Cotton Weaving and Designing / 6th Edition

by John T. Taylor

CHAPTER IV
DOBBIES

THE tappet shedding motion is the simplest and most perfect for a small number of shafts. They may be made to work an indefinite number of shafts, but it is seldom that above eight or ten are worked with ordinary tappets, and about sixteen with Woodcroft’s or other plate tappets.

With dobbies, a higher number of shafts may conveniently be worked, but it is not only for this reason that dobbies are so extensively used. They are extensively used for weaving twills, satins, and other simple weaves, on four or five shafts. The chief advantage they possess is that any number of shafts within their capacity may be used without extra trouble or cost; whereas ordinary tappets have to be made specially for each pattern; whilst section tappets, and oscillating tappets, are inconvenient.

Dobbies are made to weave up to 40 or more shafts, but 16 and 20 are the commonest numbers. Most dobbies now used are on the double-lift principle; indeed, the single-lift dobby or witch machine is almost obsolete in cotton weaving. The chief kind of double-lift dobby is the “Hattersley” or “Keighley” dobby. The principle of this machine was invented by Messrs. Hattersley & Hill, of Keighley, Yorkshire; hence its name. Since the original patent rights have expired, almost all loom makers have their own particular form of this dobby, embodying many more or less minor improvements on the original. The principle of this dobby will be understood from the lecture diagram, [Fig. 91]. The dobby is placed at one side of the loom, and is therefore in a convenient position for being attended to. The upright rod R is connected to a crank on the bottom shaft of the loom, and therefore the rocking lever AB, centred at C, will make one complete movement to and fro, every two picks. The knives D and E slide along, always retaining a horizontal position, one going inward as the other comes outward.

FIG. 91.

The shaft or stave is connected to the jack lever FGH at F, and the upright MN is fastened to this lever at H, the fulcrum being at G. The upright MN has two hooks, P and Q, connected to it at opposite ends, and suppose that when the knife D is in its innermost position, as in the diagram, the hook P is dropped on to the knife; when the knife begins to move it will take the top of the upright MN with it until MN assumes the position indicated by the dotted line M′N, and the stave is lifted. If it is required to lift the same stave for the succeeding pick, the bottom hook is then dropped on to the knife E, which at that moment will be in its innermost position just commencing its outward movement, and is taken forward by it until the upright MN assumes the position indicated by the dotted line MN′; and it will easily be seen that as the top of the upright is moving back from M′ to M whilst the bottom of the upright is moving forward from N to N′, the centre of the upright H remains stationary at H′, with the exception of a slight movement caused by the knife going further back than the hooks, and thus the stave remains up all the time. The character of the shed is, therefore, what is termed “open shed”—that is, if a stave is required up for several picks in succession, when it is lifted it remains up until it is required to come down again. This is what is meant by “open shed” as compared with “centre shed,” the characteristic of which is that the lifted stave, instead of remaining up, is let down halfway every pick and taken up again if required.

The method of dropping the hooks is as follows:—Two levers, S, T, of different shapes are employed for each pair of hooks; these levers are centred on a rod, X. One of the levers, viz. T, is bent from the fulcrum to touch the bottom hook, and the lever S projects straight out from the fulcrum, and an upright needle O rests upon it, the top hook resting upon the upright needle. When the lever SY is lifted a little at Y it will drop the top hook, and when TY is lifted at Y it will drop the bottom hook.

In a 16 shaft dobby the parts shown in the diagram are duplicated sixteen times—that is, there are sixteen uprights MN, each with two hooks, sixteen levers SY, sixteen like TY, and sixteen of other parts. The levers SY and TY are operated by lags pegged so as to lift the staves to give the required pattern. These lags work round a cylinder or barrel, which is turned round the space of one lag every two picks intermittently. Each lag operates the hooks for two picks, one row of pegs operates the top hooks P, and the other row of pegs the bottom hooks Q. The method of pegging the lags will be understood from [Fig. 92], where two lags are shown with the pegging for a two and two twill. Of course care must always be taken that the pegs are put opposite the proper levers, as when only a portion of the jacks are used, say eight, it is often preferred that the staves be connected to eight jacks in the middle of the machine.