The smoother and slower the harness can be raised and lowered, the better. Therefore, to get a high working speed, the time or the portion of a revolution of the crank shaft given to the rise and fall must be as great as possible, so that very little could be gained by using a tappet. If a tappet is to be used, a box tappet will be required, or is more satisfactory for a single-acting machine, in order to make the griffe in falling follow the tappet and avoid any plucking or jerking; sometimes a fork lever, with the tappet or wiper working between the prongs of the fork, is used for the same purpose. For a double-acting machine double wiper tappets, acting on levers or treadles, are sometimes used, the griffes falling of their own weight; the tappets are nearly round eccentrics, or like plain tappets with a very short dwell, not more than one-fourth of a revolution of the crank shaft. It may therefore be considered that the harness should always be moving either up or down, with a small pause when the griffes are at the top and bottom, to admit of an easy turn and to allow the shuttle time for its passage through the shed. With the crank drive the shed will require to be opened a little wider than if the dwell was as great as it should be, especially in wide looms, in order to let the shuttle get through freely; but it would be more desirable, and a saving of strain on the yarn, to have the dwell greater, and not open the shed any wider than is necessary to admit the shuttle.
Speaking generally, the usual rule for the time of shedding may be said to be to let the shed be closed when the cranks of the loom are at the top centre, or perhaps one-sixteenth of a revolution farther forward, and let the shed be full open when the cranks are about the bottom centre. It therefore follows that the single-acting jacquard must open the shed in a little less than half a revolution of the crank shaft, and close it in the same time; but the double-acting machine takes nearly a full revolution (three-quarters, or a little more, should do) to either rise or let fall one of the griffes. It will be evident that there is a considerable difference in the nature of the shedding. A single-acting machine requires the weft to be beaten up on a closed shed (the time of the crank in coming from the top to the front centre being taken up with rising the griffe from the bottom up to catch the hooks), whereas in a double-acting machine the case is different; though the shed is closed at the same place, or in the same position of the cranks, the griffes are in an entirely different position. The closed shed with the single-acting jacquard is when the griffe is down, but with the double-acting machine it is when the two griffes are on a level--that is, halfway up. Of course, more or less of the yarn may be at the bottom position, so that there may be no closed shed, or in no position of the griffes may the yarn be all on a level, unless none of the hooks are on either of the griffes, or if one griffe has all the hooks on it. When the single-acting machine begins to open the shed, the driving-crank of the griffe is upright or at the top centre, and in the position to give the slowest motion to the griffe; whereas for the double machine the lifting cranks are horizontal when the shed is closed, and in the position to give the griffes the quickest motion; therefore, when the lay gets to the fell of the cloth in a single-acting machine, the shed is still close, whereas with a double-acting machine it is fully half open. From this it follows that, to get a close covered cloth, the double-acting machine is the better, as the weft is beaten up in a crossed shed; but to get a clean-surfaced fabric, with the weft lying straight between the two portions of the warp, the single-acting jacquard is better. It is for this reason that the single-acting machine is preferred by many workmen for making fine damask, which does not require much covering to give it a good appearance, and there is less chance of having cut weft and of looping when striking on the open shed. Of course the weft may be cut in a hard fabric by the reed having to strike too heavily against it, and in this case striking on a crossed shed might prevent the cutting, by the weft going on easier.