The two most common causes I've encountered with the upper thread shredding or breaking are operator error.

1] As another poster mentioned . . . be sure the needle is inserted the right way. The 'slot' goes on the side away from the bobbin hook.

or

2] The upper thread coming off the spool has somehow become entangled in something [or some part of the machine] causing extra drag on the thread. Or you haven't threaded the machine properly.

I've also used my industrial Pfaff walking foot machine to sew clothing-weight material. Extra care must be used in selecting quality thread, relaxing tensions, and using the proper size needle. I've found too that I need to slow down somewhat to keep from breaking thread. The only serious problem is that if you are working on something delicate . . . the foot pressure, even when minimum, will often mark or damage the work. No wedding dress alterations here! <g>

Bottom line: if you are 'most' interested in medium/heavy projects like bike bags, back packs, etc. you are better off purchasing an industrial machine with a walking foot. It will do clothing better than a 'home type' machine will do bags/packs.

DON