Knitting technology, Third Edition

22.3
Warp let-off
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, releas-
ing 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.

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Knitting technology
In the second arrangement, the warp beams are positively driven to deliver a pre-
determined run-in. The surface speed is monitored so that, as the beam circumfer-
ence 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 knit-
ting 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 600 cpm) that are knitting fabrics from full-sized beams. The fric-
tion 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 com-
pletely 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 side-
ways, 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 cir-
cumference of the lower (driver) cone transmitting to the largest circumference of