Circular versus flat machines

On the basis of knitted stitches per minute against the capital cost of the machine, circular garment-length machines are generally more productive than V-bed flat machines for cut-and-sew knitwear. Prior to computer controls, the price/performance ratio was 1:3 in favour of body-width circular machines. Against electronic V-bed flat machines, however, circular machine builders had to move to less versatile large-diameter machines (33-36 inches) in order to achieve a ratio of even 1.2:1. There are large numbers of body-width RTR and SPJ mechanically-controlled machines still in operation, as well as some that have been retro-fitted with electronic controls.

Circular garment-length machines are mainly of the rib cylinder and dial type (Fig. 20.1) or of the double-cylinder purl type. Although more restricted in patterning capabilities than flat machines, they may offer advantages in productivity and fineness of gauge.

Many are of the revolving cam-box type whose cams, selection units and striper units are altered when their externally positioned levers are contacted as they pass by the control position on the periphery of the machine (Fig. 20.2).

The peg drum control unit for the garment-length programme is now tending to be replaced by an endless film loop that is driven by a horizontal perforated roller. The film is advanced by one row of holes for each feed or transfer section that passes per cam-box revolution. When no changes are required, an economiser rack-wheel operates.

On Bentley machines, the Mechatape Pattern Control Unit was introduced to replace peg drums or trick-wheels and provide a virtually unlimited pattern depth, faster running speeds, easier pattern preparation and more rapid pattern changes. The control unit consists of a drum whose perforations correspond to the staggered rows of punched hole positions on a plastic film loop. Each row operates through the bank of horizontal levers onto the levers of a passing selection unit. The arrangement in the selection unit is fixed for a complete circuit of the machine whilst it selects onto the jack pressers arranged around the cylinder.

Bentley Spj Circular
Fig. 20.1 RTR circular garment length revolving cam-box rib machine. The peg drum control unit and timing chain are clearly visible. Also note the slipping belt take-down mechanism which draws down the stationary fabric [Walter Bullwer].

The fabric take-down mechanism cannot be driven directly by the machine rotation as the length of fabric knitted per machine revolution can vary in different parts of the garment sequence. The slipping-belt system is an efficient arrangement that accommodates itself to the varying rates of fabric production.

The take-down rollers and the belt pulley that drives them via worm gearing are

Fig. 20.2 Close-up of RTR revolving cam-box showing the exterior striking levers (A = striper box; B = dial stitch cam adjustment and levers; C = cylinder stitch cam and adjustment levers; D = stationary striking lever post) [Walter Bullwer].

attached to a pivoted lever. The rollers are driven faster than the rate of knitting so that, as soon as the surplus fabric has been drawn away, they tend to climb up the fabric, lifting the pivoted lever together with the belt pulley so that the belt becomes slack, stopping the drive to the rollers until sufficient fabric has been knitted to lower the lever again. This self-adjustment occurs so smoothly that a consistent take-down tension is ensured.

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