Loop length is equally as important in warp knitting as in weft knitting. In the form of run-in, it is determined by the warp let-off which is either negative or positive. In the first arrangement, tension on the warp causes it to be pulled from the beam as it turns against a controlled friction. The mechanism is self-compensating, releasing warp on demand. An overall increase of run-in is obtained by increasing the speed of the fabric take-up rollers, which increases the tension.
In the second arrangement, the warp beams are positively driven to deliver a predetermined run-in. The surface speed is monitored so that, as the beam circumference decreases, the beam drive speed is increased to maintain a uniform rate of let-off. The arrangement must also be capable of catering for fluctuating let-off requirements in patterned fabrics. Tension fluctuations that occur during the knitting cycle are compensated by spring-loaded tension bars over which each warp sheet passes in its path to its guide bar.
On multi-guide bar raschel and tricot lace machines, the spot beams that supply the partly-threaded pattern guide bars are completely negatively turned. These light-weight beams turn easily and have a three-spoked star attached to one end on which small weights are placed and positioned in order to ensure balanced rotation. At the other end, weights attached to a collar provide controlled friction.
An intermittent negative-brake-type let-off may be employed on slow speed machines (below 600cpm) that are knitting fabrics from full-sized beams. The friction of a belt brake restrains the beam rotation until the warp tension is sufficient to cause the tension bar to be lowered, which in turn lifts the belt, allowing the beam to turn freely.
On high-speed raschel and tricot machines, the lightweight tension rails are completely separate and can oscillate rapidly at high knitting speeds. Each warp beam shaft has a separate positive drive and warp-speed-to-machine-speed adjustment arrangement (Fig. 22.2). A machine-driven 'nut' and a warp driven 'bolt' are 'fast screwed' together, so that when the bolt turns at a different speed it moves sideways, moving a steel ring sideways as it transmits the drive between two opposed cones (3). The slowest beam speed is achieved with the ring on the smallest circumference of the lower (driver) cone transmitting to the largest circumference of
the higher (driven) cone. As the warp beam decreases, the ring is gradually progressed towards the largest circumference of the lower cone. The upper cone shaft drives a vertical shaft through bevel gearing and its worm (4), then drives the worm wheel of the warp beam shaft.
The ring can adjust in either direction, controlled by a two-way rackwheel, dependent upon either one of two side racking pawls on a slide (7) that is moved by the 'bolt'. The bolt spindle shaft (6) is driven by a belt (5), from a metering roller driven by contact with the warp of the beam surface. The 'nut' shaft is driven at a constant machine speed by the lower cone shaft.
A change gear system (1), positioned between the main machine drive shaft and the lower cone shaft, enables the gearing to be altered to produce different run-in rates. A clutch arrangement can be employed to alter the warp drive speed, if required for patterning purposes, or to disconnect the drive in interrupted warp let-off sequences.
Karl Mayer have now developed a computer control unit that, from fabric parameters inputted via a keyboard, automatically regulates the warp let-off of the machine. The computer receives the machine data pulses from encoder emitters on the warp beam shafts and the main machine shaft. Control data computed by the system is then transmitted as pulses to the individual warp beams to drive a series-wound d.c. motor and worm gearing .
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