How much backlash do you give when tightening down the bevel gears that drive your hook?
This singer 257 has a pair of bevel gears to drive the rotary hook. The larger of the two (24-tooth) is connected to the (horizontal) arm shaft, and is made of metal. The smaller of the two (16-tooth) is connected to the vertical shaft that drives the timing belt, and is made of plastic. The dark reddish-brown plastic gear had disintegrated before landing with me, and I replaced it with a bright white new plastic gear.
The first time I put it together, I positioned the larger gear right up snug against the other gear (no backlash). But later discovered some howling from the machine at even moderate speeds.
The next time I put it together, I pressed the gears together, then backed off a smidge. Tightened the set screw. Checked for the presence of backlash. Rotated the hand wheel a little bit, checked for the presence of backlash again. Continued rotating the hand wheel a bit through a full two handwheel revolutions, checking for backlash at each stop. Now it seems to have at least a little bit of backlash at all positions. And the howl has disappeared.
I suspect that the tolerances are not as close on molded plastic gears as they are on machined metal gears, but I have no evidence.
Relevant bits from the singer-257 manual:
The manual on pp 8 (Guides for Using this Service Manual) says "The sets of gears are to be adjusted for smooth run and smallest amount of play and black lash"
and on pp 37 (Horizontal Arm Shaft Reassembly) it says "Adjust the gear mesh with eccentric G so there is not play between arm shaft gear and vertical shaft gear. before tightening screw 8"
