FIG. 92.
FIG. 93.
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.
FIG. 94.
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.
