Dobby Machines

DOBBY MACHINES

These are exceedingly useful machines for forming the shed in weaving, since they can be used for both simple and complicated weaves. There are many types in use, most of which are negative acting in so far as they only lift the healds, springs being used beneath the healds to bring them down again after being lifted by the dobby. In the cotton trade 16 up 20 jacks is usual. Dobbies in common use are known as single lift, double lift, negative, positive, open shed, closed shed, and crossborder. Single Lift - In this type there is a single knife or griffe in use to raise the heald stave. The whole of the shafts return to their original position after each pick. A fresh selection of staves to be raised is made for each pick. Looms fitted with this dobby run slower than others, about 140 picks per minute. Double Lift - These machines are fitted with double selecting and lifting parts which move at half the speed of the loom. They give an open or semi-open shed. The speed of the loom is considerably more than for the single-lift type. Crossborder - This machine is used when headings or a change of weave is required as for bordered handkerchiefs, serviettes, towels, etc. Positive - Dobby machines which make an open shed and positively lift and depress the heald staves as required by the design. Negative - Dobbies which only lift the heald staves, and require springs or other means to move the staves to the bottom position. Centre Shed - Every thread of the warp is moved for every new shed. The shed opens from the middle. Some healds ascend and the others descend. Closed Shed - So termed because all the warp threads are brought to one level after each succeeding pick as in single-lift machines. Open Shed - The type generally used for automatic looms, also the double-lift machines. After a heald stave is lifted it remains up until it is required to be down again. The warp threads constantly form two lines, upper and lower, and the only changes are when threads move from line to the other. Semi-open Shed - This shed has a stationary bottom line, and to make changes, threads pass from the top to the bottom, or from the bottom to the top. The threads which remain up for more than one pick in succession only fall halfway and then go to the top again.