The double rotary hook with bobbin case was designed by Max Karl Gritzner in 1879. Two rotations of the hook form a complete stitch, whereby the hook picks up the supplied upper thread, guides the bobbin case with the bobbin and lower thread, and is then released. The rotary hook then performs another empty movement until the upper thread is supplied again for the next complete stitch. With the vertical double rotary hook today's industrial high-speed sewing machines can achieve up to 5000 stitches per minute, and with the horizontal double rotary hook up to 7000 stitches per minute with high running smoothness. In addition, the system is not only more expensive to manufacture than the central bobbin hook, but also more complex to maintain. Precise mechanical knowledge is required for disassembly and maintenance, as the timing of the double rotary hook must be readjusted after assembly. Similar to the central bobbin hook, the mechanics of the double rotary hook are robust and low-maintenance. However, regular cleaning and light lubrication with oil should not be neglected.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 213