Every time I use a sewing machine, it’s for some really thick or tough material that requires a lot of punching power, and is easier to sew at a slow speed. Most machines run a little too fast for my purposes, and don’t develop enough power at slow speeds. They rely on speed by building up inertia in the flywheel to provide the necessary power to sew through thicker fabric. In order to lower the speed and increase the power, the motor needs to be geared down. Since most old sewing machines use a belt, this is not difficult to do using a double pulley system. My sewing machine is a 1950s Nechi BU with zigzag stitch.
Overall, this works pretty well. The only downside is due to the round rubber belts that are used as standard sewing machine belts. These belts are very stretchy, and the drive feels kind of mushy when you are sewing. It is also difficult to reverse the machine using the handwheel, in other words, it doesn’t back drive very well. I need to find belts that have less stretch, more similar to automotive v belts, or timing belts. Another option would be to use a chain drive from the motor to the large pulley on the double pulley, and then a belt from the small pulley to the sewing machine. The ideal power transmission would be to use timing belt drives, then there would be no stretch. The downside is that you want the belt to be able to slip/stretch/break if the sewing machine gets jammed. If the belt is not the weakpoint, then something else in the sewing machine will break.
New double pulley system (drive belt not shown)
A secondary double pulley is required to link the two belts and create the speed reduction. The first thing to do is create a mounting bracket for the pulleys. This bracket attaches to the sewing machine using the same mounting screws as the motor. I created slots in the mount to be able to adjust the belt tension between the motor.
The following piece is pivoted on the first, and permits the second belt going between the double pulley and sewing machine pulley to be tensioned.
The double pulley is machined as two parts, the small and large pulley independently. Both pulleys have brass bushings pressed into them. They are connected together by two pins crossed drilled into the pulleys. A shoulder screw acts as the pulley axle.
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