Wire Knitters Have It 'Sewn Up' With Yaskawa

Knitting with wire is a tricky enough process, but if the variable speed drives that control the machine perform anything less than perfectly, the task can be infuriating. Wires as thin as 0.003 inches can be used for the knitting and any snatch on acceleration or delay on soft stopping of the machine results in breakages and downtime while the machine must be rethreaded.

Leicestershire based Tritex Design & Development Limited produces machines to knit fine wire into circular tubes from 0.2 to 30 inch diameter. These are used on a whole range of industrial applications from reusable filters to exhaust seals, chemical demister pads to aircraft air conditioning filters. The company selected Yaskawa VS Mini J7 single phase inverters to control its motors. Motor sizes range from 0.4kW to 1.5kW, according to machine type.
The Yaskawa drives were selected for their exceptional starting torque which provides the smooth acceleration ramps required to ensure the wire is not stressed on start up. The same drive also has an excellent soft stopping capability enabling rapid stops to be made in less than 0.5 seconds. This has productivity benefits as well as ensuring the machine runs smoothly and fault free.

"The machine must stop without snapping the material," explained Kevin Roberts, Tritex Director. "If it does break, the machine must stop rapidly to prevent the loose end of the wire entering the main part of the web. If a hole is created in the knitted mesh, the knitting process has to be re-threaded and restarted," Mr Roberts concluded.

On machine set up, the Yaskawa VS Mini J7's jog function is used to slowly increment the knitting machine at 5 Hz to allow the machine to be threaded. Knitting head speeds vary dependant on the machine diameter up to 2000 rev/min down to almost zero speed, depending on the material used and line production speed. Depending on the customer's requirements, the machine can have between 3 and 2640 needles arranged on the periphery of the knitting head, using wire between 0.003 and 0.020 inch diameter. Wire materials can be anything from stainless steel for certain filter applications to platinum that is notoriously difficult to knit with.

The Yaskawa VS Mini J7 is a remarkably compact AC inverter that can be used with almost any AC motor. The 0.4kW version used by Tritex measures just 128mm in height yet delivers 150% at 3Hz or less. A high-speed current limiting function minimises the possibility of over-current trips and provides inrush current suppression. The inverter is simple to set up using an intuitive menu of commands and a speed control knob on the front of the drive.

Wire knitting is widely used in the manufacture of high temperature supports for precious metal catalytic converters, explosion container collars for automobile/car air bags, reusable filters for crankcase breathers, flexible high temperature exhaust seals, small demister pads for the chemical industry, anti-vibration mountings, industrial and domestic pot scourers, aircraft air conditioning filters and noise insulation pads.

Released: October 2002