The main body and core of sewing equipment. Taking a flatbed sewing machine as an example, no matter how complex or precise its mechanism is, as long as it uses a needle and thread to sew fabric, it cannot function without the combined movement of four basic mechanisms: the needle bar mechanism, the hook mechanism, the thread take-up mechanism, and the feed mechanism.
Among these four mechanisms, the mechanism that transports, retrieves, and tightens the needle thread is called the thread take-up mechanism. In each sewing cycle, the thread take-up lever is responsible for taking in and releasing the top thread and, in conjunction with the fabric feed mechanism, forming the sewing stitch. The thread take-up spring, thread clamp, and thread deflector all belong to the thread take-up mechanism. Properly adjusting the tension and take-up accessories is crucial for maintaining good stitches.
The working characteristic of the thread take-up mechanism lies in the up-and-down movement of the thread take-up lever hole. When the thread take-up lever moves downwards, it supplies thread to the descending needle and, when the rotary hook catches the top thread, supplies more top thread to the rotary hook, until the top thread passes over the bobbin frame. When the take-up lever moves upward, it is responsible for both removing the top thread from the shuttle and tightening the existing stitch in the fabric to form a firm stitch, and also for pulling the top thread out of the spool to prepare for the formation of the next stitch.
Thread take-up mechanisms are classified as follows:
1. Cam take-up mechanism: A mechanism in which a cam drives the take-up lever.
2. Linkage take-up mechanism: A mechanism in which a four-bar linkage drives the take-up lever. The take-up lever is the link in the four-bar linkage.
3. Sliding lever take-up mechanism: A mechanism in which a crank-sliding lever mechanism drives the take-up lever. The take-up lever is the guide rod in the four-bar linkage.
4. Rotary take-up mechanism: A mechanism in which one or two discs or other shaped components equipped with take-up pins rotate to take up the thread.
5. Needle bar take-up mechanism: A thread guide or clamping device fixed to the needle bar, or a take-up lever mechanism directly fixed to the needle bar.
In industrial sewing machines, the most common thread take-up mechanisms are linkage-based and needle bar-based. There are two methods: one is to tighten the previous stitch before needle insertion; the other is to tighten the thread using the tension device of the take-up device during needle retraction. Overlock or coverstitch machines (including double-thread chain sewing machines) use the first tightening method, while flatbed sewing machines use the second. The thread needed for the second needle insertion comes from two sources: one is obtained during tightening the previous stitch, and the other is drawn from the spool after overcoming the pressure of the tension regulator. Generally, the stroke and trajectory of the take-up lever are relatively fixed. Due to its subordinate position, the relevant dimensions of the mechanism are unlikely to be adjusted, and manufacturing and installation should strictly adhere to positioning requirements.
Initially, flatbed sewing machine thread take-up mechanisms included cam mechanisms and linkage mechanisms. Because linkage mechanisms have lower-pair connections, they offer many advantages, especially reducing thread breakage. Therefore, current industrial flatbed sewing machines all use linkage-based thread take-up mechanisms. The four-bar linkage thread take-up mechanism in current industrial sewing machines primarily functions to prevent the feed dogs from stretching the thread and loosening it as they advance the fabric. This minimizes interference when the needle pierces the fabric, and allows for proper adjustment of the stitch length according to the feeding and tightening process, ensuring the correct position of the needle force.
Based on the working principle of sewing machines, there are strict timing requirements for the coordination of the thread take-up lever, needle, and shuttle. The thread take-up lever, in particular, requires highly accurate and coordinated movement with the other mechanisms of the sewing machine. Otherwise, it can lead to problems such as skipped stitches and thread breakage, or even complete malfunction. Therefore, the manufacturing process of the thread take-up lever requires high precision.