Bobbin Thread Limitations

[Ogoki]

What exactly limits bobbin thread weight?  I realize that some machines are “meant” for heavier work, but which aspects of the machine explain this in terms of thread capabilities?  My twin-needle post-bed and flat-bed have no problems with heavy bobbin threads, but my little Singer 95K43 seems pretty limited.  By that I mean that it is difficult to get proper tension with heavier bobbin thread: by the time I get enough needle tension to pull the bobbin thread taught, the lock is appearing on the top of the fabric; also, the needle thread doesn’t look as nice as it does with a thinner bobbin thread; and, it seems to be finicky and require frequent adjusting.

I assume this is normal when trying to run a bobbin thread that is too heavy for the machine, but what exactly is the difference in machines that accounts for this?  I don't mean this as a question about my 95K, but as a general question about bobbin thread capabilities.

Thanks for any musings...

[Wizcrafts]

The dimensions, type of motion and clearances between the shuttle driver and shuttle, the bobbin thread slot and tension spring configuration, the needle hole in the feed dog or throat plate, the take-up lever stroke, needle stroke, crank shaft thicknesses, bearings, thread tension springs, foot pressure spring, check spring and needle size are a few things that can limit the thread size a sewing machine can properly manage.

Knots that are hard to hide or move up and down are caused by excessively large thread for the thickness of the material, or by hard to optimize thread tensions (because the machine was not built for that thread size combination). Knots always on the top are due to excessive top tension (from over-tightened upper tension disks, or thread binding off the spool or along the thread path), or a bobbin tension spring that is set too loose, or too large of a needle for the thread sizes, or a much larger thread size on top than in the bobbin. Thread knots on the bottom are from an overly tight bobbin spring, or loose top tension disks, or thread riding too high up in the upper disks, or failure to thread the check spring correctly, or too small of a needle, or much larger bobbin thread than top thread.

The 95k43 is a tailors' and seamstresses' machine, with a shuttle and bobbin that was designed to clear and tension no more than #69 bonded thread. Actually, there was no bonded nylon thread when this series came out. It is pushing it to sew with cotton or polycore button hole thread and is best used with common garment thread sizes. The clearance between the shuttle driver and shuttle will probably pinch the top thread as it tries to go around the bobbin case.

[Ogoki]

Thanks for the detailed response, Wizcrafts!  Much appreciated.