Upper fabric feed device for sewing machines

Abstract

A fabric-transport device for a sewing machine above the stitch plate converts the rotary motion of the main upper drive shaft into reciprocating vertical motion of a pressure (hold-down) foot and a feed foot. The rotary motion is converted by a crank disc connected to a bell crank via an input crank. The bell crank swings a composite connecting rod, comprising a sliding link which interconnects the bell crank with a ternary link carrying the feet and comprising a guide bar for guiding the sliding link. The ternary link is spring-loaded against the stitch plate by a swingable arm and alternatingly lifts the pressure foot and the feed foot. The system provides for fast reduction of the velocity, towards zero velocity, of the feet as they depress to engage with the material to be sewn. SU FIELD OF THE INVENTION This invention relates to a feeding arrangement above the work table or stitch plate of a sewing machine for advancing or feeding of material to be sewn. More particularly, this invention is concerned with improved means for converting and transmitting motive power from the upper main drive shaft of the sewing machine to a pressure (hold-down) foot and feed foot, respectively. BACKGROUND OF THE INVENTION A feeding arrangement, especially suited for high-speed industrial sewing machines, comprising a pressure foot and a feed foot, is known (see U.S. Pat. No. 3,935,826, issued Feb. 3, 1976) in which the feed foot and the hold-down foot are alternatingly brought into feeding or advancing contact with the material to be advanced in the sewing machine. The feet are vertically moved, in reciprocating motion, by means of a bell-crank lever which is eccentrically actuated and the bell-crank lever, in turn, actuates a ternary link. In the system of the patent, a particular transmission arrangement or assembly or linkage system is employed, this system being disposed between the eccentric actuator of the bell crank and the feet engaging the workpiece to be transported or advanced by the feeding mechanism, which will ensure decrease, towards zero, of the velocity of the feet in their respective downward-motion as they reach their lower limiting positions. For the transmission assembly or linkage system, means comprising cam or crank-actuated transmissions or combinations of both types of transmission can be utilized. In practice, the transport of material of varying thickness is of considerable difficulty because such variations will require a constant pressure adjustment by the transporting feet, the attendant linkages and transmission means. Thus the system of the patent comprises four major kinematic elements or components, namely, a ternary link or rocker in the form of a bell-crank lever fulcrummed on a fixed portion of the machine housing and having one arm articulated to a crank arm, a binary link articulated at one end to the other arm of the ternary link, a lever swingably connected at one end to the upper member of a vertically displaceable and spring-loaded frame forming a guide for the presser or hold-down foot (the binary link being connected pivotally to an intermediate location of the latter lever), and a further ternary link of triangular configuration pivotally connected at one vertex to the lower end of the latter lever, pivotally connected at another vertex to the presser or hold-down foot, and pivotally connected at the third vertex to the feed foot. These elements perform primarily swinging or rocking or oscillating motion rather than complete rotations, i.e. rotations of multiples of 360.degree.. This arrangement must be made responsive to variations in thickness of the material to be sewn. Thus, in this prior-art arrangement the vertically shiftable frame is employed to provide for the required adjustment. This will result in difficulties, however, due to limitations in space and due to complexities of the various moving elements in the sewing machine arm. The feed foot is guided in an arm swingable on the machine housing and adapted to laterally displace the feed foot and advance the fabric. Thus, while the presser or hold-down foot only undergoes vertical movement, the feed foot can undergo both vertical and lateral (fabric-feeding) movement. OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide an improved feed mechanism for feeding material in a sewing machine. Another object of the present invention is to provide an assembly for transmitting motive power to a feed foot and pressure foot, respectively, in a sewing machine, which assembly is compact and of relatively light-weight design. Another object of the present invention is to provide a transmission assembly for operating a feed foot and a pressure foot which will be responsive to varying thickness of the material that is to be sewn. Yet another object of the present invention is the provision of a feed mechanism that can be successfully employed at high operating speeds of a sewing machine. Still another object of the present invention is the provision of a feed assembly which is readily installed and serviced. An important object of the invention is to provide a system utilizing advantages of U.S. Pat. No. 3,935,826 but which eliminates disadvantages of the aforementioned frame, both with respect to structural and spatial considerations, and substitutes an improved arrangement enabling adjustment of the device to varying fabric thicknesses. SUMMARY OF THE INVENTION These objects are attained by eliminating the frame of the system of the aforementioned patent and swingably mounting the triangular link or lever, instead, upon a spring-biased arm hinged at its end remote from the triangular link upon the machine housing, the spring acting upon this arm with adjustable force to resist upward displacement of the triangular link. According to a feature of the invention, the driving vertex of the triangular link is connected, by a binary link, to an extensible swing or rocker pivoted to the housing at a first point on one of two relatively slidable members of this rocker. The binary link is articulated to the other member of the composite swing and rocker at a second point while the crank is connected, in turn, to the swing or rocker to actuate the same. The feet are brought to standstill progressively by progressive approach to coincidence of the aforementioned first and second points. In accordance with the present invention, a transmission arrangement is provided for converting the rotary motion of the upper main drive shaft of a sewing machine into the reciprocating vertical motion of the pressure foot and the feed foot, respectively. The rotary motion is initially transmitted, by means of an input or drive crank, to a drive wheel or crank disc which will actuate, in eccentric movement, a bell crank mounted on a fixed pivot. The bell crank thus performs a rocking or swinging motion which is transmitted to the composite connecting link and from thence, by means of an output crank to a ternary link. The ternary link is pressure adjusted by a link having a fixed pivot point which link is maintained under pressure by the force of a compression spring resting with one end centrally on the link. The spring is guided on a guide shaft and terminating with its other end in the upper wall of the sewing machine where provision is made for an adjustment screw for selection of the desired pressure to be exerted on the spring. According to one feature of the invention, therefore, the composite connecting link comprises a guided sliding link and a guide bar for guiding the sliding link which guide bar has a fixed pivot point. The free arm of the bell crank, hereinafter also referred to as the swing arm of the bell crank, is pivotally attached to one end of the guided sliding link, while the other end of the sliding link is pivotally connected to the output crank which connects the system with the ternary link. According to yet another feature of the invention, the guide bar is disposed in said guided link to permit pivotal swinging motion of the sliding link with respect to the pivot point of said guide bar. According to still another feature of the present invention, an equal distance is provided, at zero speed of the transporting feet, by the sum of the effective length of the sliding link plus the length of the swing arm of the bell crank and the sum of the length of the distance from the pivot center of the guide bar to the pivot center of the swing arm with the sliding link plus the length of the swing arm of the bell crank.

Dependencies are inferred from the claim text.