Knitting technology, Third Edition

22.8
Needle bounce and high-speed knitting
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
Fig. 22.3
Model of weft-knitted loop formation indicating the mechanism of ‘robbing-back’
and the build-up in yarn tensions acting on the needles.

---

284
Knitting technology
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.