Double-shed Jacquards are used chiefly in weaving heavy goods where a very large and deep shuttle is required to hold a reasonable quantity of weft. The principle of this machine will be easily understood from Fig. 121. A is connected to the crank shaft of the loom and moves the end of the lever BC up and down, the fulcrum of the lever being at E. The bottom board or plate F is therefore moved up and down, and in doing so the griffe G is made to move oppositely, the bottom plate coming down as the griffe goes up, and _vice versâ_. This is effected by the top levers PR and QS, which are centred at O. One end of the griffe is connected to Q, the other end of the griffe is connected to P. This gives firmness and strength to the machine. These Jacquards are usually made very heavy, as they are chiefly for heavy work.
Only a few hooks are shown as an example, but the machines can be made any size. When all the hooks are resting on the bottom board, which will be when the bottom plate is at the top of its stroke, the card is pressed against the needles and selects the hooks to be lifted in the usual manner, after which the griffe rises as the bottom board sinks. Thus an extra deep shed is produced without the griffe having so far to lift as would otherwise be the case. The shed produced is a centre shed, all the ends coming to the centre every pick.
Several open-shed Jacquards have been patented. That of Wilkinson’s is illustrated at Figs. 122 and 123. A and B are a pair of hooks, which are connected by a cord passing round a pulley, W. This pulley works on a pin at one end of the thin plate C, and at the other end of the plate is another pulley, X. The neck cord E passes round this pulley to the bar D, to which it is fastened. It is obvious that when one hook of the pair is lifted, say, 4 inches, and the other is at the bottom, the pulley W will be lifted 2 inches; and as the cord E is fast to D, the harness threads will be lifted 4 inches, the same as the hook.
If one hook of the pair is lifted and it is required to keep the same ends of warp up for the next picks, the hooks being connected round the pulley W, one hook going up as the other comes down will keep the harness cords stationary, and the hooks A and B can be lifted alternately one up, one down, without moving the cord E, which will all the time be keeping the warp ends up. The shed thus obtained is similar to that in a Keighley dobby; the ends, when once they are lifted, stay in that position until they are required to come down. The principle can be applied to either double-lift single-cylinder or double-lift double-cylinder machines.