaches the jaw 2 rigid with carriage 10 to clamp the workpiece 7 therebetween with a firm grip. Jaw 3 has a hook-shaped end which points downwardly toward jaw 2 in the illustrated release position but is generally parallel to the latter jaw, i.e. substantially horizontal, in the clamping position. Such a jaw has already been disclosed in a commonly owned copending application, Ser. No. 914,143, filed by me jointly with Harald Colbrunn and Heinz Fransing on June 9, 1978. 
     The swing of control valve 12 between its illustrated air-admitting position B and its other position A is controlled by two fixed stops 23, 24 respectively engaging a handle 14 of the valve in the loading position and in an unloading position at the opposite end of the track. When the handle is in position A, channel 28&#39; of valve 12 connects the interior of cylinder 33 via tube 13 and port 29 to an outlet 30 for venting that cylinder to the atmosphere, thereby allowing springs 35 and 42 to return the jaw 3 to its release position. 
     The operation of the system shown in the drawing is as follows: 
     With carriage 10 in its loading position at station 50 and clamp jaws 2, 3 wide open, solenoid valve 43 is briefly energized by means of a manual switch or a photoelectric sensor, not shown, upon the placement of a workpiece 7 on the supporting table with the left-hand end of the workpiece introduced between these jaws. As the shut-off valve 43 shifts into its alternate position, air under pressure enters the channel 31 and simultaneously flows into cylinder 20 to drive the piston 27 upwardly, against the force of spring 26, in order to press the nipple 10&#39; firmly against the sealing ring 32. A leakproof path is thus formed between channels 31 and 9, 28, allowing air under pressure to flow past check valve 11 and through control valve 12 into cylinder 33 for closing the jaws 2, 3 around the workpiece as described above. When the front end of the workpiece is inserted between presser foot 8 and feed 46, the workpiece is advanced toward the right and entrains the clamp 1-3 together with carriage 10 against the restoring force F toward work station 5. By that time the valve 43 has been reclosed so that the tip 25 of piston 27 is withdrawn from nipple 10&#39; and no air escapes from channel 31. Moreover, the volume of compressed air previously admitted into cylinder 33 is trapped by the check valve 11 and the control valve 12 which remains in position B until the carriage 10 reaches the unloading position when the setting member 14 is swung around by its contact with stop 24 to vent the cylinder 33 and reopen the clamp. The release of the workpiece allows the counteracting force F, which may be intensified at this time as described in U.S. Pat. No. 4,102,280, to return the carriage to station 50 where stop 23 restores the illustrated position of control valve 12. It will be apparent that, with piston 27 retracted, nipple 10&#39; can readily enter the socket 22 of block 4 to realign the entrance aperture of passage 9 with the discharge opening of channel 31. The rounded profile of sealing ring 32 also facilitates such re-entry. 
     Any accidental or other movement of handle 14 into position A while the carriage 10 is traveling toward station 50 with the jaw 3 released, as shown in FIGS. 1 and 2, will have not effect upon that jaw since tube 13 will not be pressurized under these conditions. 
     It will be understood that the clamping mechanism according to my invention may also be used for positively displacing a workpiece along a guide path such as track 15 instead of merely tensioning a fabric layer or the like entrained by other transport means such as a sewing-machine feed.