This invention relates in general to sewing machines and in particular to a new and useful drive mechanism for connecting a drive shaft of a sewing machine to one or more of a plurality of reciprocatable needle bars.
For sewing, embroidering or so-called tufting machines it is necessary to temporarily stop the needle bar or needle bars in reciprocation. For example, when sewing the corners of shirt collars on a two-needle sewing machine, one needle must be disconnected while continuing to sew with the second needle to the seam corner before the shirt collar is turned into the new sewing direction, around the inserted second needle. For obtaining seam interruptions, all needles must be disconnected. For shifting the embroidery frame from one pattern area to the next, also all needles of an embroidery machine must be disconnected, and in tufting machines, groups of needles are temporarily stopped to obtain tuft or color effects.
For the purpose of disconnection, the interposition of a clutch between the needle bar(s) and the drive is known (German Pat. No. 937,504, U.S. Pat. No. 2,868,152). If necessary, the needle bar is simply separated from its drive and retained in its upper dead center by a locking device. Since machines with clutch devices to be moved along have larger moved masses than machines without these disconnecting devices, and since the disconnecting occurs abruptly while the machine is running, the rotational speed of the machine is restricted to quite low limits if the inertia forces are not to become too high and if severe impacts and vibrations, or even breakage, of transmission parts are to be avoided.
Also very often the detachment of the needle bar from its drive causes errors in stitch formation after the needle bar is connected in again, and this leads, when sewing a corner on a shirt collar, to a deviation in the first stitch after the collar has been turned into the next sewing direction.
To avoid stitch errors after the needle bar has been reconnected, to obtain approximately the same quiet running of the machine with the needle bars connected or disconnected, to prevent strong impacts and vibrations as the needle bars are being separated from their drive, and hence to increase the rotational speed of the machine, there is known from German Pat. No. 1,045,756, (U.S. Pat. No. 2,824,532) an articulated drive to be switched on and off for a needle bar where the crank pin of the usual arm shaft crank is connected with the needle bar drive element by a link pair. The joint of the link pair is articulatedly connected by a connecting rod with a supporting link pivotable about an axle fast to the housing. For switching the articulated drive a double lever is used which is pivotable via an intermittent switching mechanism about a bolt fast to the housing and which comprises two lock teeth, of which the one snaps in the on position into a groove in a slide-piece and the other in the off position into a groove in the needle bar. In the on position, the drive movement is transmitted to the needle bar, and in the off position, it is transmitted to the connecting rod and the supporting link. Although one achieves thereby a relatively quiet run of the machine, the needle bar must, in this arrangement of the links, be mounted in slideways at the upper and lower ends, to be able to absorb the lateral forces acting on it. Because the links participate in the full needle bar stroke, the crank and needle bar pins are exposed to strong inertia forces, owing to which the rotational speed increase attainable as compared with the prior art drives with clutch between the needle bar and its drive is moderate. For modern high-speed sewing machines the arrangement known through German Pat. No. 1,045,756 is not suitable.
While in this device the stroke of the needle bar could be varied, this is not provided for in the patent. Besides, the needle bar lock in the disconnected position would then have to be eliminated. But the needle bar would then not stand still completely. Lastly it would be possible with this link arrangement to bring the needle bar always into the upper dead center of the maximum stroke for disconnection when sewing with a reduced stroke.