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.

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.

Dobbies constructed with single jacks, as indicated in Fig. 91, are only suitable for narrow looms. Those constructed with double jacks are preferable for wider looms, as they not only keep the healds under better command, but they also move them in a perfectly vertical plane without the tendency to a slight side movement such as occurs when the healds are controlled by single jacks, in consequence of the ends of these describing an arc of a circle as they rise and fall. One of the best adaptations of double jacks to the Keighley type of dobby is that exemplified in the “Climax” dobby made by Lupton and Place, Burnley. This is represented in Fig. 93, in which A and B are complementary jack-levers operated from the same baulk-lever, J, controlled by hooks P and Q, to govern the same heald. The distinctive features of this dobby are the construction of the outer jack-lever A in one part, instead of two parts, and its attachment with the inner jack, B, by means of a link, C. This modification is a great improvement on double-jack dobbies in which the connections are made with streamer hooks or rods, or those in which the jacks are geared by means of toothed segments, as these increase the number of parts that are liable to wear and to get out of order.

The Keighley dobby is decidedly the most popular one at the present time, but what is known as the “Blackburn” dobby is preferred by some. This is a double-lift dobby, which gives a centre shed—that is, the staves which are required up for a number of picks in succession are let down halfway every pick and taken up again. The principle of this dobby is illustrated at Fig. 94. The staves are lifted by the two jacks A and B; when B is lifted it causes A to lift the same distance. There are two hooks, D and F, for each double jack, and the lags are divided into two parts, all the odd numbered picks being fastened together, and the even picks forming another chain. The pegs in the lags press back the hooks, the back part of each of which forms a spring, so that when the hook is pressed back it leaves the stave down.

The knives lift alternately. When one is going up the other is going down, and when one hook of a pair is lifted, as in the diagram, a lag operates the other hook, and if the same stave has to be lifted for the next pick, the hook is left over the knife, and the second hook will be taken up whilst the stave is being let down, and will catch it halfway and take it up to the top again. This is the advantage of all double-lift machines over single-lift. The staves which are required up for a certain pick are being taken up whilst those which were up for the previous are coming down. A saving of time is thus effected, and the looms can be run quicker than with single-lift machines. A crank L on the end of the bottom, or picking, shaft is connected by means of a lifting-rod E to the end of a horizontal arm M, mounted on a shaft G, which constitutes a fulcrum for the arm. On one end of shaft G, at the rear of the machine, there is fixed a toothed quadrant H, which, through the medium of a small wheel, transmits motion to a similar toothed quadrant H' fixed on the end of a shaft G'; and each quadrant is connected, by means of rods K and K' to the rear ends of the respective griffe-bars D and F. On the opposite ends of the shafts G and G', there are fixed, one on each shaft, two short arms which are respectively connected to the fore ends of the griffe-bars. Therefore, as the crank L revolves, the griffes are raised and depressed alternately, and in a contrary manner.

Another thing to be borne in mind is that in a single-lift machine all the staves come to the bottom every pick, and therefore the character of the shed is different from that of a double-lift. In double-lift machines there are the “open-shed” like the Keighley dobby, and the “centre shed” like the Blackburn dobby. It is important to remember these points, as the cover and appearance of the cloth is affected by the beating-up being done in different kinds of sheds.

The loom crank is usually set at the top centre, or thereabouts, when the rising and falling staves are level, so that the shed will be partly open for the next pick by the time the loom crank gets to the front centre. In single-lift dobbies the beat-up is made when the shed is closed, and so the warp has not the same chance of being spread as with the timing of double-lift dobbies. This difference in the character of the shed when the beat-up is made is caused by the fact that in a double-lift machine the knives, being in the middle of their stroke, are moving at their quickest speed when the shed is closed, and in a single-lift the knife is almost stationary when the shed is closed. The same thing occurs in Jacquards, and the matter may be better understood by a reference to the chapter on Jacquards.

Dobbies can be made “positive” in various ways. Keighley dobbies are made with a pin fixed on the upright MN (Fig. 91) at the point H. A wire is hooked on to this pin and connected to an L lever at the side of the loom opposite the dobby; this is connected to a lever at the bottom of the loom. By connecting the bottom of one stave to this lever the stave will be pulled down as the upright MN is taken forward, and so the knife whilst taken one stave up is pulling another down, rendering the dobby positive. This, of course, will only do for certain simple patterns, such as twills, satins, and similar weaves, and would not do for patterns where different numbers of staves are lifted every pick. Positive dobbies are not much used in the cotton trade.

The ordinary form of dobby is non-positive, the stave being kept down by springs in some form or other. One reason which may be urged against ordinary springs, or “jack boxes,” is that the pull on the heald increases as the stave is lifted and the spring opened. It is obvious that this is just the reverse of what is required, as the stave is lifted positively, and the pull on it may therefore conveniently be decreased as it is lifted, and the healds would last longer. The use of the spring is to keep the stave down, and therefore it should exert its greatest force when the stave is at the bottom. A simple method of accomplishing this has long been in use. Something on the same principle has been used on hand looms for generations, and very cheap and convenient undermotions of this kind have long been available for power looms; but, strange to say, cotton manufacturers have been very slow at adopting them. An undermotion of this kind is illustrated at Fig. 96. The spring is fixed at A, and a wire hook connects the spring with the quadrant at B. It will easily be seen that as the stave lifts, the direction of the pull of the spring is gradually moved over the centre of the quadrant at C. If the stave were lifted until the spring was in a direct line from A to C, the pull on the stave would be nil, as all the force would be exerted on the fulcrum. Each stave is connected at both sides in the same manner, the springs and other parts are all arranged in a very compact manner, and the cost is very small.

Another form of undermotion on the same principle is much used in Yorkshire in the woollen and worsted trades. This is illustrated at Fig. 97, and is known as Kenyon’s undermotion. In this the springs are arranged horizontally, and therefore longer springs can be used. The quadrant is centred at C, and a strap is fast to the quadrant at D. The spring is connected with the quadrant at F. The strap passes from the quadrant under the bowl B, and then to the stave. Another quadrant serves in the same manner for the opposite side. The spring is fastened to a bar at E, and as the stave is lifted, the pull of the spring is gradually moved over the centre C, and therefore the pull on the heald gradually decreases as it is lifted.