friquant

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"

Precaud

Singer's instructions imply that these gears should not apply any pressure on each other. So it sounds to me like you did the right thing by backing off a smidge. Lots of older machines use a plastic gear on a vertical shaft that drives the zigzag mechanism. Some of them are on eccentric shafts which allow the gear mesh to be fine-tuned. My general POV is to use the tightest coupling that allows the gears to rotate freely, without any binding.

friquant

I like this. Having some way to fine-tune the gear mesh would be awesome. I was kind of in guess and check mode.

I did it based on visual observation of backlash (which was harder to do than I expected). To do it again I think I would take your view. That is, set it a little tighter than I think I can get away with, check if it turns smoothly from the handwheel, run it under power and make sure no howling. If any binding or howling, make it a little looser and try again.

Precaud

I hate it when the website software logs me out without notice while I'm typing a lengthy reply.

I think your approach will be fine. Run it at the slowest possible speed, that's where binding will be the most obvious. Howling is more likely to be from the vertical shaft that needs oiling. I'd do that before setting the backlash.

This setup was used on most of the "Stylist" series machines. Singer never did a good job of explaining the maintenance.

It's important to keep SMO and petroleum products off of that gear. Over time they will deteriorate the nylon, causing premature shrinkage and cracking. Only use synthetic grease on it and the nearby gears.

But the vertical shaft bushing DOES need to be oiled... it makes an unnerving howling/groaning noise when really dry!

friquant

Initially I put mineral oil on it, but later when I was satisfied that was operating correctly I switched to silicone grease. I didn't think to clean the mineral oil off first. :thinking:

It's tricky oiling the vertical shaft bushing. I'll have a look and make sure it's wet.

Precaud

It's actually quite easy on this model. There's a little hole in the front casting, it's just visible in your pic. Insert an oiler with a long neck down there and put a couple drops directly on the shaft.