On circular knitting machines, higher productivity involves faster needle movements as a result of an increase in the number of knitting feeds and of machine rotational speeds. On fabric machines, the machine revolutions per minute have almost doubled and the number of feeders have increased twelve-fold over the past 25 years, so that as many as 4000 courses per minute can be knitted on some plain machines, whilst on some high-speed seamless hose machines the tangential speed of the needles can be more than 5 metres per second.
To achieve this productivity, research and development has been necessary into machine, cam and needle design. The horizontal cam track sections have been reduced to a minimum whilst needle hooks and latches have been reduced in size wherever possible in order to reduce the extent of the needle movement between the clearing and knock-over points.
'Needle bounce' is a major problem in high speed knitting. This is caused by the needle butt being suddenly checked by the impact of hitting the upper surface of the up-throw cam after it has accelerated away from the lowest point of the stitch cam. At this moment, inertia at the needle head may cause it to vibrate so violently that it may fracture; also the up-throw cam becomes pitted in this section. Needles passing though in the miss section are particularly affected as their butts contact the lowest part of the cam only and at a sharp angle that accelerates them downwards very rapidly. To reduce this effect, a separate cam is often used to guide these butts at a more gradual angle. The smoother profiles of non-linear camming help to reduce needle bounce and a braking effect is achieved on the butts by keeping the gap between the stitch and upthrow cams to a minimum. For this reason, on some hose machines the up-throw cam is horizontally-adjustable in conjunction with the vertically-adjustable stitch cam.
The Reutlingen Institute of Technology has carried out a considerable amount of research into this problem and, as a result, a new design of latch needle with a meander-shape stem, a low smooth profile, and a shorter hook is now manufactured by Groz-Beckert for high-speed circular machines. The meander shape assists in the dissipation of the impact shock before it reaches the needle head, whose shape improves resistance to stress, as does the low profile, whilst the gently-shaped latch is designed to open more slowly and fully onto a cushioned position produced by a double saw cut.
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